Bottlehead Forum
Bottlehead Kits => S.E.X. Kit => Topic started by: greyhamster on November 19, 2013, 08:14:40 PM
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Hi everyone,
I've just finish my S.E.X amp. All the measurements are within 10% except voltage at C5 is 3.8V (should be 3.15V).
The tubes light up; both at the same time and illuminate quite brightly.
However, there is loud buzzing sound on both channels even at no volume, and there is popping and crackling sometimes. I triple check to make sure there is no lifted ground connection as well as bad soldering joint; and no wire touches each other neither. Please help me I don't know what to do now.
ps: I used it with a headphone.
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I'm curious about this too as mine measured 3.93v @ C5, and -2.5v @ C4. What did you get at C4?
I measured 3.06v AC @ terminal 4, and 3.05v AC @ terminal 5 on the transformer which looks perfect.
For the tubes try reseating them a few times to clean up the contacts, also if you tap them lightly while running can you hear any popping or crackling on the headphones?
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It sure sounds like a bad ground connection.
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Real Expect
A7,B7 -2.49 -3.15
A8,B8 3.74 3.15
C1 -2.51 -3.15
C2 3.77 3.15
C4 -2.51 -3.15
C5 3.94 3.15
15,25 16.9 19
A3,B3 16.5 19
other measurements are within 10%
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I have check every ground connection and re-flow them (not sure if I miss any) but the buzzing hasn't gone away.
Can the plate chokes PC-3 touching the chassis cause the problem?
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Substantial buzzing is signal ground-related. Popping and cracking could be a noisy tube, but most often it is due to a missed solder joint. Often we have posters who say they can't find it, then come back and say it was some connection that was hidden by other components. Is the buzz in both channels? If so it is probably a problem in the power supply. Filter capacitors might be a good place to start looking. If it is just in one channel it is probably in the amplifier circuit around the tubes.
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Substantial buzzing is signal ground-related. Popping and cracking could be a noisy tube, but most often it is due to a missed solder joint. Often we have posters who say they can't find it, then come back and say it was some connection that was hidden by other components. Is the buzz in both channels? If so it is probably a problem in the power supply. Filter capacitors might be a good place to start looking. If it is just in one channel it is probably in the amplifier circuit around the tubes.
Thanks Doc,
I was well aware of that so I had checked many times before posted here.
The buzzing was in both channel and very loud regardless of the position of the volume control. At first I thought it was an easy fix because there should have been a missing or bad ground connection. I was looking, again and again all for night yesterday; re-soldered every possible ground connections.
I have been checking these connections again this morning but nothing is found.
Here are some pictures. I hope you could give me a hint on what to look for next.
http://imgur.com/vCFnvBf,lXAM2Nu,88QE0JF,RuSC1Hf,0YX1P83,hh10Gj1,8VkGTI9
Best,
Dave
Ps: The popping is gone after two minutes so I think the tubes is good.
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Looks like a very clean and careful build. The only thing I can see that might possibly be an issue is that the connection of the black wire that goes from power trans terminal 10 to the center terminal of the T-strip right on front of the power trans might be a little strange at the center terminal of the t-strip. But it's not real clear in the photo and I could just be seeing things. Doesn't help that I left my reading glasses at home today!
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When I built my amp (version 2) I had the same issue. Turned out I did not extend the ground buss wire from pin 3 of the tube socket to terminal 18 and then 17 I one side and 38 - 37 on the other side. That meant the ground was not tied to the chassis plate. When I fixed that the amp was dead quiet.
I can't tell from the pictures if that is your problem but it's worth checking.
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Looks like a very clean and careful build. The only thing I can see that might possibly be an issue is that the connection of the black wire that goes from power trans terminal 10 to the center terminal of the T-strip right on front of the power trans might be a little strange at the center terminal of the t-strip. But it's not real clear in the photo and I could just be seeing things. Doesn't help that I left my reading glasses at home today!
I have checked and re soldered that connection but it doesn't solve the problem :(
When I built my amp (version 2) I had the same issue. Turned out I did not extend the ground buss wire from pin 3 of the tube socket to terminal 18 and then 17 I one side and 38 - 37 on the other side. That meant the ground was not tied to the chassis plate. When I fixed that the amp was dead quiet.
I can't tell from the pictures if that is your problem but it's worth checking.
can you please tell me more about this? I don't think there is any wire from the pin sockets that connects to the ground. And do you mean terminal 12 - 13 and 22 - 23?
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Well someone with the 2.1 version should give you the terminal numbers. Don't know if the 2.1 version is wired the same. My problem was the ground buss was not connected to ground (on both channels) until I found the problem. Your solution might be different than mine but I think Doc's advise to look carefully at every ground point in the amp is wise. The amp should be dead quiet with no music playing.
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Hi everyone,
Wanted to chip in and share some experience on the topic. This is my first post on the forum btw (alhough I have spent quite a bit of time reading posts!).
I've built a SEX amp a few weeks ago. I ran right from the start but I was also having a loud buzz with headphones. I measured about 5-6mV of hum in one channel and 2.5mV in the other (8 ohm tap) on the binding posts with the volume all the way down. Turns out the 5-6mV of hum/buzz was due a bad tube.
After replacing the tube, I still had about 2mV of hum/buzz on both channels. Reflowed nearly all solder joints (starting with ground connections). It seemed to have some effect, hum/buzz went down to about 1.7mV. Removed the felt pads of the PT (the ones under the chassis). Seemed to have some effect, down to probably 1.5mV. I did not intall the C4S upgrade yet.
After something like 100h of use with speakers, the hum is down to 0.6-0.8mV, which is I think is fine with most speakers and about 0.5-0.6mV when the amp has been on for a couple of hours.
I absolutely love the amp with speakers. I'm using a cheap ODAC for now and some OK PC monitors. Even in this less than ideal set up it sounds fantastic. Cannot wait to hear it with Orcas! And eventually with the C4S upgrade.
However, I still get a buzz with low impedance headphones (still on 8 ohm tap). Not super loud but annoying and very quickly tiring.
After doing some research turns out some people have had such problems with low impedance / high sensitivity headphones. I am using some Phonon headphones bought in NYC some time ago which I suppose are relatively sensitive and only 24ohm.
Anyway, my point is that I've spent a couple of nights trying to figure out what was wrong with the amp when in fact I was probably down mainly to the cans... I'm not even sure reflowing solder joints and my other initiatives had any effect at all, it may have simply been the effect of component break in. (In fact when I put new tubes in the amp I tend to have about 2mV of hum which goes down progressively)
Still have a couple of questions though:
- Is 0.5-0.6mV of hum acceptable for a SEX / an indirectly heated triode amp or is it still too much? Btw this is measured with no load not sure if that makes a difference (would have to try again when I have the Orcas). Paul Joppa wrote in a post somewhere that 0.3mV could be achieved although I don't have any details on how this was achieved.
- Is there a way to make the SEX silent enough for low impedance headphones? I understand there were some 120ohm resistors on the jack previously (but the jack was not shorting at the time) - how could I implement that? Is changing filter caps an option? Could also use the 4 ohm tap but I like the flexibility of the 8 ohm tap. And I could also invest in a pair of HD650 and not bother about it...
- Should I put back the felt pads of the PT? Does it make a difference in terms of sound quality? Safety?
In any case, thanks for this terrific piece of equipment! Intructions are super clear and it is a lot of fun building it
+ thanks in advance for any inputs
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I'll had my 2c's as i also have a faint hum i am trying to cure. With HD650's its barely noticeable, but with more sensitive Grado cans it can clearly be heard with no music playing. And as above i have no idea what is considered normal. (maybe i'm just being too fussy)
I've tried two sets of tubes as one of the tubes supplied with the kit has a loud bzzzzt noise all the time. And with the other tube i could hear a faint humming in the background as described above.
With the 2nd set of tubes i am still hearing the faint hum in the background so i'm pretty certain its in the amp and not the tubes that is causing it. Unfortunately one of these two tubes is also slightly noisy and microphonic so i'm trying to source yet another set, hopefully i will be 3rd time lucky.
Volume has no effect, neither does connecting/disconnecting a source to the amp so i can only conclude its power supply related. I re soldered all the ground points, heater supply, and most of the main power supply to no avail. I also measured resistance from the chassis ground point all around the amp and dont see more than 1ohm anywhere (lower limit of DMM). Also tried using crocodile jumper cables to add extra ground points to no avail.
Interestingly i have noticed it getting slightly better as the amp burns in, i wasn't sure if it was my imagination or if i was just getting used to it and blocking it out, but now you say it i'm almost certain that is the case. I probably have ~16 hours play time on it so far.
I never thought to try and measure it, i will give that a go tonight. I might even dig out my scope and see if i can see it.
Another thought i had was cable routing around the output transformers. While i was resoldering i had the amp unplugged (obviously as i'm still here) but i was still wearing the headphones and any time i moved the iron around the transformers i could hear the buzzing noise in the headphones, got me thinking maybe the issue is just the transformers picking up noise from the wiring around them, or close proximity to the heater supply diodes.
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When you say you took the felt washers under the power transformer out do you mean you took the nylon or fiber shoulder washers that go around the mounting screws out? This would be most likely to increase hum, not decrease it.
The 120 ohm resistors are a good idea, You can just remove the wire connected to the "tip" and "ring" terminals of the TRS jack and install a 120 resistor between the end of each wire and its terminal.
And yes, tubes need time to burn in.
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Mark - good to read that you are making similar observations!
Would be very interesting to benchmark noise levels to find out what the "typical" levels are.
Doc B. - thank you so much for your input, I was indeed referring to the nylon/fiber washers around the mounting screws. Will put them back then and report in sometime this week.
Regarding the installation of the 120ohm resistors, I have a shorting TRS jack - is that an issue? I thought the 120ohm resistors were not compatible with the shorting jack (I may be wrong)
thanks
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I'm seeing about 800-900 microvolts of hum on our demo unit here on the 8 Ohm tap unloaded. That number drops to 300-400 microvolts with a 10 Ohm load (most meters get a little inaccurate down this low).
This residual hum will decrease if you move to the 4 Ohm tap, which is a very useful setting if you plan to use high sensitivity headphones, and of course a little bit of burn in time on a fresh set of tubes won't hurt either.
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Thanks PB - am reassured, this is exactly in line with what I'm getting and should only get better over time
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Also potentially worth noting: I hear a pretty significant difference in noise if the RCA jacks are unconnected vs. if they are hooked up to a source component.
Without a source - notable hum, tracks with volume pot.
With a source (how you are always going to use the amp, obviously) - this disappears. We're talking at least 6dB of hum just gone, and this is repeatable. What remains is a tiny bit of noise at the highest volume settings which I am fairly certain is the source, not the amp.
I mention this in case people are testing for hum by plugging headphones in to a source-less amp. Connect a source to the RCAs first! It doesn't have to be playing.
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The best way to do it a power amp or other component without a volume control is actually to short the input. Even if you have a source plugged in you can still get noise from the source component and bad cables. Of course since the S.E.x. amp has a volum control just turning down the volume all the way will do the same thing.
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Done a bit more testing this evening and discovered the hum starts immediately when i power on, then a few seconds later when both tubes warm up and start working it doesn't change in volume. Therefore am i right in assuming that rules out the audio path / amplification?
It also sounds like its ~100hz in frequency, so wouldn't that imply the problem is somewhere around the main power supply section?
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Have you thoroughly checked the whole power supply terminal strip and components -- making sure all diodes are in the correct orientation and are not damaged, filter cap for dc filament supply ok?, and especially the soldering around the joints where the thick wires all come together from the big diodes -- those can suck up a lot of heat and can be difficult to solder well? Also, too much heat here without heatsinks on the component leads can damage the diodes and cap.
HTH,
Jim
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Can you measure the hum level? It's very difficult to help someone determine whether residual hum is normal or not without knowing what the level is.
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I'm not entirely sure how to do this correctly so i measured both ways.
Measuring between the chassis plate and L/R at the headphone jack i see .49v/.48v AC on the multimeter. Between the ground point on the jack and L/R i see .002v AC, i think that's as low as my meter goes so i'll need a better one.
I got the scope out (old analog) and with the earth lead clipped onto the chassis plate i see an AC waveform of ~.34v @20ms (50hz). Between the ground point on the jack and L/R i'm way down at 500mv/div and see an odd looking seesawing ripple ~200mv peak to peak in 10ms divisions (100hz).
Hope i did that right, and thanks again for the help..
Mark
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Done a bit more testing this evening and discovered the hum starts immediately when i power on,
Sure sounds a lot like a bad solder joint in the power supply.
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Are all the filter capacitors oriented properly?
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Ok so i assume those readings aren't right, and point to an issue in the main power supply section. Everything looks ok and has been reflowed but to be 100% i'll use a solder sucker this time and disassemble/rebuild that entire section.
Thanks again,
Mark
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Stripped and rebuilt the HV power supply section (terminal 1-5), and the two power feed posts 10 & 20 which seemed to improved things a little. Good news is the power on noise is much better, after the initial bwong noise of the transformer energizing it all goes quite before the tubes start working so we are making progress.
I think i am now down to tube noise with a light hum in one channel, and a slightly louder buzzing sound in the the other, but the noises switch channels when i swap the tubes around. The question is if the light hum is from the tube or the amp so i will source a replacement set of tubes and work from there.
Cheers,
Mark
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I have a similar buzzing problem after finishing my S.E.X build yesterday. My readings are quite similar to the other users' here, 3.83V @ C5, and -2.38V @ C4, and I have hum noises from both channels. I can live with the (barely audible) noise from one channel, however the noise from the other channel is very audible and annoying, especially with a low impedance phones such as my ATH-AD2000.
After tearing off half of my beard and going through and checking that all the soldering joints all over again, I swapped the tubes and the hum switches channels, so I guess the tubes are the problem here. I'll let my S.E.X play for a while and see if burn in will solve the humming problem. I hope those 6DN7 tubes are easy to find if I happened to have a bad tube.
PH
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PH,
If this kit was a recent purchase Bottlehead will send you a new tube if the noise doesn't settle down after 50 to 100 hours of burn in. You can just leave it on.
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I have a similar buzzing problem after finishing my S.E.X build yesterday. My readings are quite similar to the other users' here, 3.83V @ C5, and -2.38V @ C4, and I have hum noises from both channels. I can live with the (barely audible) noise from one channel, however the noise from the other channel is very audible and annoying, especially with a low impedance phones such as my ATH-AD2000.
After tearing off half of my beard and going through and checking that all the soldering joints all over again, I swapped the tubes and the hum switches channels, so I guess the tubes are the problem here. I'll let my S.E.X play for a while and see if burn in will solve the humming problem. I hope those 6DN7 tubes are easy to find if I happened to have a bad tube.
PH
If you're using low impedance phones, did you wire it up using the 4 ohm taps? If not, that's probably the easiest way to reduce your noise floor. At least on the lower noise channel, where the tube probably isn't the issue.
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PH,
If this kit was a recent purchase Bottlehead will send you a new tube if the noise doesn't settle down after 50 to 100 hours of burn in. You can just leave it on.
Thanks! I'll give the tubes 50 hours before asking Bottlehead for a replacement. I'm living on the other side of the world so getting a replacement is rather tricky...
If you're using low impedance phones, did you wire it up using the 4 ohm taps? If not, that's probably the easiest way to reduce your noise floor. At least on the lower noise channel, where the tube probably isn't the issue.
I wired the amp using the 32 ohm taps, but I installed the impedance switch today and still observe the hum after switching to 4 ohm setting. I think it's definitely a tube problem here.
On the plus side, the amp sounds absolutely delicious even with the hums! I have yet to install the C4S kit so it can only go up from here.
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Just following up on this. I plugged the same 24ohm phonon headphones in my SEX amp yesterday. After 2/3 extra hundred hours of use, suprise: the background noise is almost completely gone! (the amp is still 100% stock and running on the 8ohm tap)
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An update on my S.E.X. After a few dozen of hours my Raytheon tubes seem to settle a little bit, the buzz (caused by one of the tube) seems to be a little less audible, but it's still there if I'm using my low impedance AD2000.
I finally got a set of replacement tubes from the Queen (thanks, Eileen!). Instead of a replacement Raytheon tube I got a new pair of RCA tubes. I plugged the new set of tubes in and yes, one of the tubes gives me the nasty buzz again, while the other one is completely quiet. After a while I got fed up and mixed the good Raytheon and the good RCA and voila, no more buzz! Now I only need to explain to whoever trying my SEX why I got two different tubes on the same amp...
-PH-
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I've not had the best of luck either with these tubes, of the first three sets i bought one of each was unusable, and they were to replace the set that came with it. Gets annoying spending days at a time burning in tubes to find they wont get any quieter. I'm starting to wonder if were scraping the barrel of worldwide supply, or if i'm just unlucky.
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So, to get back to the original information, with these new tubes that are now quiet, are you still getting the weird voltages like 3.9 V and -2.5 V? I am having this buzzing issue and these voltages exactly. I just received 2 new pairs of tubes but I have not tried them yet. Please advise. Thanks in advance.
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I've not had the best of luck either with these tubes, of the first three sets i bought one of each was unusable, and they were to replace the set that came with it. Gets annoying spending days at a time burning in tubes to find they wont get any quieter. I'm starting to wonder if were scraping the barrel of worldwide supply, or if i'm just unlucky.
Did this actually end up being an issue after you installed the resistors at the headphone jack?
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Oh yeah they went beyond the normal background hum, anything from a constant bzzzzt noise to intermittent russling, and one of them kept making loud popping noises that would make you jump out of your skin. They were just bad tubes.
Karma balanced out in the end as i picked up four International Servicemaster coin base tubes from Australia for $3 each which turned out to be re branded Sylvania black plates and completely silent. That totally made my day :)
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So, to get back to the original information, with these new tubes that are now quiet, are you still getting the weird voltages like 3.9 V and -2.5 V? I am having this buzzing issue and these voltages exactly. I just received 2 new pairs of tubes but I have not tried them yet. Please advise. Thanks in advance.
Just measured again, +3.61dc and -2.49dc, with 2.99vac/3.00vac at the transformer. Pretty sure they would read evenly with no load on them.
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This has nothing to do with noises in the amp.
(if you don't believe me, unground the center tap and ground one leg of the 6.3V supply)
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We believe you Paul, it just throws people off as the voltage check list in the manual says it should be +3.15 and - 3.15, hence people query it.
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In the SEX 2.1 (with PT-7 power transformer) the heater power is only connected to ground for two short periods during the 60Hz power line cycle - just when the diodes are conducting. Most of the time it floats. For that reason, if there is a little leakage between heater and cathode, the voltage will drift away from symmetrical. You can put a couple 100-ohm resistors in series across the DC power and ground the center point if you want to see matched voltages; you are not however going to hear a difference unless the leakage is so bad the tube should be replaced.
The center tap is grounded so that the winding itself is always grounded. This allows it to act as an electrostatic shield for other windings.
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Paul,
I just tried this out of interest and it does make a difference, enough that i am going to have to wire it in permanently.
I initially tried it with just the scope connected and could see the sharp peaks were knocked off the noise with the resistor center point grounded, i then plugged in a pair of headphones (also before the 120r resistors at the headphone socket) and i could clearly hear a reduction in the higher pitched bzzzt noise any time i connected the ground. Voltages evened out a bit from +3.6/-2.4, to +2.9v/-3.1v, though i find it curious they swung the other way to be slightly more negative.
Before i warm up the soldering iron is it worth experimenting with small value caps across the heater circuit, or directly on the transformer secondary to help further reduce any switching noise?
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Interesting - thanks so much for posting!
My best guess at this point is that it's common-mode noise. Capacitors aren't going to help unless they have lower impedance at 60Hz than the two 100 ohm resistors, which are effectively in parallel. 50 ohms at 60Hz is 53uF, that's getting pretty silly.
You can replace the resistors with a pot, probably anything from 50 to 500 ohms will do. You'll want a 3-watt or greater rating. And make sure the tubes are the same make - some have series heaters, so the best balance will be off center for those. (Also make sure they are the same phase - pin 7 to pin 7, 8 to 8.) Adjust it for the lowest hum.
The Schottky diodes don't have reverse recovery spikes, but they do still have switching noise. So it's possible that a common-mode choke will have an effect. If you put one between the transformer and the rectifiers, it will reduce coupling to the other windings (i.e. the HV winding). Putting one between the DC filter and the tubes will have more effect on what noise gets to the heaters. I have, at this point, no idea which would be more effective, or whether either would be audible at all. But it's a standard approach, so there's a good chance.
Before getting too carried away, do consider getting a second set of tubes. I imagine that different tubes, especially different makes, might have different sensitivity to heater noise coupling.
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My best guess at this point is that it's common-mode noise. Capacitors aren't going to help unless they have lower impedance at 60Hz than the two 100 ohm resistors, which are effectively in parallel. 50 ohms at 60Hz is 53uF, that's getting pretty silly.
I was thinking smaller in the 10-100nf range directly on the secondary winding as i've seen it on various schematics with the explanation of preventing switching noise from propagating back through the transformer into adjacent windings.
You can replace the resistors with a pot, probably anything from 50 to 500 ohms will do. You'll want a 3-watt or greater rating.
I will look into that, from the bit of research i did i found this schematic you posted a few years back, http://usr.audioasylum.com/images/0/5625/FC-1_application_300B.GIF (http://usr.audioasylum.com/images/0/5625/FC-1_application_300B.GIF) , i'm curious as to the purpose of the RC filter, whats the advantage to that vs directly connecting the center of the pot to ground?
And make sure the tubes are the same make - some have series heaters, so the best balance will be off center for those. (Also make sure they are the same phase - pin 7 to pin 7, 8 to 8.) Adjust it for the lowest hum.
Tubes are always identical pairs, i'm a little OCD about that. I only have one series wired tube which is one of the Zeniths supplied with the kit, i never used that set as they were very noisy, but i'm now wondering if that is because one is series wired and the other is parallel wired. I cant remember which of the two had the problem so i will need to retest them.
The Schottky diodes don't have reverse recovery spikes, but they do still have switching noise. So it's possible that a common-mode choke will have an effect. If you put one between the transformer and the rectifiers, it will reduce coupling to the other windings (i.e. the HV winding). Putting one between the DC filter and the tubes will have more effect on what noise gets to the heaters. I have, at this point, no idea which would be more effective, or whether either would be audible at all. But it's a standard approach, so there's a good chance.
I'm curious so i'll keep an eye on ebay for something common-mode and capable of handling a few amps to experiment with. Unless something salvaged from a computer ATX power supply would be suitable, i have a box of those toroidal style chokes here that are rated for a few amps at 240v.
Before getting too carried away, do consider getting a second set of tubes. I imagine that different tubes, especially different makes, might have different sensitivity to heater noise coupling.
I have a bit of a collection at this stage of various different brands, interestingly one of my tubes started making an intermittent russling sound last night, i'm not sure if its just coincidence or if i finished them off while testing yesterday as i had the amp powered on/off a few times during the process. I went back to an older set that i had it my bad pile marked as "slight hum" and so far they are silent, i need to go back and re-evaluate my collection of bad sets to see if they will work fine now.
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Guys, I am still having this problem. It is a loud buzzing sound more in the left channel than the right but the volume doesn't change with the volume control. I have tried 3 different sets of tubes and my voltages are -2.5, 3.7, -2.5, 3.7 at C1,2,4,5 and A7,8, B7, 8. All other voltages and impedances are perfect. I have gone over every terminal and resoldered any that had even a hint of error. I don't know what the problem could be. Please help. Thanks in advance.
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Sorry for the confusion, there are two thread in this thread and I do get them mixed up.
Can you quantify "loud"? Like, by measuring the AC voltage at the speaker terminals? I understand that not everyone has a meter that will read a millivolt or less - I'm just trying to guess what candidate problems to look for. If your meter will do this, it could help.
If you have some clip leads (cheap 10-pack at Radio Shack; very handy!) you can clip the power section grid (pin 1) to ground. If it still hums the same amount, it's in the power section, if not then it's coming into the driver section. The 450v caps are the respective filter caps to look at - 47uF for the power section and 22uF for the driver. Check for orientation, possible bulging, and good solder joints.
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From the left channel terminal I am getting 144 mV and the right channel terminal I am getting 5 mV. This seems high so I may have done it incorrectly. Black lead on black terminal and red lead on red terminal? Once it warmed up it went to 39 mV and 6.6 mV respectively. This is so discouraging. Thanks again.
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Guys, I am still having this problem. It is a loud buzzing sound more in the left channel than the right but the volume doesn't change with the volume control.
This is likely a loose solder joint in the power supply, probably around the 22uF capacitor on the left channel.
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Ok, I am still at it but the problem seems to have shifted slightly. Now when I power it up there is a faint hum. After a few seconds it starts to get louder until it gets just as loud as it was before but now the sound is approximately equal in both channels with headphones. Without the headphones plugged in, I measure 6.6 mV on the left channel and 5.4 mV on the right. I have gone through and resoldered every terminal that looks less than perfect, especially those around the power supply capacitors and it is still buzzing. Please give me more suggestions. Tell me what else I can do. I am dying to hear this thing playing music. Thanks again in advance.
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OK then, back to my previous suggestion to ground the grid of the power section of the tubes and see how much noise is left.
In addition, you have used the terms "hum" and "buzz" in this thread. Hum means it's a nearly pure low frequency tone, usually 120Hz - not deep bass but unquestionably in the bass. "Buzz" means the tone has a lot of harmonics and a raspy nature; what we used to call buzz-saw back when there were such things. This is a useful diagnostic - power supplies with some filtering but not enough will give a hum, while capacitive coupling such as heater to cathode coupling is biased towards the higher harmonics and has more of a buzz character. Can you sort out what you are hearing?
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Ok, first things first. I did not have clip leads so I temporarily soldered in some hookup wire from each of the tubes pin 1 to ground. With that in place, it is virtually silent, only the faintest of hums. So I removed the temporary leads and resoldered the 22uf cap terminals as well as every other spot around the tubes sockets. I then put the tubes back in and tried it. Same sound, same volume, 5.4 mV left and 6.6 mV right which is what I meant to put in my previous post. Anyway, I tried a second set of tubes and the sound was still there, but lower at 3.7 mV left and 3.8 mV right. Then I tried the tubes that came with the kit, coin base Sylvanias and the sound went to 6.3 mV left and 3.7 mV right. As to the sound, it sounds to me as if it is a buzzing sound, not a hum. There are definitely variations in the harmonics. At this point, I think the tubes may be playing a role since the noise has changed in volume with each set of tubes.
Also, the tubes that measure 3.7 mV really are fairly quiet and loud sound could be due to the sensitive headphones I have 105 db at 44 Ohm Shure SRH440 for testing anyway. It worries me though because I have Denon D5000 headphones and Klipsch Heresy 1.5 speakers and I don't want that sound to color my listening. What is normal hum/buzz with the SEX? Will it decrease significantly with the C4S upgrade which I have yet to install? Thanks again.
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THIS POST HAS BEEN MODIFIED TO CORRECT AN ERROR IN THE SECOND PARAGRAPH!!
Great data! That really helps; it's not the high voltage power supply (except possibly the last stage with the 22uF capacitor) and it's not the power stage. So it's coming in at the first stage. And it's buzzy, so there is probably a capacitive coupling going on. And it's in both channels - it's always possible, but very unlikely, that there is the identical fault in both channels.
My best guess at this point is that the 6.3v winding, which is common to both channels, is not properly grounded. You can check the DC resistance from power transformer terminal 10 (the 6v winding centertap) to the chassis. It's in the middle of page 22 in the manual. I'd check the resistance from the G terminal of the IEC inlet to the chassis as well, since it's right there. In both cases, check the transformer or socket terminal to the actual chassis, which tests all the mechanical and soldered connections. Check the resistance from power transformer terminal 10 to power transformer terminals 1 and 2 4 and 5, to be certain that terminal 10 is connected internally in the correct way - I'm trying to cover all the bases here!
If that does not make a difference, then it's back to that 22uF capacitor. The lead that attaches to 12U is grounded by the wire from 12L to 13L, and 13 is bolted to the chassis as the single point connection of signal ground to chassis ground. So check the resistance of that capacitor lead to the chassis itself.
11L and 11U are presumably good connections since the high voltage power does get to the tube, and you are certain about 11U. The only way this could fail is if at 11L the two resistors are connected but the joint to the lug itself has failed. You can check the resistance from the positive lead of the 22uF to either resistor (8.2K to T6 or 150K to A5).
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Paul, thank you so much for the help. I checked the resistances around terminal 11 and the 22 uf cap and all checked out. I resoldered the ground connection from terminal 10 on the power transformer to the terminal strip 3. However, I do not know what the different resistances within the power transformer should be so I couldn't check that yet. Can you clarify that second paragraph for me? And can you clarify what an acceptable level of hum is for the SeX without C4S? Thanks again.
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Paul and others, thank you very much for your help so far. I am however still in the thick of it. I am hoping to get that clarification about what those resistances should be around the PT-7 as well as a good explanation and a picture of the 120 Ohm resistors on the headphone jack. I would love to know some more details about why one would want them, whether they will be ok if I use speakers more than headphones, and exactly how to install them. Thanks again.
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Paul, thank you so much for the help. I checked the resistances around terminal 11 and the 22 uf cap and all checked out. I resoldered the ground connection from terminal 10 on the power transformer to the terminal strip 3. However, I do not know what the different resistances within the power transformer should be so I couldn't check that yet. Can you clarify that second paragraph for me? And can you clarify what an acceptable level of hum is for the SeX without C4S? Thanks again.
THIS POST HAS BEEN MODIFIED TO CORRECT AN ERROR IN NOTE 4)!!
Normal noise for a SEX amp is 0.3mV at the speaker terminals.
Second paragraph:
1) My best guess at this point is that the 6.3v winding, which is common to both channels, is not properly grounded. You can check the DC resistance from power transformer terminal 10 (the 6v winding centertap) to the chassis. It's in the middle of page 22 in the manual.
--- Properly grounded means zero resistance between terminal 10 of the power transformer and the chassis plate. See note number three below. No transformer resistance is involved. Note that you will not be able to measure with precision fractions of an ohm.
2) I'd check the resistance from the G terminal of the IEC inlet to the chassis as well, since it's right there.
--- Sorry, on the current IEC the terminal is labelled E (for Earth) rather than G (for Ground). My bad, I didn't look it up. There are three terminals on the IEC socket, labelled E, L, and N (for Earth, Line, and Neutral). There is a wire from terminal E to the chassis ground lug, as described on page 20 of the manual. check the resistance between the terminal E and the chassis; it should be zero. See note number three below. No transformer resistance is involved. Note that you will not be able to measure with precision fractions of an ohm.
3) In both cases, check the transformer or socket terminal to the actual chassis, which tests all the mechanical and soldered connections.
--- Do not re-solder. Do not connect the meter to the solder. Do not connect the meter to the wire. Connect one meter probe to the actual terminal itself. Connect the other meter probe to the chassis plate.
4) Check the resistance from power transformer terminal 10 to power transformer terminals 1 and 2 4 and 5, to be certain that terminal 10 is connected internally in the correct way - I'm trying to cover all the bases here!
--- The actual transformer resistance of this winding is too small to measure. Meters that can distinguish such small resistances from zero cost $2000 and up. So just look for as close to zero ohms as you can measure! Note that you will not be able to measure with precision fractions of an ohm.
Clear now?
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Yes, thank you very much Paul. That clarifies things nicely for me. I will check that later tonight and report back with my findings. Thanks again.
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Ok, I finally got those new measurements done. The resistance between terminal 10 and the chassis plate started at 0.1 Ohms and climbed up to 1.3 Ohm before dropping down and hovering between 0.4 and 0.9 Ohms. When I checked again after a couple minutes it hovered between 0.3 and 0.7 Ohms.
The E terminal on the IEC resistance to the chassis plate hovered between 0.2 and 0.5 Ohms. Finally, the resistance between terminal 1 and 10, 2 and 10 on the PT did not even register on my meter, just showed O.L. I am assuming that the fluctuating resistances are a bad thing but I don't know why. Is it a bad connection? Solder joint? PT? Thanks again for the help.
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THIS POST WAS BASED ON AN ERROR OF MINE AND IS CANCELLED!! See later apology.
Bingo! I think, anyhow. Looks like the terminal 10 is not connected to the center tap of the 6.3v winding.
First off, all resistances less than 1 ohm are probably zero, it's just hard to get an accurate measurement. Those are probably not the problem.
You can fake a good grounding, or ask Eileen (email the spare parts address) fora replacement transformer.
If you fake the grounding, it will confirm this to be the problem, so it might be interesting whether or not you ask for a replacement transformer. The easy way is to connect T1 (or T2, your choice) to T10 of the power transformer. Leave the connection of T10 to the chassis.
A better way is to use two 47 ohm resistors, T1 to T10 and T2 to T10. But the easy way will at least answer the question definitively, and will probably be good enough that no further effort is needed.
I should note two things:
1) If the easy way causes the fuse to blow, then your measurements of T1 to T10 and T2 to T10 were incorrect and we're back to Square Zero.
2) If either method works and gives low hum (finally!!) then you deserve a new power transformer if you want it.
If you can get a good high resolution close-up photo of the transformer T10, showing any internal transformer wires, that would be informative for us - but no sweat if that's not easy to do.
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I would also mention that it's possible that you accidentally snipped off the wires that connect to T10 on the power transformer. If this is the case, you can reconnect them with some solder.
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Ok, I will try to get a good shot to show T10 to you guys. I am reasonably certain I did not snip those wires but I will definitely check. Barring that, I will try the 47 Ohm resistor way tomorrow and see what I come up with. Thanks a lot gentlemen. Have a great night.
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I get the same results as Mauro, T10 to IEC/chassis ground is .1ohm, but T10 to T1 or T2 has no continuity?
Are we talking about the right terminals here, T1 and T2 being the 240v mains side?
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I do not know the differences between the 240v mains and 120v mains versions. I am going to try to diagnose the problem tonight with the 47 Ohm resistors. Thanks again.
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Crap! I apologize, guys, I screwed up the terminal numbering. I'll go through and try to fix my posts.
The transformer terminal numbers are embossed on the bobbin. They go 1-2-3-4-5 across the top (the back edge of the amp) and 6-7-8-9-10 across the bottom. (Every other electronic component goes clockwise from below; transformer bobbins are non-standard).
Terminals 4 and 5 are the 6.3v heater winding, and terminal 10 is the center tap on that winding. You should have continuity from T10 to T4 and from T10 to T5.
Again, sorry - my bad.
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Ok, that makes sense. I got nothing when I connected the resistors between 1, 10 and 2, 10. I will try with the proper terminals after I check the internal connection. Thanks for the clarification Paul.
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Well, I thought we had the problem found but no such luck. There is continuity between T4 and T10 and T5 and T10. Basically 0 Ohms resistance. Back to square one as they say. Ground issue probably but I don't know where to go from here. Thanks in advance.
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I plan on working on this tomorrow but does anybody have some advice on where to start, if there are certain spots that should be looked at first, resoldered, reflowed, anything? Thanks again.
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Just reading back through your posts i noticed you have a couple of sets of tubes. I cant believe you got coin base Sylvanias with the kit, you lucky bar stool! The black plate Sylvanias are the best tubes IMO.
Of the sets you have, pick the best of them and let them run for a couple of days. I have roughly a dozen sets and i have found most of them start of very noisy to the point of being unusable but after a day or so constant use they quieten down. Some never do, but its very rare to find a pair that are quite right out of the box.
Its hard to say if you have just reached the natural noise floor of the amp, just have noisy tubes, or if you have a problem. I found that differentiation difficult at the beginning. The voltages you posted seem so high its either noisy tubes or a problem IMO, so as above leave a set of tubes burning in to rule out the tubes.
If you are re soldering i would double check all the grounding posts as somebody mentioned before the fact they are connected to the chassis plate makes them a great conductor of heat so they are hard to get a good solder on. Specifically posts 3,13,23. I would also double check all the caps on 1-5 and 6-7, and 16-17 for the power supply section. I also found the plastic insulation on the wires really annoying as it melts so easily and if your not careful you can end up with a bit of it in your solder contaminating the joint. If its not shiny i would suck it out and re solder rather than just re flowing.
Going beyond the stock build, adding the two 100ohm resistors in the heater supply that Paul suggested a few pages back made a big difference to mine so its worth trying. Basically run a resistor from C1&C2 to C3 creating a grounded center point. That would probably have been enough for my HD650's but with sensitive 32ohm cans i had to put 120ohm resistors into the headphone socket. Basically wire them in series to the left/right channel. i.e. audio signal passes through them. Ideally i wouldn't use them, but i really had no choice to get away from the noise floor.
If you get that far and your still not happy i suggest reading through my thread as i made a few other modifications that all helped in their own small way, but for now just stick to the above and get it working as it should be. Its IS worth it though when all said and done, my Grados and this amp are a match made in heaven.
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Thanks very much for the advice. Unfortunately, those coin base Sylvanias are the worst set of tubes I have. Granted, the other 2 sets are full base GE, but I do have 3 more sets coming from The Tube Store. I like the way the full base tubes click into the sockets, seems more solid almost. Anyway, I have the SEX amp running right now with some low level tunes and some old crappy Sony headphones. I may add the resistors but I do not have any 100 Ohm until tomorrow. I will report back with the results when I am finished. Cheers.
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I don't know if this was covered earlier in this or other threads but on page 22 of the S.E.X. manual a ground wire is attached from 17L to 22L with instructions to not solder. I didn't see instructions to solder 17L anywhere and found it loose on the 3rd or fourth check looking for the 1 ch hum/static I had.
Karl
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Question for Paul. Is it necessary to use 100 Ohm resistors for the C1 and C2 to C3 grounded center point or can one use similar values there?
Also, I have about 30 hours on this set of tubes now and there is no difference whatsoever to the buzz. Still measures at 3.7 and 3.7 mV.
As for the 120 Ohm resistors on the headphone jack, can someone explain which wires they go in series with? I don't want to mess anything up.
Thanks again.
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Question for Paul. Is it necessary to use 100 Ohm resistors for the C1 and C2 to C3 grounded center point or can one use similar values there? ...
The value is not very critical. A 1/2 watt rating is adequate above 68 ohms, and I'd guess 200 ohms is too much. I've not done any experiments myself, so all this is "educated guessing".
3.7mV is way too much, and the fact that it's the same on both channels gives me some doubt whether it's the tubes. So far this is still a puzzler. You say have new tubes coming, and you can do the resistor test - if neither of those does the trick we'll have to dig a little deeper.
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I agree, highly unlikely you would get the same noise level from two different tubes after that much use so i think we can rule out the tubes as a source. There has to be something fundamentally wrong, looks like were back to basics on this one..
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So my amp is now silent, but playing beautiful music as I type this. I finally took it to the one electronics guy in my city, who worked on my turntable and NAD 3020 last year. He and I did some troubleshooting together and connected it to an oscilloscope. Wow, was there ever a lot of ripple on this beast. So, instead, of doing the grounded center point with the 100 Ohm resistors, we tweaked the power supply by adding capacitance and resistance and balancing it between the two legs. Each leg now has 10000 uf and .1 Ohm resistance. The voltages levelled out to about positive and negative 3.12 v for the filaments and there is now very little ripple. With my junky sony headphones, there was no hum or buzz at all. With my heresies, the same. Unfortunately, with my D5000, there is some hum, not horrible, but I will add the 120 Ohm resistors in series to mitigate that soon. I will also post a couple pictures and the updated schematic for what we tweaked in the next day or so. Thanks again, everyone, for the help and great advice. Cheers.
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Very interesting, and a contribution to our knowledge base - thank you very much! I look forward to the schemo posting.
This has been a real puzzler; with this knowledge in hand (plus a few other recent posts) it is at least clear that the heater supply noise is more of a contributor that we thought, and/or than it was ten years ago. Perhaps power lines are getting noisier, or the available tubes are getting to be of lower noise immunity, or perhaps something that hasn't occurred to us yet.
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Looking forward to seeing this schematic.
While testing i found the added capacitance made no difference to the output noise, stock had something like 1.1v ripple vs .6v when i doubled it. In the end i put a 22000uf Panasonic TS-UP cap in there anyway.
Centering the rails via resistors did make a big difference, and with a variety of different tubes. So i believe this is the key element here, sounds to me like your guy just did it a different way by using caps between each terminal and the center ground. Please post a few pics too so we can see how he did it..
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This is a picture of the handwritten schematic. I will post a pic of the actual section when I can tear myself away from listen. Maybe later today.
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These are the pictures of the modified power supply.
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And another shot of the whole thing.
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Excellent - really clever approach! It keeps the filament power grounded at AC and centered through the big 4700uF caps.
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Yes, I was surprised with the remarkable difference in noise from before and after this change. Like night and day. Needless to say, I was also very pleased with the result.
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Thanks for posting your findings and the schematic. Its good to know i'm not the only one who had this issue with noise and solved it this way. I don't think people believed me on how bad it was.
I'm going to experiment with your version as you ended up with the voltages better centered than mine, and probably quieter because of it. I suspect having the resistor on each polarity is why.
Enjoy!
Mark
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I hooked up the amplifier to a distortion meter today and came up with some great results. My SEX measured 6% THD at 1W and 2W RMS. The signal to noise measured between 89 and 90db. It did even better than I thought it would, thanks to my power supply modification performed by Donovan Stenger of Can-Tech Microsystems here in Brooks, Alberta. He enjoyed working on it so much that he is thinking about building tube amplifiers in his spare time.
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Curious what the air quality is like theses days in Brooks? ;)
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Better than some, worse than some others. Luckily, where I live in town, is out of the normal wind path from that place you are alluding to. Have you been here before, Larry?
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Back in the late 80`s i bought a new motorcycle there, lived in Cowtown for a stretch, but thats what I remember about Brooks, the smell. :o
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Most days, there is no smell but once in a while, because of the packing plant and the feedlots attached, it stinks pretty bad. It's not a bad place to live, but certainly there are few people here that share my love of diy tube audio.
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Just reposting the schematic and pictures.
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Now pictures