Bottlehead Forum
Bottlehead Kits => Legacy Kit Products => Topic started by: atalcott on June 30, 2020, 02:17:38 PM
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A little while back I picked up a Foreplay I kit that had been sitting in someone's closet for the last 30 years. I'm just getting starting with tube electronics, and putting it together was a great learning experience. It came with the Anticipation and Sweetest Whispers upgrades, though I only installed the former. Once I had it running in the original configuration I started thinking how I could make it better for my tastes and purposes. I built a new case for it out of thinner walnut with a lower profile, moved the PT underneath the chassis inside the case, moved the input and output jacks to the back of the case, replaced the dual volume pots with an Alps Blue Velvet stereo unit, added a choke in place of the second resistor in the CRCRC network, replaced the coupling caps with Solens, added 100k resistors at the outputs to attenuate the output level a bit (without them the gain was just ridiculous when matched with my Lafayette KT550 power amp) and got a matched pair of Tung Sol reissue 12AU7s.
The thing sounds great, but it's got a pretty pronounced 60Hz hum. I tried to replicate the grounding scheme as much as possible when moving to the new chassis, but with the move of the i/o jacks I did have to make some changes. Like I said, I'm more or less a complete beginner, so I'm not sure where to start with tracking down the hum. It’s in both channels, is unaffected by input source or volume control, though there is an additional, slightly louder 60hz buzz over the top of the hum when I touch the volume pot. I’ve also tried moving things around (various wires, the Anticipation circuit boards) with a chopstick and no movement seems to make any difference in the volume of the hum.
Any suggestions for things to look at? Any obvious mistakes I've made with the grounding scheme? Pics attached. Thanks very much for any input.
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One widely used mod from back in the day was to tie the signal ground to the chassis ground. If I recall correctly, that connected terminal 13 to 14 - but it's been at least a couple decades so I might have mis-remembered. It solved most hum issues.
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Thanks Paul. I just tried that out but no luck I’m afraid.
In the original setup signal ground goes to the grounds for the volume pots, the CF/VA boards, the cathodes of the first section of each tube, and the heater center tap of socket B (pin 9), then to terminal 13, and finally a line ran from there to terminal 1. I put a jumper from 1 to 3, connecting the circuit to chassis ground at the first filter cap ground, and there only. With that configuration, I didn’t get any hum.
Now the signal ground goes to a jumped dual terminal strip near the volume pot. That strip is connected to the grounds for the volume pot, CF/VA boards, the cathodes of the first sections of each tube, and the heater center tap of socket B. Finally, a line runs from that jumped terminal back to terminal 1.
To my thinking this should be effectively the same as the original, but maybe I’m missing something. The only effective difference I can see is that now the output signal ground comes after the input signal grounds.
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Well, it was worth a try :^) Hopefully PB will join in; he has the most experience fixing these things. Meanwhile:
* If you haven't already checked the resistances and voltages, it's always a good thing to do first to be sure it's working properly.
* The second thing is to re-flow every solder joint, making sure the solder flows and wets both wires and terminal lug. It takes only a little time, and solves a great majority of issues without wasting a bunch of time figuring out where the problem is.
(N.B. If you are using the original single-crystal copper ground wire, it had an amazingly strong enamel insulation that makes soldering difficult worth checking each joint. Usually just re-soldering does the trick, but this wire sometimes needs serious scraping.)
* The third thing is to isolate the preamp from other components and cables as much as possible - shorting plug at the input are most helpful, but turning the volume all the way down is often nearly as good.
* Finally, posting some photos of the wiring is a big help.
I don't understand your 100K output resistor - can you clarify?
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With that configuration, I didn’t get any hum.
So...problem solved?
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Very nice and very original take on the vintage BH kit.
Karl
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I looked at the photos again more closely, and I see the 100K resistors, along with some resistors to ground whose color code I can't quite read - are they the original 470K? If so, the combination increases the output impedance from 1K to 82K - way too high to drive cable capacitance, and can much more easily easily pick up hum by capacitive coupling. Also, the resulting attenuation is very small. You could change the 470K to 10K, which will reduce the output impedance to a hair less than 10K, and will reduce the output by 20dB - including any hum.
The wiring layout of the original was not that great, and yours is somewhat worse, primarily because the path is longer. Ideally, the signal and ground wires should be bundled together all the way to the tube and then to the output jacks - the idea being to minimize the loop area enclosed between them. (I see that you have done this with the Anticipation boards.) That will be a big job, so try the 10K first!
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So...problem solved?
Sorry, that was unclear I guess. What I meant was that I didn't have hum in the original configuration, the first one I described. I do have hum in the second configuration though.
I looked at the photos again more closely, and I see the 100K resistors, along with some resistors to ground whose color code I can't quite read - are they the original 470K? If so, the combination increases the output impedance from 1K to 82K - way too high to drive cable capacitance, and can much more easily easily pick up hum by capacitive coupling. Also, the resulting attenuation is very small. You could change the 470K to 10K, which will reduce the output impedance to a hair less than 10K, and will reduce the output by 20dB - including any hum.
Yep, those are the original 470k resistors. I saw a couple other posts here on the forum about attenuating the output like that, but I didn't see anything there about adjusting the output-to-ground resistors, so thanks for the suggestion! Sadly, I replaced the latter with 10ks and though that does seem to have improved the bass response a bit it had no effect on the hum. The gain is still a bit much for the power amp--at 1/4 volume it's already way louder than I'm ever going to use--but it's workable. Really, the Foreplay probably isn't the best match for the fairly sensitive KT-550. But even with the hum and some imbalance in the voltages at the #1 sockets (I had to replace a couple of the LEDs and I think the forward voltage on them doesn't match), the Foreplay sounds way better than the $100 SS headphone amp + pre-out I've been using as a volume control so far.
The wiring layout of the original was not that great, and yours is somewhat worse, primarily because the path is longer. Ideally, the signal and ground wires should be bundled together all the way to the tube and then to the output jacks - the idea being to minimize the loop area enclosed between them. (I see that you have done this with the Anticipation boards.) That will be a big job, so try the 10K first!
I re-checked the resistances and voltages and all is well there (except for the imbalance I mentioned above, but I don't think that would be causing hum?). I reflowed all the solder joints, and also installed a 3-prong plug with the ground connected to terminal #3. Alas, no joy with any of it.
I'm not opposed to tearing down and revising the wiring layout. This is all part my process of learning about tube circuits and aside from just getting rid of the hum I'd like it to be right. Can you expand on the "big job" changes I might make in the wiring layout, given the custom chassis? What would be the ideal layout?
Should I be thinking about faulty components yet? I know these were good tubes since they came from a very reputable source, but who knows. I also wonder about the buzz over the top of the hum when I touch the volume pot, which does vary in volume with the pot position. What could that indicate?
Thanks again for all the help. Despite the frustrations, it's all a blast.
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I also wonder about the buzz over the top of the hum when I touch the volume pot, which does vary in volume with the pot position. What could that indicate?
That tends to indicate that the metal body of the pot ins't earthed, which will manifest as that kind of issue.
What power amp are you using and what speakers?
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I see, that makes sense. Because the pot is mounted right in the walnut its body is indeed not grounded. That’s easily fixed at least.
The power amp is a 50wpc PP 7027A Lafayette KT-550 and the speakers are ADS L520s.
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Yep, those are the original 470k resistors. I saw a couple other posts here on the forum about attenuating the output like that, but I didn't see anything there about adjusting the output-to-ground resistors, so thanks for the suggestion! Sadly, I replaced the latter with 10ks and though that does seem to have improved the bass response a bit it had no effect on the hum.
OK, it looks like we are still at square zero. I would expect the 10K to affect the treble slightly, but not the bass. And it would certainly make a substantial reduction in hum that's generated upstream of the output jacks. Does that mean it's generated in the power amp? ... there's no certain way to know without gathering more information and thinking logically about it.
At a guess, there is something wrong with the ground connection - could be cables, could be RCA jacks, could be internal wiring, could be something I didn't think of. I suppose a simple test is to see whether the hum is present with the Foreplay turned off, or with the tubes pulled out, or both. For each case, check the hum at full volume and with the control all the way down. If your meter has an AC voltage range, try to measure the hum voltages as you do these checks. That might guide you in the process of checking the wiring. There's no point in fixing small things that might be an issue until we fix the big thing(s) that are certainly an issue.
This may well be a long and frustrating project, but I assure you that the longer and more frustrating it is, the more you will have learned by the end!
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Well, I'm pretty used to troubleshooting and frustrations from my work as a software engineer, so bring it on!
I could be wrong about the effect on the bass. I had the impression it was stronger, but it sure wasn't a proper test and that impression could have been due to where I was sitting, the song I was listening to, etc. Anyway...
BTW, grounding the body of the volume pot did get rid of the buzz. Is what's happening there that a small signal from my hand is conducting into the body of the pot, and since there's no path to ground it induces that signal in the signal path?
I think you may be right about the grounding and I've played around a bit with it this morning and here's what I've observed.
- There is no hum in the speakers when I connect this other preamp I have to the power amp using the same interconnect cable.
- With the Foreplay connected to the power amp and the Foreplay powered down there is also no hum.
- With the Foreplay connected, when I power it on the hum is completely absent at first, then gradually comes up to volume as the tubes start conducting. When I power off the Foreplay the hum disappears immediately.
- The volume control has zero effect on the hum. It's exactly the same at zero volume and at full.
- The hum is exactly the same in both channels.
- I'm reading zero AC voltage at the output jacks of the Foreplay when it's plugged in.
- With the interconnect disconnected from the Foreplay, I can easily induce very similar hum by holding the jacks close to an AC source like a power cord or my DAC.
- Plugging the interconnect into the Foreplay (or the other preamp for that matter) while the power amp is on creates a lot of loud noise--hum and pops--as the connection is made
I have a sense from these latter two items that the issue might actually be with the grounding of the RCA inputs on the power amp?
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I would put a 3 wire power cord on your Foreplay. Earth the chassis plate with the power cord.
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I would put a 3 wire power cord on your Foreplay. Earth the chassis plate with the power cord.
Thanks Paul. I have already installed a 3-prong cord, with the ground going to the mount of the first terminal strip (Terminal 3). It didn’t make a difference I’m afraid.
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You have the chassis earthed back by the power supply, I would move the ground connections for the power supply caps off the chassis terminal and to the power supply negative terminal instead, then break the connection there between the PSU negative and chassis earth.
Move the ground wire that you have to the other side of the chassis so it runs up front around the two blue power supply caps.
Earth the signal ground up at the front of the chassis, probably best done on that 3 lug strip you have up front.
I can't see the second power supply filtering resistor in your build...
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Awesome, thanks for the suggestions. I'll try these maneuvers today/tonight and report back.
That Hammond 155J choke is in place of the second power supply resistor.
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No joy. After rerouting the grounds for the power supply caps and the terminal where the tube grounds attach as you describe I tried a few different configurations with the chassis and signal grounds. Without the ps grounded to the chassis things got way noisier, varying from very loud 60hz hum with the power cord ground connected to the chassis, to hum+buzz without it attached. Reconnecting the ps ground to the chassis at terminal 3 got it back to its usual very quiet hum. I tried connecting the signal ground to the chassis at the first coupling cap terminal strip and then back to the “tube-ground†terminal, but the hum was the same in both configurations.
Things are getting messy enough in there at this point that I’m definitely looking at a full tear down and rebuild once I figure out a layout that gets rid of the hum.
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Does your power amp have a 3 wire power cord?
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Does your power amp have a 3 wire power cord?
It does not, no.
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Well, adding a 3 wire cord won't hurt anything.
I finally found a diagram of the KT-550 and it's a very high gain amp (30+dB). With everything powered up and running, what AC voltage do you get at the speaker terminals?
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The KT-550 manual rates the input sensitivity at 1v RMS for 50 watt output. The pre-amp that was designed to go with it was the KT-600. Schematic here: http://www.mcmlv.org/Archive/HiFi/LafayetteKT600.pdf.
With everything up and running and no input signal, just the hum present, I'm reading 0.000VAC with my Fluke 115 across the speaker load. With an input signal the voltage varies, naturally. At low-to-moderate volume the peaks look to be about 0.6VAC. At moderately loud the peaks are up to around 1.6VAC.
I will try installing a 3-wire power cord on the power amp in the next couple days. It's been on my to-do list anyway.
The hum is pretty dang quiet, but definitely noticeable in between songs at ~8 ft. from the speakers as I sit here.
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You could definitely try some -12dB Harrison Labs attenuators at the input of the amplifier.
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Just installed the 3-wire cord on the power amp--no difference in the hum. :|
I ordered the attenuators too. We'll see how those do.
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Alright, while I'm waiting for those attenuators, a little update. I've been reading whatever I can find on grounding and hum issues, including this page http://cognitivevent.com/av_hum.html (http://cognitivevent.com/av_hum.html), which has some specific info about the Foreplay. I haven't had any big "ah hah" moments about my problem in particular but it's been good to delve more deeply into the principles. I tore down all the ground and signal wiring and rebuilt it based on what I've gathered. Pics attached. The first is a general overview with the VA/CF boards in their normal spots, the second two are closer views of the two sides of the thing with the boards moved out the way.
- Twisted pairs for the input signal wires, with grounds split off from the signal wires near the source selector switch and attached to a terminal where the volume pot ground is also attached.
- I debated about where to ground the output jacks, since the original FP has their first ground at the output jack grounds and then on to the volume pots, but I also read in several places that the output jacks should be grounded as close to the cathodes grounds of the first stage as possible. I decided on the latter, so ran a twisted pair from the output jack grounds to another terminal on the same strip. The cathode and heater center tap grounds also go to this terminal.
- The grounds for the VA/CF boards go to the third terminal on this strip.
- I then have a 18awg twisted trio from these terminals to the ground of the third filter cap.
- Shortest possible 18awg wires run from the negative terminals of the second and third filter caps to back to the negative terminal of the first filter cap.
- I've tried connecting the PS to the chassis at the third filter cap ground, first filter cap ground, and not at all. I've ended up leaving it connected to the third filter cap ground.
- Lastly, since I've got those attenuators on the way and Mr Joppa mentioned my attenuating resistors possibly affecting the treble, I removed those and the 10ks between output and ground and reinstalled 470k resistors between output and ground.
- For S&Gs I put rubber washers between the PT and the chassis plate.
- Also tried adding a .1uF cap with a 10k resistor across it between the third filter cap ground and the chassis, based on the grounding schematic on the page linked above. I confess I don't understand exactly what that's supposed to do, but I had the parts and it seemed worth trying.
I think this all amounts to a star-of-stars type scheme, with all the runs kept as short as they can be, given the configuration.
The result? The hum is about the same as it was before the re-wiring. It's definitely louder than with the attenuating resistors. It's possibly a bit louder in one channel, but it's been like that all along.
But at least my wiring is a bit more respectable now!
I hope the attenuators bring the hum down to an inaudible level, but I'd love to be able to attain a silent build of the circuit without them. This being such an extreme scenario, it seems like an excellent opportunity to fully explore the mysteries of hum and noise.
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Nothing but a shot in the dark: unbolt your choke and move it around while checking the hum. Make sure that you don't zap yourself in the process.
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Probably not related but the 1k resistor at the HV rectifier looks like it has been toasty, is it shorted across the first filter cap?
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No luck with moving the choke around. That 1k resistor connected to the HV of the rectifier isn't shorted either. I'll see about replacing it just to be safe. I plan on rebuilding the PS entirely with new parts at some point anyway.
One of the Pauls had asked earlier what VAC I was getting at the speaker terminals. When I tried to measure that earlier I wasn't doing it properly. I have 57.5mVAC on channel A (tube A) and 12mVAC on channel B at the speaker terminals. At the preamp outputs I have 6.5mVAC on channel A and 3.9mVAC on channel B.
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I made two small changes that seem to have helped a bit. First, I made the connection to ground at the terminal where the output jack and the cathodes attach. I'd read in this article from the Valve Wizard about the chassis connection being as close as possible to the outputs.
Second, noticing the imbalance between channels in the hum voltage, I remembered that I'd replaced the LEDs with newer HLMP-6000s and also had replaced the transistors on the quieter channel (at some point when I was moving things around with the ol' chopstick I had shorted that side out and fried the transistors). I replaced the transistors and LEDs on the noisier channel, and though the hum is still clearly audible, it's about as quiet as it has been. I'm measuring about 12mVAC on both speaker terminals now.
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With Harrison labs -12dB attenuators plugged into the amps, you'll probably be in good territory.
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The attenuators came yesterday and they did help a ton. The hum is still there with my ear right up against the speaker, but from normal listening distance it’s not audible. I bet the configuration could stand yet another set of -12db attenuators in series. Even with these installed the system is still louder than I’m ever going to listen to it at 1/4 volume. The Foreplay is really just not a good match for KT550, but it still sounds better than my other current option.
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I would suggest you build just a cathode follower buffer preamp to use with the KT-550. You could parallel the halves of the 12AU7 to do this. If you'd like, I could draw something up, it wouldn't be that complicated to adjust your build. This would reduce your system gain by another 24dB.
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That sounds fantastic Paul. I’d love to see a schematic and try it out. Thank you!
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There's not a ton of HV current available from that little PT, hence the super low operating current, but this will work much better with your system.
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Thanks Paul. Everything else is perfectly clear but I haven’t seen than overlapping circle symbol before and am confused there. My best guess is that they just represent the existing transistor/LED network, and the resistors on the board need to be adjusted to the values listed? Am I in the ballpark? I take it the “3mA†is the “super low operating current†you refer to.
If you weren’t tied to the original PT how would you adjust the power supply to bump that up, and what advantages would that provide?
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overlapping circle symbol
Constant current source. In this case, the C4S: Camille Cascode Constant Current Source.
cheers, Derek
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Thanks Paul. Everything else is perfectly clear but I haven’t seen than overlapping circle symbol before and am confused there. My best guess is that they just represent the existing transistor/LED network, and the resistors on the board need to be adjusted to the values listed? Am I in the ballpark? I take it the “3mA†is the “super low operating current†you refer to.
If you weren’t tied to the original PT how would you adjust the power supply to bump that up, and what advantages would that provide?
That is the CF white board that would come with the anticipation. The VA boards are not used.
If I didn't have to use that PT, I would just run more current. More voltage doesn't really matter IMO.
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That is the CF white board that would come with the anticipation. The VA boards are not used.
If I didn't have to use that PT, I would just run more current. More voltage doesn't really matter IMO.
Got it. Still not totally clear on the changes around the CF board though. Currently R2 on the boards is connected to the B+ on one side via the "C" lead, and to the gate of the MJE340 and the anode of the first LED on the other side. In your schematic there appear to be just two connections to the CF board now, with the new grid leak resistor R1k connected to the MJE340 source along with the cathode (after the new bias resistor Rbias). But that would take R2 out of the circuit, and your schematic also indicates a change in R2 from 100k to 150k, so obviously that doesn't work. If R2 were connected to R1k via lead "C" then I wouldn't think R2 would need to be 2w, since it's just ultimately connected to the grid... Schematic of the CF board attached so you don't need to look it up.
I'm thinking about building this as a separate unit since 1) seems like a shame to use both tubes when only one is needed, 2) I don't really need source selection and could make it that much more compact, and 3) I have my eye on an SE KT120 build down the road that actually needs a high-gain preamp like the Foreplay.
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I didn't draw the connection from the C4S to B+, but that does need to be made. R2 connects to B+, just as it does in the original Foreplay Anticipation upgrade.
The new grid leak resistor (rlk, which is 1,000,000 ohms, not 1K) and the 1K rbias connect to the collector of the MJE340, which is the easiest pad to identify on the board.
The B+ connection is easily identified because it will only have the free end of R2 connected to it.
The ground connection is the other connection that feeds the smaller R1 and feeds the 2N2222 transistor.
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Great, that all makes sense, thanks. If I build this as a separate unit, would any of the PCBs for the newer CCS upgrade kits work? If so, can I buy those separately?
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You could use the SEX 3.0 C4S upgrade. The LEDs need to be turned around and you'd need a pair of PN2222 and a pair of MJE340 transistors.
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Over the past month I've been ordering parts for the buffer preamp and trying to figure out how to lay it out. Today I managed to finally get the first pass built to a point where I could power it up slowly and take some voltage readings. Pics are attached for reference. So far it's just a chassis plate with front and back pieces to hold the input/outputs and volume . Not sure where it's going to end up, looks-wise.
B+ is at 115v with 120v supply. Grids are at 2.18v and cathodes are at 5.8v. Both sets of LEDs on the board fire up.
Right now I have two problems: There's no sound in one (red) channel and the PT is running very hot. The sound out of the channel that's working is pretty good, so that's encouraging at least.
I haven't been able to find any obvious bad connections in the red channel, the input isn't shorted to ground... I'll just have to keep experimenting with that one.
The overheating PT is what I could use suggestions on. The amp is pulling 240mA, and if I'm reading the AES sheet right the max rating for this PT-442 is 315mA so that should be enough headroom. What other readings could I take that might point me in the right direction?
As far as the layout goes, I wanted to try something with the tube horizontal for a lower profile, inputs and outputs on the back, volume knob on the front. Beyond that, I did my best. I was limited to 6" depth because of the steel I could find was a 6" x 18" sheet of 22ga. Seemed to make sense to have the tube in the center with the socked facing inwards for best access to the terminals, and then things just sort of filled in from there. The ground scheme is: Inputs/volume pot/chassis --> CF boards/heater center tap/outputs --> PS Caps --> DC supply negative. It's pretty compact I guess, but too cramped around the terminal strip (that holds all the connections to the B+, two ground lugs, and CF boards and cathodes/grids) and the volume pot. I appreciate any feedback if anything looks way weird or is a no-no. First try at a layout so I expect there'll be a lot to learn from mistakes here.
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You have a backwards diode in your rectifier bridge.
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Ah, thanks Paul. Bonehead move. I flipped the diode from the B- to AC so that's fixed. Evidence attached.
After the switch the PT is still running very hot. Could I have damaged it with the backwards diode?
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That's somewhat possible. I would pull the tubes, power up the transformer, then measure the AC voltage of the two windings. A damaged transformer is very likely to show problematic voltages on the unloaded windings.
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With the tube out, the AC voltages are 63v and 10.5, which is obviously not right. Looks like I fried it, so I'll get another one coming from AES.
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Got the new PT and swapped it in, but it turns out that wasn’t the problem. With the tube in or out, the new one also measures about 65vac and 10.5vac when hooked up to the rectifier bridge. Unhooked, the voltages are normal. Something must be pulling a bunch of current to drag the voltages down, yes? I’ve been over and over the rectifier bridge and I’m sure I have the diodes oriented correctly now.
Looking back through the thread here again I note you said I needed a pair of PN2222 transistors. At the time I skimmed over the fact that that part number is different from the 2N2222As in the original Anticipation upgrade, and I used new 2N2222As with the S.E.X. CS4 board. The specs on the two look the same except the collector current is 1A with the PN2222 but .625A with the 2N222A, but could that be it. Pic of the board attached. Could a damaged rectifier cause this?
Thanks again for all your help. If it's ok for me to seek help elsewhere, including posting the schematic, let me know. I'd love to relieve you of the burden of dealing with my shenanigans since this isn't an actual product you should have to support, but I've been reluctant to post elsewhere out of concerns about distributing proprietary info.
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The differences between those transistors isn't critical.
How are the voltages with no tubes in the sockets?
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Please do not post our schematics or other IP. It sounds like you may have bad rectifiers.
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Please do not post our schematics or other IP. It sounds like you may have bad rectifiers.
Roger that.
I’ll try replacing the rectifiers.
Without the (single) tube in the socket the main secondary measures about 65vac where it attaches to the rectifier bridge. The heater winding measures 10.5vac at the socket. With the tube in the primary winding starts at about 63vac then falls gradually. I haven’t run it for long enough to see how far it’ll fall but within say 10 seconds it drops to the mid 50s.
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You probably have a blown rectifier diode.
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I've been listening to this for a few weeks now after sorting out this last problem. It sounds great with the KT550 and I'm planning on sticking with it long term. There's still a tiny amount of hum through 96db sensitive speakers, so I'm thinking I'll re-chassis it with a saner layout.
If I didn't have to use that PT, I would just run more current. More voltage doesn't really matter IMO.
If I stepped up to, say, a Hammond 262D6 (12va 120V secondary @ 53 ma), that should keep the voltage in the same range and not require beefing up the PS caps, but though I understand I'd need to lower the value of the R1 to let the CCS draw more current, I'm not sure how to calculate what value needed, and if any changes would be needed in R2 and the bias resistor. Would you be willing to give me any guidance on what those values should be? No worries if not, but no hard in asking.
Thanks again for all the help with this. It's been a great learning experience.
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The 6.3V heater winding needs a ground reference of some kind, which I didn't draw into the original schematic. How did you end up dealing with that in your build? That may be all that's left to get nice and quiet.
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I just copied the original Foreplay I circuit and so have a 68k resistor in parallel with a .1mfd cap from pin 9 to the same ground point as the CS4. The input/output ground is the only thing "upstream" from that point, and is also the chassis ground.
So do you think I wouldn't see much or any improvement by running more current and moving the load 12AU7 load line up a bit?
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So do you think I wouldn't see much or any improvement by running more current and moving the load 12AU7 load line up a bit?
Probably not.
Your volume pot also works best when mounted to metal and can get noisy if mounted to wood without some kind of connection to bond it to the chassis earth.
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Alright I’ll leave the PT alone then. Thank you for the feedback.
The volume pot body is grounded to the chassis. This buffer was a separate build from the Foreplay I I posted pics of at the beginning of this thread, and there’s no wood involved in this one. The chassis I made for the buffer is just some cobbled together steel plates and it’s pretty ugly. I want to put it in a proper chassis anyway, so I’ll take the opportunity to revise the layout a bit and in the process track down that last bit of hum.
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Here she is in the new chassis. It's dead quiet now and sounds great. Calling this project a wrap.
Thanks again for all the help. It's been an awesome learning experience.