Bottlehead Forum
Bottlehead Kits => S.E.X. Kit => Topic started by: Adrian on May 10, 2014, 07:00:22 AM
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I just completed the build and carefully checked each wiring point, visually checked solder connections (I know that may not mean anything), took resistance readings, and voltage readings.
All resistances are almost exactly per check sheet values.
Tube filaments light equally.
All voltages are also right on expected values (my line voltage is 123.1VAC) EXCEPT:
A/B 7 ... -2.53/-2.52VDC vs. -3.15 (20% less)
A/B 8 ... 3.63/3.62 VDC vs. 3.15 (15.2% more)
These values place 6.15VDC across the filaments (nominal value of 6.3VDC).
C1 ... -2.57VDC vs. -3.15 (18.4% less)
C2 ... 3.66VDC vs. 3.15 (16.2% more)
C4 ... -2.56VDC vs. -3.15 (18.7% less)
C5 ... 3.86VDC vs. 3.15 (22.5% more)
C-Terminal voltages are consistent with the tube filament voltages.
I don't thnk these individual readings are a concern because the filament voltage is only 2.4% below nominal.
Is this correct or should I be concerned?
Also, I am now carefully checking all solder connections (re-solder if doubtful) because I have a bee-buzzing noise of constant volume regardless of pot setting coming out of the left channel with the phone jack fully inserted and half-volume buzzing out of both channels with plug inserted half-way.
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Those heater voltages are normal so i wouldn't worry about those.
Try swapping the tubes around and see if the buzzing noise moves channel, that will confirm if the issue is a noisy tube or something else.
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mcandmar: Thanks for the quick response.
I reflowed all the common/ground connections and components going to them, repowered, and still have the same buzz. Waiting for the tubes to cool down and I will swap them as suggested.
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Swapped tubes and the buzz also swapped channels - have to get different tubes now.
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You can also put some time on them. We have found that 50-100 hours of use on a tube that has been sitting in a box on a shelf for 50 years can help a lot.
-PB
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PB: Are you suggesting that I turn on the amp and let it sit for 2 - 4 days and then see what happens?
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Yes.
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OK
Got another problem after I connected the input.
I only have right channel sound when I put the phone plug fully into the jack.
I know only one channel is coming through because I played an analog test album that separates the left and right channel outputs and ONLY the right channel plays regardless which socket the tubes are in. (I did swap the inputs and can verify that only the right channel plays.)
ALSO, the LEFT Channel Plate Choke (the one connected to "B") runs hot and the RIGHT Channel Plate Choke (the one connected to "A") is cool - strange!
Does this point to a problem with the phone jack or the left input ... or maybe neither?
Why does the Plate Choke on the Channel that doesn't play (LEFT) get hot and the Plate Choke for the Channel that plays (RIGHT) is cool? Should the Plate Chokes get hot? - as hot as the main tranny?
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Sounds like you have a short in the left channel, somewhere between the output transformers and the headphone socket. Double check the wiring, if you cant see anything post up a few pics..
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Thank you, sir. I'm looking into it now.
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ALSO, the LEFT Channel Plate Choke (the one connected to "B") runs hot and the RIGHT Channel Plate Choke (the one connected to "A") is cool - strange!
This won't happen unless there is something extremely amiss in the voltage checks.
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PB: I checked all the wiring several times and see no evidence of shorting.
I did not check the voltages after I connected the inputs and discovered this problem and this should have seemed obvious if one channel is drawing excessive current (me bad!).
Also, on the left channel I noticed a bulge at the top of the 1000 microF cap (gain adjustment?) in parallel with the 680 ohm cathode bias resistor. If this cap is blown (bulges usually mean that), then perhaps the bias resistor is not doing its job of maintaing the proper -V bias at the grid causing the valve to conduct at maximum current (way outside its linear range). This might cause the plate choke to draw excessive current. Just my thoughts on it. But, I don't like the look of the cathode bias cap. I'll try to get a picture of this.
Going to check the resistances and voltages.
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Oh thats not good, if its bulged its done for, bin it. You definitely need to start from scratch with some resistance measurements to find the issue before powering up again.
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mcandmar: I agree. Upon resistance checks I found infinite resistance at B1 and upon trying to get this reading the grid stopper (220 ohm) broke at the body - just split apart. Probably overheated it. I was afraid that would happen. After I soldered in these four grid stoppers I checked their resistances and all of them went up (229 - 252). The now broken one read 235 after installing.
There is no reason for me to procede with any futher diagnosis.
I will remove the offending parts (grid stopper, cathode bias resistor and cap).
I've never order parts like this before so off into the unknown.
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Thinking back, I probably damaged the bias cap upon initial voltage checks.
I did touch the +VM probe to something near the left channel and terminal strip 21-25 and got a little smoke! I continued on with the checks and all were good in the end. I probably should have started over and checked all of the voltages again.
It is true: you learn alot when you screw up.
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Not convinced that was the source of your issues so keep going and check everything out.
Are you in the US or UK? I have those parts spare i can send you if your in the UK.
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I'm in the US.
Thanks for the support.
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Is that capacitor in correctly? They usually bulge when they are in backwards, and they will bulge until they explode.
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Yes
I thoroughly checked every wire and component twice and re-flowed many connections after I discovered I had a problem with the left channel.
I really think I smoked it during the initial voltage checks and failed to stop and ask the critical question: What's the worst that could have happened with this tiny puff of smoke?
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While I'm waiting for replacement grid stopper resistors (one cracked - overheated) and a cathode bias capacitor (swollen and suspect) I have very carefully checked every wire, component, and connection thoroughly. - TWICE. This makes four complete times going over every installation step. I swear I could wire this thing in my sleep. I can find no evidence of shorting or bad connections and I have re-flowed or totally reconnected many joints 'just to be sure'. So, I'm left with putting in the missing parts I'm waiting for, doing the resistance and voltage checks and going from there.
Because I was not getting any left channel through the HP jack I have carefully compared the right channel to the left channel and I have found only one variance. This is how the 1.5 microF plate blocking capacitor is oriented - the left plate choke was running hot. The two blocking capacitors are metalized polypropylene type and probably do not require any special orientation in the circuit but I have to ask: Should I reorient it to match the right channel? If so, I'll have to get another one because the leads are not the right length to turn it around (unless I solder an extention to one end).
Anyway, before the parts arrive I will get a jeweler's loop and visually check every sq cm of this sucker.
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Direction of the film capacitors doesn't matter so nothing to worry about there.
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Its worth getting an illuminated magnifying glass or loupe, plenty to chose from for only a few $ on ebay
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Adrian,
If you have gray hair like many of us here (PB not included) the illuminated magnifying glass is good to have. You should be able to get one under $40 at MCM electronics.
I've been using one since my late 30s.
http://www.mcmelectronics.com/product/21-10255
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Grainger49: Yep, grey hair pushing 62.
I use a desk mounted, lighted magnifier but a handheld device would be more useful in some cases and places.
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For a good lighted handheld, look at what the stamp and coin guys use...John
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Also consider checking out the magnifier apps for the iPhone if you use one. Saved me more than once.
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Thank you - thank you everyone -- SUCCESS! I replaced the suspect cathode bias cap and the 220R grid stopper and headphones and speakers work great, well sorta. I still have the constant buzz in one channel with a much softer buzz in the other (both HP and speakers) that swaps channels when I swap the valves. Because this moves from channel to channel with the valves I am going to assume for now that these valves need to be exercised. I'll run this unit for about a weeks run time and watch for the valves to awaken from their long slumber on the shelf. Here are a couple of pics - please excuse my out of date camera phone and generally poor photography skills.
As you can see I did the entire top in black-on-black and finished the base with an ebony stain. I like the way it turned out but probably more important the WAF is also high.
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The ebony finish looks very nice, very ebony-like.
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I'm continuing to run-in these valves hoping the bee-buzzing from primarily one channel (swaps channels with valves swap) subsides. I wired the output trannys for 8 ohms to for use with speakers and lower impedance phones. I have a pair of NAD VISO HP50s that sound great using my ZUNE player and want to also use these at home and not just commuting and at work. But the buzzing is not right and must go away.
Will installing the C4S on the S.E.X. help quiet buzzing valves?
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It is very unlikely that the C4S will affect the tube (valve) buzzing. The buzzing is probably heater to cathode coupling due to poor isolation between them. Often this will sort itself out with some running time. Give it 50-100 hours of operation - that's just a guess, there is no science to it, but it often works.
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Thank you, sir.
I'm patient and in the meantime, if I use higher impedance phones (HD600 or Pro 4AA) or even my inefficient speakers the buzz is much farther in the background and not as intrusive as with lower impedance phones.
Although I am not especially OC about this, I am aware of the unconscious effect of background noise that is consciously unperceptible. Especially when it slides in and grabs you during low volume passages or between tracks.
Buying replacement valves is ok, but if I wait these may just turn out to be the best valves I could hope for - who knows.