• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Dead channel

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:cannotbe:

I have a DIY PP using 13em7 that I just recently finished and one channel works fine while the other is extremely quiet even at full volume. Both channels worked briefly, but then I had to re-wire to move from breadboard to chassis and replace a resistor and cap along with some other changes. Now I get virtually no amplification but I do hear a signal (though I think it gets louder with the volume control, so maybe there's a tiny amplification). I've checked the wiring, voltages, resistances, etc. The dead channel voltages are slightly lower than the live one, but otherwise all seems in order and to spec.

I have an old heathkit tube checker but haven't yet figured out settings to check these tubes, but I did swap the channels and that made no difference (so I infer tubes are OK, or at least not the problem). Finally, the output of the CD works fine on other amps.

Any suggestions for where to look next? OPTs? Bad input (I do get continuity all the way to the POT)? A signal cap? I have a Fluke 12b meter that measures capacitance, but I've read about inaccuracies measuring some caps (mostly higher voltage, but still) and wonder if it'd be accurate.

Carl
 
Did you check the DC voltage on the tubes or also the AC ... I mean the signal?

Just apply a 1kHz tone at the input and try to see if every stage is actually amplyfing the signal looking at the AC voltage at its output (MIND THE HIGH DC VOLTAGES) ... so typically after its coupling cap. If you dont have a signal generator, a tune would work as well. This way, you could detect the defective stage and component.

I assume your meter can read true rms values.

Ciao
Gianluac
 
Thanks. I didn't check the signal, just the DC. I can try the signal if I can get my audio generator to work... I'll check the meter. So I'd be looking for an AC voltage increase over the input voltage after the coupling cap? If I find an increase there, then where? At the output before the OPT? After? Both?

Carl
 

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OK, I took a look at the AC at both sides of the C7 and C10 caps. Just plugged a CD into the inputs and measured, and got voltage on the right side and nothing on the left at ANY point - even at the plate of the input (preamp) setion of V1 (though I'm getting B+ at both V1 plates. So I swapped V1s between the channels and still I get a signal on the right and not on the left. I assume that means the valves themselves are OK. (?)

I also measured the input signal at the wiper of the pot where I got a signal on both channels. The signal is making it that far on both sides. I stupidly didn't measure voltage at the grid terminal to see if I'm getting a signal out of the pot, but will do so this weekend.

Assuming it isn't the pot that's causing the problem and I am getting a signal at the grid of V1 preamp section but nothing at the plate, what else could be wrong? Would a bad C7 cap cause the problem? Remember that swapping valves didn't change anything so I can't see how a bad tube section could cause the problem. I'd rather not de-solder and test the C7 cap unless I have to - it's a bit of a rat's nest in there. (My next task in this business is to learn how to make a neat layout and wiring...)

Thanks again,
Carl
 
Disconnect the power, take out tube V1 and make some resistance measurements on the socket, for instance measure from pin 4 to ground, you should get 100k with the volume pot all the way up, near zero with it down.
You should put your test lead on the top of the socket, not where the pins are soldered.. you want to make sure you're getting good connections all the way to the tube.

Now measure from pin 4 to the pot side of C6, you should see the resistance of the pot. Check C6 and make sure you've got continuity from the input jack to it.

Measure from pin 5 to the other side of the 36k resistor, and from pin 6 to ground (disconnect C8 first, measure that as well).

If all of that checks okay then hook it back up, turn it on, shut out the lights and see if you didn't do something silly like forget to connect the filament ;)
 
Thanks, Radtech,

I did all the resistance and DC voltage measurements several times, (all from tube side of sockets). Can't confirm that the tubes are making solid connections in the sockets, and I did use somewhat cheap sockets. All heaters are glowing nicely. I'll check between pin 4 and the Pot and C6 as you suggest. Note I can't get a continuity reading from input to Pot (without jumpering C6) but I did get AC passing all the way to the Pot.

I'm curious, though. If C7 were bad on the left side, should I still be able to get an AC reading at pin 5 with a signal input? Seems to me I should, but I'd like confirmation. If so that all suggests the Pot is the problem. It'll be the first thing I check today.

Thanks again for all the ideas.

Carl
 
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Make sure you haven't inadvertantly swapped a couple of connections on one of the tube sockets on the bad channel. Check for shorts. Find the last point where the signal is present.. (Wiper of pot?) Then go from there..

I had an octal socket where an extremely narrow solder splash flowed down the contact and onto the metal mounting ring.. Dead short and it was not immediately apparent. Found it with an ohm meter.
 
OK, tonight I looked at the Pot. Wiper was soldered in such a way as to MAYBE short it. Resoldered it and re-measured AC voltage. A little improvement, but not much, and the channel is still dead. But the Pot resistances read fine, so back to the drawing board.

What about my question about the coupling cap on V1? If it is bad, should I still get an AC voltage reading on pin 5 (the plate of section1, the "pre-amp") or might I get little or nothing?

Carl
 
Yes, you get AC on the plate if the cap is dead. You are using a voltmeter, aren't you? Because an oscilloscpoe would not like the high DC voltage on the plate, but the v/m is OK.

Just see if the plate has AC ... if not the grid should not have signal as you swapped the tubes and found them working. So if you don't have AC on the plate I believe you should inspect again the socket and input wiring

Ciao
Gianluca
 
Yes, using a voltmeter, and no, I didn't get any signal to speak of.

Last night I re-wired the input to the Pot and found a short in the input jack (it's a scrap project and I used an RCA output jack set from an old CD ROM drive as the input, and there's tight tolerances for the leads...). After cleaning that up and re-assembling, I went straight to playing some music, and for 2-3 minutes, all seemed well (volume was a bit lower than I remeber from when it worked briefly right after I finished it).

Then the left channel died again. I'll scrap the junker inputs in favor of some nicer ones I have around, then run the resistance, DC and AC voltage checks, button up and try again. But I'm about ready to throw in the towel... In any case, it seems like it's related to the input and not the Pot or amplification sections. I'll report back.

Thanks for all the help,
Carl
 
AAAARRRRRGGGGGHHHHH!!!!

OK, so after some work I ended up with an amp on which neither channel worked well. Puzzled, I went back to the amp this evening and found a cap (C7) had disconnected at pin 1 on V1 on the right side, and in fact may have been the wrong cap in the first place (.022 instead of .015?). I replaced it, then looked some more. Sure enough, R12 on the left side was disconnected at the far side from pin 5 of V2, so I fixed that. Then I also fixed a potential PS problem wherein C1 may have been shorted. In the end, I figured I'd fixed a ton of problems that COULD have been responsible for the problems I was encountering.

Anyway, after re-working all the obvious wiring problems (some of which were problems from the start, most of which were not) I'm left with an amp that doesn't amplify. Essentially the same problems, just now on both channels. I've checked wiring for correct layout and I'm virtually sure I've got it right now.

So...where to next?

BTW, I first built Fred's SET, so when I failed miserably on the latest attempt to fix the PP version, I put on Diana Krall's version of A Blossom Fell (from All For You album) on the SET. Beautiful.

Carl
 
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