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Old 23rd January 2007, 02:49 PM   #1
Carlp is offline Carlp  United States
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Unhappy Dead channel



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
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Old 23rd January 2007, 04:47 PM   #2
Gluca is offline Gluca  Italy
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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
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Old 23rd January 2007, 04:53 PM   #3
Carlp is offline Carlp  United States
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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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Old 23rd January 2007, 05:05 PM   #4
Gluca is offline Gluca  Italy
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You should look into deeper details at the stage preceding the first "mute" coupling cap (where you can't find the AC signal).

Any schematics?
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Old 23rd January 2007, 05:34 PM   #5
Carlp is offline Carlp  United States
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Thanks again. It's the "miniblok II" by Fred Nachbauer, http://www.dogstar.dantimax.dk/tubestuf/mnibl2-2.htm
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Old 23rd January 2007, 07:16 PM   #6
Gluca is offline Gluca  Italy
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Look after that C7/C10 cap: if you don't find AC there, the problem is in the first half. Otherwise in the second half of the valve

Ciao
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Old 24th January 2007, 04:29 AM   #7
Carlp is offline Carlp  United States
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Thanks. I'll start there and report back.
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Old 27th January 2007, 05:21 AM   #8
Carlp is offline Carlp  United States
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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
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Old 27th January 2007, 09:34 AM   #9
radtech is offline radtech  United States
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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
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Old 27th January 2007, 02:52 PM   #10
Carlp is offline Carlp  United States
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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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