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Old 26th January 2011, 01:05 AM   #1
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Default 6AU6 preamp problem

Hi

I am trying to troubleshoot a stereo amp with one dead channel.
I have found that I have 5 VDC on the plate (pin 5) of the problem 6AU6 and
2.5VDC on the Cathode(pin 7)
Pin 6 has a very high voltage (compared to the other tube) of 100 VDC.

The working channel has only about 30VDC on pin 6 and 100VDC at pin 5
I have replaced all the relevant resistors checked the tube socket, ground connections, the DC supply is fine at 243VDC.
The 6ufd bypass cap seems OK and everything is wired the same as the working channel.
I have swapped tubes all with the same result.
Help!
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Old 26th January 2011, 01:30 AM   #2
jjman is offline jjman  United States
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Cathode and plate voltages imply the tube is idling much hotter than the good side. Is there dc voltage on pin 1? Maybe the input control's wiper is bad?
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Old 26th January 2011, 02:42 AM   #3
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I'd try to help but it's too hard to read the diagram.
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Old 26th January 2011, 02:58 AM   #4
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I agree with jjman. Try putting a 1 megohm resistor between pins 1 and 7 and see if the voltages return to normal. That resistor should really be there as a safety net if the pot fails.

Gary
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Old 26th January 2011, 03:48 AM   #5
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Default No Effect

Tried the 1MΩ resistor and it had no effect.
I just don't understand why this tube would not be conducting.
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Old 26th January 2011, 09:14 AM   #6
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It's conducting like crazy, it pulls its anode voltage all the way down...could you measure the dc offset on pin 1 as jjman suggested?

Well, like crazy...2.4mA. But a lot more than the other channel, that's for sure.

Last edited by mastodon; 26th January 2011 at 09:20 AM.
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Old 26th January 2011, 12:48 PM   #7
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Solder (or connect other reliable way) the pin 1 to ground.
What is the anode voltage then ?
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Old 26th January 2011, 12:58 PM   #8
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Quote:
The working channel has only about 30VDC on pin 6 and 100VDC at pin 5
The 680 kohm resistor from supply to screen must be wrong (small).
Check this again.

If it is OK, then take the tube away from the socket and measure the voltages then.
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Old 26th January 2011, 01:08 PM   #9
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More likely the 470 resistor from p6 to ground is O/c.
Strange circuit, why is the tone control within the overall loop? put it in the negative feedback circuit by all means, but where it is the neg Fb will be fighting it....
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Old 26th January 2011, 01:09 PM   #10
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Oops, 470K resistor.
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