6AU6 preamp problem

Glowin Plates · 26th January 2011, 12:05 AM

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!

jjman · 26th January 2011, 12:30 AM

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?

bob91343 · 26th January 2011, 01:42 AM

I'd try to help but it's too hard to read the diagram.

rotaspec · 26th January 2011, 01:58 AM

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

Glowin Plates · 26th January 2011, 02:48 AM

Tried the 1MΩ resistor and it had no effect.
I just don't understand why this tube would not be conducting.

mastodon · 08:20 AM

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.

artosalo · 26th January 2011, 11:48 AM

Solder (or connect other reliable way) the pin 1 to ground.
What is the anode voltage then ?

artosalo · 26th January 2011, 11:58 AM

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.

toprepairman · 26th January 2011, 12:08 PM

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....

toprepairman · 26th January 2011, 12:09 PM

Oops, 470K resistor.

26th January 2011, 02:48 AM	#5
Glowin Plates diyAudio Member Join Date: Feb 2004 Location: Victoria	No Effect Tried the 1MΩ resistor and it had no effect. I just don't understand why this tube would not be conducting.

26th January 2011, 08:14 AM	#6
mastodon diyAudio Member Join Date: Jan 2011	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 08:20 AM.

Thread Tools	Search this Thread
Show Printable Version Email this Page	Search this Thread: Advanced Search

Similar Threads
Thread	Thread Starter	Forum	Replies	Last Post
6au6 for stereo?	TechnoBeaver	Tubes / Valves	5	9th April 2010 03:11 PM
6AU6 pentodes	TGRANT	Tubes / Valves	20	6th March 2009 12:22 AM
6au6 Cf	mashaffer	Tubes / Valves	3	21st April 2008 10:19 AM
better 6au6 alternative	nhuwar	Tubes / Valves	6	6th February 2007 06:10 PM
6AU6 Varieties	wrenchone	Tubes / Valves	2	13th January 2007 10:49 PM

New To Site?	Need Help?
Register to Participate Search Privacy Statement Contact Us	Frequently Asked Questions Did you forget your password? Mark Forums Read

26th January 2011, 12:30 AM	#2
jjman diyAudio Member Join Date: Jan 2009	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?

26th January 2011, 01:42 AM	#3
bob91343 diyAudio Member Join Date: Mar 2010	I'd try to help but it's too hard to read the diagram.

26th January 2011, 01:58 AM	#4
rotaspec diyAudio Member Join Date: Nov 2007	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

26th January 2011, 11:48 AM	#7
artosalo diyAudio Member Join Date: Feb 2010	Solder (or connect other reliable way) the pin 1 to ground. What is the anode voltage then ?

26th January 2011, 12:08 PM	#9
toprepairman diyAudio Member Join Date: Nov 2010	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....

26th January 2011, 12:09 PM	#10
toprepairman diyAudio Member Join Date: Nov 2010	Oops, 470K resistor.

Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)