I'm baffled -- 6AK6 push-pull amp - diyAudio
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Old 13th June 2008, 05:19 AM   #1
PRNDL is offline PRNDL  United States
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Join Date: Oct 2006
Default I'm baffled -- 6AK6 push-pull amp

I'm having problems with my 6AK6 push-pull guitar amp, which now has a scratchy sound at high volumes. I've gone through everything and it appears to be the output tubes.

The amp has a B+ of 290 volts, which puts about 250 volts to the anodes of the output tubes and slightly less to the screens.

Several people mentioned that 6AK6's can't take more than 180 volts (even though the manuals say 300).

The amp originally had 6AQ5's, but I changed to 6AK6's to drop the power down to around 2 watts. (I also connected pins 2 and 7).

The amp is cathode biased with a 280 ohm resistor and 25F cap.

Someone else mentioned that the problem is the 6AK6's are idling high and are pushed past their limit when driven hard, which is often in a guitar amp (we like them turned up to 11).

If I recall correctly, there was only about 2 volts on the cathode, which meant about 3.5 mA per tube. 3.5m x 280 = .98 watts

The solution, I believe, would be to increase the cathode resistor to something like 1K or so.

One option would be to use a different tube - 6AN5's have a max plate and screen voltage of 300 with about 1.3 watts.

I thought it might be a good idea to ask before buying a pair of new (NOS) 6AN5 tubes.

I've attached a schematic.

Thanks!!!

PS
Those 6AK6's do sound awesome for guitar. One concern is how the 6AN5's sound. Military JAN 6AK6's are available, as long as they can handle the voltage.
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Old 22nd November 2010, 08:29 PM   #2
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Join Date: May 2008
Quote:
Originally Posted by PRNDL View Post
I'm having problems with my 6AK6 push-pull guitar amp, which now has a scratchy sound at high volumes. I've gone through everything and it appears to be the output tubes.

The amp has a B+ of 290 volts, which puts about 250 volts to the anodes of the output tubes and slightly less to the screens.

Several people mentioned that 6AK6's can't take more than 180 volts (even though the manuals say 300).

The amp originally had 6AQ5's, but I changed to 6AK6's to drop the power down to around 2 watts. (I also connected pins 2 and 7).

The amp is cathode biased with a 280 ohm resistor and 25F cap.

Someone else mentioned that the problem is the 6AK6's are idling high and are pushed past their limit when driven hard, which is often in a guitar amp (we like them turned up to 11).

If I recall correctly, there was only about 2 volts on the cathode, which meant about 3.5 mA per tube. 3.5m x 280 = .98 watts

The solution, I believe, would be to increase the cathode resistor to something like 1K or so.

One option would be to use a different tube - 6AN5's have a max plate and screen voltage of 300 with about 1.3 watts.

I thought it might be a good idea to ask before buying a pair of new (NOS) 6AN5 tubes.

I've attached a schematic.

Thanks!!!

PS
Those 6AK6's do sound awesome for guitar. One concern is how the 6AN5's sound. Military JAN 6AK6's are available, as long as they can handle the voltage.
You don't have a resistance directly in series with the screens of your 6AK6s. I'd start around 1k.
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