6SN7 / 6V6 power amp : very odd behavour

[aardvarkash10]

cool. Bigger resistor in your PSU will do it for dropping the B+ voltage.

FWIW where did the original schematic come from? Boozhound?

 

[aardvarkash10]

Yep - boozhound. Take a look at the values on the schematic...

An externally hosted image should be here but it was not working when we last tested it.

 

[smeezekitty]

I would say they are pretty close.

 

[aardvarkash10]

nope. You have around 250V on the anode of the 6sn7 instead of 180V, the anode load resistor on your sn7 is one fifth the boozhound value and the cathode resistor on yours is half the boozhound value. Thats quite some shift...

 

[dcgillespie]

Make sure the common lead of the output transformer is grounded. That is not shown on the schematic. The fact that placing your DVM probe on the output tube plate causes oscillation likely indicates it may already be taking place with the other symptoms you are indicating. If the common of the output transformer is not grounded, depending on how the transformer is physically mounted or where it's located could allow it to become a major contributor to your problem.

Dave

 

[ray_moth]

A grid stopper of 1k for the 6V6 might help improve stability.

Also, 15uF decoupling cap after 680 ohms dropping R is not really enough; should be 10x that. However, since the 2 stages are 180 deg. out of phase it probably doesn't matter.