Power Tubes Oscillate

[gravitylow]

Hi,

I just got my 18 watt together and began trouble shooting. The preamp and phase inverter seem to work fine, but when I plug in the power tubes they oscillate from rail to rail.
I have modified the lite circuit by adding a master volume using a dual 1M pot so what I have going into the power tube looks like this:

--||----
|
/
\
/<-----------/\/\/\-------- pin 2 el84
\ |
| /
| \
\/ /
\
|
\/

Where the pot and resistor to ground are 1M and the grid resistor is the standard 8.2K. As soon as I bring the master volume off of minimum the output section oscillates from rail to rail. Any ideas on what could be causing it?


Thanks

[gravitylow]

Looks like my drawing didn't work out too well. What it should show is a 1M pot from the output of the phase inverter cap to ground. The wiper then goes to the 8.2K input the the el84 with a 1M resistor to ground.

[markh]

When you say "rail to rail" do you mean clipping/square wave behavior? Is it low or high frequemcy? Did you try removing the preamp tubes, then the PI to see if this changed anytthing? Did you try moving the wires around near the power tube sockets?

--mark

[gravitylow]

Yes, square wave from approx ground to B+ at approx 10KHz.
Pulling the preamp tube has no effect.
Pulling the phase inverter tube stops the oscillation.
Checked the power tubes on a tester and they checkout fine.
I haven't tried moving wires since I am afraid to keep it turned on for more then a few seconds to keep it from damaging something.

Thanks for your help.

[gravitylow]

I'm a bone head. More poking around and I discovered that I was over driving the **** out of the amp with my function generator. The preamp wasn't over driven but the power amp was.
The good side is that the amp seems to be working fine now.

Very embarrassed.
Doh!

[zaphod_phil]

Nothing like a bit of stress testing to make sure your amp's solid, eh?