I need help with my home brew.

[John Fisher]

Hello! I am currently building a 20 watt fixed biased amp using 2 el84's and my preamp is a Fender style with bass treble and mids.
It works good. Loud and clear but when I put the volume on max the sound tends to cave in a bit on and off. If I strum hard, the volume will momentarily decrease a bit and then recover. I don't think it's the power supply sagging as I checked the voltages during the test and they seem pretty constant. Any suggestions? Thanks!

[TransLucid]

If you have test equipment I suggest:

1. Put the amp on the bench connected to a dummy load. Connect a signal generator to the input.
2. Look at the output at the load with a scope, and confirm everything works correctly at lower power levels.
3. Crank it up and watch what happens - you should see something strange.
4. Check back in the circuit to see where the signal is getting squashed - input to the PI, preamp stages, etc.
5. Assuming the problem is in the output stage (most likely, in my opinion) disconnect the feedback and see what happens. The check the PI output.
6. Are you using large coupling caps to the output tubes? If so, could be blocking distortion...

[Tom Phillips]

...6. Are you using large coupling caps to the output tubes? If so, could be blocking distortion...

I agree. Blocking distortion was my first thought when I read the original post.

Two things you could easily try:
1) Increase the value of the grid stoppers on the power tubes,
2) Decrease the value of the coupling caps between the PI and the power tubes.

What values are you currently using?
Can you post a schematic of your build?

Regards,
Tom