Need help-Strange problem

[HiGain]

I have an express built from the two rock kit. Pretty much stock to kelly90, with the exception of a switch ahead of the third stage to toggle between stock and .0047/220k/330k voltage divider. It also has a cross line master with a .015 series cap to keep the hash down, as well as a cross line dull switch to cut out some highs. Last it has a 500k pot with a .047 and .0047 cap across the second stage plate resistor. Although I have these mods, I never really use them.

The problem is I get a high frequency oscillation (two octaves above high E on the guitar) accompanied by some loud rising and falling static. It sounds a lot like DC. Once initiated it will taper down to silent with no guitar signal after about 5 seconds.

This horrible sound is brought on by either playing at high volumes and abruptly stopping, or switching on the standby switch. Toggling all the other switches and pots does not affect the sound. Banging on the chassis at various places, including at the standby switch does not affect the sound.

I have tracked it back to the preamp, because the cross line master volume will reduce the volume of the hideous sound at the same rate as the guitar signal.

The thing that really has me puzzled is that going from standby to operation kicks in the static oscillation. I wonder is this a bad cathode cap, or even a PS cap? I used Mallory PS caps (which are not old), and good quality cathode caps.

Any ideas?

Thanks,

Jake

[HiGain]

I also hear some quiet sporatic muted static, and hear the static more loudly when I switch from operation back to standby.

Here is a pic.

Jake

[keithrick]

I would check the preamp tubes. This amp will expose a slightly microphonic tube. Try changing tube positions first.

Good luck. Nice build!

[HiGain]

I would check the preamp tubes. This amp will expose a slightly microphonic tube. Try changing tube positions first.

Good luck. Nice build!

I swapped a couple low gain Ruby tubes in to see what would happen. Same problem. Then I tried a few sovteks and EH tubes. Guess what! Problem solved. It was the first tube afterall. This is after hours of debuggin and desoldering.

Thanks for your help!!!

Jake