Parasitic oscilation on AC30 top boost channel

[Ondrej]

Hi,

having my guitar unplugged from the amp with full gain on the top boost channel, cut control fully left (= no cut), turning the treble control right (= more treble), the amp starts to oscillate at about 14:00. It's a very high-pitch sound.

As I increase the treble even more, I get a sort of tuning-old-radio sound until, at the rightmost pot position, the oscillation stops but the sound coming from the speaker sounds like decrease in power (cannot test with guitar as the amp is too loud).

The normal channel is ok. I can crank it full-power, no cut control and the amp is stable.

Also, I have a LAR-MAR PI master volume. With decreasing the MV about 10dB, this behavior disappears.

Have tung-sol 12ax7 in preamp and JJ EL84 in poweramp.

Thanx

[Reeltarded]

A good ac30 is pretty unstable. My ac picks up Japanese radio transmissions from 1941.

Uhh..

This is without a guitar? that won't do. Check it with a guitar. You'd NEVER run treble and no cut. NEVER. On dark guitars I cut about 50% or more with 70% treble or so.

Controls near the middle, volume wide effin open. Glory.

[Ondrej]

A good ac30 is pretty unstable. My ac picks up Japanese radio transmissions from 1941.

Uhh..

This is without a guitar? that won't do. Check it with a guitar. You'd NEVER run treble and no cut. NEVER. On dark guitars I cut about 50% or more with 70% treble or so.

Controls near the middle, volume wide effin open. Glory.

Thanx. Of course, and I'll rarely play full volume. But the behavior bothers me. I did a lot of chop-sticking, trying to move cables around, checking the lead dress. Not much help.

Interestingly, when I touch the chassis, the problem is eliminated.

[Ondrej]

The problem seems to be solved!

Looks like all was about shielding. More in this thread: http://vintageamps.com/plexiboard/viewt ... 10#p980210