amp good...then amp bad...then good...

[studiodunn]

Here is the "good" state of the amp - https://ampgarage.com/forum/viewtopic.p ... 10#p400610

I had maybe 10 hours on it and all good. Then out of the blue the other night during rehearsal the amp just goes into ugliness. The amp starts with notch distortion on the notes and goes into full on fart/rumble nastiness that can't be tamed with the volume knob.

I replaced the tubes with known good tubes

I've checked every connection and lead and re-flowed anything even slightly subject and still nothing. Any ideas would be appreciated.





Thought I found the cause, but it's still got this going on....

[R.G.]

Sudden fail of the bias voltage in the too-much direction, possibly.

The full rumbly nasty may be some almost motorboating.

Measure the bias voltage on the output tubes' grids, and the idle current on the output tubes. Give them two aspirin, and let us know what happens.

[statorvane]

I've had similar sounds in a SE amp that turned out to be a bad rectifier diode. If you check the voltages like R.G. suggests, I'm sure you'll narrow it down.

Let us know what you find.

[studiodunn]

CT- 394.6v

V4 - 392.8v, 64.4ohm
V5 - 392.7v, 74.2ohm

-33.7v bias voltage / 26.2mA

Scratching my head

IMG_6929.jpg

[martin manning]

...and the solution was?

[didit]

SOLVED

So all my voltages were good, tubes were good so I went back to signal path. I removed the pre tubes and eliminated the PS and output stages form the problem, i.e. no noise with the pres out.
I went back through and cleaned and re-soldered and still noise. the fact the amp was reacting to the slightest touch had me leaning towards a ground issue. I went through all the grounds and there it was. A 1mm strand of input shielding making contact at the input jack. The way it was wired it was hidden under the coax and almost perfectly camouflaged against the jack.

Thanks for the suggestions and help

Best put end of thread...

[martin manning]

Thanks, Ian. Even though this thread is only one page, viewing through the "keyhole" of a hand-held device, and entering on the last post, it's was still easy to miss.

[studiodunn]

NOT SOLVED

So all my voltages were good, tubes were good so I went back to signal path. I removed the pre tubes and eliminated the PS and output stages form the problem, i.e. no noise with the pres out.
I went back through and cleaned and re-soldered and still noise. the fact the amp was reacting to the slightest touch had me leaning towards a ground issue. I went through all the grounds and there it was. A 1mm strand of input shielding making contact at the input jack. The way it was wired it was hidden under the coax and almost perfectly camouflaged against the jack


Thought I found the cause, but it's still got this going on....

[studiodunn]

I THINK I have the amp sorted...THINK.
I finally tracked the source of a bad filter cap by lightly tapping my way through the circuit until I found something that was louder than everywhere else. That happened to be the 22u @ V1. I replaced it and the issues went away. The amp sounds great and acts like a wreck. It comes on at .5 and is wild at 3.

Thanks for the help and hanging in on a very weird run I've had with this amp.