Peavey Special 112 Oscillation and Breakup

[TomCarlos]

MEF Members -

I am working on my Peavey Solo Series Special 112. For good measure (and because I have boxes of caps), I replaced the High and Low Voltage Supply caps. There is negligible ripple on the caps. I am measuring +43.3 vdc on the positive rail and -43.3 on the negative rail.

I am using an 800 hz signal into the clean channel. With an 8 ohm load, I see approx 22.5 vrms (~83 watts) before the signal oscillates. And with a bit more signal, it will then flat line (clip) on the upper and lower ends of the wave - but the oscillation is gone. The signal seems to be back to normal looking (with the exception of the clipping).

If I try to repeat the test using a 4 ohm load, I can only get to 10 vrms before the amp goes into oscillation city.

With this test alone, is it possible that the output devices are breaking down because of heat? The original 70483100 (NPN) and 70473100 (PNP) remain in the amp.

Any hints on where to look and other tests I should run?

Thanks, Tom

[g1]

Do you have your scope grounded to the speaker (-) ? You need it grounded to chassis and the load must be floating (no other grounds) due to R110 shown below the speaker.

[TomCarlos]

G1 - thank you....

It was said "stupid is as stupid does" ... and stupid me had the scope ground on the blue speaker lead. Once I moved it to the chassis, the amp behaved nicely. I can get 120 watts rms before clipping - I'll take it!!

::: still kicking myself :::

Tom

[glebert]

Those amps are stupid loud.

[TomCarlos]

And "clean" ... don't forget the "clean." The Power Transformer is massive (compared to other combo amps of that size). My friend is building me a cab for the head. I will pair it with a 1x12 or if I can call in another favor, have my bud build me an open back 1x15 (in hopes of getting a nice jazz tone).

[g1]

::: still kicking myself :::

Guess why I remember that scenario. They say you should learn from your mistakes, but you need to remember them too.

[glebert]

And "clean" ... don't forget the "clean." The Power Transformer is massive (compared to other combo amps of that size). My friend is building me a cab for the head. I will pair it with a 1x12 or if I can call in another favor, have my bud build me an open back 1x15 (in hopes of getting a nice jazz tone).

My favorite jazz tone with a Bandit (and the Specials I have had were basically Bandits with 2x power) was putting in a 12" bass speaker and closing up the back of the cabinet. Made it a lot thicker yet still clean. I think it was an Eminence speaker out of a Peavey combo like a Basic 60 or something.