Ampeg Jet fizz J-12D

[drewl]

I know I posted about this amp the last time I worked on it bit I can't find the post.

Both 6BK11's are pretty microphonic and had oscillation issues.
Tamed that with some plate caps but now the output is fizzy- high frequency oscillation on the output.

Removing negative feedback eliminates it.
Seeing how low a feedback resistor I can put in before oscillation returns.

[g1]

Have you tried flipping the phase on the OT primary?

[drewl]

Yeah, it squeeled like a pig.

I'm starting to remember this amp
It had hum issues also
The schematic calls for pin 8 of the 6bk11's to be a heater but it has to be grounded.

[The Dude]

Datasheet pinout shows pin 8 as internal shield and filament pins 1&12. I can believe it should be grounded, but I don't see any filament connection.

[drewl]

Yup, I remember we figured it out last time.
The Ampeg schem doesn't have the heaters and the Joe Piazza version had it incorrect as pin 8 of the 6bk11's as a heater instead of pin 12.
So, when I connect the feedback resistor to the output tranny, it's putting -1.5v on the speaker!
The other side of the 6.8k feedback resistor has around plus 1.5v which rises slightly with the feedback disconnected.
Why? I don't know yet.
Sorry I can't post schems right now, computer is hosed and phone won't do it right now.

[drewl]

Something about those 6bk11's agghhh.

Just did an older M12a yesterday with 6SL7's but otherwise very similar and it was dead quiet.

[g1]

Here's the real schematic.

[Randall]

Wow, that's an interesting design. I don't understand how V1 is working, especially the second 2/3 side. With cathodes and plates connected and one side grounded no less. How does that work? Or for that matter, how does that circuit make tremolo?

[drewl]

By sheer will it makes tremolo!

V2 is causing the oscillation, but even in the v1 spot.
shame cause it has more gain and sounds better than the other one or the 6K11 I have.
Oh well, I did see some on Ebay that weren't too expensive.

[drewl]

Dammit!
I can get it clean on a scope and load resistor but with guitar and speaker the high frequency oscillation is there.

It's giving me a freaking headache.

Think tone caps could cause it?
Every other cap has been replaced in it's lifetime.

[g1]

Anything odd about the speaker, like negative terminal connected to frame?
Maybe try reversing phase at the speaker, just for kicks?