High-Frequence Noise at Roland Bolt-30

[Gokarna]

Hello,
I got a 80is Roland Bolt-30 amp here, that doesn't work right.
The amp has a transistor-based preamp and a tube-driven poweramp with a phase-inverter tube 12at7.

I changed the powertubes from the the original 7591A tubes to the 6l6 (5881) tubes including the different wiring of the tube-sockets.

I did also some mods from the bolt-60 amplifier page
( Roland Bolt 60 )

but changed nearly everything back to the original standard.

Now I have a very high-frequency squealing, when I turn the amp on - it becomes louder and louder very fast, so I only dare to power-on the amp for some milli seconds.

I disconnected the wires from the "feedback-Loop" of the phase-inverter pcb (pos.1), then the amplifier works fine.

I attached the schematic of the amp to this message, the disconnected wire is marked red.
I also checked (roughly) the output transformer, the speaker- and the headphone-plug and the belonging wiring of these plugs, but can't find the problem.

Can anyone give me some help, where the problem might be ?

Thanks a lot,

[Enzo]

When you rewired the power tube sockets, you get the wires from the output transformer backwards. That red wire is normally the NFB - negative feedback - line. But when the wires are reversed, that turns it into a positive feedback wire. SCREEEEEE.

You have pin 3 marked on your drawing. That is the plate of each 6L6. Swap wires going to each pin 3 to the other socket. I'll bet my lunch money that cures it.

Well, tomorrow's lunch money. At the moment, I am getting ready to go find dinner.

[Gokarna]

When you rewired the power tube sockets, you get the wires from the output transformer backwards. That red wire is normally the NFB - negative feedback - line. But when the wires are reversed, that turns it into a positive feedback wire. SCREEEEEE.

You have pin 3 marked on your drawing. That is the plate of each 6L6. Swap wires going to each pin 3 to the other socket. I'll bet my lunch money that cures it.

Well, tomorrow's lunch money. At the moment, I am getting ready to go find dinner.

Hi Enzo,
thanks again, and Yes, that was it.
I've just figured it out by myself2 minutes after posting my message, resoldered everything and everything works fine.

Thanks again for your answer,

Another question according this amp:
I like to install an extra "Master-Volume" between the output of the "phase-inverter"-pcb (pos.8 & 10) and the 1.5k resitors grids of the power-tubes.
I found a schematic for this in a german book for tube amplifiers, but it didn't work.
The idea of the book was a 1 Mega-ohm stereo poti with two 0.022µF caps for each tube, one at the ccw-side and one at the wiper, cw connected to ground.
The explanation in the book wasn't very good, so I haven't found a own solution.
Is it possible to drop the caps at the ccw-side or at the wiper-side ?
Or do I risk a short when the potentiometer is at minimum ?
Or do I need special (tone)caps ? (Don't think so...)
Do you have a solution for this ?

Best regards,

Till A