Rivera knucklehead 55 reverb weird power tube wiring?

[Ariel Meyer]

Hey all,

I've had an amp come in for repair with a bad burnt smell (hopefully OT is ok, fuse blew, we'll hope and see), to note, the amp has a triode/pentode switch, the screen grids resistors smoked completely and took out the power tube board with it with lots of scorching marks,
I've traced most of the wiring myself, but there are a couple of oddities (for me) (like why is HTR connected to a flyback diode then to pins 2 and 6 and then to a GND point ? to prevent voltage entering HTR), also pins 1 are connected with each other with no GND reference ?

I wish to wire this on a chassis mounted socket rather than PCB, but before i revert to what i know i'd like to keep this original, any help would be appericiated since i can't find a readable schematic that shows what i see on the board

[martin manning]

Isn’t this your schematic?
https://schematicheaven.net/newamps/riv ... lehead.pdf
Power tube pins 2 (heater) and 8 (suppressor) are grounded with the flyback diodes. Pin 1’s are grounded only when a plug is inserted into the speaker 1 jack for OT protection.

[Stevem]

It’s so darn wonderful how when fiberglass starts to burn the resin becomes a truly amazing conductor!

[Helmholtz]

From the schematic power tube cathodes ( pin 8 ) are grounded via the speaker jack.
So no power tube current without speaker plug inserted.
Makes sense and protects the OT from no-load condition.

[Ariel Meyer]

I see, after your comments i went thru the schematic and understood it better, thanks !

[Roe]

From the schematic power tube cathodes ( pin 8 ) are grounded via the speaker jack.
So no power tube current without speaker plug inserted.
Makes sense and protects the OT from no-load condition.

This is a clever solution but it isn't optimal to ground the secondaries together with the tube sockets if you use NFB applied at the phase inverter?

[martin manning]

This is a clever solution but it isn't optimal to ground the secondaries together with the tube sockets if you use NFB applied at the phase inverter?

The power amp includes the phase inverter and the feedback loop, so it's not a problem.

[Ariel Meyer]

I wired after quadruple reading the schematic and your input, amp works perfectly once again, thanks

[martin manning]

Great to hear, thanks for following up!

[Jered]

Hi all! In the above schematic, what are switch 3A and 3B doing? Ive never seen that before, was just curious. Thanks

[LOUDthud]

On the last schematic dated 01.03.92, S3A is a High-Low Power switch which lowers B+ to half Voltage. S3B changes to bias for the lower B+.