Ampeg Gemini II 2 - 7591 tubes red plating and what not

[Intergalactic Tourister]

This is the mid-60s version. All point to point (the control panel does not have a pcb either).

Caps - all new, everywhere.
Resistors - all check good.
Transformers - check good.
Power Supply - new diodes, caps, everything.
Bias pot/cap/resistors check good. Cap polarity observed (+ going to ground)

Here's the thing: It was working perfectly 100% (after the restoration)... then it started red plating. Bought some new JJ 7591 tubes and still red plated.

PROBLEM:

When I measure the B+ my DMM starts clicking profusely like a Geiger counter and I got 513v at A as well as at B. Then I got like 460v at F. Measurement was taken with tubes pulled out. At this time a bright small pinpoint spark appeared just above the rect. diodes and 220K resistor feeding off the HV leg. FWIW, I also measured about 20v B+ when in stand by mode.

Control Grid (on standby/no B+) = -22v is the other measurement I got before turning it off.

I really want to avoid turning it back on so... any ideas what could be wrong?

Question:
What is up with the .1 cap tying the heater center-tap to ground?


Thanks!

[Enzo]

Your B+ will naturally run higher with the tubes removed.

The cap to ground for the heater center tap is just a way of grounding the AC without a direct connection. Hum or ripple will flow through the cap.


A small brighgt spark is an arc, and will have to be removed. A spark jumps between two places, and the plasma of the spark then acts as a conductor. It will carbonize the surface where it travels. Carbon is conductive, so every trace of that must be ground away. If it happens on a tube socket, it is usually easier to replace the socket. I use my Dremel for grinding away carbon.

Two tubes red plating or just one?

[Intergalactic Tourister]

Both tubes red plating. Takes them about 5 seconds to start glowing.

I don't see any carbon or trace of the spark. Resistor and diodes still test good... I'll replace them anyway.

[Intergalactic Tourister]

Problem solved!

After moving some connections around it finally works now. One of the eyelets on the board was conducting from the eyelet next to it- The last filter cap and the anode of the bias diode were apparently conducting through the board material. I simply connected them to different eyelets.

Thanks for responding Enzo.