'66 Ampeg B15N - very high voltages...

[boroman]

My friend gave me beautiful 66 Ampeg B15N that haven't been played for a long time. For a check up, clean and some minor service. I already cut "death cap", replaced caps (3x47uf + 1x33uf), cleaned pots and everything. Hooked it up to a cab and sound is VERY weak, like 1/10 overall volume. It's not the tubes, not the speaker, no the bad EXT 8 OHM jack I suspected first, not the speaker cable as well.

I have checked voltages and I have approx 440V on the plates - looks good, 5V recitfier and 6.3V filament taps also. But more than 200V on the preamp tubes (PIN2 to ground), and 440V on 6SL7 phase inverter (PIN2 to ground & PIN5 to ground). Schematic calls here for like 225-270V and I have almost double that! I have worked on many B15Ns but it's the first time I'm having that problem...

Any ideas?

[Enzo]

OK, so phase inverter. You have high plate voltage. SO is the cathode voltage up to snuff? Or is it sitting close to zero? Print says 2.2v or so.

Same thing on the preamp tubes, have we got 2v or so on the cathodes or is it close to zero?

[Helmholtz]

Seems the 6SL7 is not conducting current.
Without PI current the preamp voltages will be high.

[boroman]

OK. Found the problem.

(Why it's always like that - that you think and work all day... then write down the post here, go for a walk and think again)

Someone took the "speaker protection" out from the circuit. Red and white cable that runs along with +/- speaker cables down to the cab. If no speaker load is connected, no signal is passed to output tubes (since phase inverter is non-operational if 1K is not grounded). I joined red and white together and viola! Will leave this topic here in case someone would like find out what I did