135w UL Vibrosonic, plate voltage tanking, help!

[BluegrassDan]

Been working on a 1977 135W ultralinear Vibrosonic that has been a real nightmare.

What might cause the plate voltage to drop significantly during warmup? All pin 3s measure around 500v then drop down to 3v after a few seconds. The UL taps stay at 500v. Bias is running 50ma, which is too hot.

New Hammond OT, wired for UL. New tube sockets. Everything appears to be wired correctly. Tubes test good and work fine in another amp.

[sluckey]

Bad OT. Wiring error.

[Latole]

Vibrosonic Reverb 135 watts schematic is same a the Twin reverb 135 watts ultralinear

Or I' wrong ?

[BluegrassDan]

It’s the same, except the output wiring is a bit different for 8ohm configurations.

[pdf64]

Seems weird, what’s the bulb wattage in your light bulb limiter, how brightly is it lighting?
Take the valves out until you can prove the sockets can provide them with suitable voltages.

[tdvt]

Have you found this schematic below already?

Many thanks to Delrefugio Amplification for drawing it. I more or less used this to rebuild my UL Pro Reverb, as it is actually readable.


The Fender schematic from that era is a confusing nightmare.

[Latole]

Have you found this schematic below already?

Many thanks to Delrefugio Amplification for drawing it. I more or less used this to rebuild my UL Pro Reverb, as it is actually readable.


The Fender schematic from that era is a confusing nightmare.

You are right ; here the original

[AlNewman]

All 4 tubes?
I'd be suspicious of the matching pot or the 33k resistor.
Is it all stock?

[Latole]

All 4 tubes?
I'd be suspicious of the matching pot or the 33k resistor.
Is it all stock?

It is original , look Fender schematic

[sluckey]

All pin 3s measure around 500v then drop down to 3v after a few seconds. The UL taps stay at 500v.

I can't think of any fault that fits these symptoms. Are you very sure of the voltage readings?

[PRR]

An unsoldered joint at OT CT *might* power-up a voltmeter probe and go open when the 6L6es start sucking. But I also favor misreading or misreporting.

[tubeswell]

What voltage on Pin 5 with the output tubes pulled out?