Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: BluegrassDan on April 30, 2023, 07:37:22 pm

Title: 135w UL Vibrosonic, plate voltage tanking, help!
Post by: BluegrassDan on April 30, 2023, 07:37:22 pm
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.

Title: Re: 135w UL Vibrosonic, plate voltage tanking, help!
Post by: sluckey on April 30, 2023, 07:55:57 pm
Bad OT. Wiring error.
Title: Re: 135w UL Vibrosonic, plate voltage tanking, help!
Post by: Latole on May 01, 2023, 04:11:51 am
Vibrosonic Reverb 135 watts schematic is same a the Twin reverb 135 watts ultralinear

Or I' wrong ?
Title: Re: 135w UL Vibrosonic, plate voltage tanking, help!
Post by: BluegrassDan on May 01, 2023, 10:54:01 am
It’s the same, except the output wiring is a bit different for 8ohm configurations.
Title: Re: 135w UL Vibrosonic, plate voltage tanking, help!
Post by: pdf64 on May 01, 2023, 11:47:55 am
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.
Title: Re: 135w UL Vibrosonic, plate voltage tanking, help!
Post by: tdvt on May 01, 2023, 02:47:56 pm
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.
Title: Re: 135w UL Vibrosonic, plate voltage tanking, help!
Post by: Latole on May 01, 2023, 02:58:40 pm
Quote from: tdvt on May 01, 2023, 02:47:56 pm
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
Title: Re: 135w UL Vibrosonic, plate voltage tanking, help!
Post by: AlNewman on May 01, 2023, 06:30:34 pm
All 4 tubes?
I'd be suspicious of the matching pot or the 33k resistor.
Is it all stock?
Title: Re: 135w UL Vibrosonic, plate voltage tanking, help!
Post by: Latole on May 02, 2023, 02:20:58 am
Quote from: AlNewman on May 01, 2023, 06:30:34 pm
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

Title: Re: 135w UL Vibrosonic, plate voltage tanking, help!
Post by: sluckey on May 02, 2023, 07:42:56 am
Quote from: BluegrassDan on April 30, 2023, 07:37:22 pm
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?
Title: Re: 135w UL Vibrosonic, plate voltage tanking, help!
Post by: PRR on May 02, 2023, 12:34:11 pm
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.
Title: Re: 135w UL Vibrosonic, plate voltage tanking, help!
Post by: tubeswell on May 03, 2023, 01:40:17 pm
What voltage on Pin 5 with the output tubes pulled out?