Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: centervolume on April 17, 2020, 11:24:28 am

Title: Vibrolux Rvb AA 964 V5 no cathode voltage
Post by: centervolume on April 17, 2020, 11:24:28 am

Greetings all

rebuilding a 73 vibrolux rvb amp, using AA964 as reference after tear down. I was able to salvage most of cab (replaced particle board baffle with cleats) but apart from transformers, the chassis is more or less from scratch.

Powered up last night to check voltages which are all as per expectations (about 20% higher B+ 465 as opposed to 410 on schematic). This is with the exception of V5 which is part of the trem circuit. My opto isolator is dead and I am waiting on replacement. As it stands, the V5 plates are 400+ volts but neither of the cathodes are showing any voltage (I'm looking for 2+ on pin 3 and 16? on pin 8). Will these cathode voltages appear once a good opto isolator is installed? 

Title: Re: Vibrolux Rvb AA 964 V5 no cathode voltage
Post by: pdf64 on April 17, 2020, 11:50:31 am

The oscillator and buffer triodes are pulled deep into cut off until the trem is switched on. Try a shorted rca plug if no footswitch handy.

Title: Re: Vibrolux Rvb AA 964 V5 no cathode voltage
Post by: centervolume on April 17, 2020, 10:07:53 pm

Ok thanks this makes sense now that you mention it. Most of my limited experience is with bias vary style trem.

Best wishes and stay well :)