Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Big chief on March 19, 2020, 07:33:21 pm

Title: Fender Vibrolux reverb silverface
Post by: Big chief on March 19, 2020, 07:33:21 pm: Need help very loud hum volume does not change hum
Have replaced filter caps, grid resisters in pre amp power tube sockets all new tubes
New resisters on power tubes. Cleaned Turett board replaced by pass caps
Any help would be great
Title: Re: Fender Vibrolux reverb silverface
Post by: tubenit on March 19, 2020, 08:20:01 pm: Can you post some hi resolution photos of the inside of the chassis. And post voltages of tubes, please.

This will help us help you.

With respect, Tubenit
Title: Re: Fender Vibrolux reverb silverface
Post by: shooter on March 20, 2020, 07:05:00 am: Quote
Need help very loud hum
start with NO input, pull V1 power up listen for hum, any?
Yes, leave V1 out, pull V2, any?
Yes............
Title: Re: Fender Vibrolux reverb silverface
Post by: Big chief on March 20, 2020, 09:53:55 am: Pulled v1 hum was a little less Pulled V2 still had hum listed below are the voltages on tubes
V1 pin 1 183.4 pin2 0 pin3 1.5 pin4-5 3.1 pin6 188.1 pin7 0 pin8 1.4 pin9 3.1
V2 pin1 190.0 pin2 0 pin3 1.3 pin4-5 3.1 pin6 193.4 pin7 9 pin8 1.5 pin9 3.1
V3 pin 1 362.4 pin2 0 pin3 O pin4-5 3.1 pin6 366.4 pin7 0 pin8 5.2 pin93.1
V4 pin1 184.2 pin2 0 pin3 .5 pin 4-5 3.1 pin6 189.4 pin7 0 pin8 1.5 pin9 3.1
V5 pin1 316.0 pin2 12.6 pin 3 0 pin 4-5 3.1 pin6 297.5 pin7 12.6 pin8 0 pin9 3.1
V6 pin1 53.4 pin2 53.4 pin3 99.5 pin4-5 3.2 pin6 256.8 pin7 54.1 pin 8 99.5 pin9 3.1

Power tubes
V7 pin1 35.9 pin2 3.1 pin 3 371.4 pin4 371.2 pin5 35.9 pin6 374.4 pin7 3.1 pin8 0
V8 pin1 38.9 pin2 3.2 pin3 381.3 pin4 370.1 pin5 38.9 pin6 376.1 pin7 3.1 pin8 0

Can not send photo at this time Please help
Title: Re: Fender Vibrolux reverb silverface
Post by: jjasilli on March 20, 2020, 10:30:27 am: Check bias supply caps.
Title: Re: Fender Vibrolux reverb silverface
Post by: shooter on March 20, 2020, 11:32:11 am: +1
also check bias voltage at PA tube sockets

https://el34world.com/charts/Schematics/files/Fender/Fender_vibroluxreverb_aa964_schem.pdf
using this for reference, pull all tubes EXCEPT PI and PA, still hum?
Title: Re: Fender Vibrolux reverb silverface
Post by: Big chief on March 20, 2020, 02:05:25 pm: Pulled power tubes no hum
Adjusted bias pot reduced hum maybe have to live with the hum
Don’t know how to check bias on pre amp tubes (Sorry)
Title: Re: Fender Vibrolux reverb silverface
Post by: shooter on March 20, 2020, 02:30:09 pm: Quote
Don’t know how to check bias on pre amp tubes
nobody but a few odd folk do :icon_biggrin:
the easy method, measure VDC at the tubes cathode, that is basically the bias.
typically 1-3 volts. You can use that VDC and the resistor cathode to ground to solve for I (Vk/Rk) = Ik, multiply that by plate VDC and you have a ballpark tube dissipation.
and if you find that enjoyable, you too can be an odd folk :icon_biggrin:
Title: Re: Fender Vibrolux reverb silverface
Post by: Big chief on March 20, 2020, 06:19:35 pm: Thanks so much for the lesson I will enjoy as I’m already an old folk
Title: Re: Fender Vibrolux reverb silverface
Post by: shooter on March 20, 2020, 06:27:24 pm: I had a few smart folk here drag me along kickin n screamin, then I tried n liked it, complex math not so much :laugh: