Arbutt 7/12/2018 2:33 AM
Fender virbolux reverb amp noise issue
Hello to you all
this is my first post on here.

I do amp repairs for a bit of fun and to keep me busy now i don't work anymore . I have a basic understanding of audio and valve amps
I understand about the high voltages

this vibrolux amp i was given to look at by a friend has a noise fault. after mostly 20 - 30 mins it will start. sounds like a bad tube or noisy resistor

i have swapped the tubes out for new ones and still the same. I replaced all the anode resistors still noisy

I went and got another new set of tubes just to be sure.

the noise goes when i remove v4, but if you listen very quietly you can just about hear it
i can see the noise at the PI on the 82k and 100k on the scope ( crt type)

but i can't see the noise elsewhere . i have replaced the 82k and 100k and just about every other part in the pi stage.

All the voltages are correct apart from one Pin 3 the schematic says 17.4v i read about 70v

the noise is not hiss or a mobile phone or interference

Just to re - cap on what i have done
Changed all the tubes twice
Changed all the anode resistors
changed all the parts in the pi inc tube base
Removed V1-2-3 noisy
remove v4 the amp is quite, but you can just hear the noise,
Noise can only be seen at the PI on the 82k - 100k.
All parts are changed for new parts only


I would welcome any help, i have run out of ideas
thanks in advance
Jazz P Bass 7/12/2018 4:17 PM
Check the Vac ripple on the node 'Y'.
If it is excessive, you may have a faulty filter capacitor.
The Dude 7/12/2018 4:32 PM
Welcome to the place!

1) You say removing V4 kills the noise. Does turning the reverb all the way off (wiper at ground) have any effect on the noise? That should tell us if it's the V4A circuit or V4B circuit that's causing the noise.
2) The place where you have 70V instead of 17.4. Is that at TP17? Check the values of R37 & R39.
3) What sort of noise is it? Is it hum, hiss, buzz, crackling, etc.
Arbutt 7/13/2018 1:49 AM
Thank you for your help and reply
The noise is not a hum or a hiss.. I can only explain it as the normally sound you get with a noisy tube or anode resistor.
when i test the volumes are turned down and Reverb turned down and vibro controls off.
TP17 i have about 70v on it, using a DMM.
I have changed all anode resistors and changed R36-37-38-39-40-60
the 560pf cap has been changed I only had a 500pf 500v silver mica
I have checked the filter caps all look ok. There is no ripple on them..

Replaced the 6L6 for a known good pair

The amp is working correctly and plays and sounds good.

If i leave the amp on test with just power tubes and PI tube and v4 after about 20mins + the noise will start
If i leave the amp on test with just power tubes and PI tube without v4 , the amp is quite. But if you listen with your ear to the speaker. you can hear the noise. It is not hiss i can hear its the intermittent noise ( like a noise tube )

when i if got amp to me it was a clear case new tubes or a couple of resistors . i was clearly wrong

Can i upload a audio clip on here ??
thank you once again
g1 7/13/2018 4:50 PM
That 17VDC at the cathode of the PI must be a typo. When I see weird voltages like that on schematics I compare plate and cathode circuits. Here in the plate circuit you have approx. 360V at point Y feeding the plate resistors. Then about 260V at each plate. 100V across 100K is 1mA. Times 2 plates is 2mA total. That is going through the cathode circuit which is approx. 40K. So I expect around 80V at the cathode. I think they erroneously wrote 17 instead of 77 volts at TP17.
stokes 7/16/2018 5:02 PM
Quote Originally Posted by Jazz P Bass View Post
Check the Vac ripple on the node 'Y'.
If it is excessive, you may have a faulty filter capacitor.
This.I would just go ahead and change out the crap caps Fender uses in these and most of their more modern amps.I've had so many newer Fenders with various symptoms and in just about every case it was a bad filter.If you have a good quality cap,I like F&T or Sprague Atom.If you can find BMI,it is a lower priced Sprague Atom,made in the same factory on the same machines.Anyways,get one new cap and sub it,one by one,til you find the offending cap and then change them all.
Arbutt 7/17/2018 5:21 AM
Many thanks again for your help. Can filters caps cause this type of fault. With my knowledge I always thought it was hum. The 560pf cap I have changed for a silver mica one. It is a bit micro-phonic is this normal?. The anode resistors have been changed. For ones that I have used before. Can you recommend any type that is best.. many thanks arbutt
Arbutt 7/17/2018 5:29 AM
Also as soon as I scope pi side of r36 the noise goes. Is there any other component that can cause noise like a bad tube
1ampman 7/17/2018 10:15 AM
Is this a reissue or old amp. If it's old could be conductive circuit board.
Arbutt 7/17/2018 10:25 AM
Many thanks for the info. this amp is an reissue. Do the boards go conductive on these amps