Fender 6G8 hum

[uki]

Hey guys !!

I built this amp about 12 years ago, and with the help of the good folks around here got the amp working properly!
This is the amp I use for giging!

But now the amp is humming what looks to be 60hz,  I may be wrong thou

PA tubes apparently are working hotter, but it is summer and its very hot

Hum increases as volume goes up either channel

I did pull the PI tube to listen if the hum would stop or stay, it does keep happening w/o PI, although with very low volume
Amp does not have master volume

Here some voltage readings, it did not chance from years back

Bias -43   

Node A 405 vdc
          B 404 vdc
          C 380 vdc
          D 242 vdc
          E 294 vdc
          F 275 vdc

Some guidance to fix this hum down would be appreciated, thanks in advance!

[uki]

Also i did a bulb(100w) test

At power on it did flash full light, then faded down to a dimmed light, then with standby on the light increased.
no input signal

follow pictures of the bulb

thanks

[AlNewman]

Maybe one of you're power tubes is not as balanced as it once was.  If you pull the PI and there's still hum it points towards the power section.

[uki]

Maybe one of you're power tubes is not as balanced as it once was.  If you pull the PI and there's still hum it points towards the power section.

I did found the old notes, it is from 1ohm cathode resistor, to compare with today.

old: 25.8mv, 26.2mv, 24.6mv, 24.8mv

today:  V7 20.8mv, V8 20.5mv, V9 20.9mv, 21.5mv

also I've compared the grid resistors:

old: V7 1607ohms, V8 1632ohms, V9 1636ohms, V10 1569ohms

today: v7 1666ohms, V8 1840ohms, V9 1844ohms, V10 1622ohms


Screen resisitors:

I have no notes from before, those read today:

V7 477ohms, V8 498ohms, V9 568ohms, V10 477ohms

All resistors in the amp are carbon composition.

Any help is appreciated, Thanks
 
picture from 2015

[bmccowan]

I agree with Al, tubes first. Got spares? You must if you gig with it. If swapping tubes does not reveal anything, my next step would be to check all the grounds. Easy after 12 years for something to come loose or build up some corrosion. Clean the pots and all plug/jack connections? Filter caps should last 12 years, but Anyways, that's some easy stuff.

[AlNewman]

Quite a bit less current now than before.  What are your plate voltages at the power tube plates now?  The grid voltage hasn't changed?

[uki]

I agree with Al, tubes first. Got spares? You must if you gig with it. If swapping tubes does not reveal anything, my next step would be to check all the grounds. Easy after 12 years for something to come loose or build up some corrosion. Clean the pots and all plug/jack connections? Filter caps should last 12 years, but Anyways, that's some easy stuff.

No spare tubes, yet.  Indeed i found some loose stuff, but nothing related to the hum thou, grounds are good 

Quite a bit less current now than before.  What are your plate voltages at the power tube plates now?  The grid voltage hasn't changed?

It does read the same as years back
 
v7-10  408 vdc


Also there is a reading from pin 5 to ground in ohms, its different now

old: v7 294k, v8 294k, v9 283k, v10 283k

now: v7 275k, v8 275k, v9 258k, v10 258k

the power tubes does feel hotter, i may be wrong thou

thanks

[AlNewman]

Are you measuring with the light bulb still in circuit?
If so that would reduce voltages from operating conditions.

Otherwise, it seems strange there would be less current but similar voltage at the plate.  Perhaps it's a leaky filter cap.  Also, what's your DC voltage at pin 5 of the power tubes?

[uki]

Are you measuring with the light bulb still in circuit?
If so that would reduce voltages from operating conditions.

Otherwise, it seems strange there would be less current but similar voltage at the plate.  Perhaps it's a leaky filter cap.  Also, what's your DC voltage at pin 5 of the power tubes?

No light bulb while reading voltages

Pin 5 of PA tubes -42 vdc

Thanks

[AlNewman]

Probably want to check resistances across your filter caps with the amp off and drained.  Try with the stand by switch open, as that will isolate the reservoir caps from the other caps in the chain.

[uki]

Probably want to check resistances across your filter caps with the amp off and drained.  Try with the stand by switch open, as that will isolate the reservoir caps from the other caps in the chain.

I've disconnected the + side of the reservoir caps from the circuit and measured uF and  resistances

cap 1 - 21.98uF ,  there is ohms and raising, did read it until 4-5m 
cap 2 - 21.58uF ,  there is ohms and raising, did read it until 4-5m
DMM can read 200m ohms

there was a moment when cap 1 was reading like 160m ohms and raising(confusing), then after reading again it gave the reading above.
DMM battery is new

also i did this test and result seen to be the good case

Also i did a esr test with scope(hope i did this right) follow on pics

Thanks

[AlNewman]

What about the filter caps on the pre amp side?  You shouldn't have to lift one end unless you get low readings, there should be no other paths to ground. 

[uki]

What about the filter caps on the pre amp side?  You shouldn't have to lift one end unless you get low readings, there should be no other paths to ground.

Ok got new readings w/o disconnecting the caps.

6m ohms for the power section
16m ohms for the preamp section

Did play a bit with amp to hear what is going on,
amp start with low noise level, the noise increase as the amp warm up,
did play for about 20 minutes.

Thanks

[AlNewman]

Maybe the power amp caps are weak?  Maybe it's time to focus again on a tube change.  The fact that one of the screen resistors is off spec is interesting.  You could pull 2 tubes, one on each side of the output transformer, check for noise, and then check  the other 2 tubes the same way, and see if you can isolate where the noise is coming from.
The screen resistor should probably be replaced either way.