1965 Bandmaster hum

[Bombacaototal]

I checked all ground connections first physically and then with the meter. They are all fine

I have just measured the heaters and they are indeed unbalanced. One side is about 3.63V and the other around 3.08V

[Stevem]

You already know that tube wise V1, V2 and V3 are not the issue so let’s narrow this down more .
Please take the time to unsolder the Plate output cap on V4 one at a time, does the hum then stop when doing this?
If it does stop then you have one of those caps that are leaky, and if that’s the case then your dealing with 120 hz hum and not 60.

Also note that in terms of 120 hz hum tons of Fender amps can be easily improved.

The ground path in the dog house from filter node C as on the schematic is daisy chained off of the high current power supply filters for node B and A, and that’s a bad thing !

The ground wire in between node C and B should be cut open and the a new ground wire run feeding node C and D.

This new ground should go to the ground on the vibrato channel input jacks, or if you have a real big ass iron land this new ground wire about 1.5 inches away from where the two PT center taps are landed now.

[Bombacaototal]

Many thanks Steve but i don't think it's a 120hz hum, to me it seems like 60hz, and please refer to the recording for your own assessment.

Will look into your suggestions, many thanks

[Bombacaototal]

You already know that tube wise V1, V2 and V3 are not the issue so let’s narrow this down more .
Please take the time to unsolder the Plate output cap on V4 one at a time, does the hum then stop when doing this?
If it does stop then you have one of those caps that are leaky, and if that’s the case then your dealing with 120 hz hum and not 60.

Also note that in terms of 120 hz hum tons of Fender amps can be easily improved.

The ground path in the dog house from filter node C as on the schematic is daisy chained off of the high current power supply filters for node B and A, and that’s a bad thing !

The ground wire in between node C and B should be cut open and the a new ground wire run feeding node C and D.

This new ground should go to the ground on the vibrato channel input jacks, or if you have a real big ass iron land this new ground wire about 1.5 inches away from where the two PT center taps are landed now.

I was checking the supply grounding. The 3 caps for the stack and PI are grounded at the buss bar (pictured) and the power supply ones are close to the bias pot at the chassis (pictured)

So either a leaky capacitor or the hum is due to the unbalanced heaters. I will try the humdinger circuit https://robrobinette.com/Generic_Tube_A ... #Humdinger

[johnnyreece]

One thing to keep in mind - the "center tap" wasn't always located dead-center. It might be sufficient to disconnect the CT, then create the artificial center tap with two resistors. Just a thought!

[Bombacaototal]

Yeah I just removed the CT and created the humdinger circuit but the hun is still there, made no difference.

I therefore removed it and grounded the CT of the filaments again

[johnnyreece]

Sounds like it might be time to check for leaking DC. Those blue molded caps are usually pretty sturdy, but things fail sometimes...Another thing to maybe try - clip in another parallel cap on the different nodes on your power supply. It could be that one of the replaced caps has given up. Stevem's power supply grounding suggestion seems sound, as well.

[Bombacaototal]

Sounds like it might be time to check for leaking DC. Those blue molded caps are usually pretty sturdy, but things fail sometimes...Another thing to maybe try - clip in another parallel cap on the different nodes on your power supply. It could be that one of the replaced caps has given up. Stevem's power supply grounding suggestion seems sound, as well.

Thanks Johnny. Yes the filter caps grounding is in line except that the tonestack and PI caps are grounded at the busbar instead of the tremolo jack. Should I try moving them?

Yes, next will be the coupling caps and then back to the filter caps. I will try your suggestion first to clip a parallel cap and see if it will solve

[johnnyreece]

Just to be clear: Are you meaning the filter caps for the PI and Preamp are grounded at the busbar, or the grounds for those sections themselves (cathodes, pot grounds, etc.) are on the bus bar?

[Bombacaototal]

Just to be clear: Are you meaning the filter caps for the PI and Preamp are grounded at the busbar, or the grounds for those sections themselves (cathodes, pot grounds, etc.) are on the bus bar?

The filter caps and I posted pics of their respective grounding wires and where it is grounded. THe Tonestack x 2 and PI is the black wire that comes from under the chassis

[johnnyreece]

Got it. Stevem's suggestion is to move the B node's ground over to the PS ground, if I'm understanding correctly. Not sure that's the source of your hum, though.

[mojotom]

Take a look at the solder joint on the lower left on you filter caps photo, I will check that one and resolder if necessary but like I said earlier I would put some new 100uF 350V and check on hum. If it’s fine you can even resolder the current ones but at least you’ll know.
Then check caps on the signal path for leakage and redo grounds points on the chassis itself (and between pots copper bar and chassis).

[Bombacaototal]

Ok here’s a test that takes all of 30 seconds.
With the amp in play mode remove the the 12AT7 next to the output tubes.
if the hum goes away then we need to narrow down what’s going on in the preamp section even more.

To do this one channel at a time locate the two 220K mixer resistors that feed the 12AT7 input grid cap.

Next unsolder each resistor one at a time and if in doing that the hum goes away I will then bet that on the input side of that resistor you have a DC voltage that should not be there.

This voltage will be from a leaky coupling cap.
If the vibrato channel is passing the hum then the .1 cap off of the plate is bad, if the normal channel shows DC voltage/ hum then the .047 cap off of that tubes plate is bad.

Just did this test, basically removed the tonestack wire coming to the 220K mixing resistor before the entrance of the PI, each one at a time (first the normal channel and then the vibrato channel). Hum is still present

[Bombacaototal]

You already know that tube wise V1, V2 and V3 are not the issue so let’s narrow this down more .
Please take the time to unsolder the Plate output cap on V4 one at a time, does the hum then stop when doing this?

I've unsoldered the .1uF (after the 100K and after the 82K plates of the PI), one at a time and hum is still there, maybe worst

[Bombacaototal]

When I received the amp it was blowing fuses, basically because of a 6L6 that went bad during shipping. Could my 470R resistors get bust because of this?

I have just re tinted all PI connections