1965 Bandmaster hum

[Bombacaototal]

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 will attempt this one tomorrow. Just to double check wouldn't it be risky having the supply grounding at the input jack which can come loose overtime?

[Bombacaototal]

Today I replaced the PI supply capacitor, removed the ground daisy chain and grounded the PI supply to the vibrato channel input jack, leaving the two tonestack supply grounds at the original place. Unfortunately still hum

[Bombacaototal]

I just pulled out the 220K from the vibrato side and the hum is still there.

I guess with the experiments and changes we can rule out:
-Normal Channel and Supply
-Vibrato channel, tremolo and supply
-PI supply and supply ground
-Pre amp and power tubes
-filament unbalance and CT
-bias electrolytic cap

I guess only left are the 100uF supply caps and the coupling caps on the PI?

EDIT: just removed the .1uf of the PI from the 220K side now and had more hum

[Bombacaototal]

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 realised I had done it the other way around with PI to vibrato input and the two channels to the bus bar. I have reversed it as per suggestion above but unfortunately still hum

Maybe time to swap all PI blue caps?

[Bombacaototal]

So I grounded each side of the 500pF cap in the PI entrance. At the 220K channels mixing resistors side there is still hum, but if I ground the other side of the 500pF (pictured) the hum is gone. I thought it might have been the cap and replaced it but hum is still here

[martin manning]

Try lifting the heater leads up away from the cathodes on the PI socket. Arrange them so they are going straight up when they leave the lugs.

[Bombacaototal]

Try lifting the heater leads up away from the cathodes on the PI socket. Arrange them so they are going straight up when they leave the lugs.

Hi Marin, just did as suggested, a bit hard to picture tho, but unfortunately no change.

[martin manning]

That looks better, anyway. Did you try a new tube there?

[Bombacaototal]

That looks better, anyway. Did you try a new tube there?

Hi Martin, yes I tried another 12Aat7 and the hum is the same. Unfortunately I have no more spares but I did also try a ax7 which is reading about 50% and the hum is present too

[martin manning]

Have you cleaned the connections between the speaker jack bushings and the chassis?

[Bombacaototal]

Have you cleaned the connections between the speaker jack bushings and the chassis?

Just did that. Also re did the OT ground at the jack as the original lead looked fairly small. Still hums

[Bombacaototal]

I just analysed the hum frequency and its indeed 120HZ and not 60HZ as I first had assumed.

I am a bit out of ideas on what to try next. The 100uF supply caps? Or the 3x 0.1uF at the PI?

[Stevem]

You have previously posted that only when V4 is removed does this hum stop right?

If that’s the case and since you have tried different tubes and ruled out power supply filters then you have either a PI plate decoupling cap that is leaking DC down stream to a output tube, or that tube socket itself has become conductive and is bleeding plate voltage into the grids.

[Bombacaototal]

You have previously posted that only when V4 is removed does this hum stop right?

If that’s the case and since you have tried different tubes and ruled out power supply filters then you have either a PI plate decoupling cap that is leaking DC down stream to a output tube, or that tube socket itself has become conductive and is bleeding plate voltage into the grids.

Filter caps arrived, I replaced the 100uF and hum is still here. I think it was lower, as in less loud, but still present. But doghouse would not close with the 500V. I just ordered some 450V

I guess only the PI caps left..

[Stevem]

Interestingly I guy / customer just got back in touch with me about his blond Bassman.
I spent days on this amp last year with the same issue as yours that I narrowed down to being in the PI section, just like yours seems to be.

This guy needed the amp back to gig with so I had to let it go as it was with its hum issue unresolved months ago, but In my case with this amp and with yours I now think that due to the process of elimination that I had gone thru leaves nothing but the component board has being conductive and is bleeding power supply voltage into the grid of that 12AT7.

When I get the amp back in my hands next week this is what I will look into with this guys Bassman.