1965 Bandmaster hum

[martin manning]

Look for AC voltage on the board around the high voltage eyelets and grid leads.

[Bombacaototal]

Look for AC voltage on the board around the high voltage eyelets and grid leads.

Thanks Martin. Between the PI plates I have 0.157V AC as I get closer to the end of the board, which is closer to where the plates connect to the 0.1uF. closer to the plates supply is lower like 0.131V AC

Around the 1M is 0.078V AC

Around the 220k that go to the bias from the PI it is 0.11VAC

Around the vibrato channel plates it's 0.091V AC

I'm using the buss bar as ground for the MM probe

[martin manning]

What AC voltage to you have across the PI coupling caps, one side to the other, plate end and power tube grid end?

If you see significant voltage there, try this experiment to remove the HT ripple from the board. Note this will bypass the standby switch.

[martin manning]

What happened with this? Curious to know if it turned out to be a leaky board causing the hum.

[Bombacaototal]

Apologies Martin haven't had a chance to look into this amp again. Will try today or tomorrow and will report back

[Bombacaototal]

What happened with this? Curious to know if it turned out to be a leaky board causing the hum.

Just took it out of the headshell

On the 220k side I get 0.137VAC

On the plates side I get 0.121VAC

Is this significant?

[martin manning]

I would definitely try the experiment above to get the ripple off the board and see if the hum goes away. If it does, then you need to decide if it's something you can live with or if you want to do something about it.

[Bombacaototal]

I would definitely try the experiment above to get the ripple off the board and see if the hum goes away. If it does, then you need to decide if it's something you can live with or if you want to do something about it.

Thanks Martin. That is what I have been dwelling with a bit. Ideally I would need to replace the boards and balance the filaments to make this amp "correct", but I really like how it sounds now and the "mojo" it has. I have been wondering if these "issues" are contributing to the final outcome of how it sounds. And funny enough, these transformers are meant to run at 117V. I have tried at 117V and at 121V (biasing for each seperately) and I prefer the snap and top end of the 121V. I am also using the 4ohms OT into an 8ohms speaker. So I guess this amp is "all wrong" but in the end sounds good. The noise (hum) to signal is not too bad, although the constant low end frequency is a bit annoying...I would like to address it if I could. That is where my mind is at now...would love to get your thoughts

[martin manning]

If you can confirm that the HT ripple voltage is the cause of the hum, and you want to eliminate it, I would consider a minimally-intrusive approach as shown below.

[Bombacaototal]

Thanks Martin, I really appreciate all the help. I will try to remove the ripple as per your first post bypassing the standby and will report back. I hope tomorrow I can do it

[Bombacaototal]

If you can confirm that the HT ripple voltage is the cause of the hum, and you want to eliminate it, I would consider a minimally-intrusive approach as shown below.

Sorry for the delay Martin, have been struggling a bit with covid, but hope to be back at it soon

[martin manning]

No problem, take care of yourself, and get well soon!

[Oddvar R]

Would be nice to hear if this has a solution. I have the same problem with my Bandmaster, allthough this is a clone build. Whatever I do, no change in the hum.

[angelodp]

I suppose you have tightened all the pots and jacks and any bolts that hold down T's.

[Bombacaototal]

Just wanted to give an update. The hum was getting inserted from the bias supply. Changed bias cap to higher value, 47uF, and an additional cap across the pot, plus the standby pot was loose and this was replaced

Hum reduced greatly and the amp is fairly in good shape now