1965 Bandmaster hum

[Bombacaototal]

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.

Thanks for the note Steve and very interesting that you found an amp with similar issues. I'm curious on what you find once you have the amp back. How are you planning on checking the conductivity of the board? I am happy to try on mine

[Bombacaototal]

THis is the hum with the new 450V filter caps in. Peaks are at 99HZ, 148HZ and the short one on the right is at 201HZ

In the room it feels less loud than before but at my DAW the DB is s follows: Previous at -46.8db, current at -42.1db

Any point in replacing the other 22uF caps. I love how this amp sounds and trying to keep it roughly as close as I can while mitigating the issue

[martin manning]

Maybe consider replacing all of the old rectifiers? That would be a good idea for reliability anyway.

[gldtp99]

I installed new 1N4007's in the rectifier (Bias, too) when I got my #2 '65 Bandmaster up and running when I first got it

I re-did the whole Rectifier/Bias board when I replaced the blown PT it came with

When I tested this one out it sounded almost as good as my #1 '65 Bandmaster (my favorite amp) so I decided to keep what was initially going to be a "Fix and Flip" amp

[Bombacaototal]

Thank you Martin and GLD.

I will look into replacing the diodes. It seems they added goop to the joints? All of them look black. Also the diode there is a RCA 35604. The schematic doesn't specify. Is the 1N4007 a correct replacement? I always found different diodes will feel different under the fingers.

I have some NOS RCA 1N5408 which I could use. They are SK9010 3-Amp 1000Volt

[martin manning]

1N4007 would be the standard replacement. Your 1N5408's would be more than adequate. The black paint on the eyelets suggests that the existing diodes are not original.

[Stevem]

I have not started getting back into this humming amp yet that I have , but what I would suggest to would be to yank out the PI tube and with your meter look for low level DC and AC voltages that should in no way be found on certain pins in the tube socket.

[Bombacaototal]

1N4007 would be the standard replacement. Your 1N5408's would be more than adequate. The black paint on the eyelets suggests that the existing diodes are not original.

Thanks Martin, any idea on why someone would add the black paint to the joints? Anything I should do before unsoldering them?

My 1N5408 have the leads slightly rusty. What would you recommend to apply to clean them up?

I have not started getting back into this humming amp yet that I have , but what I would suggest to would be to yank out the PI tube and with your meter look for low level DC and AC voltages that should in no way be found on certain pins in the tube socket.

Thanks for the suggestion

[martin manning]

... any idea on why someone would add the black paint to the joints? Anything I should do before unsoldering them?

Sillyness, really. You could try to remove most of it with a cotton swab dipped in solvent, or just desolder as usual and remove as much of the old solder as you can with braid.

My 1N5408 have the leads slightly rusty. What would you recommend to apply to clean them up?

They will just continue to rust. I would get new 1N4007's.

[Stevem]

The dab of whatever color marker just shows that the solder connection was inspected after it was made.

This Ampeg amp in my photo has original red marker dab’s to show a separate inspection has been made.

If you have any white Vinegar at home soak a section of saturated paper towel around the rusted leads, if the rust is not too heavy then after about 1/2 hour the Vinegar should neutralize the rust and the steel will take solder again.

This is if you have nothing better to do because even with today’s prices 1N1007s are cheap!

[Bombacaototal]

Thanks Steve and Martin.

I appreciate the insight on the colour markings and thanks for pointing the Ampeg which had a similar approach

Yeah, will get some new diodes, seems like the best way to go about this.

[martin manning]

The dab of whatever color marker just shows that the solder connection was inspected after it was made.

But was it really necessary for the tech who replaced those diodes to paint the joints?

[Stevem]

Well i agree if this amp was working normal.in this case with this Bandmaster I think that whoever was checking things out the last time may have just made those marks to keep track of what solder connections where reviewed in the trouble shooting that was done, as the Fender factory would never do that even though a final inspection was done.

[Bombacaototal]

Will nail polish remover be enough to remove the paint?

[martin manning]

It's acetone. Give it a try.