Bassman AB165 static

[yaryaryar]

Hi all,

I've got what should be a simple problem, but I can't seem to narrow it down. It's a '67 Bassman, AB165, and it's got some low-level static going on at idle. It's independent of the volume or tone controls. I've cleaned all the pots and sockets. I've tried known-good tubes in all locations, so it's not a bad tube. When I pull them one by one, the static goes away when V2 is pulled. So, I assumed it would be the plate resistor on V2, so I replaced that, and the problem remains.

Any ideas on what to try next? Thanks so much for any suggestions.

[Leo_Gnardo]

Typically I replace all the C's & R's in the V2 circuit. Sometime that's not enough. If noise persists after replacing the parts what's left? The circuit board, and we know that black cardboard develops leakage paths sometimes after decades of aging & absorbing moisture. Heck even brand new builds on black cardboard have turned out to be leaky.

Enzo's method, and you might give this a go before swapping any parts, warm up the circuit board area with the V2 R's & C's on it with a hair dryer or heat gun. That may drive out enough moisture to get the noises to stop. If not, do what I do: replace the circuit with new parts but built on tie strips instead of stuck on that leaky ol' board. Works a charm, though it does take a good deal longer than the hair dryer / heat gun method.

[Justin Thomas]

Okay, I don't normally advocate just changing out "this part" or "that part," but, mine had a similar issue. I tried the usual plate load fixes. What it ended up being was one of the 220k mixing resistors. So, it may be one of the not-expected resistors. Takes a few minutes to change out; just don't trash the resistor in the process, and replace it if it doesn't fix the issue. Leo's advice is good - check ALL the parts associated with the stage. But I would recommend doing it one at a time... Anything that doesn't help should be put back to original, IMHO. Good luck! They're sweet amps!

Justin

[yaryaryar]

These are great suggestions. I actually don't have a hair dryer or heat gun, so I put the chassis out on the patio for a dry, sunny afternoon. No change. I'll see if I can borrow a hair dryer in the next couple days. Then I'll try swapping out the mixing resistors, but I'll have to order those resistors anyway, so I'll have to wait a bit. I love the idea of building out that mini-circuit on a separate board... but like you said, I'll only do that if all else fails.

Possibly related: I'm poking around with a chopstick at the components and solder joints in that stage, and I noticed that one of the ceramic disk capacitors (.01uf, leading into pin 7 of V2) is microphonic. It doesn't test for continuity, so I know it's not completely shorted. But none of the other caps are microphonic that I can tell. Could this be the source of static?

[Randall]

Doubt that cap is the problem, disc caps sometimes are a little microphonic if they are tapped. One thing you could try while you are waiting for parts is to re-flow all the solder connections to rule that out. Not too much heat or time with the solder iron, just enough to reestablish those connections. I have gone so far as to remove old dull looking solder from vintage eyelets and replace with new to quiet an old creaking groaning amp.

[Tom Phillips]

...I'll try swapping out the mixing resistors, but I'll have to order those resistors anyway, so I'll have to wait a bit...

Or...just swap the two mixing resistor locations. If the static moves to the other channel then you have found the problem. If the static stays in the same channel then you have ruled out a problem with the mixing resistors and there is no need to move them back to the original location.