AB165 Bassman crazy microphonic V1(2)

[MORE_Guitar_Solos]

Hi guys,


I have done some searching on this but can't find answers to specific questions I've got.


I've recently resurrected this amp that I've had for ~25 years now. Recapped it and removed some silly mods I'd done in my youth, it's basically an AA864 now but with guitar-centric tone stack values in the bass channel and the deep switch is a standard bright switch now.


It's now pretty quiet with no hum but V1 and to a lesser extent V2 are insanely microphonic no matter what tube I put in there, and in fact I have 12AY7's in both positions to lessen the effect. It's classic tap on the faceplate and get pings, tap the tube(s) and it's pretty loud. As mentioned, I've done endless tube swaps, so it's probably not the tubes, I've replaced just about every component in the circuit including all tone caps, the plate resistors, the slope resistors, all coupling caps, the grids are shielded wire, every solder join in the amp has been sucked out and reflowed. I noticed that not only the tube but the plate wires were microphonic, so I replaced those too, and that did reduce the problem a little bit.


I have two questions:


1. I've replaced virtually every component except the tube sockets. Is it possible for tube sockets themselves to be microphonic? Is it worthwhile trying to change these?


2. The voltage on the entire pre-amp is high. I note on the schematics the plate voltage of V1 is meant to be around 235v, but I'm seeing north of 260v. When I recapped the amp I replaced the power supply node resistors with stock values, but would it be worthwhile increasing the first node (after the choke) resistor form 1k up to say 1.5k, hopefully lowering global voltage for all the small tubes? Is this over voltage likely to increase microphonic issues in the preamp?


Many thanks in advance, and it's been a few years since I've posted, so glad this fantastic resource still exists!
Dave

[stratomaster]

I have two questions:
1. I've replaced virtually every component except the tube sockets. Is it possible for tube sockets themselves to be microphonic? Is it worthwhile trying to change these?

2. The voltage on the entire pre-amp is high. I note on the schematics the plate voltage of V1 is meant to be around 235v, but I'm seeing north of 260v. When I recapped the amp I replaced the power supply node resistors with stock values, but would it be worthwhile increasing the first node (after the choke) resistor form 1k up to say 1.5k, hopefully lowering global voltage for all the small tubes? Is this over voltage likely to increase microphonic issues in the preamp?

1. Yes. It's possible for there to be a structural issue with a socket that makes it noisy or microphonic.  Far more likely though is some other cause, but it seems you've eliminated most of those.  I'd advise to reflow the solder joint between the two input jacks as this tends to crack and become microphonic. Add a bit of fresh solder or a dab of flux and use high heat.  You can sometimes even hear the jacks shifting and releasing the residual stresses between them.  Recheck for microphonics. If you've still got them, then move on to the sockets.

2. What is your incoming AC voltage?  What is your filament voltage?  If those are in line and you want to still reduce the preamp voltage then yes, changing the dropping resistor is a good move. Otherwise if the other voltages are also high then a voltage bucking circuit is probably the more direct fix.

[MORE_Guitar_Solos]

Thanks for your reply, Stratocaster.


I neglected to add that I'd also redone all the inputs too, so those are probably kosher. I'd rigged input 2 on both channels to be a tonestack lift via footswitch, that's cleaned up now. Youth is wasted on the young!


I'm in Japan so I'm using a step-up transformer from the wall and right on 120vac is hitting the PT. I'll do some proper voltage readings again when I have time but I believe the heaters, power section and bias were pretty close to ideal at about 425v on the plates and biased pretty conservatively at around what should be ~60% dissipation (32-34mv at the cathodes). Finding decent tube sockets over here will take a while so I'll try lowering global preamp voltages a little and see if that helps.


Thanks again!
Dave

[dcsambrook]

Have you checked the fiber board for DC voltage between eyelets? Cloth wire insulation can also become conductive, especially runs under the fiber board.