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.

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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.

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Hey DCSam,


Thanks for your reply.


I did clean the board pretty thoroughly and didn't measure any DC leakage, and aside from the plate wires I replaced (those very likely were problematic as microphonics went down a bit) under board wires have been generally replaced or removed as art of converting from AB165 spec to AA864. I suppose I could just replace all wires in the vicinity, but those plate wires were the only ones that actually seemed microphonic.


I appreciate the thought, cheers.
-Dave

[dcsambrook]

Going to an FG board and replacing the grids with shielded solved the problem on my 67 AB165. I could chopstick anywhere between the inputs and grid connection at the tube and generate very unpleasant sounds. Sounds like your problem is localized at the tubes?

Before I did all that, I'd cleaned and retensioned the tube sockets and done pin drags on each. You?

[MORE_Guitar_Solos]

I've done most of these, but not sure what "pin drags" are, could you please explain?


Cheers

[dcsambrook]

You need a single pin the size of the pins on a tube that fits the socket you're testing. Push the pin into each hole and pull it out. You're checking that each pin is tightly gripped.

Hope that makes some sort of sense.

[MORE_Guitar_Solos]

You need a single pin the size of the pins on a tube that fits the socket you're testing. Push the pin into each hole and pull it out. You're checking that each pin is tightly gripped.

Hope that makes some sort of sense.

Got it, I'll try to rig something up like that, thanks.


Just took some voltages, and things are globally high, including the power section, by about 20vdc. across the board. 120VAC on the nose coming in with the step-up transformer (100-120VAC), heaters seeing 6.7VAC, 6L6 plates are at 440v, and little tubes are all over by a similar amount, so obviously mods to the power rail resistors won't really help.


On a whim I tried plugging straight into the wall at 100VAC, and got heater voltage at 5.7VAC and 6L6 plates at ~380v with correspondingly low voltages on all the small tubes. This is lower than ideal obviously, but do you all think 5.7VAC is sufficient to light the tubes? It was less microphonic at the reduced voltage...


In the meantime I'm searching for a local supplier where I can find some decent tube sockets...


Thanks

[dcsambrook]

Have you checked the sockets for shorts between contacts or between contacts and shell? Other than pin fitment, most of the socket issues I've seen were caused by heat damage or carbon traces from arcing. Stuff happening not caused by the sockets themselves. Should be able to diagnose a socket as good or bad without replacing. Good contact, no unwanted connections.

[dcsambrook]

All jacks and knobs tight? Shielded wires grounded at one end only? Solder on input jacks good?

[glass54]

Hi Dave,
Regarding Tube life, etc see attached (interesting read and I believe Mr Didden is a respected guy).
Personally, I prefer 6.3V AC +/-5%. I have encountered many Marshall's in the 6.8 to 7.3V over the years (required buildout resistors)
https://audioxpress.com/article/the-internal-life-of-vacuum-tubes
Regards
Mirek

[MORE_Guitar_Solos]

Thanks for all the replies, everyone.


Somewhat embarrassingly, after I did a thorough job of retensioning all the sockets, the problem was mostly gone. I think the V1 socket must have had some sloppy slots, probably on the plates.


There is still a bit of noise with a 12AX7 in V1, but I think this may just be due to the design with that second gain stage, and the fact that I removed the many high frequency snubbers from the circuit to open up the top for guitar use. With a 12AY7 in V1 it's dead quiet, no hum, and sounding great.


Happy to have this thing back in fighting shape, thanks for all the assistance, guys.


And Glass54, I'm running it at the 120VAC level and things seem to be pretty happy.

[tubeswell]

It's classic tap on the faceplate and get pings, ...

I know you've tightened the socket pin clamps etc, but if you're getting residual microphonics when you tap the chassis, it could be something like a loose input jack tip switch contact that isn't firmly closing on one of the input jacks when there is no plug inserted (in which case you can try carefully re-tensioning the jack tip contact)

[dcsambrook]

Make sure all pots and jacks are tight and have the correct lock washers.

[MORE_Guitar_Solos]

Make sure all pots and jacks are tight and have the correct lock washers.

All good on those measures, thanks. Amp is now pretty healthy and I'm happy with it.


Thanks again to all for your suggestions, dudes.
-Dave