Microphonic Wires? - Vintage Fenders

[bbmade]

I have had quite a few vintage Fenders amps seemingly have a microphonic tube only to notice the wires seem to be the real issue.  I've tried re-soldering the connections, trimming the lead back slightly and re-soldering but nothing seems to help.

I have an AA864 on the bench right now and it sounds great but V3 seems to be slightly microphonic.  Change to different tubes and it got worse (old USA tubes I have) or better (newer 12AX7).  Tap around with my probe and the wires are like microphones. 

Am I onto something or on something here?  Any advice?  Is there a resistor to ground or something for the grids that could help? 

This amp sounds great so it's not a deal breaker but I can imagine selling it and having the buyer hassle me about V3 being noisy.

[acheld]

I think you're on to something, though I bet there will be differing opinions.

Wire does age, and in my vintage test equipment becomes quite stiff and brittle.

The question is:   what to do about it?    If you want to keep the vintage look, I bet you could desolder the offending wire, remove the cloth insulator, reinstall the insulator on new wire, and replace.

Or, simply replace the wire with some of Hoffman's Fender type hookup wire, keep the old part in a baggie with the amp as sign of authenticity, and call it a day. 

[tubeswell]

Tap around with my probe and the wires are like microphones.

Are you tapping the wires? or the chassis? Or by ‘wires’, do you mean reverb pan cables? You described it as ‘vintage fender’, but what sort of vintage fender is it.

[bbmade]

Tapping on the wires from the board to the tube sockets.  I've had old Fender amps where the board was like tapping on a microphone but other amps where the wires seems to be where the noise is coming from.  Obviously the grid wires are the loudest.

It doesn't matter what tube I put in V3 on this Bassman AA864.  The wires are still making noise when tapping on them.  I've tried gently moving them closer to the chassis or away from plate wires etc.  Doesn't seem to matter.

[shooter]

move any wire through an electro-magnetic environment and it'll conduct.  might better look for the radiating source.


is your board the multi-layer warped looking thing?

[tdvt]

 

I have had quite a few vintage Fenders amps seemingly have a microphonic tube only to notice the wires seem to be the real issue.  I've tried re-soldering the connections, trimming the lead back slightly and re-soldering but nothing seems to help.

Am I onto something or on something here?

I have recently had the same experience with two different vintage Fenders, a '61 Showman & '65 Princeton Reverb. It was definitely the wires & not the components they connected to or the board itself.
With the Showman I was able to find a happy medium with lead dress.

It was worse with the Princeton, particularly a couple of the under-board wires. Not the jumpers that are completely under the board, but the ones that pass through a hole on the edge of the board & head to an eyelet somewhere towards the middle of the board. Also, that board was noisy in a couple areas.

I used several applications of isopropyl alcohol, with gentle heat each time, to dry out the wires & the boards (upper & lower), which helped quite a bit. One wire I had to just replace & I ran it on top of the board,  through the air, which Fender did in a few places on that amp anyway.

[HotBluePlates]

I have had quite a few vintage Fenders amps seemingly have a microphonic tube only to notice the wires seem to be the real issue.  ...

Am I onto something or on something here? ...

It is probably impossible for the (single conductor, solid core) wire itself to be microphonic.  That's because there is no mechanism for it to generate an output voltage.

However, tubes and capacitors DEFINITELY can be microphonic, and there are known physical mechanisms that cause them to deliver an output AC voltage when the parts of either move relative to each other.

Most likely, when you tap a wire the mechanical energy is transferred to a tube (via the socket pins) or a capacitor (at the board eyelet) that actually makes the racket.

... It doesn't matter what tube I put in V3 on this Bassman AA864.  The wires are still making noise when tapping on them.  ...

Replace the wire then.

I suspect you will still have the noise afterward.

[bbmade]

I finished up the amp, didn't replace any wires.  With that particular amp, a lot of work had been done previously and I was trying to do the bare minimum to get it up for sale.