Robbing the Vibrato Tube for Input Tube on Fender amps

[nevetslab]

When I'm going thru initial check-out on Fender Combo amps, such as Deluxe Reverb or Fender Twins, for example, and come upon a very noisy tube in either the Normal or Vibrato Ch, I'll go to Standby, and remove V5 Vibrato Circuit Tube and substitute that tube in the offending input channel's tube (V1 or V2), just in case I luck out and find a perfectly quiet/working tube in the Vibrato circuit. While that didn't work in the previous 65 reissue Deluxe Reverb I had on the bench, it DID work on the one that's up there now..

Over the years, I must confess to having used this trick and saved $$ on preamp tubes. It's often like having a spare working tube on board that can be used. Is it as good as a fresh new 12AX7/ECC83S? Sounds like it is for the most part. I wouldn't do this for an input mic tube on a tube condenser mic, of course. And, if it doesn't sound up to snuff, I THEN replace the tube.

Am I the only one pulling this one, or is this a widely used trick out there?

[xtian]

Same deal, swapping sensitive V1 tubes with the PI, which is usually far less sensitive to microphonics.

[The Dude]

Yep, I'll usually try that first with microphonic tubes, if the tube is good otherwise. The circuit has a lot to do with whether a tube is microphonic or not.

[Helmholtz]

It seems logical to use the least noisy/microphonic tube in the input stage as its noise gets amplified most by the following stages.
Tube noise doesn't matter in the tremolo circuit at all.

[Enzo]

DOne that many times. Even if I don't leave it there, it is an effective troubleshooting move.

[g1]

Yes. And I always try them all in V2 position, then put the worst tube into the Trem position. If the Trem still works, it stays there.

[drewl]

Do it all the time.

[Leo_Gnardo]

Sometimes I think Leo - Fender that is - arranged the circuit so that field analysis of problems could easily be done. The technique we're discussing is an example. Also, on occasion I find some prime kind of tube relegated to the vibrato position. I've even found Telefunkens there, hiding their light under a bushel so to speak. Whenever possible I try to save the customer some $$ through tube swapping.

On rare occasions I've found tubes in the vibrato position that send noise into the audio. I know it seems improbable, and I was taken aback first couple times I ran across it. Swapping a new or known good tube into the vib position, noise gone, proof enough for me. When troubleshooting hard to find noises don't neglect this possibility. To be fair, I've encountered it only a very few times in a long career of amp wrenching. The tubes that misbehaved in vibrato function always sounded horrendous in preamp positions, clearly defective. OTOH tubes that sound a little "off" do often function well for vibrato.

[Enzo]

I know it seems improbable,

But of course that is the thing. It takes only a few seconds to swap a tube around, and then we know, on way or the other.

[Helmholtz]

Of course a defective tube can cause troube in any position.