Tubes for Fender Vibrato / Tremolo

[mresistor]

Just curious - are there any other tubes besides 12AX7 that work well in the Tremolo slot of a Fender amp such as a Vibro Champ?

[P Batty]

I read that it needs to be "strong" whatever that means.  I usually end up putting a good tube that is microphonic in that spot.

[mresistor]

Yeah - I was thinking a vintage Telefunken or Sylvania would be a "tone" waste in the tremolo position. I think I will try some other 12 series in there and see what gives. I ordered up some unlabeled EH 12AX7s for this slot in my amps.

[HotBluePlates]

I read that it needs to be "strong" whatever that means.  I usually end up putting a good tube that is microphonic in that spot.

Reading an old tube book (from the 60's), the author stated oscillators need a tube with good transconductance so that it will start oscillating.

Transconductance (Gm) is the ability of a tube to respond to a voltage change on the grid with a current change at the plate. If Gm is insufficient, the tube can't "get going" oscillating.

Seems that this works because a tiny voltage change on the grid of a tube with strong Gm yields a plate current change which flows through a plate load, making a bigger voltage output which is then fed around to the grid. And away we go.

Interestingly, a 12AX7 has the lowest Gm of any of that series of tubes. So any 12A_7 should work, unless the other triode is used in a way where you need the gain of the 12AX7 and not the other type you plugged in.

Of course, it's safe to try 'n see.