Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: mresistor on July 11, 2012, 03:53:11 pm
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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?
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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.
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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.
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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.