Princeton tremolo + bias setting question

VMUNIX

Member
2009-09-09 2:55 am
south
So I built this, and sounds really well, super clean with 410 volts at the plates and with the bias at -34 volts, pretty much I did exactly the schematic, the only mods are a trimpot for the bias, some screen resistors, and a 5y3 instead of gz34 and a few other things.

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The problem is when I switch the tremolo on and vary the "intensity" pot, the current draw goes beyond what is considered the "safe" point for class AB 6v6's , so I'm not sure what to do, one thing would be to put the "intensity" at maximum and then adjust the bias so it stays in the safe zone even when the vibrato is maxed , the thing is I'm starting to think this bias based vibrato is a terrible idea, to have a front panel pot that affect this much the longevity of the tubes, any one have experience with this ?
 
This circuit has been around for donkeys years with no issues. The Vibrato modulates the grid voltage but should not over cook the grids. Is the coupling capacitor the same 100n value? There will only be a maximum of 10V peak to peak from the vibrato stage. Trimpot for the bias is the problem. Where did you put it and is the 25u cap still on the -ve supply output side going to the grids?
 
The instantaneous tube current might peak over whatever limit you are considering, but the average tube current should remain about the same. having the trem signal increasing and decreasing the tube current is no different from the music signal doing the same thing.

And more importantly, what Jon pointed out: This circuit is not the first of its kind, and it is over 50 years old. All those amps with this design have been cranking out the music for decades. I don't think it needs to be changed now. If you look up the specs on the 6V6, and see how it likes those 420 volts on the plate.
 

VMUNIX

Member
2009-09-09 2:55 am
south
This circuit has been around for donkeys years with no issues. The Vibrato modulates the grid voltage but should not over cook the grids. Is the coupling capacitor the same 100n value? There will only be a maximum of 10V peak to peak from the vibrato stage. Trimpot for the bias is the problem. Where did you put it and is the 25u cap still on the -ve supply output side going to the grids?

Yes, values are the same as the schematic, except the 27k bias resistor has been replaced by a 10k resistor and a trimpot, but the voltage is still -34, gives around 20mA per tube.
The problem is the intensity pot with the vibrato "off" , varying to the max the bias voltage rises to -25 (that operates the tubes at 24 or so mA), at the minimum setting it drops to -40 and there's a slight decrease in the output volume but that is not really a big problem, I'm more concerned about leaving the por at maximum, it is still about 10 watts each tube but may be the tubes will not last for long, I'm using nos GE.
 
One saving grace is the valves are not over driven for long, just the positive side of the colpits oscillator output. Some Fenders use a neon and LDR, the LDR is in series with the drive to the phase splitter, it doesn't sound so good though. If you are worried, why not place a 47R in series with the cathodes to ground, that will limit the current drawn by the 6V6s. Tweaking the bias voltage will give you the desired quiescent current. Or just increase the 1M resistor that feeds the vibrato to the bias.
 

VMUNIX

Member
2009-09-09 2:55 am
south
One saving grace is the valves are not over driven for long, just the positive side of the colpits oscillator output. Some Fenders use a neon and LDR, the LDR is in series with the drive to the phase splitter, it doesn't sound so good though. If you are worried, why not place a 47R in series with the cathodes to ground, that will limit the current drawn by the 6V6s. Tweaking the bias voltage will give you the desired quiescent current. Or just increase the 1M resistor that feeds the vibrato to the bias.

Thanks Jon for the tips, I've seen the ldr-neon version and thought that it was a sort of improved design, now that people tell me the Princeton bias tremolo is fine I possibly leave it as it is.

are you familiar with this amp? what do you think of it ? the tremolo is connected to the PI, however is a LTP.

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