Balanced triodes....really needed???

[andershoeg]

Hello everyone!
I'm a little in doubt here. When I buy new preamp tubes, at least the places where i buy Them, there are no preamptubes with the "balanced triodes" for the phase inverter to buy. They are all just labeled 12ax7. My question is; is it really necessary to buy special tubes for the phase inverter slot, or can I use any of the normal 12ax7's??
B. r. andershoeg

 

[94Tremoverb]

No, it is absolutely not necessary. The phase inverter circuit in almost all amps is not symetrical anyway, so a balanced tube does no more than to lock in whatever imbalance - some due to the component value tolerances, most due to the inherently different circuits in each half - is present anyway.

There are a very few positions in a tiny number of amps where it might make a difference - these are parallel driver stages that typically come *after* the phase inverter in many very high power amps (200W and above mostly, eg the Marshall Major), but again it would require the rest of the amp to be accurately matched (which it isn't) before it made any real difference... and then the result would be to *eliminate* distortion. Some hi-fi amps are in fact designed like this, with very careful matching, exactly for that reason.

Save your money and just buy better *quality* tubes, not hi-fi thinking incorrectly imported into guitar amps! Anyone selling balanced triodes for phase inverters in guitar amps has a good line in snake oil.

 

[andershoeg]

Hey 94Tremoverb!
Thank you for your fast answer. You confirmed my thoughts about this subject
Glad to hear that I don't have to shop for something which I wouldn't know where to get....
B.r. andershoeg