Tubes and GAIN on Cary Rom-808

The NOS Black plate 12BZ7 tubes and the 12AX7 tubes are possibilities for GAIN portion of my Rocket 88.

I have little understanding of these. I have them both. I check the bias when I roll them.

Does anyone have knowledge enough to expound on the subtle differences/changes that can be listened for when choosing either one… if there is any?


128x128Ag insider logo xs@2xmoose89

The gain or amplifying power of a tube is called the "mu" factor. You can look this up in any tube manual. I don't have my GE tube manual at hand so I can't comment on the difference in mu of the 12BZ7 and 12AX7. I recall the 12AX7 is considered a high mu tube (100 mu).

Bias refers to the operating current of the output stage of your tube amp. The driver tubes (12BZ7, 12AX7) use cathode resistors. This is called "cathode bias" and is not usually adjustable, so no need to worry about it.

I recall a good bias current for a 6L6GC is about 35 milliamps. This sets the amount of current (in watts) the plate/cathode can safely sustain. Excess current on the plates will cause "red plating" and will shorten tube life. 

The manual for the OP's amp states preamp tubes as EL84 and 6922(6DJ8)?

Same goes for reviews on the amp.