Tubes and GAIN on Cary Rom-808

Thank you!

There are a few factual errors in Jason’s posts. Use of a cathode resistor does not necessarily limit the ways in which the tube is biased . Current is measured in amps, not “watts”, but if you multiply plate current by plate voltage (voltage from plate to cathode) you arrive at the power in watts that are dissipated by that tube in that particular circuit. Manufacturers publish an upper limit for plate current, voltage, and power dissipated. It’s a good idea to observe those limits. Until now I’d never heard of the 12bz7, but specs reveal it is a high mu, low transconductance dual section triode. Not much different from a 12AX7. 

The first iteration used 6922. The R second series uses 12 series tubes . I still have my Rocket 88R and love that amp paired with an SLP 98 P. Such a great sounding good value they are. From the manual...

"Bias levels will change with time of day and power usage in your home. In fact during the 
summer months when the air conditioner is running you may have less AC line voltage 
available and in turn the bias will be slightly lower. This is not critical. In fact the Rocket 
88 R is designed to operate in the range of 180 to as high as 240 mA DC current. The 
best sound will generally be produced in the 195 to 225 mA region. You may wish to 
experiment if you desire. NEVER ADJUST THE BIAS SETTING WITH A VOLTMETER 
SET TO THE VOLTAGE POSITION"

" The basic premise I started with on the Rocket 88 R was to develop a truly linear, wide 
bandwidth, stereo vacuum tube power amplifier. Also, in direct response to Rocket 88 owner’s 
comments about input sensitivity we wanted to ‘harmonize’ the input sensitivity level of the 
amplifier to match other components used in an audio system. I was seeking an amplifier design 
that would precisely mimic the input signals up stream from the source signals while being more 
sensitive. In the design, the front end stage EL-84 tubes are used as current sources for the high 
gain 12BZ7 driver stage and phase inverter tubes, with one used for each channel. (12AX7 or 
12AY7 tubes may be substituted for the 12BZ7.) The EL-84’s do not do any of the amplification. 
They allow the 12BZ7 phase inverters to operate in an equal balanced fashion. 
From the first incarnations of this project to the final result it was an intriguing concept, to say 
the least. We found that changing the input circuit to accept the ‘12 series’ vacuum tubes caused 
a dramatic drop in total harmonic distortion. Overall circuit dynamics and frequency bandwidth 
improved with the new driver tubes as well. We have come to the conclusion that the limiting 
factor in the previous Rocket 88 design was the 6922 tube we used in the front end stage. The 
‘12 series’ tubes certainly fixed that!"

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

Same goes for reviews on the amp.

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. 

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.

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).