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!"