1959 model Zenith SRS12W amplifier from a stereo upgrade speaker unit containing a 12" woofer, a 5" high frequency speaker, and an 11dBW push/pull 6BQ5 amp with an anode follower phase inverter. I have removed the amp from the speaker so it can be used separately. I completely removed the tone controls from the circuit and removed the 6AT6 cathode bypass capacitor to reduce it's gain somewhat. I then reduced the feedback resistor from 22k to 15k to further reduce gain and I experimented with bypassing the cathode resistor of the 6BQ5s but found it unnecessary. I have acquired a second identical amplifier with matching codes on the transformers in order to have a stereophonic pair. The schematic of the modified design is here. The amplifiers use a 5Y3GT rectifier, two Amperex Holland "D" getter 6BQ5 output pentodes, one 12AU7 driver and one 6AT6 input tube. If building from scratch one could use a 12AX7 or 6EU7 rather than the 12AU7 so there would be no need for the additional gain provided by the 6AT6. Wanting to keep as much of the original Zenith circuit as possible, I decided to retain the 12AU7 and 6AT6 setup and left the 6AT6 out of the feedback loop as Zenith did. One can argue that some improvements could be incorporated to improve the amplifier's specs a bit and this is true, but if you can forget about specs for a moment and just listen with your ears you will find it quite enjoyable just the way Zenith designed it. Zenith selected 1k and 2k2 resistors for the cathodes of the 6AT6 and 12AU7. In theory 2k2 and 1k5 respectively should be more ideal but I tried this change and ended up putting it back like it came. It should be noted that this amplifier inverts absolute phase. This could be changed if you desire by switching the appropriate leads on the output transformer. The output transformers are Zenith number 95-1623.
This amplifier, like many 6BQ5 amplifiers, has a remarkable presentation which will come as a surprise to those who would judge the amp on the size of his valves.
The anode follower inverter, in my opinion, gives this amp a magical characteristic that is missing in many other amplifiers. Gabe Velez discusses this very thing in detail on his Magnificent Magnavox page.