Heathkit UA-1 (or UA-2 or AA-61)
This is a small mono power amplifier. It uses a pair of EL84 (6BQ5) output tubes running in Ultralinear mode. It uses a 6AN8 as the input and phase splitter, and a EZ81 (6CA4) as the rectifier. This little amp was quite an eye opener for me. It has fantastic output transformers, I'm talking just plain amazing. The amp's -3 dB points are at 8 Hz and 65 KHz at full power. Plus the Square wave response at 20 Hz (5 watts) is very clean, it looks like there isn't a transformer there at all. After all the most critical part of all tube amps are the output transformers. These transformers (I believe that they were made by Chicago Standard), are larger than the transformers that are on the VTL 50/50 amp, but this amp is only rated for 12 watts! The amp was originally designed with two feedback schemes. The first is the traditional voltage feedback scheme, where some of the output signal is taken from the 16 ohm tap on the output transformer, and fed back into the cathode of the input pentode. The amp sounds amazing in this mode. The second scheme uses some current feedback as well as voltage feedback. With this second scheme, the current from the speaker is sensed via a small value resistor and fed back into the cathode of the input stage. The voltage feedback from the output of the amp is reduced by about half to keep the total feedback about the same. This may have worked well with old speakers, but doesn't sound very good with my Mission bookshelf speakers. And it sounds just awful with the Quad ESL's. You can flip a switch on the front of the amp to select either the great sounding traditional all voltage feedback, or the bad sounding current/voltage feedback scheme. When I rebuilt these amps, I removed that feature completely from the amp, and used the voltage feedback scheme. I'd say that this amp is a real sleeper, and you should buy a pair of them if you get the chance. The UA-1 and UA-2 amps are essentially identical. The only difference between the UA-1 and UA-2 are that Heath increased the value of the cathode resistor on the output tubes because they were running them WAY too hot, and burning them out quickly. The UA-1 had a 130 ohm resistor in the cathode at first. They changed that pretty quickly though, and the schematic here reflects that change. They also reduced the value of the screen grid resistor on the input stage to 680 K ohms. I have tried both the 1 meg and 680 K resistor and there is no difference that I can hear or measure. The AA-61 is a cosmetic change, Heath went from a gold colored chassis with a raised plastic "Heathkit" logo to a black chassis with a painted on logo. I have a pair of the AA-61's.
This amp slightly edges out the Fisher SA-16 as my favorite for the Quad ESL's.
I designed a capacitor board which replaces the original twist-lock capacitor in the amplifier. The board also provides a convenient location to mount the tweeter saver and feedback components. Most importantly, the board also provides a line fuse and a b+ fuse; needed safety features that were neglected in the original design.