I did as you suggested xtian; I did not find any info on dedicated EL84 stand alone power amps from the Building Blocks thread of the site you mentioned, which is shame as the AO-39 power amp would take a minimal amount of effort to convert. Some of the modifications that would have to be addressed are as follows:
• The "PI" is really just a two input dual phase driver. The actual dual phase signals are developed using a transformer located on the preamp chassis (which I don't have). The driver has a signal balance adjustment (left end of chassis).
• The output tubes have a HUM BAL adjustment in the cathode circuit that actually balances (matches) the idle current through each output tube.
• The two NFB loops are taken from the plates of the output tubes and loop back to the cathodes of the PI.
The power and output transformers could stay right where they are, the 5u4 rectifier tube could stay seated also to hold down the volume and create sag (Fat Fender sound).
It would be helpful if someone designed and tested a circuit for the AO-39 at some point. They could sell circuit boards (populated and blank) for a business. The AO-39 is juicy, full and fat sounding through out its entire volume range; it was designed for a clean, full frequency organ sound. Where the AO-39 really shines is in guitar lead work using a modern all tube guitar preamp, the notes flow together like drippy sweet liquid. A friend of mine's father who was a Ham Radio guy converted one of these for his son. The only thing I remember was he moved the 12AX7 closer to the output tubes and rewired it to become a mixer/driver, can't remember much else. I remember how alive the conversion sounded. His Dad passed away years ago, the secret on how to convert the AO-39 was lost with him.
I can't Blue Print the amp as it was stolen while my friend was on tour. I guess no one is interested in (dedicated stand alone) AO-39 power amps at this time, to bad as I have three of them I saved from the junk heap over the years hoping some one would discover how to convert them as they are. Know of any last ditch efforts I could focus on regarding my quest.
![Sad :(](./images/smilies/icon_sad.gif)