I just bought this amp for my first venture into push pull.
The intergrated has already been modified to work as an amp only. First thing I did was plug it into some 89 dB speakers to hear how it sounded since this was a working amp when bought . Right off the back I wasn't crazy about the tone of this amp and noticed around 95 dB at listining level a little crackling in the highs. Next step was to dig into it to see what was going on in it.
First pic is the factory schematic with the pre and tone controls removed and what I found is in the second pic.
Stock
Modified
Quite a few mods have been made to the original circuit (opinions?). I noticed a couple of other things going on. Resistance values in parentheses are out of spec actuals. The two caps have ? marks because the values are unknown. I measured them with two differant meters and could not get any readings. All other caps and resistors are within specs.
Here is the 6AN8 tube data sheet.
http://frank.pocnet.net/sheets/127/6/6AN8A.pdf
The can cap in the PS looks to have been replaced and checked out good with an ESR meter but the first section reads 85uF off the rectifier and the second section 33uF. The 4.7k resistor reads 5.3k.
Stock PS
If the modified circut is acceptable (big if since I haven't seen a pp schematic before that resemble the mods) my plan is to replace the can cap with a 50+50uF JJ cap I have, then replace the resistors that are out of spec and the two questionable caps. I would also like to change the bias to a 200 ohm resistor and 150uF cap for a pair of EL84's. Right now the quad shares one bias circuit. Remove the hum pots from the H+ too.
I would appreaciate some help to get the best out of what I have to work with here. Something is telling me that the EL84's should sound better than what I hearing with this amp so far.
Brad