As you may remember, the amps we built were flawed because the tubes were not reliable, and the bias regulator zener diodes blew up about half the time. So I just cut them out and built the amps without them. Since I did not have good tubes with me, We tested the ones I did have and the ones that seemed okay were the ones we used.
These desperate decisions yielded an amp that was okay, but not as spectacular as it should or can be!
Here is the latest:
I tested lots of output tubes and found out something I did not know. I found out that some EL84/6BQ5 tubes are rated for 550 volts, and others are rated for 300 volts.
Most American 6BQ5 tubes are good for 300 volts. I thought a 6BQ5 was a 6BQ5 and good for 550 volts. Not true. This amp runs at 425 volts, and if you use a 300 volt tube, they tend to run away and glow cherry red. Not good! Amperex are the best, and can easily take even a thousand volts, whereas the ones we used at Carverfest are good for only 300 volts. Even the great Telefunken tubes are good for only 300 volts. So don't use them. You will be tempting fate. Mullards are good for 550 volts.
The tubes to use are the brand new Sovtek 6BQ5/EL84 tubes sold by Electro - Harmonix. I have tried them for extended time periods, fifty of them to be exact, without a single failure. That compares with about one in three failures for the Red Army tubes (300 volts) that I got for a buck or so. These EH tubes sound great and are superb tubes. You can get a matched quad for about $29 dollars, or about $7.25 dollars each. They are almost as good as Amperex. Buy them for your Carverfest amp! eBay item number 290613742187.
The next item is the bias regulator. Get a single 15 volt, 5 watt zener and install it across the 220uf/35 volt cathode bypass capacitor. This capacitor is in the cathode circuit of the four 6BQ5 output tubes. Solder the cathode end (the band) to the + end of the capacitor and the anode end to the - end of the capacitor. The schematic you have may show two 7.5 volt zeners, or I might have erased your copy. In any event, the circuit should be a 15 volt, 5 watt zener across the 220uf cap.
Buy it at any on-line place like Parts Express or Mouser. The brand is Micro - commercial, part number 1N5352B - TP, 5 watt zener. Cost about 55 cents.
These changes will make the amp sound the way it should. The bias regulator especially! Without it the bias floats all over the place and the amp sounds mushy. With the regulator working the amp gets a better soundstage, greater depth, sweeter imaging; even the presentation will be wider and more detailed. I wish the ones I brought to Carverfest had worked. They got to me the day before I left, and I did not have the time to try them, but I thought they would work. Oh well.