Bruce,
I initially read about this interesting cct at the DIYparadise site, then found that you have taken the ball and run with it. I have a pushpull 6L6GC Chinese stereo amp that I would like to try the output stage principles on, it is point to point wired so easy to modify (compared to a PCB-based design). If possible, could I use a part or parts of the existing preamp cct to save major mods in the first instance? Maybe drive the output pair of valves from the first half of the 6N1 only? Attached the current schematic below.
This amp worked well for a while with the 6P3 (Chinese 6L6) valves, good bass, sounded excellent at times, but a bit too close to a tranny amp sound at other times. Then it would occasionally blow a fuse, then one valve flashed up, and the fuse blew again, so I bought some EH EL34's, they sounded very strong in the midrange but no good overall. In hindsight this was not the EH EL34's fault, as there was probably a fault in the amp. I then got some TAD 6L6's and put them in, still didn't sound too good, and one of them flashed up and the tube was white inside with lots of white ash inside it after the amp was turned off, or the fuse blew, whichever happened first. All this time I had had a few fuses blow, several weeks apart, amp in daily use, so decided there was an intermittent fault and temporarily retired it. Recently I did a lot of inspecting (with x 2.5 reading glasses) to look for visual clues, and drew up a schem while I was at it, and tested most of the resistors and compared resistances between similar points on each channel. I could not find anything wrong. The amp uses fixed bias and I was suspect on the bias pots as they were the only component in the whole amp that looked cheap, and when I was biasing for different valves they were difficult to adjust, so process of elimination, remove fixed bias and give it cathode bias. After 2 days running 2 x TAD 6L6 and 2 x 6P3 it is sounding quite good and hasn't blown a fuse - yet...So maybe now is a good time to move on ahead and try the Oddwatt cct, the EH EL34's survived this amp with no casualties and sound great in an SE amp I am running, so maybe I could do the Oddwatt conversion and run the EL34's while still on holidays and have the time. As you can see from the schem the bias is proly on the cold side, I am having difficulty matching 1960's datasheets "typical operating conditions" to modern day UL connected pentodes, am still a newbie. Sorry for the long rave.
Attachment:
Ming Da MC34B schematic mod 1.gif
Ian.
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