I was given an AGS stereo receiver recently and discovered an interesting biasing scheme for the output tubes (6BM8s push pull). The cathodes of the four 6BM8s are wired in parallel with one end going thru the heaters of a pair of 12AX7s, which are the phono preamp section, and then to ground. So the 12AX7 heaters in series provide the DC biasing for the 6BM8s. The reason I started investigating this is because one of the 6BM8s would glow red hot when I switched the unit on. I tried swapping tubes but this didn't solve the problem. I looked at voltages around the offending tube and found the plate voltage was about 20 V lower than the rest, so I thought maybe a problem with the transformer. After about the fourth power cycle, everything seemed to work with no repairs done. The only problem was now there was about 8 V on the 12AX7 heaters, not enough to run them. Suddenly the heaters came up and a different 6BM8 started to glow. Thereafter a different tube would glow everytime it powered up. I'm not really interested in repair as I will be using the receiver for parts, but I wonder if anyone would like to comment on this.
On a prevous post I talked about an AGS PA amplifier. The power supply is one long daisy chain of resistors and caps with the result that voltages to the last tubes were very low and dropped with the slightest load, not very good design. I spoke to a fellow about it who used to have a repair shop, and he assured me that AGS was crap. Judging by the number of repairs to the receiver I have, and the quality of construction, I would have to agree. However, it has 16 tubes and some other goodies so will make a good organ donor.
Don