NewYorkDave
Well-known member
There was a thread here on Prodigy about these amps, including some great analysis by PRR, but the thread seems to have disappeared. Bummer.
Schematic
I rescued several of these from the scrap heap. I was going to take the transformers and junk the rest, but I'm kind of reluctant because they're nicely-made amps (and, for what it's worth, apparently rare).
Looking at the circuit (which I traced myself, no schematic was available) I see a couple of areas for improvement. First is that terrible input arrangement, with the bias current for the transistors being drawn entirely through the wipers of the level pot. (As you would expect, this caused a lot of noisy if not downright jarring intermittents when the amps were still in service). The step-down ratio of the input transformer seems a bit excessive except for the fact that the pot value is so low--and I might get rid of that pot, anyway. Beefing up R11 (higher power rating) and C1, C2, C3 (higher voltage) and increasing B+ by a few volts is an obvious thing to try.
Bypassing R11 might be worthwhile; the net signal voltage across is is zero in theory but that's only if the output pair is perfectly balanced (never). My experiments with push-pull outputs in tube line amps showed me that whether the cathode resistor was bypassed or not could have a measurable effect on the distortion content--with more higher-order distortion products with cathodes unbypassed, at least in the circuit I was working on.
It does seem odd that the DC operating points are arranged so that R11 has to be so big; half the supply voltage is dropped across it. But as I recall PRR pointing out, this does make the output section hard to blow-up.
Schematic
I rescued several of these from the scrap heap. I was going to take the transformers and junk the rest, but I'm kind of reluctant because they're nicely-made amps (and, for what it's worth, apparently rare).
Looking at the circuit (which I traced myself, no schematic was available) I see a couple of areas for improvement. First is that terrible input arrangement, with the bias current for the transistors being drawn entirely through the wipers of the level pot. (As you would expect, this caused a lot of noisy if not downright jarring intermittents when the amps were still in service). The step-down ratio of the input transformer seems a bit excessive except for the fact that the pot value is so low--and I might get rid of that pot, anyway. Beefing up R11 (higher power rating) and C1, C2, C3 (higher voltage) and increasing B+ by a few volts is an obvious thing to try.
Bypassing R11 might be worthwhile; the net signal voltage across is is zero in theory but that's only if the output pair is perfectly balanced (never). My experiments with push-pull outputs in tube line amps showed me that whether the cathode resistor was bypassed or not could have a measurable effect on the distortion content--with more higher-order distortion products with cathodes unbypassed, at least in the circuit I was working on.
It does seem odd that the DC operating points are arranged so that R11 has to be so big; half the supply voltage is dropped across it. But as I recall PRR pointing out, this does make the output section hard to blow-up.
) for video amplifiers that plugged into what appeared to be the same frame. The supply voltage on those was +18V. I measured +15V on the supplies I have here, but it's possible that they may have drifted low after 30+ years. The PSU circuit is old school, all discrete, with trimmers and whatnot. Anyway, I decided to try +18V.
