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In the Forum: Off Air Audio
In the Thread: Forward in the past with old radios.

Post Subject: Your new tubesPosted by JJ Triode on: 3/16/2011

The ECL82 = 6BM8 was used a lot in low-end amps, both push-pull and single-ended. Actually, according to your radio's schematic to which you posted a link, they are not being used "one per channel," rather it is a mono radio (nothing wrong with that) and they are a push-pull pentode pair, class AB and good for around 8 watts. One triode section is a gain stage and the other triode section provides phase inversion for one output pentode only, the other pentode being driven directly off the first triode; this is the so-called paraphase splitter topology. Most PP people prefer the concertina, long-tailed-pair or transformer splitters (I recently converted a small EL84 amp from paraphase to concertina with good results) but in view of the somewhat collectible status of this unit I would not change it. The same applies to the option of connecting the output sections in triode, which would reduce power by about a factor of 2 with little likely improvement to the sound.
Ironic that your radio came to you "fully serviced" but the audio output tubes were not checked!
Note the text on the Italian web site says it is a cathodyne phase splitter, but it is not. Cathodyne = concertina, with one output section driven from the plate and the other from the cathode of the splitter section. This one is definitely paraphase. There is also global feedback around the power amp, I would say it is about 12 dB, which is reasonable. There is a shared cathode resistor on the outputs, so matched pentodes would be preferable but I would say not critical. The paraphase splitter has so-so phase balance anyway, as it depends on a voltage divider (I think it is resistors 88 and 89 on the schematic) to cancel the gain of the inverter triode, and this will never be exact without hand-trimming.

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