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Vintage threads from the first ten years

geloso schematic & EL503tubes?

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2/25/2000 1:38 AM
Marc	geloso schematic & EL503tubes? Hi! I am looking for any info on a power amp I picked up on eBay; It's a mid 1970's Geloso 6.1/4113 and it has a quad of EL503's for around 100w. Nice heavy amp, if I can get it to work... Anybody know a source for the EL503's? Any help would be appreciated. thanks, Marc

2/25/2000 8:26 PM

Hi Marc:

According to the GE book, the US equivalent for EL503 is the 8278 tube. When the book was printed in 1973, 8278 was not indicated as available from GE at the time.

Looks like it would need about half the load impedance in the output transformer compared to an EL34. If it was my amp and I couldn't get replacement power tubes, I would probably replace the sockets and rewire for EL34s and their biasing requirements, then connect half the speaker load that the original amp was using, for instance a 4ohm speaker cab to the 8ohm output, that should do it. Then watch for any overheating of the filament transformer since the EL34 supposedly will draw 0.3A more current each than the 8278 was specified at.

Hope this helps,
Mike

2/26/2000 2:16 AM
Marc	MB: I was thinking along those lines too, but I think it'd require reworking the PS for any kind of power since right now it's sitting on about 325V for the B+. Kinda low for an EL 34 isn't it? might have to go to find the factory in Milan..... thanks Marc

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