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Old 22nd April 2008, 07:18 AM   #1
m6tt is offline m6tt  United States
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Default Strange circuit - heater hum opt feedback?

I am working on a new project lately, for which I cannot find a schematic (it doesn't matter--pretty much a redesign), however I have been tracing out the circuit to make some sense of the old design. It was a three-channel power amp from an organ, 2x6l6 transformer coupled, 2x6v6 transformer coupled, and 2x6v6 which are setup normally but no transformer, instead a huge non-electrolytic cap (almost a coke can 630v 6.8uf--why?) between the two anodes and the anodes ran to what seems must have been another chassis.

The interesting circuit is the 6l6 part, because although there are the normal connections to the transformer, there are some anomalies. The p-p primary leads come off the plates, the B+ goes to the center tap, etc. However, two wires (primary?!) come off the heaters too (2 & 7 pins). The heaters on these tubes are run in series of the 12.6v (13.2v) supply. These connections to the opt are the terminations of the heater chain. The transformer is most certainly an OPT, based on its connections, its location on the chassis, and its connection to the speaker outputs. Further, the 12.6v supply is provided by a large transformer on the chassis, and it is clearly large enough to provide this voltage for many, many tubes.
I have a few theories, one of which is that somehow the heaters have been wired into the OPT in a humbucking configuration, i.e. out of phase with the primary. Or is part of the OPT being used as a regulating choke? Has anyone ever seen anything this weird?
I am wondering what exactly might be going on here?
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Old 22nd April 2008, 05:43 PM   #2
EC8010 is offline EC8010  United Kingdom
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Occasionally, heater power was derived from the HT supply by connecting valve heaters in the place of output valve cathode bias resistors; it saved a transformer winding, an expensive cathode resistor, and was a cheap way of getting DC heaters. That might be what you have here.
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Old 22nd April 2008, 11:04 PM   #3
m6tt is offline m6tt  United States
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ok, it was less serious than I thought! since the amp is point to point, and the OPT and heaters use a brown wire, I got a bit confused! I rewired it a bit and had it running el34s or 6l6gbs cathode biased and sounding pretty good. I still don't understand why one set of the 6v6s in the original amp had that huge oil cap between the anodes–I imagine the OPT for these was on another chassis, so I guess this was to prevent some kind of RFI? I've honestly never seen a non-electrolytic cap which was that large. Unfortunately it was covered in ancient electrical tape and so it's hard to read all the values–it's white with a red dot, and I can clearly read 6.8 and 650v. I'm assuming that'd be µF based on the size of the thing.
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Old 23rd April 2008, 06:13 PM   #4
EC8010 is offline EC8010  United Kingdom
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6.8uF at those impedances would be short circuit.
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