Cap-Lifted Filament Center Tap

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Ethan
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Cap-Lifted Filament Center Tap

Post by Ethan »

Greetings 18Watters,

I was a regular here about ten years ago and then life got busy and I disappeared for a while.

A friend's Ampeg Reverberocket got me back to the electronics bench recently and I'm reaching out with a question: The filament center tap is isolated from ground by a .1uF film capacitor, which I've seen in other Ampeg models from around this time, and I'm really not sure why.

The model is a GS-12r from late 1965 (12AX7, 6U10, 7199, 7591x2). It's cathode biased and the cathodyne phase inverter has the cathode running at 50v (probably a little higher at modern mains voltages). The only reasons I can think of for such a cap would be either to filter out high frequencies from the heater circuit (doubtful that's why it's there) or to allow the heater DC reference to float with the cathodes - an odd choice but plausible.

As of now, I haven't even fired the amp up yet. It's in terrible condition, sat for many years and was worked on previously by someone who shouldn't have been in there at all - ever. I'm currently going through it and cleaning up everything that's obviously wrong before I apply power. I'm undecided as to whether I leave the cap in the circuit (replaced with a nice, new one, of course), remove it and ground the center tap directly, or lift it with a voltage divider but I'm curious to know what the group's thoughts are regarding why the cap would be there in the first place. Any takers?

Thanks,
Ethan
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TriodeLuvr
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Re: Cap-Lifted Filament Center Tap

Post by TriodeLuvr »

The schematic doesn't indicate a need for the cap in terms of DC isolation. My WAG is that it might prevent a catastrophic failure in the event of a heater/cathode short in one of the tubes.

I strongly recommend that you remove the death cap at the transformer primary and install a three-wire grounded line cord.

Good luck with the restoration!
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Ethan
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Re: Cap-Lifted Filament Center Tap

Post by Ethan »

Oh, the removal of the “death cap” and the 3-wire cord are absolutely a given! Doing a lot of cleanup in there but that’s top of the list.

Your theory re: the isolation cap is as solid as any. Could be right...
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