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Author Topic: 811A: Center tapped primary?  (Read 217 times)
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KW6T
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« on: February 06, 2025, 05:14:48 PM »

Good day.
A quick question.  For my Johnson project, I am running a pair of 811A's in the modulator as the manufacturer intended.  The available transformer's 6.3 volt winding lacks a center tap.  Is it possible to run these tubes without a filament center tap?   Otherwise, another $350.00 transformer order will be necessary.
Russ
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Russ

"Cheap" is rarely the least expensive.
K8DI
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« Reply #1 on: February 06, 2025, 06:16:27 PM »

I was up this same creek a while back and a few folks here reminded me I don’t need the center tap to carry more than the plate current. A one amp 6.3v ct transformer in parallel with the one you have now will provide the reference. DON’T connect the primary to power. It’s really a couple opposing inductors to ground. Added bonus: there’s 120v on the primary. Induced there by the filament voltage, it can only provide a small current…just enough for some grid bias! Extra added bonus— it doesn’t have to be a 6.3v transformer. Try a 12.6vct unit. It’s still two inductors to ground, although you’d only get 60v on the primary to swipe bias voltage from..

Ed
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Ed, K8DI, warming the air with RF, and working on lighting the shack with thoriated tungsten and mercury vapor...
MikeKE0ZUinkcmo
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« Reply #2 on: February 06, 2025, 06:48:19 PM »

I agree with Ed's recommendation for a 1 amp 6 volt transformer.    I did the same in my DENTRON years ago for the same AC hum cure.  Lots cheaper than the $320 unit you mentioned.

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KW6T
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« Reply #3 on: February 06, 2025, 11:29:46 PM »

Mike, Ed,
Many thanks!
73,
Russ
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"Cheap" is rarely the least expensive.
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