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Title: 811A: Center tapped primary? Post by: KW6T 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 Title: Re: 811A: Center tapped primary? Post by: K8DI 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 Title: Re: 811A: Center tapped primary? Post by: MikeKE0ZUinkcmo 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 (https://mikeharrison.smugmug.com/Dentron-Clipperton-L-Amp/Rework-and-repair/i-6qKmgvs/A) years ago for the same AC hum cure. Lots cheaper than the $320 unit you mentioned.
Title: Re: 811A: Center tapped primary? Post by: KW6T on February 06, 2025, 11:29:46 PM Mike, Ed, AMfone - Dedicated to Amplitude Modulation on the Amateur Radio Bands
Many thanks! 73, Russ |