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In Reply to: RE: Cathodyne VS LTP posted by Triode_Kingdom on May 1, 2021 at 20:22:10:
I'll try this first with a design similar to the 30W RCA circuit in RC-30. This circuit creates positive feedback between the pentode and cathodyne, significantly increasing the gain. Max Robinson discusses this circuit on his site and (thank you Max!) provides data regarding gain. HK used this technique also, although it was a triode-triode front end. In fact, I was involved in discussions right here a few years back regarding this technique in the HK A500.I don't know why my SPICE simulation indicates issues with the 'AU7 cathodyne, but I think there must be an error in the model. With that in mind, moving forward with the cathodyne rather than the LTP seems to be the simplest approach. At this power level (35W), the 5881s need about 60V pk-pk at each grid for full output. This drive level is well within the capability of the pentode-cathodyne with the voltages I'm using, so long as there's enough gain.
Here's the RCA circuit redrawn by Max. It's unusual in that it provides negative *DC* feedback to the screen of the pentode, while simultaneously providing positive *AC* feedback to the cathode. The former helps to stabilize the operating conditions of the cathodyne, which was Max's focus in the article. He measured the gain at about 450 (!), an increase of more than 6dB.
Edits: 05/01/21
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