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i have RDH3 and 4, should it be in their?
Tube MANUAL:
; from the guys who wanted to sell more tubes.
However using small triodes transformer-loaded was out of fashion when 6CG7 was introduced, so they don't give you a suggestion.
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how the altec engineers calculated the impedance of the 6CG7 stage
Transformer coupled triode power stage: select load 2*Rp or higher, play with the curves for biggest swing with lowest distortion. Then try to find ANYthing close.... Altec surely used whatever transformer was handy in stock.
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i have RDH3 and 4, should it be in their?
Yes, both have the complete work-up of push-pull triode power amplifier. If you can wade through it (it aint easy), re-do it with 6CG7 curves.
However the fact they used 6CG7 means it may not be a POWER stage, cuz the 'CG is a small bottle. Actually it can probably do a whole Watt push-pull, whereas a studio line doesn't need even a half-Watt. So exact analysis is not needed.
And here's the thing. 6CG7 is 7K(?). Load with 14K per tube. In push-pull, the output winding will be specified as 56K. But there are NO 56K power OTs. 10K, 20K, maybe 30K.... but the cost rises and the treble performance droops. So it isn't so much about the tube as about the limits of practical transformers.