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I do not

...have the ability to test output power.

I tried the 6P3S-E in my ST-70 based on frequently posted recommendations by Mike Samra to try them in the VTA, as well as in other amps. Along with the K40Y-9 PIO caps, Mike was a big cheerleader for some of the surplus Soviet stock. Perhaps he was a Putin pawn, eh? (Mike would see the humor in that, I'm sure)

I bought my 6P3S-E back when Jim McShane was still selling them, and as noted, they were the most pleasant for me to listen to in that circuit, when compared to the original Dynaco Mullard EL-34, and reissue Gold Lion KT-77s. Before I set off a firestorm from KT-77 lovers, the key words are "for me" and "in that circuit".

I talk to another VTA ST-70 owner, and if I remember correctly (given those particular email discussion were a couple of years ago) he liked actual 6L6 (or it could have been KT-66) better than the EL-34s in his amp too.

As I'm sure you recall from your Dynaco repair days, the original ST-70 manual said "In emergencies, ...the 5881 and KT-66 can be used in place of the EL-34's..."

I'm not suggesting that that was an optimal substitute, as your 3.8K vs 6.6K transformer difference indicates, only that Mike Samra and a few others of us thought it "sounded good".

Is the main result of mismatched tubes to transformers limited to reduced output power, or does it also increase distortion, or cause some other detrimental measurement? I consulted my 1964 Basic Electronics text book, but if the answer is in there, I missed it.

Edit: I found something for me to study regarding output transformer impedance (link below). The second sentence says, "If the wrong transformer is used, the results can be low output and loss of tone quality. ". The loss of power is obvious, but the "loss of tone quality" is more subjective, at least the way it was written. Perhaps the resulting loss of tone in the VTA ST-70 is what some of us found pleasing. Or am I drawing the wrong conclusion?



Edits: 07/09/21 07/09/21
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