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In Reply to: RE: 6L6 Outputs posted by Rod M on July 09, 2021 at 15:55:03
Definitely not plug 'n play. The pinouts aren't even the same.
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Buy Chinese. Bury freedom.
Follow Ups:
The pin-outs are indeed different (no G3 on KT66) but the original ST-70 manual recommend them as emergency substitutions. I'd be comfortable at least suggesting Rod try a KT66 or 6P3S-E, perhaps both a bit more robust tubes than a standard 6L6.
Am I missing something?
Yeah, it looks like not a great idea. I've got a set of Westinghouse 6L6GB and GE 6L6GCs that I using now in the Mac. Given the cost of replacing these NOS tubes, it doesn't seem worth the risk.
-Rod
As posted in the original Dynaco ST-70 manual, KT-66 was an acceptable "emergency" substitute for the EL-34, but the KT-66 is a more robust tube, voltage wise, than the average 6L6.
If you're really curious, you might run the idea by Jim McShane. He handles the Gold Lion KT-66, of course, but he also has some tilted bottle 6L6GC-STR Tung-Sol reissues on sale at the moment for the bargain price of $35.50 per pair. If Jim thinks they'll handle the ST-70s 410 VDC on the plates, they might be interesting to experiment with.
If he gives you the green light, and you don't care for them in your ST-70, they'd be good back-ups for your Mac.
Yes, my mistake. Substituting a 6L6 type for the EL34 is indeed possible. It's the other scenario that's different and more difficult.
One thing that concerns me about all this is the screen rating of the 6P3S-E. Within the 6L6 family, only the 6L6GC is rated to withstand more than 400V on the screen. Many 6L6 types are rated 300V or less. It's apparent the 6P3S-E isn't the equivalent of the 6L6GC in terms of anode dissipation, and that raises the question of what the maximum screen voltage and dissipation might be. No one really seems to know, and the Russian data sheet, which shows a maximum of 250V on the anode, is no help.
I'm aware that many owners have installed the 6P3S-E in their ST-70, presumably without immediate failure. However, there are reports of the sound changing over a period of 50 or 100 hours, and that's a concern. What's happening in these tubes to cause that? Could it be some aspect of screen or anode integrity? No one seems to have made even the most fundamental electrical measurements, such as output power, so there's really no way to know.
My current project will use the 6P3S-E with more than 400V on the anodes. However, screens will be regulated down to about 300V. I don't know if this is absolutely necessary, but it will at least provide peace of mind and the ability to use most 6L6 types in the future.
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Buy Chinese. Bury freedom.
I posted the Dynaco KT-66 note more for my own clarification rather than to correct you.
I did a search for 6P3S-E and read some of your previous exchanges with Mike Samra regarding that tube. I copied and pasted one of his replies to you below.
"I have made many measurements but mostly in 6L6 and KT66 amps. Most of the disputed argument I presented in past posts was to reiterate the fact that these tubes can handle much more than 350v rating claimed in the Data by the Russian military"
I know I haven't put 50 to 100 hours on those tubes in my ST-70, which puts about 410 V on the plates, so I can't give you any longevity results. Probably more like ten hours, enough to determine that I liked them better than EL-34 based tubes, but not enough to keep the amp in my system any longer than that.
Mike was sometimes casual regarding voltage specs. I remember him once good-naturedly telling me I had no balls, since I expressed concern about running a 200 volt rated Russian Teflon FT-3 cap in excess of that. Mike swore they were good to 600 V (or more), but I admit that I usually try and keep within set guidelines. Losing a $10 (or whatever the 6P3S-Es went for) tube was one thing, but creating a fire and / or shock hazard by running a cap 300% over rated voltage was something I admit I wasn't comfortable with.
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