the 6AS7 - Nick - OTL Asylum

the 6AS7

81.31.233.243

Posted by Nick on July 13, 2008 at 07:26:48

First a question about what the max current we can safely expect from the cathode of these tubes? I've done some experimenting and with no grid current I get about 500mA out of the cathode, but with grid current I can get up to 1.5 amps out of a single cathode!! That's with about 110volts at the plates, in both cases mentioned. about 120mA was neede for 1.5 amps, and it didn't help to increase the grid current, no more cathode curent even with more thatn 600mA grid current.
So do anyone know what I can pull out of these tubes before I damage the cathode?
Second I must mention the durability of these tubes amaze me. I have an amp which is kinda just slapped together for testing out ideas, and it has been my main amp for about a year now. Normally I leave my amps on 24/7. Well for a while I noticed it was really hot, and though it runs hot on purpose (SE class A type), this was really really hot. But being lazy I didn't bother taking it out of the rack to measure bias and it ran like this for at least two months. Well finally I got my act together and pulled it out and checked the idle situation. Well turns out the bias had shifted a tad, and now each triode was running at 28watts! Thats 56watts continous per tube!!! (The tubes are driven by 5687 directly coupled.) The sockets were brown, the plastic bottom of the tubes hard and brittle, the silver getter is half gone, and there are gray spots on the plates. But they still perform! I redid the bias circuit and rebiased to 12watts per triode, and after a month the amp still sounds great. I have replacement tubes, but have decided to keep the old ones untill they are dead. And btw these 6AS7s are the dreaded american straight GT bottle types(without the grid cooling fins).
This experience is the reason why I've decided to try and get more out of the tubes with grid current, but wonder how far I can go.

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Topic - the 6AS7 - Nick 07:26:48 07/13/08 (1)

poor 6AS7 :-( - Coresta 01:09:43 07/16/08 (0)

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