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In Reply to: RE: Nature Sound Permalloy OPT posted by Wojciech on June 03, 2014 at 08:59:28
Glad to help. If their OPTs are anywhere near as nice as their permalloy chokes, they should sound wonderful! I would recommend keeping current draw below 75 mA (though that's probably not a concern with either the 2A3 or VT52).FWIW: I just finished up a VT52 amp - used a 5k OPT - a great tube!
Edits: 06/03/14Follow Ups:
Yes
It's a wonderful tube and my personal favorite. Unfortunately, I have only one pair of Sylvania Vt52 and a pair of Sylvania 6a3 which I will use for everyday duty (and vt52 for special days ) I guess VT52 is a single plate version of dual 6A3.
If it's not a secret what operating point did you use in your circuit for VT52?
The VT52 is also known as the 2C45 and/or the '45 Special.'
See the link below for more info.I'm running mine with 7.0 vac on the filaments; 340 vdc plate-to-ground with an R(k) of 1k35 ohms (two 2k7 12 Watt Mills resistors in parallel). I'm going on memory here, but I believe the bias is approx 55 vdc.
As you might expect, the Western Electrics are the best sounding VT52s, although the other triple-V filament VT52s are almost as good.
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From the information in the link, I speculate that the higher (7v) filament rating was used specifically for transmitters because they are intermittent - the loss of lifetime is not a problem if you run it less than 60 seconds at a time. They may well have been identical to the more conservatively rated 6.3v filaments. The specifications are not really independent of the intended application.
FWIW, 7 volts is a standard voltage for three lead-acid cells under continuous charge, as they would be in an automobile, airplane, etc. The same cells used as storage batteries would give a standard 6.3 volts until they become discharged.
Filaments and heaters were often designed to accommodate a range of heater voltage for these reasons - dry cell and hearing aid tubes were especially subject to filament voltage abuse, and presumably tube manufacturers had the technology to deal with it.
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