This was a bit of an eye opener for me. I didn't realize how different the Russian tube heaters are. They seem to be only safe to run under constant voltage then they all, for the same tube number, consume the same amount of power 4 watts in my case. NOS US and European tubes are +-5% and consume 3.78 watts. Under constant current my Sovteks were very consistent in that they where under heated at 3.34 watts. I plan to use regulated voltage with these tubes. Thanks everyone for all the informative comments..!
It's been discussed before. Lower noise. PRR mentioned it long ago, was used in commercial mic preamps and the like. Different properties of course.
Frankly, stuff like that is one of the reasons I disagree with so much that has been said in this thread. There's nothing better about deviating over a spec than deviating under a spec.
I have.
It happens, on rare occasions.
Including heater-to-cathode shorts.
But then again, I've serviced thousands of tube type equipment over the decades.
Unfortunately I have, and it is a common failure in vintage Mullard 12AX7A/ECC83 if you do not limit the warm up current. It's not a common failure I will admit.
I had a 22 month old JJ 300B that went open filament back in June, I was to put it mildly "displeased" Currently running a 20 yr old spare JJ as its replacement. First failure I have encountered in several dozen of these tubes I have used over the past couple of decades. (I suspect this was caused by a mechanical problem, but I was not able to see anything amiss.
DHTs I use constant current heating and adjust to the nominal value after a few minutes of operation. I mostly now use Rod Coleman's filament regulators - I can't do much better on the cost so, and they're good.
Low level audio I use voltage based heating with regulated 6.3V supplies, for power amplifiers I went back to AC heating over a decade ago.
I had a 22 month old JJ 300B that went open filament back in June, I was to put it mildly "displeased" Currently running a 20 yr old spare JJ as its replacement. First failure I have encountered in several dozen of these tubes I have used over the past couple of decades. (I suspect this was caused by a mechanical problem, but I was not able to see anything amiss.
DHTs I use constant current heating and adjust to the nominal value after a few minutes of operation. I mostly now use Rod Coleman's filament regulators - I can't do much better on the cost so, and they're good.
Low level audio I use voltage based heating with regulated 6.3V supplies, for power amplifiers I went back to AC heating over a decade ago.
i have seen tubes marked 6SN7 that i can swear to be a 6H8S, they look identical in real life, that that was the tube i mentioned was running ac at 5.2 volts....
i was repairing an Antique Sound Lab 300b set, the amp came with the Takatsuki tubes, realising those were very expensive tubes, i asked to owner to take them back and give me old tubes instead if only for testing....
i did encountered tube arcing with two out of six tubes that he gave me...
up until then i never realized that dht's can arc over...
All of my Sovtek 6SN7’s are also marked “6H8C” (S). I’ve had good luck with them over the years as a 6SN7, except for one that was badly out of balance. They do draw a little more current than 0.6A, but I’ve never given it much thought.
Considering the datasheet is the Soviet 6H8C sheet, and has Sovtek 6SN7 seemingly taped over top and photocopied? Yup!
https://shop.ehx.com/catalog/addimages/6sn7-sovtek.pdf
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