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In Reply to: RE: Solution? posted by Naz on March 24, 2015 at 03:27:20
I lost three KT88's over a year due to 6.6-6.8v filaments. Filament burnout was virtually unheard of in the old days but the new Russian and Chinese tubes seem very critical about filament voltage.
What i did was to bring out the filament transformer primary to a separate AC connector ad used a Sola transformer to feed it.
Follow Ups:
at the opposite end of the scale I once tested a 845 filament (on an otherwise dud tube) to destruction. 50V on a 10V filament for an hour did not break it!
I have amps rated at 220V-230V AC input running various tubes with my mains varying from 230-260V and have never had a single filament failure. Admittedly, these days I always use inrush current limiters.
FWIW, I've run Svetlana KT88 filaments happily at 6.8V.
Naz
While I admire your audacity in running your tubes under such challenging conditions, the tube manufacturers did stringent research into filament voltage and tube life and the results are not good once you move past the design center the lifespan starts taking a steadily steepening nosedive. Unless, one has deep pockets for buying tubes, keeping the filament voltage at whatever the design center is should be an important issue. cheers, Dak
that's just me. Note that I said I run inrush current limiters these days and have done for years. The few tenths of a volt drop under normal operating conditions keeps all of my filaments well within spec even under elevated mains voltage conditions.
So, to clarify my response to the initial poster's question I don't think anything needs to be done to the rest of the amp to cater for slightly higher mains voltage but I would still add a few cheap inrush current limiters to the filaments.
I would of course check the ratings of caps and the like but I'd be surprised if anything needed to be done in that respect.
Naz
Hey Naz, if you check in my first post, that was primary recommendation. I found that you can knock the primary voltage back 3 to 5 volts depending on the CL used. Dak
If I test the filament voltages and find them above 6.5v I install 2 watt metal oxide dropping resistors to bring the voltage down. I found that about .22 ohm to .33 ohm in each line will bring the voltage from 6.5 to about 6.1 with the .33 ohm resistors. Works great. cheers, Dak
PS, this was for a SE amp running 2 7591 and 1 12ax7. For an amp running higher filament currents you may need to use higher wattage dropping resistors.
Edits: 03/24/15
If that is happening I would use a different brand of tube. That's less than 10% over and there is no way they should burn out from that.
Svelanta (sp?) is gone AFAIK. I am currently running Gold Lions. But that experience made me gun shy about anything over 6.3v.
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