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In Reply to: RE: Extend tube life options.. posted by Paul Joppa on November 24, 2015 at 13:41:19
Thanks.
If we are not talking about switching anything off, would increase the bias and lower the filament voltage extend tube life? I can adjust bias to about -110v and filAment voltage to about 1.7 volt.
B+ will be left on for the BG's
Would this help, or are we still running into cathode poisoning and/or other problems?
NC
Follow Ups:
The filament voltage cannot be reduced very much while maintaining a decent electron cloud. The only thing quantitative I've seen was filament voltage proportional to the 13th root of the current ratio, e.g. at 10% of normal current you would need a minimum of 4.2 volts. That rule is of questionable validity since it was developed for pure (non-thoriated) tungsten filaments.
I would speculate that with a very negative bias, with or without appropriate reduction in filament voltage, the destruction of the cathode surface would proceed as if the amp were operating normally. The more negative bias provides a bit of protection, being more attractive to positive ions, but the reduced current will likely increase the power supply voltage making the ions more energetic and destructive.
Portable tube radios would sometimes switch only the filament power, leaving the high voltage connected. However, these were mostly 45v or 67.5v supplies, and nobody expected these tubes to last more than 1000 hours or so.
I have heard that the best modern 300Bs have ten times better vacuums than WE-produced NOS 300Bs. No idea how valid this information might be, but it's the residual gasses that become destructive ions so I would be extra cautious about leaving the high voltage on such difficult to replace tubes!
Van der Bijl said that oxide-coated filaments are protected from filament volatilization, so lifetime is mostly determined by ion damage to the coating. That was in 1920 though - might be out of date? If that really is the case, then mechanical heating and cooling stresses from cycling the filament on and off may be the bigger danger - suggesting that removing the high voltage with little or no filament voltage change might be a good strategy.
Thanks Paul for this valuable information.
For longer periods I (of course) switch the amp off. I'm looking for a solution for at the end of the day (I mostly listen to music every day).
Looking at your info, it seems best to lower the current a bit when the amp is not in use during the day. Switching B+ is not an option because of the BG's, and all other options are probably introducing more stress on the tubes then just leaving them on all the time.
Thanks,
NC
Prolly easiest thing to do is pull the tubes, when you aren't going to be using the unit for a period of time. B+ will rise a bit with the tubes out, but it seems that the PSU can handle it. May even like higher voltage? I assume this it has a solid-state rectifier.
The only issue is wearing out the socket and extra handling of those tubes. And hot tubes, too.
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