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In Reply to: RE: Tube life considerations. tt verses oxide posted by amnesiac on August 07, 2017 at 05:17:49
It's important to understand that the 10,000 hour rating for the C3M is more of a minimum rating.
Some time ago, I built a pair of single ended EL34 amps for my father's TV rig. I never turned them off and I found that in that amp, the 6SN7's didn't really seem to show any wear, nor did the 5U4's. The Russian EL34's lasted about two years in constant service before getting flat and showing low emissions. Chinese EL34's went about 7 months. Fancy reissue GL KT88's went gassy in 3 months.
Tube life depends a lot on operating voltages and heat cycling. I could make you a 300B amp that would eat 300B's (run the filaments at 5.5V and plates at 40W dissipation) or would make them last for ages (run the filaments at 4.9V, plate dissipation at 25W). Unfortunately the average consumer will buy a 9W amp over a 7W amp and the finals will suffer.
Follow Ups:
yes I will definitely be running them conservatively.I dont think the new tubes have purity of metals quality or even the same alloys used in the earlier tubes and possibly the glass quality's . The build quality. The technology. The selection standards. everybody here knows this I am sure.
I put resistance in my dc supplys so they start up slow. The stc manual states with larger tt and pure t filament are important to turn on in steps and never exceed more then 150% of running current at startup. and this is where series resistance is going to be costly to implement. Perhaps I will have to switch it in. Ok this might be excessive for smaller tubes but the the fun of it isnt it? buying nos tubes of pricey ebay sellers ..not so much fun..
of note the manual is strong on the importance of running tt 5% to spec voltage is very important to keep filament temps stable to keep filament chemistry operating correctly.that thorium being lost and replenished from within the wire. Good read for anyone thanks to Franks.great database. back tomorrow..
You'd have to actually measure what a piece of gear can deliver in terms of filament current during startup. If you have a DC supply for your tungsten filaments, additional current will cause voltage sag, and voltage sag will cause reduced current. To measure this, you could put a 0.01 ohm resistor in series with one end of the TT filament, then hook a scope up across that resistor and setup a peak hold to grab the highest DC voltage across that resistor.
Some DC regulators also have current limiting capabilities. The L200C is one that comes to mind.
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