[quote author="PRR"]> limit this "cold current" to 365mA maximum.
So?
> If the filament begins to deteriorate... hasten burnout
When was the last time we saw an indirect-heat receiving tube heater burn out? (I agree that $300 DHTs are a different beast.)
There was my H-P 200AB, but I suspect abuse (toss into dumpster) even more than the decades of power-on hours it lived through.
Before that, I have to remember series-string TV sets, in which the small tubes got "constant current" from the large tubes. If you got tubes from opposite ends of the current-draw spec, the big tubes over-fried the little tubes. That don't happen in constant-voltage feed.
I think constant current drive is a bad idea. Constant-voltage drive is fairly self-limiting for a filament with resistance that increases with temperature, constant current leads to much wider deviations.
If you think the start-up surge hurts tubes, try it. Put a tube on a 10 second timer, off-on-off-on..., and wait for it to fail. I bet a buck you can't kill a tube in a decade this way; one failure is a fluke. I'll bet $20 that you can not kill two tubes in one year this way. Plain receiving tubes like 12AX7 or 6DJ8 or 6L6, not battery tubes, not WE300B. Your choice of timing: maybe 2 seconds and more cycles, maybe 20 seconds so the heater goes full-cold on each start, I don't care. Total population say a dozen (I don't want to hear that you abused 10,000 tubes and two failed). Open-heater or H-K-short failures only. 1935-1985 US/EU/Japan production or a good USSR factory, not the worst Chinese low-bid junk.[/quote]
Well, maybe with tubes it is not as severe a problem. Brad Plunkett did get burned badly years back, according to his glowing account (sorry somebody stop me!) by driving pilot lights with constant current.