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Old 30th September 2007, 07:38 PM   #31
anatech is offline anatech  Canada
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Hi Merlinb,
Quote:
As a technician, it's a little gem of information I always like to spout!
Well, I do a lot of reading in technical publications, and have over 35 years on the bench. Considering signal tubes, a slight reduction in heater voltage can bring about lower noise. Power tubes used as a signal tube are not something I know a lot about. I will then assume their performance is similar to signal tubes on a different scale.

I can say from experience that when my heater supply in my Fisher 400 preamp was failing (selenium rect.) the voltage drop caused very odd behaviour. I had unstable gain that was very severe in the area of voltage you are talking about.

In the interest of learning something here, where did you get this information? My own experience seems to contradict what you are saying. Also, beware your rp is going to go way up by doing this.

-Chris

Edit:
Quote:
I suspect most high end designers simply aren't aware of the effect of underrun heaters, or if they are, they know eveyone ELSE isn't, and will therefore probably shun their designs if they dared deviate from the sacred 6.3V.
I doubt they could resist the temptation to crow about forever tube life. Many new tube "designers" have not got the foggiest idea how a tube works. Running heaters way down in voltage would seem to be a perfect way to relieve the prospective buyer that he is in for maintenance hell.
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Old 2nd October 2007, 08:37 PM   #32
Merlinb is offline Merlinb  United Kingdom
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Quote:
Originally posted by anatech
Hi Merlinb,

In the interest of learning something here, where did you get this information? My own experience seems to contradict what you are saying. Also, beware your rp is going to go way up by doing this.

You're right, rp will go up- though not always by as much as you might expect, until you start operating below about 80% Vh. Obviously you would need to decide whether any increase in rp is actually a problem. In a preamp it usually won't be, whereas in power triodes it might be, and power pentodes perhaps not, unless it goes so high as to affect damping.

I can't remember where I FIRST heard about this effect, but here are some links where it is mentioned. I Know somewhere on the web is a study someone did of the ECC83 at different heater voltages, clearly showing the increase in linearity, but I can't seem to locate it on google at the moment.

http://www.diyforums.org/MAX/MAXheaterResistor.php
http://www.tubecad.com/july2000/page10.html
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Old 2nd October 2007, 09:00 PM   #33
anatech is offline anatech  Canada
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Hi Merlinb,
What they failed to mention in the first link was that certain tubes were designed to be run in an automotive environment. The heaters were designed to run over a wider range. Locktals are the same. I'll keep my eyes open, but I am pretty sure that normal HiFi tubes are designed to have their heaters run within 10% for the listed values.

If you run the heaters lower, you essentially have a unknown tube type. None of the specs are valid anymore and you should then test and rerate those tubes at your lower voltages. In particular, the maximum cathode current will be reduced for sure. This may or may not be an issue, but you should be sure.

Also, different brands of tubes (different runs actually) may behave totally differently to reduced heater voltage.

-Chris
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Old 7th October 2007, 06:33 PM   #34
Merlinb is offline Merlinb  United Kingdom
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Quote:
Originally posted by anatech
[B] am pretty sure that normal HiFi tubes are designed to have their heaters run within 10% for the listed values.
To acheive their rated spec, granted.

Quote:
If you run the heaters lower, you essentially have a unknown tube type. None of the specs are valid anymore
But for small decreases the changes aren't that wild. Bare in mind that even at the rated voltage, valve specs (gm, ra and mu) are only accurate to +/- 40%! Now I'm not suggesting we run the heater at half voltage or anything- then we really would get crazy performance. But I'm happy to go as low as 80% in non-super critical circuits.

Quote:
In particular, the maximum cathode current will be reduced for sure. This may or may not be an issue, but you should be sure.
I think I mentioned that in the first place, but as you say, in audio we shouldn't be running valves anywhere near saturation in any case.

Quote:
Also, different brands of tubes (different runs actually) may behave totally differently to reduced heater voltage.
Modern valves all seem to behave differently anyhow!
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