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Old 25th September 2007, 09:46 PM   #21
hihopes is offline hihopes  South Africa
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Hi Chris. Thanks for the prompt reply.

My understanding is that each LM317 should be able to handle a minimum of 1.5 amps. (According to National specsheet, the TO220 is guaranteed to handle at least 1.5 amps at voltages below 15v, with average of 2.2 amps and max of 3.4 amps. Since there are only 2 valves, each rated at 1.5 amps heater draw, I should have had current to spare. I used 4 devices instead of 1 or 2 because of heat issues.
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Old 25th September 2007, 10:45 PM   #22
anatech is offline anatech  Canada
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Hi hihopes,
The thing is that you are pushing each device above it's ratings. They would need to be on a large heat sink to run cool enough. You will learn that you can't run things at max or over. Life just isn't like that.

That is why I recommended a pair of switchers rated at 5 A each. Your heat loss would be greatly reduced. You may want to consider running each bank of two tubes off one secondary to the center tap. That should give you approx. 6.3 VAC. You can fine tune that with a resistor in each leg (not in the center tap).

-Chris
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Old 26th September 2007, 07:02 PM   #23
Merlinb is offline Merlinb  United Kingdom
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Hihopes, dont worry abotu your heater voltage- it is fine. Some people say that if you run the heater less than 10% below rated voltage, they "suffer". This demonstrates gross misunderstanding of how a valve actually works.

Running OVER voltage obviously shortens the life of the heater, like it would in any lightbulb.
But running BELOW voltage EXTENDS life. However, it REDUCES saturation current. Now, in some circuits, like transmitters, that's a bad thing. But in audio amps the valves are NEVER pushed to saturation anyway, so the reduction in saturation current is irrelevent.
Once you get below 80% rated voltage, gm does start to fall, but only a small amount- and liek I say, it usualyl increases linearity, so what you lose in one thing, you gain in another. But a low heater voltage DOES NOT shorten the life of a valve. I routinely run my heaters between 5V and 6.3V.
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Old 26th September 2007, 07:49 PM   #24
hihopes is offline hihopes  South Africa
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Hi anatech,
Thanks for the headsup. Maybe I haven't explained myaself clearly enough. There are no "banks of tubes" - only 2 x EL34's (total expevcted current draw circa 3A). Second, there is no centre tap. This is a shop-bought 12v supply with only a +12v output and nothing else. I got it years ago to power a car-audio type active crossover, because it was cheaper than building one.
I still don't understand why the tubes failed.
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Old 26th September 2007, 07:54 PM   #25
hihopes is offline hihopes  South Africa
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Merlinb, thanks a lot for that constructive contribution. If you run yours as low as 5v without distortion, then that confirms that the distortion is not coming from a low heater voltage. I was thinking about replacing the trafo, but now I think I will leave it alone.
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Old 26th September 2007, 09:20 PM   #26
anatech is offline anatech  Canada
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Hi hihopes,
Quote:
There are no "banks of tubes" - only 2 x EL34's (total expevcted current draw circa 3A).
Well, about 3.2 A at full voltage. Okay, that explains better what things look like.

Quote:
Second, there is no centre tap. This is a shop-bought 12v supply with only a +12v output and nothing else.
Okay, you may be able to adjust it internally. Either that or just pic up a straight transformer and use that. Lastly, you could use a switcher type supply. Either way there is a way to correct the situation. Increasing the regulated voltage would cool the regulators a little as well. You should be fine with 4.

Quote:
I still don't understand why the tubes failed.
The schematic will help here.

Hi Merlinb,
Rest assured that we are in complete disagreement. If what you say is true, most the high end designers would be doing that. As a technician, I don't believe you at all.

-Chris
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Old 27th September 2007, 01:42 PM   #27
Bergo is offline Bergo  United Kingdom
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Manufacturers rate their valves for a combination of long life with decent emission. If the heater is not heated to the correct temperature the emission will not be as designed.

In the late sixties I conducted a series of experiments with output valve heater voltages. The trial amplifier was a stereo Mullard 5-10 using EL84 output pairs.
Operating the output valve heaters at 6.3V AC centre tapped and connected with twisted pair produced clean response with no apparent induced hum.
Earthing one side of the supply and removing the centre tap to produce a heater voltage of 6.3V one sided increased the hum significantly.
Rectifying the supply to provide 6VDC to the output valve heaters did not improve the audio performance. (Running the input EF86 valve on 6VDC significantly improved hum performance)
If the supply to the output heaters was taken from the 6.3V heater winding and rectified and the voltage fell to 5.3VDC even with a 6,800uf reservoir (the rectifiers were large germanium power diodes on heatsinks). At this voltage distortion of peaks when playing orchestral and opera passages was extremely noticeable. As the amplfiers were U/L AB this is not entirely surprising.

HTH

Bergo
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Old 27th September 2007, 06:47 PM   #28
hihopes is offline hihopes  South Africa
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Thanks for the input. Perhaps if these were being used as output valves, they might be working harder, but in their current application, driving a pair of mosfets each, there are no distorted peaks. Distortion only occurs with very complex waveforms, and is apparent even at quite low levels, so I think the low heater supply is not the cause.
It is definitely on my to do list to improve it though. I will stick to AC I think. (Unless I can figure out why my attempt at DC killed my valves, or some kind soul can come up with a feasible explanation)
I am thinking of replacing the trafo. I can get one wound for as little (or less) than buying one. I was thinking of going for a 50VA. Any idea how much voltage drop I should expect from a 50VA? Should I specify higher than 6.3v? Will a 50VA be OK in the long term?
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Old 30th September 2007, 04:42 PM   #29
Merlinb is offline Merlinb  United Kingdom
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Quote:
Originally posted by anatech


Hi Merlinb,
Rest assured that we are in complete disagreement. If what you say is true, most the high end designers would be doing that. As a technician, I don't believe you at all.

-Chris
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. It is so rarely touched upon in textbooks, after all.
As a technician, it's a little gem of information I always like to spout!
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Old 30th September 2007, 05:11 PM   #30
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Quote:
Originally posted by Merlinb

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

SNAP ..You won't catch me out on this one. The last comment you make quite rightly...
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