new idea of DHT heater supply - diyAudio
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Old 14th February 2005, 09:23 PM   #1
kekso22 is offline kekso22  Germany
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Default new idea of DHT heater supply

Heaters for direct heated triodes can be DC or AC (50/60Hz).
AC is often used because of more linaerity and more lifeness, but it also modulates the input signal with the line frequency and its harmonics.
DC is prefered to eliminate the hum on the output, but loads the cathode unsymetrically.
I have a new idea which I will try in my next design: single stage AD1 or 2A3 (I do not know if this is really new, maybe somebody has it already done).
Why not use AC without hum?!
The idea is to use a high frequency powered heater above the audible band, lets say about 200 - 500KHz. A simple oscillator and a full bridge motor driver IC can do the job. Devices like
National LMD18201 or
ST L6202 should do it.
What do you think about it? All ideas appreciated

Thanks Emil
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Old 14th February 2005, 10:32 PM   #2
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Everything is worth trying at least once I guess.. I try hard to eliminate sources of rf interference from my amps rather than introduce them though.
The problem with rf noise is that although you can't hear it, the tubes in your amp are more than capable of operating at those frequencies.
At higher frequencies screening is much more difficult to acheive as wiring and components make quite good aerials.

I have played with switchmode power supplies in transistor amps - the result is usually good but not great. Fine detail is compromised and there's a hardness to the sound which you don't get with a linear supply. I suspect you might get similar results here.

Good luck - I would be interested to know how it all turns out if you go ahead with the design
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Old 15th February 2005, 12:38 AM   #3
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Hi,

Jim Hagerman of Hagtech is working on a similar supply for DHT's.
So far I think he's working on something in the region of 50KHz.

What I'm wondering about is that, as far as the tube is concerned, the higher you push the pulse frequency of that heater supply the more it's going to look as if it's a DC supply, isn't it?

Cheers,
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Old 15th February 2005, 12:40 AM   #4
EC8010 is offline EC8010  United Kingdom
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Hmm, or do I mean screee? If you apply a 200kHz sine wave (don't let's even think about square waves) across the cathode of a DHT, any imbalance of capacitances will mean that you are driving a grounded grid stage from the cathode. Thing is, grounded grid stages have lots of bandwidth because Miller efect isn't an issue. Imbalances in the output transformer (almost a certainty at that frequency) mean that you will modulate the HT supply. Which will have non-zero impedance at 200kHz...

I see where you're coming from regarding 50/60Hz problems, but going to RF won't be a cure-all. You'll have all sorts of RF problems to solve because of the hidden imperfections of real components. I'm not an RF whizz, so I don't consider that to be easy.
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Old 15th February 2005, 06:00 AM   #5
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Quote:
I do not know if this is really new, maybe somebody has it already done.

It has been done many times. I find the idea daft but many diyers seem happy with the result so there probably is something in it

Ultrasonic Heaters
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Old 15th February 2005, 06:12 AM   #6
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I agree with EC8010. If you're a good RF angineer and have some decent measuring facilities, then it might be possible to make it work really well. But I don't think it will be plug-n-play and each individual amp installation might need to be tweaked to make it work to it's best.
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Old 15th February 2005, 09:52 AM   #7
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I don't see what the problem with DC is?

If you use two equal valued resistors to connect each side of the cathode to the circuit, provided they are low enough, this should not be a problem. Ok the resistors may have to be a higher wattage, but there is plenty of heat in a valve amplifier anyway, so what's a little more heat?
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Old 15th February 2005, 10:10 AM   #8
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Quote:
Originally posted by real
I don't see what the problem with DC is?
A hatred of silicon?
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Old 15th February 2005, 10:21 AM   #9
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Quote:
A hatred of silicon?

NOt necessarily

A Japanese gentleman has come up with nice DC heaters for his 300B amp
Attached Images
File Type: jpg 300b tube rect heater.jpg (40.7 KB, 1126 views)
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Old 15th February 2005, 10:31 AM   #10
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"A Japanese gentleman has come up with nice DC heaters for his 300B amp"

A mercury vapor rectifier in the filament supply? A little strange.
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