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Old 20th December 2007, 09:09 AM   #1
noyan is offline noyan  Turkey
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Default Switching power supply for tube filaments possible???

http://www.meanwelldirect.co.uk/prod...12/default.htm

I wonder if i use a switching powersupply unit for tube amp heaters, how will be the hum result ? Any other unexpectable problem? Please advise,
Thanks
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Old 20th December 2007, 09:26 AM   #2
jnb is offline jnb  Australia
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Hum is not generally an insurmountable problem with heaters because thermal inertia maintains a reasonable consistency of temperature.

A heater is also capacitively coupled to the cathode. This is a stray capacitance and is too small to be an issue with hum, but it can convey higher frequencies.

Switch-mode supplies involve higher frequencies though I don't see why this couldn't be successful with care.
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Old 20th December 2007, 09:38 AM   #3
noyan is offline noyan  Turkey
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Default PS unit

Thanks. This unit switching at 83khz. I think this will not be audible. Actualy i am planning to use the 24v version for the GM70 output tube heaters only. This tube needs 20v-3A for filament.
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Old 20th December 2007, 02:35 PM   #4
rdf is offline rdf  Canada
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You might find this interesting: http://www.pmillett.com/hf_fil.htm
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Old 20th December 2007, 03:57 PM   #5
316a is offline 316a  England
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In my experience I find just about any junk on the filament supply has an audiable impact , heaters on the other hand are another matter altogether . I have often thought about using switchers , 7N7 reminded me of their usefulness a while ago **after** I'd built a huge 5V @ 10.5A fialment supply for Eimac VT127A . Those cheap low voltage lighting 'transformers' look like a good place to start . I'd use these with a hefty laminated choke to provide some isolation from the supply . I'm sure this would make an excellent supply for firing up 845/211/713 etc

cheers

316a
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Old 16th September 2008, 06:50 AM   #6
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Quote:
Originally posted by 316a
In my experience I find just about any junk on the filament supply has an audiable impact , heaters on the other hand are another matter altogether . I have often thought about using switchers , 7N7 reminded me of their usefulness a while ago **after** I'd built a huge 5V @ 10.5A fialment supply for Eimac VT127A . Those cheap low voltage lighting 'transformers' look like a good place to start . I'd use these with a hefty laminated choke to provide some isolation from the supply . I'm sure this would make an excellent supply for firing up 845/211/713 etc

cheers

316a
hello,
any news to report from your VT127A experiments?
cheers,
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Old 16th September 2008, 10:42 AM   #7
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hey-Hey!!!,
There are a few potential paths with switching filament supplies. One is to leave the AC square wave, and build a CT with a hum pot as is customary with 60 cps AC. Rectify the stuff comming from the wall through an isolation TX, wind a small step-down TX and switch the primary at 150kHz through a SE MOSFET. The gate-driver chips look quite easy to use. You want open-loop 50% duty cycle. The small, HF transformer will do a good job of isolating the filament from everything...no need for CMC as is often suggested for DC supplies.
cheers,
Douglas
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Old 16th September 2008, 12:30 PM   #8
316a is offline 316a  England
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Quote:
Originally posted by korneluk


hello,
any news to report from your VT127A experiments?
cheers,
Been and gone , torn down months ago . Tried VT127A zero bias at 600V with local cathode feedback . 7236 choke loaded CF driver and 809 input stage . Sounded nice but the output transformers ran on the ragged edge , really need to run 100mA 650V instead of 75mA at 600V . LCL filament supply weighed around 80lbs for both channels , so this project would never make it into a chassis

cheers

316A
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Old 16th September 2008, 04:24 PM   #9
kevinkr is offline kevinkr  United States
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I've used switchers for both DHT filament heating and indirect heated types without issues. I do recommend some additional filtering on the output however to really kill the ripple at the output. Basically this consists of a small common mode choke right on the output of the supply followed by a cap, a normal choke, and another cap - these caps should have good performance at high frequencies, some small film types should be fine. (Place larger ele types in parallel - but do so only if you have a scope and look at the behavior at the ripple frequency. Typical CM chokes will be a few hundred uH to a couple of mH, and the normal mode choke a couple of mH typically. You actually can often buy these if you hunt around. The parts are compact so you are not looking at a lot of size or weight, but for best results you will need a scope and some patience to iterate the circuit for best results.
You can get ridiculously low levels of noise out of a switcher this way, and if you need even better performance use a standard linear regulator (!choke on input!) to get you to the precise voltage you need at the filaments.
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Old 16th September 2008, 07:17 PM   #10
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I have access to a bunch of scrapped/obsolete/not-for-
production Class-D amps that technically "work". I think
some of them could fake a decent sine wave at 50KHz
or higher. The actual switching frame rates are way up
there.

I had my eye on abusing them to light a pair of 814's.
Needing 10V and 3.5 amps apiece.
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