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ideas on variable filament supplies?

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Posted on July 21, 2015 at 13:15:06
vinnie2
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 I know I can vary the voltage by using a variable transformer, but is there any less expensive way to make adjustable filament voltage supplies? I need to build a couple for some 1.5 volt amd 2.5 volt dht tubes.

 

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RE: ideas on variable filament supplies?, posted on July 21, 2015 at 13:23:42
Alpha Al
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 If you have a separate filament transformer, you can connect a variable resistance in series with the 120VAC primary side. Value depends on load current, try a 10 Ohm pot, wired like a rheostat. A 2 watt pot should be sufficient.

 

RE: ideas on variable filament supplies?, posted on July 22, 2015 at 03:55:08
vinnie2
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Joined: September 28, 2013
 Since each tube heater pulls anywhere from 1 to 1.75 amps, I would probably have to use one per tube. The pots are fairly easy to come by but not the filament trans. I have a couple of different trans that could supply all the tubes so I would like to use one of them, but I am not hot on the idea of spending 60 bucks for a 10 amp variac.
I suppose I could put two each on a 5 amp pot and only have to use2 trans. Thanks for the idea, I will have to roll that one around a while.

 

RE: ideas on variable filament supplies?, posted on July 22, 2015 at 04:51:44
amnesiac
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 I have built but not tested yet other then on the bench into resisters. I really neat little quad dht filament supply. It uses the technology that has been made incredibly cheap and efficient for led supplies and solar I think.

My supply uses 4 neat little dc to dc convertors 95% plus efficient. They cost 5$ each. There are many to be had. They can be bought in voltage regulated current regulated or both designed for battery charging.

I know anyone reading this is probably thinking omg super noisy but they are so efficient in converting dc to dc with out the need for a noisy r and so much less demanding on the power trans and rectification these hi tech units with a little extra filtering might be very good indeed.

Maybe I am missing something really obvious because I cant think why I havent seen them used yet for this use.

 

RE: ideas on variable filament supplies?, posted on July 22, 2015 at 05:19:00
Alpha Al
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 In the primary side, current is low. A low power pot or rheostat would work fine there. It is common practice in high-power radio transmitters to adjust filament voltage, where filament current can be 20-30 amps. A low current adjustment in the 120VAC primary side is the way to do it.

 

RE: ideas on variable filament supplies?, posted on July 23, 2015 at 04:54:15
vinnie2
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Location: North Carolina
Joined: September 28, 2013
 Do I just hook up one pot on one leg of the primary? Does it matter which leg?

 

RE: ideas on variable filament supplies?, posted on July 23, 2015 at 07:49:34
BofService
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Posts: 741
Location: Atlanta
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 There are certainly plenty of surplus 5v dc power supplies around these days. About a bazillion phone chargers, plus the power supply in every pc (which also offer 12v btw).

Alex Kitic has done some experimentation with cheap halogen supplies. See his blog: http://rh-amps.blogspot.com/

 

RE: ideas on variable filament supplies?, posted on July 23, 2015 at 08:13:07
Triode_Kingdom
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Location: Central Texas
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 The problem lies with potentially audible artifacts from the switching noise. I use a switcher on my 211 filaments, but it's modified to operate above 60kHz. Most of the cheap (and not so cheap) PS units operate too close to the audio spectrum to be viable for this use without significant filtering. I'm not sure it's worth going this route by the time you include the extra work. Filament transformers from Hammond and Edcor aren't very expensive, and eBay lists numerous adjustable regulator boards and kits if DC is needed.

 

RE: ideas on variable filament supplies?, posted on July 23, 2015 at 09:31:09
Alpha Al
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Yes, in one leg of the primary as shown. You may need to determine how much resistance is needed by experimenting.

 

RE: ideas on variable filament supplies?, posted on July 23, 2015 at 10:04:08
vinnie2
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Location: North Carolina
Joined: September 28, 2013
 What I found on the web for using a pot for a variable resistor shows one of the AC wires to terminal 3 and terminals 2 and 1 connected to the one of the primary wires of the filament transformer. A bit different than what you show. Does it sound right to you?

 

RE: ideas on variable filament supplies?, posted on July 23, 2015 at 11:26:13
Alpha Al
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 Only two terminals are needed on the pot. You could connect the third one to the wiper (center)terminal, that would not matter.

 

RE: ideas on variable filament supplies?, posted on July 23, 2015 at 12:30:12
vinnie2
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Location: North Carolina
Joined: September 28, 2013
 Well it looks like a 20 to 25k pot (using just two terminals) is just about right for a 5v tansformer. It gives me a nice smooth adjustment range from about 0.9 to 5.7 vac with no load. May need different values for other transformers, but probably not for the common ones.
Now I am sitting here trying to figure out why I would not want to install one of these little dudes on every filament suppy in every piece of equipment I have or ever plan to build. Seems to me a person could dial in the exact filament voltage needed while the amp was fired up. No more trying different resistors clipped to the filaments and then never getting it quite right anyway.
It also means a person could use any filament transformer that they wanted as long as it was at least as much as what they needed. Opens up a lot of options for using surplus and hamfest finds. Am I missing something or is this really cool?
Thanks for the tip Al, I can see lots of use for this one.

 

RE: ideas on variable filament supplies?, posted on July 23, 2015 at 22:42:51
amnesiac
Audiophile

Posts: 717
Joined: August 21, 2002
 I have got some of the halogen trans. The dc to dc converters are variable so I can try them on anything. I not sure what frequency they run on.I will get to the halogen supplies when I build my big amp. I am finding it hard to find a good reason to stop listening to my 10y amp at present.

actually I do know now Working Frequency:300KHZ

below is some of the bigger ones I bought.

 

RE: ideas on variable filament supplies?, posted on July 24, 2015 at 04:34:43
Alpha Al
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 Offhand, 20-25K sounds way too high, bit I have not done the math. It really depends on how much current is being drawn from the AC line.

One thing for sure-it will not boost voltage. If you have a 5V Xfmr, 5V is the most you will get when the pot is fully closed. You could use a 6V Xfmr, and dial it back slightly for 5V.

 

RE: ideas on variable filament supplies?, posted on July 24, 2015 at 06:09:38
lugnut1
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Posts: 132
Location: Austin,TX
Joined: September 24, 2011
 Some good info here. I too am interested in using a 6.3VCT @10A filament transformer for 2 866A rectifiers in series at 5v@10A. Any thoughts greatly appreciated.

 

RE: ideas on variable filament supplies?, posted on July 24, 2015 at 12:09:52
vinnie2
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Posts: 4481
Location: North Carolina
Joined: September 28, 2013
 Roger on that. Quite often you will find 5 and 6 volt filament transformers cheap at hamfests, and you could dial them back to anything from 5 volts on down. Very useful for some of the dht triodes that have the odd filament votages like the 26 and 27 tubes I am trying to work with now that have filament voltages of 1.5 and 2.5 v.

I tried several different values of pots, starting with a 100 ohms and working up. Didn't get any really useful range until I got up to 20k. Not sure if it will heat up too much; that is the next thing I want to try. I will let one run for a while under a filament load and see how it does.

 

well rats!, posted on July 24, 2015 at 14:13:26
vinnie2
Audiophile

Posts: 4481
Location: North Carolina
Joined: September 28, 2013
 You were right about it being way too high a value; that was because I did not have a load on the transformer. Duh! With the tube filament connected into the circuit we need to get down to about 2.5k for the pot value to get a useful range of voltage. Trouble is then you get a hot pot after running it a few minutes. Going to have to break down and do a little math and figure what rating I need and theh shop around for a rheostat with the proper specs. It may not be the universal solution I first thought it could be, but it might still work for giving me an adjustable filament supply on my test mule bread board. Hopefully this will still be a simpler, cheaper solution than a $60 variac.

 

Rod Coleman's regs are adjustable, posted on July 27, 2015 at 04:41:59
andy evans
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 Rod Coleman's regs are adjustable, cheap and as good as you can get. Ideal for DHTs and the market leader right now.

 

RE: Rod Coleman's regs are adjustable, posted on July 28, 2015 at 18:40:59
vinnie2
Audiophile

Posts: 4481
Location: North Carolina
Joined: September 28, 2013
 Interesting units. I did find some 25 watt rheostats on ebay for $10 apiece and odered a couple of theose. I will keep these in mind if the rheostats don't work out.

 

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