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Old 16th October 2008, 11:13 AM   #1
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Default Switch-mode PSU for tube heaters?

Have anybody give some thought on using a switch-mode PSU for the heaters in a tube amplifier? In these days with greenies going nuts about energy efficiency and the cost of copper isnt it a bit more sensable to look into designing and building a switchmode PSU to power the heaters in a tube amplifier.

IE: If you would like to build a beefy OTL amplifier using a few 6C33C. Say you use 8 6C33C tubes per channel then the heater supply current needed would be around 60 amps. Just imagine the huge clunky transfo needed just to heat up the tubes!!

What do you think of this?
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Old 16th October 2008, 11:56 AM   #2
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You can try a switch-mode PSU for low voltage Halogen Lamps.
To prevent noise injection from the PSU into the cicuit you should add a good Filter between PSU and Filament...
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Old 16th October 2008, 11:59 AM   #3
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Current itself is not a measure of energy consumption, 8 6C33C tubes consume a total of 328W which doesn't need much of a transformer. This 328 W can be delivered by voltages between 6.3 and 100V depending on what power supply is available.

I would be hesitant to use a switch mode supply for 2 reasons, heater inrush is normally 3-4 times the steady state current so the power supply would need to be designed for that. 2nd reasons is that I would expect leakage of high frequency noise from the heaters coming in to the audio circuit. Although the leakage is low for line frequencies the leakage increases due to capacitive coupling when the frequency gets higher.

A final note about effciency, it is questionable if you would gain anything by using a switchmode supply as power tubes heaters are normally powered by AC and an ordinary transformer have very high efficiencyu already, it is rather likely that the efficiency using a switchmode power supply would be lower.

Regards Hans
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Old 16th October 2008, 10:25 PM   #4
kevinkr is offline kevinkr  United States
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I think that this is not quite the case if you are talking about tubes requiring dc at higher voltages and currents. Losses in rectification and regulation, not to mention winding losses would tend to make switchers look attractive when more than 10 - 15W of heating power is required. Add to that the much lower weight, smaller size and potentially higher efficiency particularly if the switcher has PFC under these situations, the switcher might be the better choice. Simple common mode and diff mode filtering can improve output noise performance of most switchers by 20dB or more at minimal cost.

Heating a pair of 211 requires 2 x 10V @ 3A, so 60W delivered to the load. Figure that each regulator requires 3V at low line (-10%) for drop out, and figure a power factor of 0.7 for the bridge, figure in other losses and you are already probably over 110W at nominal line to produce 60W of heating power.. This is an efficiency of about 55%, most line operated switchers can manage at least 85% efficiency and with PFC might save you nearly 30W in mains power and weighs perhaps 0.5kg - 1kg compared to up to 5kg or more for transformers and heat sinks.

IME a ~400VA transformer to heat 8 x 6C33 is going to weight quite a lot - mine weighed over 7kg each.
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Old 16th October 2008, 11:29 PM   #5
JesseG is offline JesseG  Canada
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Using SMPS for tube heaters works fine.

See here:
http://www.diyaudio.com/forums/showthread.php?s=&threadid=111714

I have been using this preamp for over a year. One of the comments to the above post said that my tubes would fail since I am using a 5.5V SMPS - so far they have been working fine, with no sign of shortened tube life.

Just remember to shield the SMPS to prevent RF from intruding into the cct.

Jess
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Old 17th October 2008, 06:22 AM   #6
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Kevin,

I can see that there are benefits of using switchers when there is a need to use DC for heaters, you probably gain both in weight and efficiency. However the original poster used 6C33C as an example and for that and other indirectly heated power tubes I can see no or very minor benefits of using a switcher.

Regards Hans
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Old 17th October 2008, 02:39 PM   #7
kevinkr is offline kevinkr  United States
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Quote:
Originally posted by tubetvr
Kevin,

I can see that there are benefits of using switchers when there is a need to use DC for heaters, you probably gain both in weight and efficiency. However the original poster used 6C33C as an example and for that and other indirectly heated power tubes I can see no or very minor benefits of using a switcher.

Regards Hans

Saving 10kg not a benefit?
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Old 30th October 2008, 07:13 PM   #8
JesseG is offline JesseG  Canada
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Another advantage to using SMPS... there are hundreds of old 5V/3A or 9V/3A wall-warts out there in junk stores.

Why a 5V or 9V SMPS? See Steve Bench's article on starved filament supply vs distortion.

http://members.aol.com/sbench102/dht.html

Jess
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Old 30th October 2008, 11:59 PM   #9
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Default SMPS

I have a small pile of failed SMPSs'...You would have to design an SMPS to cope with the inrush surge which may offset the size/weight advantage...next up is TRYING to smooth out all those harmonics...and SMPSs' are chock full of harmonics....put your scope on an SMPS!
Using AC is fine for heaters, the one application where you might use one is on a sensitive front end....but it really is too noisy.
Simplicity........"Keep it simple stupid" The more complex, the more there is to fail! It is a statistical certainty!
Go SE, two perhaps three stages, AC heat.
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Old 31st October 2008, 09:07 AM   #10
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Jesse, Steve Bernch's observations are only valid for DHTs!
The 6C33 is a indirectly heated tube.

I agree with Richard... keep it simple- use AC heating.

Boris
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