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Bud Barnez 23rd September 2012 07:01 PM

Tube power supply and hot regulators
2 Attachment(s)
I have a power supply PCB from Lite Audio called LS-57. It has an adjustable HV regulated supply and two low voltage output which can be used for heater supplies.

I have tried to attach the schematic, hope it shows.

I'm using this pcb to power my first DIY project which is a tube I/V stage for a DAC. The tube stage is a Broskie Unbalancer and I've use 6SN7 tubes.

Everything seems to work and I'm pleased with the result but have some concerns:

1. The heater regulators get seriously hot - I measured the heatsinks at 95deg C (203F). The regulators are LM317 and I am running with 6.3V with heaters wired in parallel. There are four tubes in total. Hater current is said to be 0.6A per tube and I'm using one of the LM317s per channel so powering two tubes = 1.2A. I initially ran the heaters at 12.6V series but read that that is wrong for 6SN7 so reverted back to parallel 6V. Regs ran much cooler with 12.6V. I'm not sure if I have damaged the tubes by running for about 6hours at 12V but mainly concerned the regulators are too hot presently - Please could someone advise if this is normal/dangerous/need bigger sinks?

2. Second issue is that the 6X4 tube used for rectification is followed by a 180uF cap, and I read that maximum value of capacitance following that rectifier is about 16uF. Is this a problem - should I replace with lower value?

3. From what I read chokes are good in valve power supplies, is there a way I can implement a choke in here (replace R1?) or is there no point in a regulated supply?

4. I added 200uF at output of HV for each channel is that a good idea or should these be removed or would increasing further provide any benefits?

Hope someone can help.


kevinkr 23rd September 2012 07:31 PM

Those large electrolytics at the output of the HV supply may destroy the rectifier and/or 6BQ5/EL84 over time, (a single arcing event will do it immediately) I'd not go with more than 40uF on the output.

Yes you can replace R1 with a 5 - 10H choke and achieve a large reduction in output ripple.

I'm not sure who told you that a pair 6SN7 can't be wired in series for 12.6V operation, but the other option might be to get some 12SN7 instead, they're much less popular and some fairly exotic ones can be had for much less money than the 6.3V variant.

Bud Barnez 23rd September 2012 08:05 PM

Thanks for the reply Kevin.

Ok I will look to replace C1 with smaller value cap. Does this apply to C2 as well, or if I replace R1 with a choke can I keep C2 as is?

Regarding the hot heater regulators, would you consider 95deg as too hot - presumably their life will be short too? Would I be better off running at 12.6? I had hoped to power all the heaters from one regulator but after seeing the heat I thought I should spread the load over both heater regulators. It would be handy to use the other to power the source components.

Also is it good or bad to add additional caps after the HV output is there a maximum value or would the value installed have been calculated for optimum performance? Presently it's a 47uF?

Minion 25th September 2012 12:25 AM

You should use heatsinks on the regulators if your not allready useing them .... depending on your starting DC voltage you would dissapate less heat with 12.6v as opposed to 6.3v .... if your input AC heater voltage is 12.6v AC that would give you about 17v DC which would cause less heat with 12.6v AC than 6.3v AC ......

Bud Barnez 25th September 2012 11:04 PM

1 Attachment(s)

Originally Posted by Minion (
You should use heatsinks on the regulators if your not allready useing them.

Yes the heater regulators have heatsinks about 2" high (picture attached). The temperature I measured was of the heatsink (95deg C).

I am going to adjust it back to 12.6V as you recommend.

I'm still not quite clear about the caps, I should have been clearer- I mentioned large caps (that came as part of the power supply) on output after 6X4 rectifier (C1 & C2) and also ones I added after the final output (after C3). So should I reduce value of C1 & C2, or just C1 if I add a choke in place of R1?
...and should I remove the capacitors I added on the final output too (additional 400uF total)?

Minion 25th September 2012 11:18 PM

Remember that you will have to run the heaters in series not paralell with 12.6v DC , I would leave the caps alone and try it at 12.6v DC and see if it solves the overheating problem ...... it should dissapate considerably less heat as when running at 12.6v dc ....

kevinkr 25th September 2012 11:51 PM

Unless you like cooking your 6BQ5 due to extremely excessive inrush current I'd recommend removing most of the output capacitance you added - if the regulator is doing its job they are not contributing much in the way of noise reduction and they really over-stress the pass element (6BQ5) during warm up.

My previous comment I thought was relatively clear.. :D

Bud Barnez 26th September 2012 12:14 AM

Thanks guys. Yes will run heaters in series.

Kevin, your comment would probably be perfectly clear to a non-noob. I wasn't sure if you were referring to C1 & C2 after the rectifier or my ones after the output.

So can leave C1 & C2 be?

kevinkr 26th September 2012 12:23 AM

Hi Bud,
I think C1 should be no more than 10uF based on the data sheets I have, apparently the designer of this board was not too concerned about exceeding the inrush and peak current rating of the rectifier. C2 can remain at 220uF with the addition of the choke ahead of it.

Bud Barnez 26th September 2012 12:36 AM

Thanks for clarifying - perfectly clear now.

I just found a thread where someone questioned the same issue about the cap following the rectifier, and Kevin I see you responded the same -you must be the power supply man!

While the power supply isn't identical it looks very very similar. And recommendations were made to increase equivalent of R3 by factor of 10 and increase C4 to something around 0.047uF. Would these modifications be good here while I'm at it, or anything else I should consider?

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