Tube power supply and hot regulators

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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.

Thanks
 

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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.
 
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?
 
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 ......
 
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)?
 

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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 ....
 
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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
 
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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Hi Bud,
If the installed error amplifier tube is indeed a 12AU7A and you don't wish to make a lot of changes I would just change C4 to 0.047uF, if OTOH you are up for a bit of a challenge I would make the following changes which must be done together:


  • Change 12AU7A to 12AX7A to get roughly 30 - 32dB open loop gain.
  • Cut etch between cathode of 6BQ5 and R3
  • Remove R3 and replace with 221K
  • Solder wire from free end of R3 to pin 9 of 6BQ5.
  • Add 220uF cap from pin 9 of 6BQ5 to ground.
  • Add 0.01uF from pin 9 of 6BQ5 directly to ground - keep leads short.
  • Change C4 to 0.047uF
  • Change R5 to 68K
  • Jumper R7
These changes will implement something close to the design I started with 20 some odd years ago, and should be a much better performer than the design currently implemented on the board. (Better regulation and lower noise.)


Search on some of the other tube regulator threads over in tube / valves for my high performance 300V regulator. I still use variants of that design today.
:D

Note that if my suggestions create some "issues" I will be more than happy to help you get them sorted so don't hesitate to ask here or send me a PM.

:cop: Reference to pin 6 corrected to pin 9
 
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<snip>

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!
<snip>

Yes, I'm somewhat notorious with the local audiophile community here in New England and on the forum as an advocate for tube based voltage regulators. :D
 
Wow, thanks so much for those instructions, that's great. I'm up for the challenge.

I'm not sure what the pin 6 is for on the 6BQ5, looking at data sheet it is described as "internal connection".

Will order in the required bits, couple of questions though:

For the 10uF cap for C1, is there a particular type that would be best or any cheapish electrolytic with correct voltage?

Reading about chokes, seems multiple smaller are better than single larger one. I have lots of 100uf caps so no problem to add two stages, so am I right in thinking:
C-L-C-L-C (of 10uF-2H-100uF-2H-200uF) would be better than
C-L-C (of 10uF-10H-200uF)?
Thinking 100ma choke rating ok?

Also I have an oscilloscope and for my education would be keen to see the results if they are observable. Should I be able to see ripple with it? Looked pretty clean when I tested initially but may not have been checking correctly: I probed DC on final output.
 
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I'm not sure what the pin 6 is for on the 6BQ5, looking at data sheet it is described as "internal connection".

Will order in the required bits, couple of questions though:

<snip>

Hi Bud,
Sorry about that, that should of course been pin #9 which is the screen grid. I'll fix it in the post so it doesn't trip anyone up down the road. :D

More in next post..
 
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Hi Bud,
Use a good 10uF electrolytic for the first capacitor in the pi- filter.

In terms of chokes for this application I think the 10uF - 10H choke - 220uF cap will fit the bill nicely. I think you would be well beyond the point of diminishing returns. The critical location is actually the screen supply, but the ripple there is unlikely to be more than a few mVpk.

Yes you can use the scope to measure ripple at various points in the design which will give you an idea of how well the supply performs.

Since you are up to a challenge if you can fit it a simple LPF between the zener and the input [grid] of the error amplifier could net you a good improvement in noise performance. Say 470K and 0.1uF.

There are all sorts of other things you can try and if you take a look at my Muscovite thread over in analog source you can see my latest power supply design. This design can be implemented with a 6BQ5 but would probably be best left as another project.
 
Hi Bud,
Since you are up to a challenge if you can fit it a simple LPF between the zener and the input [grid] of the error amplifier could net you a good improvement in noise performance. Say 470K and 0.1uF.

Ok, so to do this I connect 470k between pin 2 of (new) 12AX7A and point between diode and zenner (D1 & D2), then 0.1uF between pin2 and ground?

Sorry for being layman. I think it will be safest for me to sketch out amendments when I can get to a printer tomorrow.

I will look again for some of your designs but didn't find them when I looked yesterday.
 
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Ok, so to do this I connect 470k between pin 2 of (new) 12AX7A and point between diode and zenner (D1 & D2), then 0.1uF between pin2 and ground?

Sorry for being layman. I think it will be safest for me to sketch out amendments when I can get to a printer tomorrow.

I will look again for some of your designs but didn't find them when I looked yesterday.

Yes, you make the connection as described, just imagine you snipped the connection to pin 2 and installed it as you described.

Yes, definitely post with changes.

They're here, but posted ages ago, the Muscovite thread is the best bet for my current approach. I'll provide a link:
http://www.diyaudio.com/forums/anal...ovite-6s3p-tube-phonostage-7.html#post3112139

Most of my power supply posts have been in the tube / valve forum, but it's been a while since I posted one there.

I removed all posts relating to pin 6 except the original since the whole confusion was my fault when I did not fix all instances, and I wanted to minimize confusion going forward. :D
 
Most of the parts required for the modifications suggested by Kevin arrived in the post this morning.

My interpretation of the instructions is attached. I'd be grateful if someone could confirm I have it correct. There are two bits I'm still not quite sure about these are asterisked in green.

1. Am not convinced I have the low pass filter correct on the 12AX7A as Kevin implied I need to make a snip? But not sure where. Also I have plenty of 0.047uF caps would these be OK instead of 0.1uF which I don't have?

2."Add 0.01uF from pin 9 of 6BQ5 directly to ground" - By directly to ground, does this mean the closet ground point or directly to the star ground I have for this amplifier - I have no ground connected from the transformer at present. All grounds go to a star point for each channel and then connect to ground of DAC output. I'm not sure if this is the correct way to wire the grounds. Initially I didn't connect the ground to the DAC and I got a lot of noise but on connecting to DAC it is super silent, but am worried all that noisy hash is going into the DAC?
 

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Hi Bud,
Looks good for the most part, I'd probably just delete D2 and replace with a jumper.

The 0.01uF on the 6BQ5 screen should go to the power supply circuit ground as directly as possible. (Where the other supply caps are connected if in close proximity.)

Keep me up to date and I will help you debug as necessary.

:D
 
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