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Old 26th September 2012, 01:56 AM   #11
kevinkr is offline kevinkr  United States
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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.


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.

Reference to pin 6 corrected to pin 9
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Old 26th September 2012, 02:02 AM   #12
kevinkr is offline kevinkr  United States
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Quote:
Originally Posted by Bud Barnez View Post
<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.
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Old 27th September 2012, 12:23 AM   #13
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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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Old 27th September 2012, 12:33 AM   #14
kevinkr is offline kevinkr  United States
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Quote:
Originally Posted by Bud Barnez View Post

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.

More in next post..
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Old 27th September 2012, 12:44 AM   #15
kevinkr is offline kevinkr  United States
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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.
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Old 27th September 2012, 01:33 AM   #16
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Quote:
Originally Posted by kevinkr View Post
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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Old 27th September 2012, 01:51 AM   #17
kevinkr is offline kevinkr  United States
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Quote:
Originally Posted by Bud Barnez View Post
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:
The "Muscovite" 6S3P Tube Phonostage

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.
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Old 6th October 2012, 12:13 PM   #18
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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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File Type: pdf Power supply mods.pdf (148.3 KB, 34 views)
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Old 6th October 2012, 07:29 PM   #19
kevinkr is offline kevinkr  United States
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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.

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Old 6th October 2012, 09:35 PM   #20
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You are dropping quite a lot of volts across the regs to get 6 volts so I am not suprised they get hot.
Using 12 volt heaters would be a better solution.
You can usually run 2 off 6 volt heaters in series from 12 volts.

I run a ECC83 off 12 volts with no problems.
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