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Old 4th December 2004, 01:21 AM   #1
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Default Has anyone used a triode as a shunt regulator?

I'm trying to think of fun things to try with a pair of unused ECC99 halves. The other halves are CCS loaded and drive SE 2A3s. Someone mentioned a shunt regulator, which I had thought about a long time ago when I didn't have spare triode halves. I don't have the space to add VR tubes, but I could do something with the triodes. I've read the TubeCAD article, where he starts out with just a triode and ends up with a fairly more complex hybrid regulator. I would prefer to keep this on the simple side if possible.

Has anyone actually done this? Is it worth doing, or does the CCS load somehow make the regulator superfluous or redundant? If it's worth trying, what would be a simple but effective way to build a shunt regulator with the triodes? Also, how does one pick the operating point to use for the regulator tube? Should I set it for the same DC current as the tube it's 'regulating'?

This is the simplest one on TubeCAD that uses the triode's u, and doesn't use a Zener or CCS:

Click the image to open in full size.

Should I just build this one? The article says that it's flaw is the absence of a good voltage reference. Would replacing the R||C under the cathode with a battery (like I've done with the drive triodes) take care of that?

Thanks,
Saurav
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Old 4th December 2004, 02:58 AM   #2
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@ Saurav

I haven't tried any myself, just series regulators. But here's something on Steve Bench's site you might find helpful:

http://members.aol.com/sbench/reg5.html

Wayne
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Old 4th December 2004, 03:29 AM   #3
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Quote:
I don't have the space to add VR tubes, but I could do something with the triodes.
Sorry I missed that! I would suspect that without some kind of stable voltage reference you could expect poor performance. Used in a low current demanding circuit it may work ok such as an ECC83 @ less than 1mA or so..., but I don't think it would work well with your CCS loaded ECC99 driver, i.e. you have to divide up the current between the two. Are you sure you can't find space for VR tubes? Maybe mounted horizontally in the underside of the amp? Don't know if you can mount VR tubes that way (heat build-up for one thing). Zeners (blasphemy!)? I know you are just dying to use those unused halves.
Sorry I couldn't be of more help.

Wayne
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Old 4th December 2004, 03:40 AM   #4
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Default Re: Has anyone used a triode as a shunt regulator?

Quote:
Originally posted by Saurav
This is the simplest one on TubeCAD that uses the triode's u, and doesn't use a Zener or CCS:

Click the image to open in full size.

Should I just build this one? The article says that it's flaw is the absence of a good voltage reference. Would replacing the R||C under the cathode with a battery (like I've done with the drive triodes) take care of that?
The trick is to get a fixed Voutput (Vo) to grid voltage and a fixed cathode to ground voltage. This results in regulation being equal to the voltage required to cover the current range, Gm in effect. This could be done with a negative supply to sink current through the reference from Vo to grid to neg., and the cathode grounded; or the cathode floating on a small reference (of voltage a bit greater than the expected cutoff voltage) and coupling grid to Vo with another reference. This requires two refs, but no negative supply. (In effect, the two forms are electrically equal, the negative supply providing the extra cathode boost that is otherwise provided by a ref.)

The schematic shown above will only function for AC obviously. Fine if you want a little dynamic regulation.

Oh, and this is the shunt form of the pass regulator known as - dun da da duhh - a cathode follower! If you bias a CF with a reference voltage on the grid, it will have the same (well, inverse due to the arrangement) regulation characteristics as this circuit.

In both cases, clearly, for tubes with low gain (which are required much of the time, to sink the power or current demand), regulation is very bad, demanding a second error amplifier.

Tim
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Old 4th December 2004, 03:41 AM   #5
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I think Verdier published a shunt regulator circuit in Sound Practices, as did Chimera Labs (around a 211 based circuit, I think). These references and the TubeCad and Steve Bench references are all I've found about shunt regulators. Oh, and I think you'll find something on the Audio Note website commenting on the sonic properties of shunt regulators.

From what I've read about both tube and SS shunt regulators, shunt regulation sounds better for class A and A/B circuits. Tube shunt regulation has seemingly seen very little implementation. Verdier and Lamm are two commercial enterprises using shunt regulation. Lots o' scope for developing this breed of PSU, methinks.
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Old 4th December 2004, 04:17 AM   #6
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I didn't realize I was in semi-uncharted waters

The cathode-to-ground reference could be done with a battery, right? That should be a fairly stable reference independent of the current going through it. Not sure how to do the grid-to-Vo reference easily.

Maybe I should just think of something else. Like a hybrid CCS with the triode cascoded with a FET or the IXYS part or something like that.
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Old 4th December 2004, 11:22 AM   #7
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Saurav

I have added shunt regs to my SE amps in the manner set out on Tubecad as you have shown above. this was purely to improve the ac reg of the psu. I found it made a significant difference and improved the sound particularly when added to the output stage. It improved the amps to the point that further psu tweaking had very little difference on the sound whereas before the shunt reg every tweak had a clear sonic signature...

I found that with PP amps it made a difference but nothing like as much.

I have not used the shunt reg with CCSs so can't comment on their effect.

ciao

James
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Old 4th December 2004, 03:49 PM   #8
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James, I'm not surprised to hear your results. We regulate *every* other supply, why not regulate the output? Especially on an SE circuit given the lesser current requirements. Can you tell us more about your setup and possibly post a schematic? How, in particular, do you tap the AC reference fed to the grid or cathode of the gain amplifier, assuming you use such amplifier?

So far as my neophyte thinking on shunt regulators has progressed, I've wondered if:

1) using the same shunt tube as the tube the shunt feeds gives an overall better (linear) distortion spectrum (by linear, here, I don't mean "less" but straighter rising distortion line or curve) and

2) the AC reference fed to the gain amplifier could be tapped from a transformer that in an unregulated circuit would be an inductor---thus to add a secondary winding to a PSU inductor to derive AC/DC fluctuations (elegant?). Using a transformer, as such, could eliminate the otherwise necessary shunt resistor, would avoid capacitive coupling (a weak link methinks) and could possibly simplify DC voltage requirements?

Any comments?
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Old 4th December 2004, 04:06 PM   #9
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Ditto, a schematic would be very helpful. I thought (and I could be wrong) that the shunting element needs to be able to handle the same amount of current as the element that's being shunted? If that's true, then regulating the outputs is out, because I can't put 60mA through the ECC99. Though I realize that regulating the SE output stage would provide a bigger benefit than regulating the driver.

James, so did you use the exact circuit that's in the image I posted? What values did you use for the resistors and capacitors, and how did you arrive at these values?

Thanks,
Saurav
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Old 4th December 2004, 10:18 PM   #10
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Default Answers are difficult things...

I guess a little background will explain how arrived at the scheme I'm using. Two things influenced my thinking - 1) I'm a push-pull DHT guy - these are my favourite amps and I have had some success in making them accurate and transparent and with good tone - but I noticed that as I made them more transparent then the microdetails didn't quite sound right. In particular the microdetails didn't sound as good as through my ECL82 SE amp or my sons 2A3SE amp. Everything else was better but microdetails were not... 2) When last redesigning my sons amp I completely changed everything from the WAD kit that he had bought and in the process decided to change all the PSU caps for Blackgate types (as BG recommend) to say I was surprised by the extra transparancy and lower moise floor this gave would be an understatement.

These two things lead me to look at SE and particularly SE psu in more detail. I suspected that the SE OPT having a good static mag. level helps the low level details relative to PP and the ultra clean psu that the BG produced really allowed the microdetails to flourish - but BG are very expensive so what else to do to help minimise psu noise? Note: I was not after regulation per say or ripple reduction but removing noise from the power lines. This thinking lead me to ac shunt reg. Further thoughts lead me to use the same valve for the shunt reg as the signal valve it was regulating for and to bias it at the same (or close) Op point. So for an ECC99 gain stage I use an ECC99 shunt stage - I actually use the second half of the same valve to keep the shunt as close as possible to the signal path. I use the Tubecad ac scheme as shown above with the bias and cathode resistors chosen to give the same op point or the same anode current at least for the signal and shunt valves. I then chose the capacitors that couple the ac noise into the valve for shunting to match the lf turnover point that the amp is designed to support - typically 4Hz. If necessary I will bypass the caps to maintain a flat bypass to VHF frequencies - probably not necessart for valve SE but a hangover from my SS days...

I'll try and post a schematic tomorrow.

gotta go - my better half is calling...

ciao

James
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