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ErikdeBest 6th February 2013 07:32 AM

'High power' shunt regulator
 
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Reading through the DIYaudio threads and a little own experience it looks as if shunt regulators really excel in ripple rejection and ‘subjective’ music quality. There are some proven designs, like Salas SSHV, still these don’t do ‘high power’ demanded by output stages. With the availability of high powered tubes for little money (like 40W GU50 for some USD) I am willing to experiment with ‘high power’ shunt regulators.

For example a PS capable of 300V at 220mA, with 160mA to feed 4 EL84 output tubes and a spare 60mA through the shunt tube, a triode strapped GU50. For CCS I am thinking about Gary Pimm’s CCS. I understand that if there is no load on this PS the GU50 will burn 220mA * 300V = 66W, which is more than max plate dissipation, but seems this tube can take this (even more) for short whiles. As such I know that shunt regulators produce additional heat, but that is nice for the winters when I am back in Europe.

I attach a proposed schematic for this regulator, and this is really the part I need assistance. A differential pair fed from a ‘negative’ regulated negative supply (since I read 4th edition of VA I already know how I will make this one) supplies the differential pair. Reference voltage is taken from a potential divider (R1 and R2) and control voltage is developed over R3. R4, R5 and P1 sample the output voltage, R6 is tail resistor (is a CCS, maybe LM334, also needed if it is about DC?).

If B+ voltage goes up, V on base of T2 increases, T2 conducts more, thereby T1 conducts less, less voltage is developed over R3, grid of shunt element comes closer to zero, shunt element conducts more, reducing B+ voltage.

Questions:
- Do you think it will work?
- What about using some HV bipolar transistors (MPSA 42, if I am not mistaken). How does ‘base current’ impact performance? Or better use a Vref (like TL431) instead of R1?
- Some decoupling needed, for better performance and or stability? Speed-up capacitor from B+ output to gate of T2?

DUG 6th February 2013 12:10 PM

Do you need 90V swing on the grid?

50V will reduce plate current to <5mA in a GU50

Changing R1 / R2 value will change the feedback ratio to a lower number...Will probably help response time.

i.e. You are using a 10V reference right now. Your feedback ratio is 39:1. [(300+90):10]
and your grid swing is close to 90V
If you change the R1 / R2 to 30K and 70K (30Vreference) then the feedback ratio is 12.33:1. [(300+70)/30] and your grid swing is 70V...still enough to drive the plate current low.

This would also reduce the power dissipation of T1, T2...can't be a bad thing.

Select a nominal operating point for T1 and T2 that gives them similar power dissipation for similar operating temperature. (play with R3 value)

IMHO

Some things to think about.

:)

ErikdeBest 6th February 2013 12:44 PM

Many thanks for the reply! You are right that -90V is a lot, specially as I also mention the possible use of a 6P15P (for maybe a driver stage) that will cut off with less than -15V. This is more a concept, I think, and values of resistors can be changed according to final application. But I had overlooked the aspect of feedback ratio, which is a good one, thanks!

Another thing, based on tubecad: I would probably test with a 22k (or larger) resistor in series with the grid, and AC connect B+ to the grid with a cap, so that the tube handles AC that is coming from the amp part, while decoupling gate of T2 with an elco.

trobbins 6th February 2013 07:26 PM

Re you going to put a shunt regulator on the 100V supply to give similar regulation?

ErikdeBest 6th February 2013 07:58 PM

Quote:

Originally Posted by trobbins (Post 3358764)
Re you going to put a shunt regulator on the 100V supply to give similar regulation?

that is indeed the idea. The negative 100V, or less (in case less voltage is enough) will be provided by a shunt regulator, as demonstrated in Valve Amplifiers 4th edition, by Morgan Jones. I don't think, however, that it is right to publish that regulator here? But it is a very quiet one!

Rod Coleman 6th February 2013 08:27 PM

Hi Erik,

Yes. differential bipolar transistor error amplifiers are the way to go.

If the grid current of the pentode is say <10uA, you can run the differential pair at 500uA. This in turn will keep the base current low. The tail can remain a resistor if the -100V is regulated - it will be constant current anyway, and CMR is not a real problem.

As for transistors, please try the excellent NXP PBHV series of 500V parts in SOT23 and 1W SOT223:

PBHV8540Z :: NXP Semiconductors

A "lead" cap can be used from B+ to FB, but it may be necessary to compensate the diff pair with a cap, also.

Hearinspace 7th February 2013 08:01 PM

Quote:

Originally Posted by Rod Coleman (Post 3358851)
Yes. differential bipolar transistor error amplifiers are the way to go.


Why ?

Rod Coleman 7th February 2013 08:19 PM

Quote:

Originally Posted by Hearinspace (Post 3360196)
Why ?

Hi Ian!

Compared to op-amp error servo:

1. no need for separate power supply, or risk of damage to opamp from HV.

2. open-loop gain and bandwidth can be finely controlled, so you can tailor the transient response to perfection. All stages can be separately frequency compensated.

3. cost is almost zero.

Hearinspace 8th February 2013 05:42 AM

Hi Rod, thanks !

So, you would run a shunt regulator based on the NXP transitors across the HV power supply outputs without using an additional minus supply ?

I see in the above referenced tubecad article Broskie's well known 431 / tube hybrid shunt reg. and if I remember rightly, I think you said once that the 431 wasn't exactly the chip of your dreams (my paraphrase) so now I am wondering about the inverse. If you were going to use the BJT pair as the error amplifier, would you put them under a tube or would you go for a high voltage BJT, MOSFET etc. ?

Rod Coleman 8th February 2013 12:06 PM

I use BJTs and enhancement-mode FETs for my shunt regulation, but if you use a triode, the grid supply must (of course) be negative (compared to its cathode anyway).

431 can work OK, but the noise generated by their internal bandgap reference is not helpful. It varies from one manufacturer to the next, which may make for individual testing (yuk).

I think the best path is to start with one of the existing public circuits for a SS shunt regulator, and see if there is anything wrong with it, for your application.

I do prefer an error amplifier though (but not op-amps) - for transient control over voltage - and I don't like noisy references!


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