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Old 23rd Sep 2019, 11:40 am   #1
Diabolical Artificer
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Default HV regulation.

I'm looking at knocking up a HV regulator for a valve preamp, not for voltage regulation per se', but for ripple suppression, which as this PSU will provide power for a phono preamp, needs to have very good ripple suppression of over 90dB if possible. The PSU also needs SC protection but not be overly complicated. I can't throw lots of capacitance at this problem, tied a 820u cap/RC filter, it takes too long to charge the cap.Another reason for wanting to regulate is that the tfmrI intend to use looks nice, IE shiny brass but is less than ideal. EG full wave rectified DC V + 470v, falls to 350v loaded @ 10mA, sec is rated @ 60mA.

I've never built a HV PSU from scratch but looking at various HV PSU regs as used on existing commercial and DIY preamps, there seems to be several approach's.

The simplest is in pic one, which is very simple and uses a mosfet and another Q for current limiting.

Another approach is to use a regulator IC like a 317 with elevated ground, a similar approach is to elevate a series pass element using a string of zeners - pic 2, which to me looks dodgy. A string of over 10 zeners seems to me to be problematic, from a temp co POV and possible failure of one zener in the string, not to mention the space numerous zeners would take up on a PCB.

The last approach I've seen is to use a regulator IC, like the 723 with an external series pass element using a HV trannie like one of the BU***, the few circuits I've seen use two tfmr windings though to power the reg Ic and look overly complex. Pic a PSU from an audio research preamp doesn't use a low voltage rail though, but hope you get the idea.

So what to do, some of my thinking might be a bit muddy, any ideas welcome. Andy.

Lastly is possible I'd like to use what parts I have, I have a small bag of BU208
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Last edited by Diabolical Artificer; 23rd Sep 2019 at 11:46 am.
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Old 23rd Sep 2019, 12:26 pm   #2
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Default Re: HV regulation.

I may be barking up the wrong tree here, but I think that the BU208 is a bit of a one-trick pony, with parameters such as current gain sacrificed in the quest for high-voltage robustness. In its usual TV line-output topology, it's driven with a hefty blip of base current to drive it rapidly into saturation in its role as an efficient switch, as opposed to linear amplifier. As a series regulator, it would need to be driven with a reasonable amount of base current by a device that as a consequence would be a HV power regulator in its own right. In the past, I've used the BU326A as a linear HV pass element, I believe originally orientated at efficient off-line SMPSUs, which has a happy combination of good current gain and yet decent voltage capability. That approach would probably be seen as old-hat now with loadsa HV power FETs around, aimed for the now-ubiquitous SMPSU world.
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Old 23rd Sep 2019, 12:46 pm   #3
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Default Re: HV regulation.

Ehm , that's a shame,but makes sense, I was tried driving it with a ZTX605 Darlington at lower voltages before cranking up to wkg V/HV, OP current was really weedy, sorry, don't have figures to hand. Anyway a ZTX605 looks no good at full voltage as it has a Vcb0 of 140v, Vceo 120v.

How about if I configured the 208 as a Darlington or parallel it up? I have a few pulls from TV's of HV LOPT Q's, as you say I may be better off with a mosfet, have a few of those.

Andy.
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Old 23rd Sep 2019, 12:51 pm   #4
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Default Re: HV regulation.

A LM317 with elevated ground would work, but this IC is rated for maximum 40V between input and output. In a HT stabiliser, it is possible that this may be exceeded (it almost certainly will in current limit).

Better would be a TL783, very similar but rated for 125V in-out.

A string of Zener diodes need not be huge - 100V Zeners exist and are cheap. I would go for something like your first circuit, with the 500k potentiometer replaced by 100k between C1+ and R1, and Zeners between R1 and C1-.

This will give fairly good stability. And you can improve ripple rejection by a few μF across the Zeners. (However, I would raise the 3.3Ω to 120Ω so that current limiting occurs at about 5mA).
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Old 23rd Sep 2019, 1:01 pm   #5
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Default Re: HV regulation.

Will the Tubecad PS-1 described here be enough for your needs?

https://www.tubecad.com/2008/11/blog0151.htm

You can raise its voltage by using the IXCP10M90S as described in the text.

/Martin
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Old 23rd Sep 2019, 1:08 pm   #6
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Default Re: HV regulation.

I have a good selection of zeners, so possible to elliminate a big string.

I was going to replace the pot as you say with a fixed resistive divider... " I would raise the 3.3Ω to 120Ω so that current limiting occurs at about 5mA). " but if full load current demand is up to 60mA is this not wrong? The original circuit is supposed to have a I of 100ma, I'm missing something here I think. AFAIK Q2 in the circuit of pic is off under normal operation.... Ah, think I've twigged, the 5mA you mention is at the base of Q2, therefore this is "magnified by it's hfe, sorry, transistors aren't my strong point.

I'll have a play and report back. Andy.

Edit, typed before Martins reply... I had a look at that circuit Martin, but dismissed it because of the use of the IXCY10M45S, but I suppose I could use any HV rated mosfet yes?
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Old 23rd Sep 2019, 1:45 pm   #7
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Default Re: HV regulation.

Hi Andy

Take a look at Duncans's Mosfet regulator. I have built a couple of these years ago and they are still running. I was expecting things to blow up but the protection diodes seem to do their job very well.

Cheers

Eddie
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Old 23rd Sep 2019, 2:16 pm   #8
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Default Re: HV regulation.

If it's a valve pre-amp you shouldn't need 100mA! A couple of mA possibly, which is why I suggested 5mA.

Keeping the resistor at 3.3 ohms will give you 100mA-plus before current limiting kicks in. This will give 30W-plus dissipation in the MOSFET, which is likely to make it fail (this is after it has got stinking hot). It's generally good practice to have protection levels comfortably above normal running, but not excessively - who wants to burn their fingers on a transistor when they are looking for that short-circuit?
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Old 23rd Sep 2019, 2:17 pm   #9
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Default Re: HV regulation.

Thanks Eddie, how you doing!? Good to hear from you and know your still out there alive and well : ) I'll have a butchers at that, just knocking up a few circuits on the bench.

"If it's a valve pre-amp you shouldn't need 100mA! A couple of mA possibly, which is why I suggested 5mA" It won't, I mentioned the 100mA in reference to the 3r3 resistor, which I know isn't fixed but is dependant on load. The preamp/PSU will power a phono stage and line amp, which will only draw 10mA tops, however there will be cathode followers after both of these IP stages to "drive" the connecting cable capacitance which may draw more than that. I havn't got to that bit of the design yet, so as the tfmr sec is 60mA I'm using that as my max capacity, but this isn't fixed in stone.

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Old 23rd Sep 2019, 3:25 pm   #10
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Default Re: HV regulation.

If you're intent on using a bipolar transistor as the series-pass element, consider something like a MJE340/MJE360 - they'll handle a couple of hundred volts with ease.

Alternatively, what about 'keeping it vintage' and using a couple of VR-tubes? 85A2 (85V) 0A2 (150 volts) and 0B2 (108 volts) are available cheaply.
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Old 23rd Sep 2019, 4:49 pm   #11
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Default Re: HV regulation.

Had a quick play this afternoon and killed a tranny when ramping up the OP current, have to get that bit right by more experimentation. BTW, what sort of formula should I use to calculate Re of Q2 in the first circuit?

I was using a G25N40D mosfet as the series pass element, which luckily survived. I'm not fussed about whether the series pass Q is bipolar or fet, just trying to use what I have rather than buy extra components. I have a few MJE340's, so they are an option.

Andy.
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Old 23rd Sep 2019, 5:25 pm   #12
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Default Re: HV regulation.

I confess that I don't like using solid-state devices (apart from rectifiers) in power supplies for valve equipment. I see too many blown ones come in for repair. I'm not at all surprised you've already snuffed a transistor out. If they fail short-circuit then that can put a lot of volts onto valves which might not be expecting them.

As G6Tanuki has suggested you can take a valve-based approach. They can fail too but because the supply and the load are operating on more equal terms it seems to happen less often. The noise from VR-tubes takes a little handling but it's perfectly possible to achieve very good performance. The two things I would say are i) that the regulator can draw significant current itself, which might be an issue with a weedy mains transformer, and ii) that depending on the circuit chosen one or more valves might need isolated heater supplies, which again can raise mains transformer issues.

Cheers,

GJ
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Old 23rd Sep 2019, 7:01 pm   #13
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Default Re: HV regulation.

I have used MPSA42 and PHE13009 in a darlington configuration in a capacitance multiplier (simplistically transistor regulator with no voltage reference and a large (100uF or 200uF) capacitor to the base). I used to use them often when working with higher voltages (even sometimes lower ones). Transistors were from RS or Rapid I think. There are circuits around using modern Fets too.

Although I did develop PSU's for over 500V I never worked with more than about 250V per stage so the driver transistor was Ok. I used the darlington arrangement as HV bipolar transistors all tend to have low gain. I generally used voltage multiplication for the final voltage so could smooth and add voltages for the result but have found capacitance multipliers really do work well. Effective capacitance is base capacitance x transistor arrangement current gain.
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Old 23rd Sep 2019, 7:26 pm   #14
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Default Re: HV regulation.

2N3585 is another worthy HV transistor to look out for as series regulator if there's not too much dissipation- again, combining its high voltage rating with decent current gain so that its error amp doesn't have to use too much brawn. Comes in a quaint but "proper" metal package (TO66) but, as so often with superannuated and unusual transistors, may be pricey and tricky to track down.
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Old 23rd Sep 2019, 11:52 pm   #15
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Default Re: HV regulation.

I have one of these Andy. Its from Rainer zur Linde. its a stereo version with a common bridge rect ( not fitted) delivers 50 mA each channel.

I made a few of these up and they work well. I have spares if you want one.

Input on the left. two outputs on the right. In the picture you can half see a couple of resistors across the output, they are to float the heaters above ground.

Joe
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Old 24th Sep 2019, 11:22 am   #16
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Default Re: HV regulation.

ionburn's PHE13009 suggestion sounds like a really useful affordable device- I've used a few MJE340s for low dissipation HV applications but this transistor seems like a good'un. As CFLs fade into history, devices like this might become less common-place. I'm highly sceptical of the datasheet's claimed 80W dissipation from a TO220 package, though- maybe if clamped to a slab of silver the size of the Moon....
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Old 25th Sep 2019, 11:18 am   #17
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Default Re: HV regulation.

Thanks all for the useful suggestions. I've various VR valves but had discounted a valve based approach because of the noise issue associated with VR valves, however as mentioned using valves has the advantage that full HT is delayed.

My idea of using three legged fuses stems from wanting to learn more about them and also having several boxes of things including several HV types pulled out of TV's in the past.

There's another approach which is to make a "ripple gobbler" or capacitive multiplier. I'll look into each and do a bit of testing.

Andy.
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Old 26th Sep 2019, 7:37 pm   #18
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Default Re: HV regulation.

Quote:
Originally Posted by Diabolical Artificer View Post
as the tfmr sec is 60mA I'm using that as my max capacity
If your transformer is rated at 60mA, the DC current limit should be at most 40mA (the max. DC current is a factor sqrt(2) below the max. RMS current).
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Old 28th Sep 2019, 5:05 pm   #19
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Default Re: HV regulation.

"If your transformer is rated at 60mA, the DC current limit should be at most 40mA (the max. DC current is a factor sqrt(2) below the max. RMS current)." Thanks, didn't know that. The power tfmr HT winding is really weedy, unloaded and rectified etc gives me 470v, depending on how the RC filters are configured. Any sniff of a load, the tfmr lifts it's skirts and runs off crying, EG HT drops dramatically, IE by 100v or more.

Anyhoo... tried a "ripple gobbler" AKA capacitive multiplier today, and whilst not a regulator it's supposed to nobble ripple, see att 1. Whilst it does reduce ripple as advertised I'm having an issue,see pic 2, I think with VHF oscillation. Looking at the trace in more detail, with the scope cranked up on it's fastest time setting - 2uS/div, four peaks fit in 1 div, I calculated this as 2Mhz ish.

I tidied up the layout, shortened leads and popped a ferrite bead on the lead from the MJE340 emitter to BU208 base, this improved things a tad, but the problem was still there. Also tried a 0.047u across the 820u cap, as well as a 1p and 10r "snubber". I suspected the choke, but there is no oscillation at the junction of L and the two 22u caps, IE first LC filter. However note the trace at the two collectors of the Darlington pair - pic 3, oscillation is on the pos and neg part of the cycle.

Andy.
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Old 29th Sep 2019, 11:25 am   #20
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Default Re: HV regulation.

Try putting a resistor of a couple of hundred ohms - ideally wirewound - between the emitter of the MJE340 and the base of the BU208 (where you've currently got your ferrite bead) - followed by a 1nF cap between the base and emitter of the BU208.

That should reduce the VHF gain sufficiently to stop it squealing, while having essentially zero effect at 50/100Hz.
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