6B4G - The Resurrection Shuffle

[IslandPink]

OK, so for example in the F4 amp, the Mosfets are a push-pull pair so the voltage is varying across each mosfet all the time when there's signal ?

[jack]

... And finally you will probably have a nice cap of your choice across the shunt to prevent HF instability anyway.

There's a 10uF @ 400V polypropylene MKP cap across the shunt in the MJSR.

The CCS in that design will deliver up to 200mA but those zeners won't do that - max rating ISTR is 100mA, i.e a shunt of 50mA max.b

Need bigger zeners and bigger board. I'm on it.

[Cressy Snr]

That sounds like a plan Nick.
Half an amp at 350V would be just the job.

[jack]

That sounds like a plan Nick.
Half an amp at 350V would be just the job.

That's a BIG ask for an MJSR-style device...

I'll have a look, though it may just be a wired prototype first out.

That baby going to cook. Ooooh yes.

You'll need at least a 1A CCS (should be no problem) but the Zener string is going to cook rather...

I'll do some sums (I know folk here hate that sort of thing ), but luckily I put all the hard stuff to do with the PWL modelling in a spreadsheet last time out, so we shall see what happens!

[Nick]

OK, so for example in the F4 amp, the Mosfets are a push-pull pair so the voltage is varying across each mosfet all the time when there's signal ?

Ar ewe talking about a shunt regulator or a push pull output stage?

[jack]

So, to get 350V with 5.6V zeners (the lowest noise ones), 1N4734As look like reasonable candidates (1W, cheap and low noise).

With a zener current of 100mA and allowing them to get to 80C at the lead (they're good to 200C), the PWL corrected average rating is 5.85V meaning that 60 will get you to 351V when under operating conditions +/- 1%.

Still not convinced this is a good way to go due to the variable nature of the load - if quiescent current is low, the zeners will take the full CCS output which moves them well out of their SOA.

Need to think about this.

[Nick]

Half an amp at 350V would be just the job.

What are you going to do with the 175W of heat? If you want such a shunt, then I would make it active, not a string of zeners, and look at something like this as the power device, then worry about driving it.

http://ixapps.ixys.com/DataSheet/DS9995 ... N50L2).pdf

[Cressy Snr]

Oops! just realised sorry Jack
if separate shunt regs for each valve, then it only needs to be 310V at 150mA capability.

I'm a silly b'stard.

[jack]

Half an amp at 350V would be just the job.

What are you going to do with the 175W of heat? If you want such a shunt, then I would make it active, not a string of zeners, and look at something like this as the power device, then worry about driving it.

http://ixapps.ixys.com/DataSheet/DS9995 ... N50L2).pdf

The full 500mA shouldn't be going through the zeners, so under operating conditions it should be more like 100mA, but I'm not at all sure that an MJSR is right here.

I would use the zeners with a CCS as the Voltage reference, and then a power shunt device as t'other Nick is suggesting.

[IslandPink]

OK, so for example in the F4 amp, the Mosfets are a push-pull pair so the voltage is varying across each mosfet all the time when there's signal ?

Ar ewe talking about a shunt regulator or a push pull output stage?

I'm talking about the Passlabs F4, so the PP output - just want to get straight in my head about what I don't like about the HF in the F4.

[jack]

Oops! just realised sorry Jack
if separate shunt regs for each valve, then it only needs to be 310V at 150mA capability.

I'm a silly b'stard.

That's pretty much within the capabilities of the current boards - you'd need a second extension board as, using 1N4734As, the maths shows an adjusted value of 5.656V, thus needing 55 zeners to get 311V (each board holds 40 zeners).

Using the original BZX55C5V6 zeners, you need one less, i.e. 54 (adjusted they are 5.74V), to get 310V.

I still like the idea of using the MJSR board to provide a stable HV reference for a power shunt device...

This kit comprises a PCB and all components to build the adjustable precision Constant Current Source
(CCS) and the associated Zener diode string. Test points are provided to accurately set the total output
(JP1) and Zener (JP2) current; mounting holes are provided for M3 fixings.

The board is made from FR4 with a black solder mask and an ENIG (Electroless Nickel Immersion Gold)
finish – this is more expensive and more resistant to corrosion. The real reason for using ENIG rather than
an equally technically good (and cheaper) HASL finish is, of course, that black-on-gold looks way cooler…

The kit may be configured in three ways:

1. Full MJSR
   All components - CCS and Zener string installed – up to 224V nominal, 230V under operating conditions.
2. MJSR voltage expansion board
   Used in association with a full MJSR board to extend the maximum regulated voltage available.
   Increases output to 448V or 460V under typical operating conditions.
3. Stand-alone adjustable precision CCS
   Just the CCS components installed (no Zeners or capacitor)

Electrical & Mechanical Characteristics

• Nominal regulated output of up to 224V or 448V with expansion board
  When adjusted for typical operating conditions the real output is up to 230V or 460V with the
  expansion board.
• Recommended input (Vin) vs. Output voltage (Vout) : 15V to 25V
  Minimum of 15V – do NOT make this voltage too large else the CCS will dissipate a lot of heat –
  the CCS will shed (Vin – Vout)*Itotal Watts – there is no point is wasting energy and stressing
  components unnecessarily, e.g. for 224V Vout I would use no more than 250V Vin.
• Maximum total output current (Itotal) - 200mA
• Minimum Zener string current (Izmin) – 10mA
  This is the minimum current that Morgan Jones recommends for use in the Zener string to get
  good noise performance and good regulation.
• Maximum Zener string current (Izmax) – 20mA
  As Zener current (Iz) increases, the noise generally falls off, however above 20mA this effect is
  much reduced and the heat generated can move the diodes out of their safe operating area.
  10mA to 15mA is a good operating point.
• Recommended Zener type & voltage – BZX55C, 5V6
  This is an excellent “true” Zener to use – above this voltage diode operation becomes a fusion of
  avalanche and Zener characteristics and individual noise increases; below this voltage and you’ll
  need a lot more diodes plus the effect of junction temperature (Tj) on the Zener voltage reverses.
  Stick with BZX55C5V6 Zeners for best results.

[jack]

What's that Q1 thing in the circuit above ?

It is indeed a Mosfet - the "Maida" regulator is not an invention of Morgan Jones - it's named after Michael Maida - who wrote an application note back in the 1980s - see http://www.ti.com/lit/an/snoa648/snoa648.pdf - he was at National Semiconductor at the time, now TI.

The above app note explains it all in detail - the pre-regulator tracks the output and ensure that the LM317/LT3080 never exceeds its SOA wrt. Vin-Vout and limits current too. Note that the pre-regulator series-pass device WILL get hot and needs a heatsink - whilst the original app note used BJTs, modern versions tend to use FETs. Quite a few examples around.

A Maida regulator is a pretty good series SS HV PSU...

Note: Only works for floating regulators like the 317 & 3080 etc.

[Cressy Snr]

Found half a dozen PCL86s I'd forgotten about.
These triode/pentodes will make fine active shunt regs,

So rather than using the dual mono power supplies, I was scratching my head over, we will use independent stereo power supplies, one stereo PSU for input stages, the other for the output stages, with each valve then fed from a PCL86 active shunt reg. So there will be four shunt regs altogether, dividing the regulating task between them, rather than one whacking great series reg doing the lot.

Getting somewhere now I think. Solid state regs at the moment are a step too far for my limited knowledge. I'd never have got this thing off the ground trying to use those.

Nick and Nick, you have made my mind up for me, shown me just how much I don't know, but interestingly you two caused the little bit of lateral thinking that allowed me to come up with the power supply scheme described above. Can't see any reason why it wouldn't work but.....

PSU drawings over the next couple of days, then we can see if there is anything I need to be aware of that might make it fall on its backside.

[Paul Barker]

yes i tend to stick with valves for high current as they bring their own heatsink and dont self destruct when conditions get outside design. which they tend to do in the real world as such circuits are built. so the valve survives a mistake to live another day. solid state pft.

[Cressy Snr]

OK,
Here's a general idea of the PSU and shunt regulation arrangements, using PCL86s with the triode section as error amp and pentode as shunt valve (Blencowe 2010)
Final PSU caps will be 80uF Eyuda oil caps that have been gathering dust for years.
Possible small cap to adjust choke input supply output will be high quality Mundorf silver/gold oil:

Please point out any show-stopping faults: