Power Supply Questions

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Cressy Snr
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#1 Power Supply Questions

Post by Cressy Snr »

I, like everyone else into building amps, was told early on, that the power supply is everything.
I've read dozens of books about them, but find the maths involved hard work. It isn't that I cant do the maths, its just that I'm so damned slow and getting slower. Being a Mac user, I have no access to PSUD, so I have developed a tendency after reading Dr Gizmo's writings, to slug with big caps and hope for the best.

Recently I have learned, that, just because a powerful SS amp has massive capacitors, it doesn't mean to say that a valve amp should also have them. I made the mistake I usually do with anything technical; that of thinking simplistically, that if something applies to one situation, then it applies to all situations.

As an example, the PP monoblocks could store enough energy to blow a hole in the wall and the bass was slammy as hell on speakers with an easy load, but turned to mush on "real world" speakers.

This PSU feeding my pink SET
Image
sounds fantastic, but I have no idea why. I also admit to building it on a slug and hope basis.

I read a bit of Herb Reichert from the turn of the century, who argued that a SET PSU needs to spend its time reacting to the music signal as fast as possible and not spend its time charging up dirty great caps, or being a giant subsonic bell.

So using my supply as an example and leaving active regulation out of it for the time being, what's good about it, but more importantly what's bad about it? And perhaps the more expert among us could give some input.

Thanks in advance.
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Paul Barker
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#2 Re: Power Supply Questions

Post by Paul Barker »

Isn't there a wine your thing fir macs? Psud runs ok in wine.
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ed
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#3 Re: Power Supply Questions

Post by ed »

Cressy Snr wrote: Sun Jan 14, 2018 12:06 pm Being a Mac user, I have no access to PSUD, so I have developed a tendency after reading Dr Gizmo's writings, to slug with big caps and hope for the best.
Can I suggest a browse in your local 'cash converters' or similar for a used netbook. They fetch about £50 here, sometimes less. It would probably open up a few more interesting doors as a bonus.

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#4 Re: Power Supply Questions

Post by Neal »

Or just buy a raspberry Pi3B and run this ;)

https://eltechs.com/product/exagear-desktop/
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Cressy Snr
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#5 Re: Power Supply Questions

Post by Cressy Snr »

So I guess the short answer is that nobody bothers "designing" valve amp PSUs these days, they just use PSUD as it is not worth the bother of doing it with slide rule and log tables :lol:

So let's rephrase the original question: do I really need all that capacitance in my pink SET power supply?
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andrew Ivimey
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#6 Re: Power Supply Questions

Post by andrew Ivimey »

Its a very simple thing to experiment with surely. I know some argue that fewer mfds means a faster sharper (better) amp. But it also may mean more hummmmmmmmmm.
Philosophers have only interpreted the world - the point, however, is to change it. No it isn't ... maybe we should leave it alone for a while.
Cressy Snr
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#7 Re: Power Supply Questions

Post by Cressy Snr »

Aye probably Andrew.
Anyway I downloaded WINE for OSX and xQuartz, the X11 windowing software needed to run WINE (found a tutorial on YouTube that was pretty straightforward) so have now, after I installed both applications ended up with this.
Image

Just need to learn to use it now. :)

I had to overcome an intense paranoia that nearly managed to stop me in my tracks about allowing the damn things to install.
Are they phoning home right now?
Are the apps sending my bank details to the Russians even as I am writing this? :shock:

Anyway The bloody things are in there now, so let's just relax.
WINE was a .pkg file, from WineHQ, and xQuartz also a .pkg from xQuartz.org
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pre65
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#8 Re: Power Supply Questions

Post by pre65 »

With PSUD input values as accurately as you can.

Things like transformer secondary resistance, choke DCR and capacitor DCR.

It's relatively easy to use though. :)
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#9 Re: Power Supply Questions

Post by Cressy Snr »

Hmmm

Looks like the supply eats tube rectifiers, which is why I have had all that bother with flashovers and sparky chokes.

The SS diodes in the hybrid bridge will stand the power-up but the valve bit is not so lucky.

Rethink on the cards methinks. Capacitance in the supply is way too high.
The problem is the split off into two, after the first cap. A single valve element just won't cut it.

Solutions are:

Twin damper diodes as the valve elements in the bridge.
Go solid state completely with the rectification
Lower some of the cap values.

At the moment, the PSU is a problem waiting to happen.

Slug and hope might not be the ideal method of makng a PSU. something learned then,,at least I think.
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pre65
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#10 Re: Power Supply Questions

Post by pre65 »

Steve, did you realise that the rectifier has a specified anode resistance, and if half the secondary transformer winding resistance is less than that you have to add resistance ?

From memory the last time I did that it was about 70R extra in each anode lead.

For example, one data sheet for GZ34 says Rt = 2 x 125R.

"In addition to the maximum AC voltage and DC current ratings, valve rectifiers have two other ratings that must be observed: maximum allowable reservoir capacitance, and minimum current-limiting resistance. These two limits are interelated and serve to keep the peak ripple current below a certain (unstated) level. The bigger the reservoir capacitance, the more limiting resistance you need. The GZ34 data sheet quotes a maximum capacitance of 60uF, although you can, in theory, exceed this if you increase the limiting resistance proportionately. However, this incurs extra voltage loss and wasted heat which is why the manufacturer assumes no one would want to do it.
The total limiting resistance (per anode) in the actual circuit is the combnation of transformer resistance, plus any resistance we add ourselves: "

From

http://www.valvewizard.co.uk/bridge.html
Last edited by pre65 on Sun Jan 14, 2018 6:40 pm, edited 1 time in total.
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Cressy Snr
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#11 Re: Power Supply Questions

Post by Cressy Snr »

Yes I understand that Phil, but as we have a choke input supply, then, in theory there shouldn't be a need to add resistance to the rectifier. At least that's what the old data sheets say. But of course they never envisioned, whacking great 600F of caps would be used on each channel :shock:

I'll try it later with the cap values halved. This is of course easy to do in PSUD, but on my amp it's also easy as my JJ caps are dual cap cans, so it's just a matter of cutting the copper straps, I have linking the two halves. They then go from 200uF to 100.
Last edited by Cressy Snr on Sun Jan 14, 2018 6:44 pm, edited 1 time in total.
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pre65
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#12 Re: Power Supply Questions

Post by pre65 »

Is it still choke input with that 20uf motor run as per the circuit ?
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Cressy Snr
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#13 Re: Power Supply Questions

Post by Cressy Snr »

Yes, the 20uf forms the LC filter at the start of the chain.
The voltage output at the 2A3 matches the voltage predicted by PSUD almost exactly.

It's bloody good software isnt it. :)
It'll certainly improve my PSUs on the evidence so far.
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#14 Re: Power Supply Questions

Post by Cressy Snr »

Well I've done the cap surgery with the positive terminal links, and halved the values in both channels of the power supply.
Startup in PSUD works OK now, and the amp makes less of a thrum when when the rectifier begins to conduct.

Hum has increased slightly.
The TV however is going through the amp at the moment, so It won't be until a bit later on tonight that I can audition the thing with music.
What a ruddy mess :mrgreen:
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#15 Re: Power Supply Questions

Post by Cressy Snr »

OK,
Using a combination of the PSUD and Morgan Jones, I have got the ripple back to where it was with the over capacitised supply, simply by changing the series resistor feeding the capacitor where the 2A3 is taken off.
Applying 1:10 stepped currents at intervals, 1 second after each other, at each of the current taps gives a nice fast response with no ringing provoked in one filter by the preceding one.

I'll do the resistor change in the actual amp tomorrow and will put up the traces off PSUD for the panel to have a look at.
Maybe I can get some suggestions as to how to speed up the step-response further, or maybe it's fine.

It sounds fine on music; no ill effects at the bottom end, which is what I was afraid might happen.
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