2A3 "Pink SET" Amp Rebuild.

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Cressy Snr
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#106 Re: 2A3 "Pink SET" Amp Rebuild.

Post by Cressy Snr »

Ultrapath was connecting the B+ to the cathode of the output valve. The Ultrapath cap and the normal cathode bypas cap are supposed to form a capacitor potential divider so that a portion of the power supply noise gets injected into the cathode, amplified by the valve and cancels said noise.
I suppose when everyone started getting uV noise levels by regulating the bejesus out of everything, the Ultrapath became surplus to requirements.

10uF......never been that low.

Anyway I heaved the SET to the upstairs boxroom (now my back aches) and changed back to the DIY NVA amplifier to get a reality check and I'm damned if I can put a fag paper between the two in terms of music making ability.
So is it the heavy, steam engine SET or the clean simplicity of the solid state electric train?
It's nice to be able to choose either and not feel I'm missing out.
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Nick
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#107 Re: 2A3 "Pink SET" Amp Rebuild.

Post by Nick »

The Ultrapath cap and the normal cathode bypas cap are supposed to form a capacitor potential divider so that a portion of the power supply noise gets injected into the cathode, amplified by the valve and cancels said noise.
Well, no, that sounds like you may be thinking of what JB does with his Akido shape. The Ultrapath creats a AC current loop through the valve, output transformer and ultrapath cap so that the power supply and cathode components are not traversed by the signal.

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simon
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#108 Re: 2A3 "Pink SET" Amp Rebuild.

Post by simon »

If I understand correctly you now have some kind of LC or CLC PS Steve, with the last C of 600uF now the decoupling's been removed? Which has now been reduced to 300uF?

How much C and L do you have before the three halves of JJs? In the past I was advised to use a small last cap (obvious candidate for PIO) but I've always found a larger last cap and smaller first has modelled better in PSUD.

If you have 300uF after a first cap of say 200uF that's a heck of a lot of C I reckon - perhaps try L 20uF L 200uF? And then try them the other way round. And if it's a pi filter forget all that :lol:.
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#109 Re: 2A3 "Pink SET" Amp Rebuild

Post by Cressy Snr »

There was 1200uF, now it's only down to 600. :shock:

What it really needs is a complete rethink. In my own head I mean not a rebuild. I've done that too many times and it doesn't solve anything. You might as well as PB once said, get a load of twenty pound notes and throw them on the fire.

My conceptual problem is that the Pink SET sounds absolutely fab, and that bothers me because I'm damned if I can work out why.
I could do with someone to look at it who understands and can explain what the fookin 'ell I've done. :lol: :lol:

The fook up in my understanding was born when I decided at the beginning to split the cap banks off down each side of centre after the choke. After that fateful first design decision I've had nothing but battles with the damn' thing, and that layout is the root cause of the trouble as well as the capacitance issue, which we can take as read.
Last edited by Cressy Snr on Fri Apr 13, 2018 7:54 pm, edited 4 times in total.
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#110 Re: 2A3 "Pink SET" Amp Rebuild.

Post by Cressy Snr »

Nick wrote: Fri Apr 13, 2018 6:25 pm Well, no, that sounds like you may be thinking of what JB does with his Akido shape. The Ultrapath creats a AC current loop through the valve, output transformer and ultrapath cap so that the power supply and cathode components are not traversed by the signal.
Aye, got it mixed up with the Aikido.
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#111 Re: 2A3 "Pink SET" Amp Rebuild.

Post by Cressy Snr »

Re the post above my answer to Nick, I think it might be more helpful to post a block diagram showing the actual physical layout of the PSU and its wiring, rather than a schematic, then we might get somewhere. Hopefully I can then with the panel's help, get this monster under control.

I'll get something up later tonight.
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#112 Re: 2A3 "Pink SET" Amp Rebuild.

Post by simon »

Yeah, block diagram would be helpful. And how it's physically wired.

Can't help thinking 600uF is still huge!
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#113 Re: 2A3 "Pink SET" Amp Rebuild.

Post by Cressy Snr »

Too right its huge, but the caps were all I had at the time

Block diagram below, coupling caps and grid resistors left out for clarity:
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#114 Re: 2A3 "Pink SET" Amp Rebuild.

Post by Ant »

You all forget the little red el84 pp amp wot he did. That had a 2200uf epcos thing the size of a dustbin in it.....
Also starring Rex Hamilton as Abraham Lincoln

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simon
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#115 Re: 2A3 "Pink SET" Amp Rebuild.

Post by simon »

Mahoosive!

How many Henries? 10H? Dunno without modelling and actually building it but you might need all that C to get a quiet enough pre stage as there's no sectioning. I find it really interesting that you've found an improvement in musicality as I certainly wouldn't build a PS like that.
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izzy wizzy
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#116 Re: 2A3 "Pink SET" Amp Rebuild.

Post by izzy wizzy »

Normally you would get some motorboating or at least some LF instability without adequate RC decoupling between stages. But maybe the sheer amount of capacitance is getting past that. Many SET builders use an order of magnitude lower capacitance but we all have our own style.

Cheers,

Stephen
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#117 Re: 2A3 "Pink SET" Amp Rebuild.

Post by Cressy Snr »

simon wrote: Fri Apr 13, 2018 11:53 pm Mahoosive!

How many Henries? 10H? Dunno without modelling and actually building it but you might need all that C to get a quiet enough pre stage as there's no sectioning. I find it really interesting that you've found an improvement in musicality as I certainly wouldn't build a PS like that.
The choke is 10H
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Cressy Snr
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#118 Re: 2A3 "Pink SET" Amp Rebuild.

Post by Cressy Snr »

[googlevideo][/googlevideo]
izzy wizzy wrote: Sat Apr 14, 2018 7:48 am Normally you would get some motorboating or at least some LF instability without adequate RC decoupling between stages. But maybe the sheer amount of capacitance is getting past that. Many SET builders use an order of magnitude lower capacitance but we all have our own style.

Cheers,

Stephen
This is what I'm having trouble with conceptually. By any normal valve amp power supply standards this PSU is all wrong, there are no redeeming features. It is basically, chuck in a load of big caps and brace for impact. It's like the mutant result of a horrible piece of interbreeding between a transistor and a valve power supply with the worst of both worlds being the only logical outome. Apart from the size of the caps, it had been pretty conventional, with a cascade of filters down each side and take-offs for each section; one following the other.

And yet the resultant sound is great and bass heavy music doesn't faze it. In fact, what is coming out of it is the best SET sound I've had to date. Very odd and rather troubling. I blame Dr Gizmo for this. I've applied his "pentodosity" and Moscode hybrid power supply ideas to a SET. Nobody but me would have been daft enough to do it.
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Cressy Snr
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#119 Re: 2A3 "Pink SET" Amp Rebuild.

Post by Cressy Snr »

Such fun!
Just going with it now and sod the objections. Small increase in the clarity of the low level instrumentation and tune playing has been gained by putting, in the Ultrapath position, a 30uF poly motor run cap on each channel.

Checked the low frequency stability with the Claptone remix of Gregory Porter's "Liquid Spirit" at a robust listening level.
Cone flap not present and all tinkly bits behind the thump of the drum and bassline keep going throughout.

Will try some warped vinyl later on to see if I can poke it into misbehaving.
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Cressy Snr
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#120 Re: 2A3 "Pink SET" Amp Rebuild.

Post by Cressy Snr »

simon wrote: Fri Apr 13, 2018 11:53 pm I find it really interesting that you've found an improvement in musicality as I certainly wouldn't build a PS like that.
Neither would I have.
The only reason I gave it a try, was because of the DIY NVA amp build.
I don't know whether the input, driver and output stages are decoupled from one another on the boards, as I don't know enough about solid state. All I can see on the NVA boards in standard form is a continuous positive and a continuous negative rail.
There are links within the rails that can be removed in order to (as Richard does in his commercial amps) allow multiple power supplies to be used with the boards.

Now maybe in a bipolar power supply, any LF oscillation would cancel because the oscillations would be in opposing phase, whereas in a monopolar valve supply there would be no such cancellation with the stages all talking to each other.
But in my case there seems to be none of that tendency to LF instability we would normally expect. Maybe I just got lucky and this is not repeatable elsewhere. But I can't ignore the fact that the whole musical presentation is all-of-a-piece, natural and makes sense.

What it really needs is for somebody else to lash up a mono SE, DHT three stager, with a high capacitance supply and cheap disposable speaker and see what happens.
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