Poundland 45

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IslandPink
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#76 Re: Poundland 45

Post by IslandPink »

Yes. Noise levels are fine at the level of a driver stage. As Nick says, a Zobel will help a little, I found it cleaned up the HF slightly on my phono stage. So this is a C+R across the pair of VR tubes instead of using the single capacitor. There is no formula for finding the optimum values ( as Gary Pimm found ) but try 0.1u & 390R as a starting point.
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#77 Re: Poundland 45

Post by Cressy Snr »

RhythMick wrote: Wed Sep 08, 2021 8:27 am Side question - what software are you using to produce the circuit diagrams ?
It’s the LibreOffice suite. https://www.libreoffice.org/

I use a ‘Draw’ document to do the diagrams. Over the years I’ve built up a little library of objects that represent circuit symbols, such as resistors, electrolytics, film caps, pentodes triodes, rectifiers, VR tubes, diodes, zeners, DHTs, DHPs ad infinitum. I just drag and drop them out of the gallery and join them up with lines.

Neal put me onto LibreOffice years ago after Apple had ceased making their AppleWorks software, and replaced it with iLife, which was frankly a pile of shite.
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#78 Re: Poundland 45

Post by Cressy Snr »

IslandPink wrote: Wed Sep 08, 2021 11:14 am Yes. Noise levels are fine at the level of a driver stage. As Nick says, a Zobel will help a little, I found it cleaned up the HF slightly on my phono stage. So this is a C+R across the pair of VR tubes instead of using the single capacitor. There is no formula for finding the optimum values ( as Gary Pimm found ) but try 0.1u & 390R as a starting point.
Cheers Mark,
I’ll look into that this afternoon, if I can find the right resistors. I know I’ve got some 390s knocking about somewhere.
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#79 Re: Poundland 45

Post by Cressy Snr »

OK,
I changed the cap to a Zobel with the aid of a 390R resistor. It was pretty subtle, but there was a slight increase in the clarity of the treble. Every little helps as they say.
I also added a heatsink to the first transistor in the CCS cascode. I prefer the belt and braces approach when valve electronics and solid state cross paths. Instead of being a bit hot, the first transistor is now quite warm, if you know what I mean. :)

Updated schematic:
45PSUVRshunt+CCS.png
45PSUVRshunt+CCS.png (84.46 KiB) Viewed 2489 times
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Nick
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#80 Re: Poundland 45

Post by Nick »

Yep, heatsinks are good mkay :-)

BTW, careful with DN2540 they are easy to kill (or degrade) with static.
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#81 Re: Poundland 45

Post by Cressy Snr »

Nick wrote: Wed Sep 08, 2021 3:30 pm
….BTW, careful with DN2540 they are easy to kill (or degrade) with static.
Thanks Nick,
Better dig out a zap strap for any future work then.
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#82 Re: Poundland 45

Post by Cressy Snr »

Fed up with the loud crack through the speakers every time the shunt VR tubes strike when the valve rectifier has ramped up, I’ve changed the rectification to a pair of solid state SiC 1200V 3A Schottky diodes. Now the VR tubes ignite before there is any amplification happening, meaning that startup is completely without incident.

I’ve also put a 1nF/1k Zobel network across the power transformer HT secondary winding to snub any residual spiky bits. The Black Gate WKz, 2 x 220uF capacitor has been welcomed back to the fold and sticks up through the hole where the rectifier valve used to plug in.

It’s all worked out very well and sounds great. No sonic ill-effects at all from the rectifier heresy, in fact it’s all very quick, dynamic and tonally neutral. One transformer less and 15 fewer Watts drawn from the wall too, as there is no 5V 3A rectifier heater to contend with. It’s win, win as far as I’m concerned. I was a bit loath to throw solid state bits at this amp, but I must say the DN2540 CCS fed shunt reg on the input stage, and the SS rectification have both worked beautifully.
E9411C07-16B6-4697-8A6C-2F0952F59FC0.jpeg
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Paul Barker
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#83 Re: Poundland 45

Post by Paul Barker »

Nice
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#84 Re: Poundland 45

Post by Cressy Snr »

Power supply:
45psuv6.png
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#85 Re: Poundland 45

Post by Cressy Snr »

Audio:
Screenshot 2021-09-30 at 20.04.59.png
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#86 Re: Poundland 45

Post by Cressy Snr »

I’ve at last got around to ripping out the temporary lash-up of the DN2540, cascode CCS that feeds the VR tubes that regulate the input stage HT.
I built a new one up on a piece of prototyping board, using electrostatic protection protocols to prevent any possible degradation of the pair of MOSFETs. Of course there was no guarantee that the MOSFETs were perfect in the first place but you have to start somewhere.
Anyway it all worked first time, once it was adjusted up, so the amp is now Owston ready and sounding good. :)
Solid state rectifier and first cap can be seen on the far right. CCS board to the left of the VR tubes.
DA795A36-158E-4E69-9FF4-1D9476F7A5C8.jpeg
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#87 Re: Poundland 45

Post by steve s »

Owston ready... I'm impressed Steve
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#88 Re: Poundland 45

Post by Cressy Snr »

steve s wrote: Thu Oct 07, 2021 7:45 pm Owston ready... I'm impressed Steve
Well you have to make the effort. :lol:
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#89 Re: Poundland 45

Post by Paul Barker »

nice
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#90 Re: Poundland 45

Post by andrew Ivimey »

I'd put the vr tubes out on top and if they eeren't glowing I'd get them to draw current so they do. What's the point if valves if you can't make'em look spectaculeer or even dangerous.
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