Finished and working - 12 valve all DHT PP 26-71a-45

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RhythMick
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Re: Finished and working - 12 valve all DHT PP 26-71a-45

Post by RhythMick » Tue Jan 01, 2019 5:39 pm

I'm a happy boy.

The 26s are both grounded Grid with droppers now. Sounded good but still buzz.

30uF cap from 45 b+ to where the virtual cathodes join and the worst of the hum disappeared. Still a buzz on the right channel.

Cap from ground to right 71A bias did nothing, but touching it to one of the grids dropped the buzz, the other made it worse. Sure enough dodgy connection on the interstage. Resoldered, buzz gone.

The amp isn't perfectly silent, but it's close. Given it's driving old Sony bookshelf speakers the sound is really good. I'll get it listenable now as it is (45 caps are just clip leaded) and put it on the lowthers. Then I can listen while I redesign the layout.

THANKS! You guys are awesome.

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Re: Finished and working - 12 valve all DHT PP 26-71a-45

Post by RhythMick » Tue Jan 01, 2019 6:06 pm

Interestingly, the same cap (b+ to cathode) had no effect on the 71A and increased buzz on the 26. I don't yet pretend to really understand this.

I went back to one of the articles I found really inspirational... Lynn Olson's Karna. http://www.nutshellhifi.com/triode2.html
There's a lot in there I didn't understand, much I still don't though some of it is clearer. He also talks of oscillation potential, though his is grounded grids also. His diagram has that cap on there and re reading that made me try it.
Screenshot_20190101_180103.jpg

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Re: Finished and working - 12 valve all DHT PP 26-71a-45

Post by Mike H » Wed Jan 02, 2019 4:38 pm

Excellent. :thumbleft:
 
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Re: Finished and working - 12 valve all DHT PP 26-71a-45

Post by izzy wizzy » Wed Jan 02, 2019 4:46 pm

RhythMick wrote:
Tue Jan 01, 2019 6:06 pm
Interestingly, the same cap (b+ to cathode) had no effect on the 71A and increased buzz on the 26. I don't yet pretend to really understand this.

I went back to one of the articles I found really inspirational... Lynn Olson's Karna. http://www.nutshellhifi.com/triode2.html
There's a lot in there I didn't understand, much I still don't though some of it is clearer. He also talks of oscillation potential, though his is grounded grids also. His diagram has that cap on there and re reading that made me try it.
Screenshot_20190101_180103.jpg
Good work. Hopefully all going in the right direction now.

I understand the theory with the cap from HT to cathode but it's never really done anything I can hear when I've tried it.

Cheers,
Stephen

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Re: Finished and working - 12 valve all DHT PP 26-71a-45

Post by RhythMick » Wed Jan 02, 2019 4:47 pm

What's the theory?

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Re: Finished and working - 12 valve all DHT PP 26-71a-45

Post by izzy wizzy » Wed Jan 02, 2019 5:05 pm

It's meant to keep the plate to cathode path a tight local loop. Effectively it's the difference/imbalance signal only so to my mind, minimal. If this was single ended, then I get it and can see it making a big difference. It could be called ultrapath, Western Electric bypass or some other similar thing. A cathode bypass does something similar but this is meant to be less complicated from an AC loop perspective.

Because it's never made a big enough difference for me, I've not bothered with it or explored it much. I'm sure there's more to it and hey, my theory has been called into question in this thread so maybe there's a better explanation.

Cheers,
Stephen

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Re: Finished and working - 12 valve all DHT PP 26-71a-45

Post by IslandPink » Wed Jan 02, 2019 6:10 pm

Well one point was that you can put a higher-quality cap in here because it's only something like 20uF rather than 100uF or more in the power supply ; so you can get a bit less cap-induced distortion for bigger signal levels.
But having that link there then allows you to use some series resistance in there, the so-called 'harmonic equaliser' as incorporated in some Western Electric amps. Lynn and Gary ( probably John Atwood found it ) started talking about this around the time the Aurora and Karna were being developed. I tried their recommendation on the PP 300B Aurora , and it did provide a subtle improvement in vocal tone - definitely worthwhile.
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Re: Finished and working - 12 valve all DHT PP 26-71a-45

Post by izzy wizzy » Wed Jan 02, 2019 6:35 pm

IslandPink wrote:
Wed Jan 02, 2019 6:10 pm
Well one point was that you can put a higher-quality cap in here because it's only something like 20uF rather than 100uF or more in the power supply ; so you can get a bit less cap-induced distortion for bigger signal levels.
But having that link there then allows you to use some series resistance in there, the so-called 'harmonic equaliser' as incorporated in some Western Electric amps. Lynn and Gary ( probably John Atwood found it ) started talking about this around the time the Aurora and Karna were being developed. I tried their recommendation on the PP 300B Aurora , and it did provide a subtle improvement in vocal tone - definitely worthwhile.
That's a good point I should have expanded on. I've only tried this in my 813 amps which run on 1000V. So I used a motor run cap and "tuning" this is not something I fancy so maybe I was not going at this with a positive attitude. I don't tune my circuits much for tone with different components that much.

Cheers,
Stephen

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Re: Finished and working - 12 valve all DHT PP 26-71a-45

Post by Mike H » Thu Jan 03, 2019 4:36 pm

RhythMick wrote:
Wed Jan 02, 2019 4:47 pm
What's the theory?
Ultrapath 101
 
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Re: Finished and working - 12 valve all DHT PP 26-71a-45

Post by Mike H » Thu Jan 03, 2019 4:41 pm

I see ultrapath caps in this circuit, so is especailly relevant to this type of circuit:


Image

Basically all the b— rail does is supply power.
 
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Re: Finished and working - 12 valve all DHT PP 26-71a-45

Post by RhythMick » Thu Jan 03, 2019 4:49 pm

Mike H wrote:
Thu Jan 03, 2019 4:36 pm
RhythMick wrote:
Wed Jan 02, 2019 4:47 pm
What's the theory?
Ultrapath 101
Thanks for that. I love reading Broksie's articles, hadn't seen that one.

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Grounding question for rebuild

Post by RhythMick » Mon Jan 07, 2019 11:58 am

I have a question regarding where to "ground" the PSU circuits in my redesign. While I'm rebuilding seems a good time to check my logic re placement of 0v references and get advice from the group.
PSU.PNG
The diagram shows the redesigned circuit, which is identical for all 3 valve stages on both channels. The key points from the redesign.

1) the tail of each valve pair is greatly simplified, with only a simple cathode resistor to set the bias
2) all grids (centre-taps of transformers) are grounded to the bottom of the cathode resistor
3) dropper resistors in the PSU bring the B+ voltage down to where I want it

So, the question of where to ground. By "ground" I do not mean return path for PSU current. I mean 0v reference - there should be neither supply current nor signal on this wire. I create a STAR GROUND point for each channel, both of which connect (by 12AWG copper) to mains earth. In the diagram I've placed 4 green blocks denoting options, but there's a fifth which I'll start with as we've discussed it before.

NO GROUND
Each valve stage is galvanically isolated from the others. There is no requirement that I can see for voltages to be referenced to each other. The only argument I've heard which makes sense was from Nick, that the mains transformers need a low impedance drain path for electrostatic charge built up.
If that is true then such current WOULD flow through that 0v reference wire - is that correct ?

OPTION A - MAINS TX CENTRE TAP
If the only requirement for a 0v reference on the PSU is to drain charge build up, wouldn't it make sense to place the 0v reference at the Mains TX centre tap ? I've always avoided doing this thinking it would be the noisiest possible place.

OPTION B - BOTTOM OF FIRST LC CAP
I can't see any advantage in placing 0v here as it's bound to be a noisy spot with lots of rectification going on, but it's an option.

OPTION C - BOTTOM OF SECOND LC CAP
This one I can see as an option as ripple should be very low here.

OPTION D - CATHODE GROUND
This is where I currently place connections to 0v. I currently run large gauge wires from here to star ground for the channel.

SIGNAL GROUNDS
- XLR input pin 1 connects to 26 cathode ground (bottom of cathode resistor)
- Input Transformer output centre tap connects to 26 cathode ground
- Interstage secondary centre taps (carrying signals to 71A grids) connect to 71A cathode ground
- Interstage secondary centre taps (carrying signals to 45 grids) connect to 45 cathode ground

So - I'd welcome comments and opinions, even if it's just "I'd try them all".

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Re: Finished and working - 12 valve all DHT PP 26-71a-45

Post by izzy wizzy » Mon Jan 07, 2019 12:27 pm

Your layout isn't too far removed from Lynn Olson's Karna. It takes a bit of looking to work it out. Try that for starters but don't earth points A or B coz they aren't earth or 0V.

Cheers,
Stephen

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Re: Finished and working - 12 valve all DHT PP 26-71a-45

Post by RhythMick » Mon Jan 07, 2019 12:31 pm

izzy wizzy wrote:
Mon Jan 07, 2019 12:27 pm
Your layout isn't too far removed from Lynn Olson's Karna. It takes a bit of looking to work it out. Try that for starters but don't earth points A or B coz they aren't earth or 0V.

Cheers,
Stephen
Thanks, yes I'm going to stick with D unless I get advice to the contrary.

I got my head in a mess about grounding until I read that Karna article. He talks about the Morrison book but there's one key line... "Don't think of ground, think of return current paths". That turned my thinking around, about lots of things.

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Re: Finished and working - 12 valve all DHT PP 26-71a-45

Post by izzy wizzy » Mon Jan 07, 2019 2:01 pm

RhythMick wrote:
Mon Jan 07, 2019 12:31 pm
I got my head in a mess about grounding until I read that Karna article. He talks about the Morrison book but there's one key line... "Don't think of ground, think of return current paths". That turned my thinking around, about lots of things.
Yes and something I need to think about as well and your post is a timely one fpr me. Look at the input grid point. That's referenced to earth and then to both the other stages star points albiet the 2nd stage shares the 0V and supply. I imagine your 2nd and 3rd stages would be referenced to the input grids as he has done with his 3rd stage.

Cheers,
Stephen

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