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Got some money, going to buy OPTs for PP GU-50
Got some money, going to buy OPTs for PP GU-50
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Old 19th November 2020, 06:28 PM   #1
rongon is offline rongon  United States
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Got some money, going to buy OPTs for PP GU-50
Default Got some money, going to buy OPTs for PP GU-50

OK, the sky is not the limit as far as funds go, but I have enough for a pair of Edcor push-pull OPTs.

I have a small collection of GU-50 tubes and sockets, which I'd like to try. Yeah, they're beastly, but they look cool, I think.

I have a gigantic power transformer from an early 1970s Traynor bass amp, designed for 4x 7027A. It's massive. I can use that for the power supply for a gigantic stereo amp.

I don't need a lot of output power. 10 strong watts per channel would suffice. I would like good damping of the loudspeaker. So I'm thinking of going with a B+ of about 420V and cathode bias. Triode-wire the GU-50s, so they should see about 360V plate-cathode. That should be a good spot.

So, what plate-plate primary impedance should I go for? I'm thinking 10k ohms. The Edcor CXPP30-10K looks good.

Or would I be better off with the ever-popular CXPP25-7.6K ?

It looks like Zpri of anything from 7.5k to 10k should work well. See attachment.

The loudspeakers are Snell Acoustics E/III or Snell C (original version). Both are nominal 4 ohms, not 6 or 8.

Thoughts?
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File Type: jpg GU50 350v100ma_loadlines.jpg (77.6 KB, 381 views)
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Old 19th November 2020, 08:00 PM   #2
TG is offline TG  Ukraine
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8k is quite enough. You can tie g3 to the plate, you know...
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Old 19th November 2020, 08:24 PM   #3
rongon is offline rongon  United States
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Got some money, going to buy OPTs for PP GU-50
Well, for push-pull class A triodes:

Higher value of Zprimary yields less power, lower distortion, better damping ratio.
Lower value of Zprimary yields more power, higher distortion, worse damping ratio.

Somewhere there is a 'most desirable' point for the particular designer's taste, intent.

I'm hoping to make the GU-50 triodes 'sound good' with a minimum of NFB. Maybe no global NFB loop if I can get away with it. That's why I was thinking of a high load impedance like 10k:VC.

My understanding about tying g3 to the plate along with g2 is that it doesn't make a huge difference from the usual configuration with g3 to cathode and g2 to plate. Or am I wrong about that?

I don't want to get into 'left-handed' vs. 'right-handed' vs 'CrazyDrive' and all that. Just plain triode wiring the old-fashioned way will be sufficient for my purposes.
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Old 19th November 2020, 09:16 PM   #4
TG is offline TG  Ukraine
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Tying g3 to the plate along with g2 drops GU50's Rp by about 15% and makes it quite a bit more linear and nicer sounding (ask Wavebourn - IIRC he had some curves, I can't find them now unfortunately). It needs about the same 15% more bias (and hence more drive) though.
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Old 19th November 2020, 10:25 PM   #5
rongon is offline rongon  United States
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Got some money, going to buy OPTs for PP GU-50
Oh, NICE! That's exactly the kind of info that goes under the radar, but can make a big difference.

A 15% reduction in rp is pretty big.

I can give it drive. I'm thinking of using a Williamson-style driver circuit with a diff driver pair made of 6V6s in triode. I have a couple of PCBs for that, which I might as well use.

Thanks.
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Old 19th November 2020, 10:39 PM   #6
Wavebourn is offline Wavebourn  United States
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CXPP100-10K
It is what I used in my Pyramid amps 12 years ago.
850V B+, 270V regulated G2, +12V regulated G3.
100W max, driven by LTP on 6P15P with local parallel feedback from Gu-50 anodes.
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File Type: jpg web-7.jpg (73.1 KB, 329 views)
File Type: jpg 66237_441589739232666_395742114_n.jpg (118.6 KB, 325 views)
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Last edited by Wavebourn; 19th November 2020 at 10:45 PM.
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Old 19th November 2020, 10:58 PM   #7
rongon is offline rongon  United States
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Got some money, going to buy OPTs for PP GU-50
Thanks for reminding me. You got a ton of power out of those by running them at very high plate voltage (700V?), but much lower screen voltage (200V?). I remember well the 6P15P-triode drivers you used.

Do you think a 100W rated OPT is necessary if I'm going triode-wired with relatively low voltage B+/high plate current?

I'm thinking the operating points will be about Vp-k = 400V, Ip = 80mA, so grid bias will be in the neighborhood of -63V, each GU50 dissipating 32W.

I figure I should get about 15W per channel, no? That would be plenty for my small living room with big speakers. Maybe even with no global NFB loop. I will probably put one in, make it adjustable somehow.

Would a 30W capable output transformer not be enough? Go for the 50W one? I'm afraid of the big transformers having a lot of insertion loss and more leakage inductance, causing the higher frequencies to sound worse. I've never used one of these Edcor OPTs, so I don't know...
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File Type: png GU50-Triode_Loadline_5K_400V_80mA.png (371.3 KB, 310 views)
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Old 19th November 2020, 11:05 PM   #8
Wavebourn is offline Wavebourn  United States
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With 400V B+ try triode A2. 30W is possible.
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Old 20th November 2020, 04:58 AM   #9
rongon is offline rongon  United States
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Got some money, going to buy OPTs for PP GU-50
So, would the CXPP30-10K be a good choice for GU-50-triodes PP Class A2?
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Old 20th November 2020, 06:13 AM   #10
Wavebourn is offline Wavebourn  United States
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I don't know, find what you prefer, by load lines.
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