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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 20th November 2020, 10:00 AM   #11
Kay Pirinha is online now Kay Pirinha  Germany
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What's your gigantic PT's plate winding voltage?
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Old 20th November 2020, 04:30 PM   #12
rongon is offline rongon  United States
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Location: Across the river from Rip's big old tree...
Got some money, going to buy OPTs for PP GU-50
I measured 840VCT unloaded, so I figure 800VCT under load.

I figure I can get about 450V DC out of it by using a couple of 5R4GA rectifiers in parallel to both withstand the current demand and drop the voltage. They'd probably drop about 40V.

The transformer came out of a huge Traynor bass amp 'head', but I don't remember the model number. I thought YBA-3, but I may be wrong.

The transformer's only identifying marker is the number "1969" on a sticker.

The laminations measure 11 cm x 9.5 cm x 9 cm, or 4.5" x 3.75" x 3.625"
--

Or maybe I should sell it! There's one on eBay with an asking price of $520 USD. That's amazing...

Last edited by rongon; 20th November 2020 at 04:44 PM.
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Old 20th November 2020, 05:10 PM   #13
rongon is offline rongon  United States
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Got some money, going to buy OPTs for PP GU-50
I believe the transformer is still being made by Hammond as a replacement for the original. It's their number 291YX.

https://www.hammfg.com/files/parts/pdf/291YX.pdf

The physical dimensions match perfectly. It's chunky. Weighs over 11 lbs.

Pretty hefty specs:

800VCT 534mA
6.3VCT 6A (I might use this winding for damper diode rectifiers)
120V primary
466VA

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Last edited by rongon; 20th November 2020 at 05:14 PM.
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Old 21st November 2020, 08:15 PM   #14
rongon is offline rongon  United States
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Got some money, going to buy OPTs for PP GU-50
OK, so loadlines...

I'm no expert at this, but it looks like the triode mode plate curves I found published on the web have the screen grid and suppressor grid tied to the plate. Comparing this set of curves to the triode curves copied from the LS-50 data sheet, it certainly does look like the plate resistance is reduced and gm is increased (mu stays the same). I've attached that set of curves with my loadlines drawn on.

I drew the loadlines at Vp = 350V and Ip = 90mA (Pdiss = 31.5W).

It looks to me like a 3.3k loadline might be the best compromise between power and low distortion. That would mean I'd choose a 6.6k p-p Push-Pull OPT. Does that make sense?

It looks to me like the 4k loadline is flatter and will result in lower distortion, but not that much lower. Is it because the current stays more the same as the voltage swings that the higher load resistance results in less output power?

The 2.5k loadline looks usable, but definitely would result in more distortion. You can see how quickly the right side of the loadline dives down toward 0mA plate current.

So what looks better at that operating point? 4k? Or 3.3k?

(I think 3.3k, but I'm unsure...)
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File Type: jpg gu-50_penintriod_g3_350V-90mA_loadlines.jpg (179.7 KB, 148 views)
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Old 21st November 2020, 09:27 PM   #15
TG is offline TG  Ukraine
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To me it all boils down to the desired damping factor.
99.99% of the time the amp will deliver just a couple of watts, so max power output and THD at that output are kinda irrelevant (and since you're building PP amp, even order harmonics compensate anyways).
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Old 21st November 2020, 10:21 PM   #16
rongon is offline rongon  United States
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Well... I've found that I like the sound of Class A push-pull triode amps. I had a push-pull 2A3 amp running with a 5k OPT that I thought sounded quite good, even though it's more common to see a 3.5k primary used for push-pull 2A3s. I only got 6 watts per channel out of it, though. I'm hoping to get a bit more out of this beastie. 12 watts would be nice.

In the curves graph in the previous post, at Vp = 350V and Ip = 90mA, it looks like rp is only 800 ohms.

Now, normally you'd say that 1600 ohms (2*rp) would be the load that would produce the most power output from the triode, so a 3.2k ohm push-pull OPT.

3*rp = 2400 ohms. so there's the 5k p-p OPT.

4*rp = 3200 ohms, so there's the 6.6k p-p OPT.

I don't know how to calculate damping on the speaker or Zout from the proposed amplifier, so I guess the best thing to do would be to err on the side of caution and go for the higher p-p primary impedance (i.e., 6.6k ohms rather than 5k ohms). But it looks to me like they'll all work.

Now where did I get the idea that I needed a 10k p-p primary? Maybe that would be if I wanted to run the tubes at the highest possible plate voltage and with cathode bias.
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Last edited by rongon; 21st November 2020 at 10:24 PM.
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Old 21st November 2020, 11:21 PM   #17
rongon is offline rongon  United States
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Got some money, going to buy OPTs for PP GU-50
Argh, I got it all wrong.
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Old 21st November 2020, 11:40 PM   #18
rongon is offline rongon  United States
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OK, here we go...

I took too small a sample along the -69.8V grid curve line in the plate curves. That's why I wound up with 800 ohms as the rp. But that is clearly wrong. It's too low.

I did it over, this time taking a much larger sample of the -69.8V grid curve, from Vp = 280V to Vp = 380V. The Ip changes from 16mA at Vp = 280V to 98mA at Vp = 380V.

98mA - 16mA = delta of 82mA.

delta of Vp / delta of Ip = 100V / 82mA = 1219.5 ohms.

Now the choices of OPT load make more sense.

A 5k push-pull OPT would present a load of roughly 2*rp to the GU-50-Triode.
A 6.6k p-p OPT would be 2.7*rp.
An 8k p-p OPT would be 3.28*rp.
A 10k p-p OPT would be 4.1*rp.

All would be reasonable choices.

5k would yield the most power, but least damping on the speaker.
10k would yield the least power, but most damping on the speaker.

The other two would be intermediate between 5k and 10k.

Decisions, decisions...
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File Type: jpg gu-50_penintriod_g3_Calculate_rp.jpg (179.0 KB, 88 views)
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Old 21st November 2020, 11:57 PM   #19
TG is offline TG  Ukraine
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The Rp is calculated from the tangent to the plate curve, not from the curve itself.
So actually you were right the first time
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Old 22nd November 2020, 12:05 AM   #20
rongon is offline rongon  United States
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Got some money, going to buy OPTs for PP GU-50
Wait...

What??

I thought I did what it shows here (attached graphic)...
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And then there's this, from Triode Valve / Tube: Formula & Theory >> Electronics Notes:

Anode resistance

The anode resistance or plate resistance is more exactly described as the dynamic anode or plate resistance. It represents the resistance that the anode circuit offers to a small change in voltage.

Therefore when a small increment in anode voltage ΔEb produces a small change in anode current ΔIb the anode resistance can be calculated as follows:
rp = ΔEb/ΔIb = δEb/δIb
Where:
rp = dynamic anode resistance


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And so the question remains, is the rp of a GU-50-Triode 800 ohms, or is it 1200 ohms? There's a big difference...
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File Type: png TCJ_TriodeConstants_graph.png (19.1 KB, 86 views)

Last edited by rongon; 22nd November 2020 at 12:22 AM.
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