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Old 24th June 2007, 01:26 PM   #11
disco is offline disco  Netherlands
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Quote:
Originally posted by sorenj07
...B+ is around 380V...
Assuming the four triodes take 20mA, the four penthodes 200mA, Rdc of the transformer is about 100 ohms, Rdc of the 10H is 82 ohm, you could try this setup for better regulation and less stress on the rectifier.

Put a 3uF/630V (oil) cap between the 5AR4 and the choke ;
replace the 500uF with a 100uF ;

Ripple on the 380V B+ is about 1.0Vtt;
ripple on the 344V B++ is about 0.02Vtt;

Stability should be investigated when all currents are known.
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Old 24th June 2007, 01:58 PM   #12
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Default Re: Snub that choke.

Quote:
Originally posted by EC8010
You have a choke input supply and haven't snubbed the choke. When the choke switches off (and it will) it generates a back EMF which you see as an arc in your rectifier.
by snubbing i assume you mean a small cap from the rectifier to ground, maybe .47uF or something? i'll want a KV value right
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Old 24th June 2007, 02:01 PM   #13
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Quote:
Originally posted by disco


Assuming the four triodes take 20mA, the four penthodes 200mA, Rdc of the transformer is about 100 ohms, Rdc of the 10H is 82 ohm, you could try this setup for better regulation and less stress on the rectifier.

Put a 3uF/630V (oil) cap between the 5AR4 and the choke ;
replace the 500uF with a 100uF ;

Ripple on the 380V B+ is about 1.0Vtt;
ripple on the 344V B++ is about 0.02Vtt;

Stability should be investigated when all currents are known.
I'd give it a try but I don't have a cap like that on hand and time is kind of short.. I'll try simulating the values that you give above, it might well be worth it. It must be said though that on my 98dB Cerwin Vegas, ripple is pretty much nonexistent.
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Old 24th June 2007, 02:39 PM   #14
SY is offline SY  United States
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Quote:
I think it's the LM334. What do you think?
Not that. If the 334 blows, the worst-case current is 2x B+/82k. Assuming something like 350V for B+, that's only 8-9mA.

EC8010 has suggested a good idea and it will probably fix the problem, but the basic problem is that you're running a rectifier on its edge with a fundamentally unstable supply. Snubbing will help, as will dropping the 500u cap to a more sensible value (ripple isn't super-critical for a p-p design like this). Don't worry about fancy oil caps or whatever on the input- use high voltage polypropylenes or motor-run, whatever's convenient.
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Old 24th June 2007, 05:27 PM   #15
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I do have a .9uF 1800VAC oil cap from a microwave. Would that work OK connected from the first 47uF cap to ground? There's plenty of room in the chassis, and if it helps, I'm happy to do it.
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Old 24th June 2007, 05:43 PM   #16
disco is offline disco  Netherlands
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Quote:
Originally posted by sorenj07
I do have a .9uF 1800VAC oil cap from a microwave. Would that work OK connected from the first 47uF cap to ground? There's plenty of room in the chassis, and if it helps, I'm happy to do it.

Tuning the choke improves its effect. See: http://www.diyparadise.com/tunedchoke.html

In my previous answer I suggested the use of a higher voltage capacitor which I named 'oil type'. The rated voltage is more important than the dielectricum because there's real highish voltage on it.
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Old 24th June 2007, 07:25 PM   #17
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Using some guesses for resistances and running a PSUD2 sim, it looks like a 0.47u input cap is too small and the instability is still there. 3u really makes the voltage soar (450V) but stabilizes things. 1u gets you about 380V and a stable supply in the Sim World.

In the real world, you'll have to experiment with the cap size to get the tradeoff right; you'll want to bounce the current deamnd on the rail a bit and see what the response looks like.

At the same time, the inductive kickback is not your friend, especially with 1200V across that poor rectifier tube. You'll have to snub that.

L-input supplies have their virtues but they're not trivial to design and implement.
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Old 24th June 2007, 09:08 PM   #18
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Quote:
Originally posted by disco
Tuning the choke improves its effect. See: http://www.diyparadise.com/tunedchoke.html
I'd read elsewhere that tuning the chokes in this way compromises their performance in other areas, HF hash maybe?

Quote:
Originally posted by SY
At the same time, the inductive kickback is not your friend, especially with 1200V across that poor rectifier tube. You'll have to snub that.
Umm, how exactly would I enact said snubbing?
edit: I bet Mr. Jones has something to say about that.
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Old 24th June 2007, 09:13 PM   #19
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Grab your copy of "Valve Amplifiers" and the whole procedure is explained step by step in the power supply chapter.
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Old 24th June 2007, 09:21 PM   #20
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Yeah I just read it. Two .22uF 1KV caps, one from each end of the choke, connected to each other and then to ground with a single wire. I can do that
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