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Old 23rd December 2006, 06:38 PM   #1
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Default valve regulated psu

I've got a valve phono, a Ming Da which is pants. I'm going to build a circuit off here, probably Thorstens Valve El Cheapo, into the case, 'cause the case is really pretty

Will someone be kind enough to look at the attached psu schematic to see if it's worth keeping this, or rebuilding with another circuit.
I've looked at the circuit and to be honest I don't understand it, I'm learning a bit about valves but this is beyond me, so any help will be much appreciated.

Any recomendations for a phono stage for me to build,or is the Valve El Cheapo a good un?
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Old 23rd December 2006, 07:29 PM   #2
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it'd be great if the numbers were legible. i assume this is from resizing the image?
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Old 23rd December 2006, 08:05 PM   #3
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Connecting grids to anodes should pretty well ensure destruction of those triodes wired as rectifiers when they switch off. Suggest you use 6CL3.
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Old 24th December 2006, 03:01 PM   #4
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try this one for size, it's a bit bigger.
Kev
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Old 24th December 2006, 03:14 PM   #5
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Quote:
Originally posted by EC8010
Connecting grids to anodes should pretty well ensure destruction of those triodes wired as rectifiers when they switch off. Suggest you use 6CL3.

The data sheet I found shows it as a half wave rectifier with only one anode, is there such a beast that may fit in my nine pin socket with 2 half wave rectifiers in one tube?

Kev
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Old 24th December 2006, 08:05 PM   #6
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EZ81
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Old 24th December 2006, 08:26 PM   #7
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Quote:
Originally posted by EC8010
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Thanks
The datasheet shows maximum size of first capacitor as 50uF, is this per anode or as I suspect total for the valve? I have a 100uF in there at the moment will it need changing?

Does the rest of the circuit look OK?

Kev
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Old 24th December 2006, 09:35 PM   #8
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Default Re: valve regulated psu

Quote:
Originally posted by KevinTams
I've looked at the circuit and to be honest I don't understand it, I'm learning a bit about valves but this is beyond me, so any help will be much appreciated.
It's a fairly typical series pass regulator with a differential error amp. That looks OK, but I sure wouldn't use triodes for rectifiers. Not necessary these days, with so many full wave units out there already.

However 100uF across VT rectifiers is just plain wrong. These guys can't source the current needed to charge such a big filter capacitor (though it's not a problem for solid state to handle). Oh sure, they got away with it back in "the day" because they could count on an atechnological public-at-large to not notice that replacing FUBAR rectifiers every six months or so was not a normal condition. Nor was it a big deal with replacements available at nearly every grocery or 7 -- 11.

I'd get that out of there and use not more than 20uF or so. The regulator will help considerably in suppressing any residual AC on the DC rail.
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Old 24th December 2006, 09:57 PM   #9
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Not to threadjack, but in general, is going a few mA over the limit of a tube with a very low input cap (4.7uF) better than staying within the current limits but using a huge input cap? I assume it varies...
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Old 24th December 2006, 10:32 PM   #10
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Quote:
Originally posted by sorenj07
Not to threadjack, but in general, is going a few mA over the limit of a tube with a very low input cap (4.7uF) better than staying within the current limits but using a huge input cap? I assume it varies...
The rating you need to look at is the surge current limit. With a capacitor input filter, this is the one that's exceeded all too frequently. Even though a big capacitor has less sag, it also requires more charge for any given voltage (Q= CV) and there is less time to pump it in. (More Q, less t means bigger I). I don't see how you'd exceed the surge current with something as samll as 4.7uF unless you're using some sort of high frequency oscillator.

Since VTs are low current, high voltage devices, even something as beefy as the 5U4GB, with a surge current limit of 1.0A/plate falls way short of the 10A rating of even a small silicon diode. This is why you need to watch that input capacitance.

If you use a choke or resistor to limit that surge current, you can use bigger filter capacitors.
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