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Old 28th September 2005, 05:40 PM   #1
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Default Does this look right to you?

I built Eric Barbour's "Brute Force" pre-/headphone amp this spring (it sounds great!) and I'm finally going to finish it with a nice (that's what I'm hoping...) chassis. Before I transplant everything, I'd like your opinion on the power supply.

I'm using 6 x 51V zeners for regulation at the moment, but is it as simple as just replacing them with 0D3 tubes as I've done on the schematic? Nothing else have been changed.

In short: will this work?
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Old 28th September 2005, 06:43 PM   #2
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Hmmm.... the schematic didn't work. I'll try again:
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Old 28th September 2005, 07:34 PM   #3
edl is offline edl  Hungary
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Sorry, nothing yet
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Old 28th September 2005, 08:27 PM   #4
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Strange... I'm sure I attached it both times. Anyway, I also put the schematic here:
Click the image to open in full size.
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Old 28th September 2005, 08:28 PM   #5
SY is offline SY  United States
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As a new user, your posts go through a manual moderation process. Sometimes it takes a little while for the attachments to post. Sorry about that.
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Old 28th September 2005, 08:31 PM   #6
agent.5 is offline agent.5  United States
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It works fine here.
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Old 28th September 2005, 08:34 PM   #7
PRR is offline PRR  United States
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It will probably work.

Several dubious things.

250VAC will give 350V raw B+, not counting rectifier loss. If your wall-power varies +/-20% it gives 295VDC to 424VDC. Allowing 50V loss in rectifier and 50V-100V drop in the 6AS7, it is not going to regulate except when utility power is high.

If we pretend we get +350V raw B+, and OD3 is 150V(?), then we have 350V-300V= 40V across the 20K resistor, which is 40V/20K= 2mA, which is lower than the suggested current in OD3 tubes (5mA-35mA, IIRC). 50V 0.5W Zeners work OK to lower current than gas tubes.

If it lights (you need more than 150V to start the gas discharge), at 2mA it will not be quite 150V and will not be stable.

Gas tubes are far noisier than Zeners. The 0.1uFd helps, but that is an absolute maximum (any more and it oscillates). You have a 10K to the 6SA7 grids, toss a 0.1uFd cap from grid to ground to swamp audio noise.

Your load is only a couple 6BL7 and some 6SN7? Not a lot of current. What is the grid-cathode voltage for 6AS7 at such low current? It may be high, which means your output is trying to go much higher than 300V. However your raw supply voltage is not much higher than 300V, at about 350V (300V-400V). You could end up in a range where the grid has less control than the plate. In that case, the 6AS7 is just a very expensive resistor.

What _I_ would think: starting with +300V output, I would pencil 100V across the pass-tube and 50V across the rectifier, +450V raw supply. I'd check my expected power-utility variation: maybe I need 10% more raw B+, maybe the pass-tube will work to much less than 100V drop and I can let it handle brown-outs. Then +450V raw B+ suggests a 315VAC winding (320V-0-320V if you use the twin-rectifier). Given the small size of that 10uFd input cap, I would check where the bottom of the ripple lay, and maybe add another 5%-10%, 330VAC or 345VAC.

OTOH, if it "sounds great!" and has satisfied for months, maybe you are OK on gross voltage. I'd still run the OD3 current up to 5mA-10mA, and add a noise-filter to the 6AS7 grids.
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Old 28th September 2005, 09:55 PM   #8
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Quote:
Originally posted by PRR


Gas tubes are far noisier than Zeners. The 0.1uFd helps, but that is an absolute maximum (any more and it oscillates). You have a 10K to the 6SA7 grids, toss a 0.1uFd cap from grid to ground to swamp audio noise.
That's not what I'm finding.....Unless I've a batch of good OA2s/OB2's....100V+ zeners in comparison are bxxy dreadful.

richj
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Old 28th September 2005, 10:17 PM   #9
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PRR:
Thank you very much for the reply - this is my first build, so I'm quite new at this, but if I understand correctly:

The problem would be getting enough B+ to light the 0D3s. So if it doesn't light up, some of my options could be:

- bigger input cap to bump up the voltage
- rectifier with lower voltage drop, say a GZ34 instead of the 5Y3 I have now?
- perhaps replacing the 20K resistor with something lower? 10K?

Also, you mention that gas tubes are noisy. Will it be audible hiss or 100Hz hum or something else? Will I be better off with the Zeners noise-wise?
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Old 28th September 2005, 10:26 PM   #10
agent.5 is offline agent.5  United States
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The ground of the 0.1uf bypass of the 0d3 should be tied to the cathode (ground) of the second 0d3 instead of just starground to ground.

I am not sure about this one, but don't you need separate grid resistor for each pass element?
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