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Old 3rd July 2006, 05:01 PM   #1
zobsky is offline zobsky  India
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Default pentode driver sanity check - before i build

Does this look workable. I want to use a 6bq5 in pentode mode (driven by a CD player output) to drive a 46 or 2a3.

My B+ should be around 280 - 290 V (theoretically).

Ballpark calculations (using a grid bias of -8V) indicate a plate resistor of around 8K and 400 ohm cathode resistor.

I'm thinking of using a simple resistor divider network across B+ to set the screen voltage, but am not sure what to set the nominal voltage at.

If my basics serve me right and we assume that a CD player has +/- 2V output swing, the plate voltage will swing from 50V to 250V when operating on the red load line (grid bias at -8V). What do I want the screen to operate at then (the curve is for a screen voltage of 210V)

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Old 3rd July 2006, 05:03 PM   #2
hacknet is offline hacknet  Singapore
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you would need to bypass the cathode to get the maxium swing because the resistor acts as negative feedback.
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Old 3rd July 2006, 05:29 PM   #3
zobsky is offline zobsky  India
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Quote:
Originally posted by hacknet
you would need to bypass the cathode to get the maxium swing because the resistor acts as negative feedback.
yes, i realized that (just didn't mention it in the post)

thanks
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Old 4th July 2006, 03:25 PM   #4
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I'd use the lowest screen voltage I could get away with and still have enough input headroom. I'd use 150V, 110V, 90V or 75V so I could use a VR tube to supply the screen. Look for a screen voltage that would allow a grid bias of bias around -3V or -4V.

Also, use the smallest load resistor you can that still has enough swing to drive your output tube. You don't need anything like 200V peak to peak for a 45 or 2A3, but the gain of 50 is probably about right. Also at the screen voltage shown, you're operating the tube too close to cutoff at -8V for my taste.
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Old 4th July 2006, 07:31 PM   #5
zobsky is offline zobsky  India
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Quote:
Originally posted by jeff mai
I'd use the lowest screen voltage I could get away with and still have enough input headroom. I'd use 150V, 110V, 90V or 75V so I could use a VR tube to supply the screen. Look for a screen voltage that would allow a grid bias of bias around -3V or -4V.

Also, use the smallest load resistor you can that still has enough swing to drive your output tube. You don't need anything like 200V peak to peak for a 45 or 2A3, but the gain of 50 is probably about right. Also at the screen voltage shown, you're operating the tube too close to cutoff at -8V for my taste.
Good stuff. I couldn't find any curves for Vg less than 200V though (sample datasheet at http://www.retrovox.com.au/STC6BQ5.pdf). How would I get around that ?

I've thought about using a VR tube on the screen. In fact, I have an unused 200V tap (have to look up the exact value) on the power tranny that I could rectify for this purpose.
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Old 5th July 2006, 02:55 PM   #6
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I think it should work OK as you have it described. Higher B+ might be better - probably less 2nd order distortion. But it may very well turn out that you'll get a good amount of 2nd order cancellation with the output tube. Pretty much you have to build it and try.

Going from 210V to 150V screen voltage will lower the plate current curves, so you may have to lower the cathode bias resistor. For the EL84 I don't know if I would go below 150V, but again you can experiment with it.

I have some data on pentode drivers here:

http://www.pmillett.com/pentodes.htm

For what it's worth, I've found that the operating point that you come up with experimentally that measures and sounds good may not look very good when you plot a load line. So I'd encorage you to breadboard something and play with it.

Pete
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