C3G PX25 DRD - Will it work - diyAudio
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Old 27th June 2007, 04:55 PM   #1
ianc13 is offline ianc13  United Kingdom
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Default C3G PX25 DRD - Will it work

Hi,

I am thinking of putting together a C3G/PX25 DRD amp based on the attached schematic.

The heater arrangement is not shown but will be AC on the C3G lifted about 30V above ground and 4V DC on the PX25's.

I am expecting to run the PX25's at 400V/62mA biased at -31V and the C3G's at 170V/12mA biased at -2.5V.

The only error that I can see in my schematic is the biasing resistor on the PX25 needs to be smaller as the bias voltage will be the voltage dropped across that resistor and also the DRD choke - not just the resistor as the calcs show.

Anyway the question is - will it work. Are there any problems with the schematic.

All critique welcome.

Thanks,

Ian
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Old 27th June 2007, 07:17 PM   #2
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The only observation is, Why on earth, a PX25.....?

Hideously expensive antiquated bottle.....few new made variations etc, Unlike the more traditional 300B.....

Guess you must have a few kicking round rather than paying the silly money these bottles command!

600V in the schematic Vp on the PX25 is rather high.....but otherwise it should work, How well, only time, dosh and a good O/P Tx can say!
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Old 27th June 2007, 09:08 PM   #3
ianc13 is offline ianc13  United Kingdom
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Hi Alistair,

I managed to pick up a pair of Golden Dragon branded KR PX25s on eBay for next to nothing (relatively speaking) about the price you'd pay for a single JJ 300B.

Always fancied doing a DRD amp and worst comes to worst it should be very simple to convert to a more conventional 300B implementation by changing the cathode resistance to hit say 360V and -70V on the grid.
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Old 27th June 2007, 09:51 PM   #4
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Just noticed your PSU....

Rather 'odd' to have a bridge which is Half Valve and half silicon....

If I was you, I would try and arrange either completely one or the other........

Just seems a recipe for hum in the making somehow....

Also, Where is the driver getting its anode-voltage from.... Its not connected to the junction between the cathode resistors of the PX, which I assume it should be....(500ohm and 3.6K to 15 mic, and choke...?)
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Old 27th June 2007, 10:07 PM   #5
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Quote:
Originally posted by Alastair E
Rather 'odd' to have a bridge which is Half Valve and half silicon....
No it's not. I do it all the time.

Why are you being such a downer for his project? DIY is at least in part about doing things a little differently simply because you can. So, I think a c3g to px25 DRD is a great idea. And diyhifisupply.com has TJ px25's on sale now for cheaper then their 300b's, so it seems like a reasonable choice to me.
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Old 28th June 2007, 09:49 AM   #6
ianc13 is offline ianc13  United Kingdom
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I have seen the hybrid bridge used in a number of different projects - a couple of DRD projects too so thought I'd use it here in typical Frankenstein amplifier design

The Anode voltage of the C3G will come from the potential divider between the cathode resistors with some voltage dropped across the DRD choke.

At that point if the biasing works correctly I should get 12mA flowing up through the DRD choke and through the C3G and then ~50mA (62mA - 12mA) flowing through the lower cathode resistor.

Note that the cathode resistors are all variable to a certain degree so I should be able to tweak to get reasonably near the operating points that I wish. The top one 0-1K5 and the lower one 3K1 to 4K6.

This current should then sum back together to go through the lower resistor set - that sets the bias for the C3G - before returning to ground.

Ian
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