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Tubes Asylum Questions about tubes and gear that glows. FAQ |
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Tubes Asylum Questions about tubes and gear that glows. FAQ |
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In Reply to: Damn! posted by Tom S. on March 28, 1999 at 13:57:56:
Hey, don't give up that easy!
Give the circuit just a little more analysis-If the plate of the 12AX7 is sitting at 260V, you have 40V of compliance above that, that is still within the 300V Vmax of the C4S. I don't imagine the 12AT7 needs anywhere near the 40V peak of grid drive that would be available...
How about installing a 52K 1W or better WW resistor (use a convenient value close to this, err on the high side) in place of the 66K. This will drop your B+ to 300V at that node when 2.7 mA is drawn. Put a 10 mfd or bigger bypass cap to ground at this node, and attach the top of the C4S there. R1 should be .95/.0027=352 ohms 1/4W, R2 should be about 300K 1W, because with only 2.7 mA going thru the load, you only want about 1 mA of bias current thru the LEDs.
The 12AX7 curves say this is not that great an op point, at a cathode bias of 1.25V you are in the space charge region, but we'll stay with it. Your cathode resistor should be around 1.25/.0027= 463 ohms. Eliminate the cathode bypass cap across the cathode resistor if there is one.
I would have to see the schematic to go any further than this, but there may well be ways to work the C4S into the driver stage too.
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Follow Ups
- Re: Damn! - Doc B. 08:55:13 04/01/99 (1)
- Re: Damn! - Doc B. 16:12:32 04/01/99 (0)
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