Re: couple paramour questions... - Paul Joppa

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Re: couple paramour questions...

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Posted by Paul Joppa on July 26, 2001 at 22:38:02

In Reply to: Re: couple paramour questions... posted by GDaddy on July 26, 2001 at 20:10:44:

I would try for at least 70v across the C4S to leave a little headroom, and a minimum plate voltage of 150v. (That would be 70 volts above the lowest plate voltage of 80v for a 12AT7 at 4mA current and -0.5v grid).

If the top of the C4S is at 300v, then the plate voltage can be anywhere from 150v to 230v. Ideal would be 300v at the top of the C4S and 190v at the plate, which allows a peak output of +/-110v.

There is a reason for this variation from tube to tube when using a current source. The tube is operating essentially in fixed bias, and consequently does not have the operating point stabilization (DC negative feedback) that would otherwise go with a cathode resistor. It would be possible to add a servo circuit to stabilize the driver operating point, but it would add a transistor in the cathode, which would have to be bypassed with a large capacitor to reduce the impedance. And a capacitor in the cathode circuit just does not sound as good as an unbypassed resistor. More complication, more cost, worse sound? No thank you!

For the really tweaky, you can short the bias resistor and check the plate voltage. It will nominally be 50v, at zero bias. Add 30v for a negative 0.5v on the grid (to minimize possible grid current) plus 70v for output swing with a 10v margin, to get nominally 150v as the desired minimum plate voltage. If your zero-bias voltage is moe than 100v, you may have an out-of-spec tube. Since this tube was made by so many different manufacturers who all published the same curves, I expect some will be out of spec...

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Follow Ups Full Thread

Topic - couple paramour questions... - nasaman 19:15:11 07/26/01 (11)

Re: couple paramour questions... - MattSkeen 22:01:21 07/26/01 (0)

Re: couple paramour questions... - GDaddy 20:10:44 07/26/01 (5)

Re: couple paramour questions... - Paul Joppa 22:38:02 07/26/01 (3)

Now I've rested... - jaybug 14:49:17 07/27/01 (0)

You rock, Paul. Thanks. - GDaddy 14:28:07 07/27/01 (0)

Thank you Paul. now I will rest.<nt> - jaybug 02:10:46 07/27/01 (0)

Why do we need to see ~2.0vdc at the cathode (B8)? <nt> - jaybug 20:15:14 07/26/01 (0)

Re: Why do we need to see ~2.0vdc at the cathode (B8)? <nt> - VoltSecond 22:13:14 08/01/01 (0)

I am very curious as well <nt> - GDaddy 20:50:04 07/26/01 (0)

Blue stuff OK, but MORE ABOUT HIGH PLATE VOLTAGE...(ahem) - jaybug 20:10:40 07/26/01 (3)

OK, I'll jump on the bandwagon . . . - eedork 22:30:41 07/26/01 (2)

No no -- lower value of existing resistor from B8 to ground - Patrick 23:13:25 07/26/01 (1)

I wouldn't change anything... - GDaddy 14:36:40 07/27/01 (0)

Follow Ups

Re: couple paramour questions... - Paul Joppa 22:38:02 07/26/01 (3)

Now I've rested... - jaybug 14:49:17 07/27/01 (0)

You rock, Paul. Thanks. - GDaddy 14:28:07 07/27/01 (0)

Thank you Paul. now I will rest.<nt> - jaybug 02:10:46 07/27/01 (0)

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