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In Reply to: RE: I just read that... posted by artsybrute on May 06, 2009 at 07:02:09
over on Tube DIY. (Aside: when did DIY equate to playing with capacitors?) Looking at the garter circuit, I can't see any way that there's any DC on that primary.
It looks like getting the Rks spot on will require a bit of experimenting. You are going to get some bias voltage (maybe 40% to 50% not accounting for insertion losses) from the extra B+ voltage the garter draws. I'm thinking you want to match the resistors exactly and not get into achieving values through additional series or parallel networks, since the resistor match is critical. The extra B+ through the garters makes up the rest with any residual shunted to ground. For example, biasing a 45 at Va = 250V, Ia = 30mA, Vg = -50V gives a Rk of 1k67. I would think two 500r0 resistors in series with the 100k garter tied between them *should* get the current draw about right and balance the tubes. Raymond Koonce has told me that he got the bias correct when he used two 600r0 resistors for the 71A. He's running at the data sheet OPs of ~180V, 20mA, -40V which would require a Rk of 2k0 in standard cathode bias. But apparently 1k2 works.
More cypherin' to do. Later.
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