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round and round we go ..........

Posted by vinnie2 (A) on January 23, 2016 at 17:54:45

Well I think I know what a dog feels like chasing it's tail now...talk about shooting at a moving target.
As the next step in building my deuce26/45 amp I tried using the psu from my old preamp for the B+ supply. Had to make a few changes to the cathode resitor value to try and get the operating points back where they should be per palustris's post from a while back. (See the attached link). He suggested plate volts of 220vdc, grid bias of -40vdc and plate current of 6.25vdc. I tried the following different values of cathode resistor with the results shown below.
All other things were equal for all of these readings except the cathode resistors.

cathode 1800 ohms
grid volts 48.8vdc
plate volts 238vdc
plate current 27.1mA

1500 ohms
44.5
228
29.7

1200 ohms
39.7
218
33.1

1000 ohms
35.5
209
35.5

As you can see from the numbers, the last two are the closest to the target op's, but each is off on one or another point. My question is which of these values is of the most significance? In a previous post Jim McShane stated that the bias should be whatever is required to get the proper plate current; as long as the current is close the bias is whatever it is. The 1k cathode is very close to the proper plate current, but the 1.2k cathode has the a closer plate voltage. Am I correct in assuming I should go with the 1k cathode resistor and call the plate volts close enough? Is the plate current more important to have correct than the plate voltage. If you look at the load line in the attachment, the op's from 1k and 1.2 k carhode resistors both are fairly close to the load line.
I would really like to hear any opinions on what should be given the most consideration.
Thanks!

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Edits: 01/23/16 01/23/16

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