From cign–(at)–elios.phy.OhioU.Edu Tue May 7 10:27:24 CDT 1996
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From: cign–(at)–elios.phy.OhioU.Edu (Dave Cigna)
Subject: Re: Converting fixed bias to cathode bias
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In article <318DED33.6E5--(at)--n.com>, Jack A. Zucker wrote:
>I was wondering if anyone had any suggestions of what
>value resistor/cap combinations I should start out with (for the
>cathode of the power tubes? This would be for either 6L6 or EL34s
>currently running with 430V on the plates (and 35ma measured using the
>transformer shunt method). I realize that the values would probably
>different based on which tube I selected.

Hi Jack. The basic idea is to develop the grid-cathode bias as the
voltage drop across the cathode resistor. So if you measure the grid-
cathode voltage at, say, -48V with fixed bias, and you measure
the *cathode* current at, say, 40mA, then you can use Ohm’s law: R = V/I
or R = 48/(.040A * 2) = 600 ohms. This is for one cathode resistor shared
by two tubes. Remember that the cathode current is the plate current
(which you measured by the transformer shunt method) plus the screen
current, which is likely to be ~5mA or so. Better to start out with
resistors a little on the big side and substitute smaller ones until
you get the operating point that you want. Make sure that your resistor
has a big enough power rating. You can calculate the power dissipated
as I^2 * R or V^2 / R. In the case of a pair of 6L6 the maximum
current is likely to be as high as 250mA for the pair, so:
.250^2 * 600 = 37.5 watts. You might want to use one of those big
50W jobs that will bolt right to the chassis. I think you can get
them at a fair price from Mouser. Another option is to use mixed
cathode and fixed bias. Start with a smaller cathode resistor and
‘make up the difference’ with fixed bias as necessary. I think that
this works pretty well on amps running class AB.

— Dave Cigna

 

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