Carl -
Ron's method is very accurate, as long as you verify the resistance of the
cathode resistors.
Part of a restoration usually involves replacing the meter shunt resistor, (R36, with a 1W,) and
then selecting a calibration resistor, (R38,) to make the panel meter read 70 mA (little mark
between 6 and 8 on the meter) when confirmed by Ron's method. Once this is done, you can then rely
on the meter to set the bias.
Yes, you always set for the idle CURRENT, regardless of the BIAS voltage. GOOD PA tubes will just
about all fall within the -60 to -65 volt range. The fact that yours take -48 indicates that one or
both tubes are weak. Once the 15 ohm Cathode resistors are confirmed, you can use those voltage
readings to 'match' a pair of tubes, they should be within 5% of each other for best results.
73, Garey - K4OAH
Glen Allen, VA
Drake 2-B, 2-C/2-NT, 4-A, 4-B, C-Line
and TR-4/C Service Supplement CDs
<www.k4oah.com>
Ron wrote:
Carl,
You should adjust for the correct current. That said, the meter is at best
difficult to read accurately.
The better way is to measure the voltage drop across the cathode resistors. R32
and R33 on my T4XB schematic. First see that they are both still 15 ohms, if
not replace. Then measure the voltage across those resistors while adjusting
the bias. According to ohms law, they should have about .5 volts to be at 33
ma. So 2 tubes each at 33 ma each is 66 ma, which is very close. The little
bit off is still far more accurate than the error in reading the meter.
73,
Ron WD8SBB
--- On Sat, 2/11/12, Carl Maniscalco<kj6...@cox.net> wrote:
From: Carl Maniscalco<kj6...@cox.net>
Subject: [Drakelist] Setting the bias on a T-4X
To: drakelist@zerobeat.net
Date: Saturday, February 11, 2012, 6:46 PM
Hello all--
I'm still new to the Drake world and just today got the T-4X
I've been restoring over the past few months working thanks
to one of Garey's CD-ROMs. I've got a question for the
experts, however: I've read the instructions and set the
bias control on my AC-4 so the plate amperes meter on the
T-4X reads .07A. However, when I check the the actual bias
voltage with a DMM, it's around -48 volts rather than the
expected -65 volts. If I adjust the voltage to -65 V, I get
about .02A on the meter.
My question is, should I adjust the bias control for the
specified voltage or the specified amperage?
--Carl Maniscalco, KJ6BSO
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