----- Original Message -----
From: "Peter Ravn" <[email protected]>
To: <[email protected]>
Sent: Tuesday, September 18, 2012 11:08 AM
Subject: [Drakelist] TX4B cathode resistors
The two cathode resistors in the finals in the TX4B should
be 15 ohms each,
but R32 is 30 ohms and R33 16 ohms with the finals
unplugged, so R32 is
definitely bad. The difference in voltages on the cathodes
of the two finals
is, however, minimal. The voltage across R32 is 0.85 and
0.7 across the R33.
What are the causes and consequences of the differences in
the cathode
resistors?
Thanks.
OZ8CTH
Peter
I am not sure what the intended purpose of the resistors
is. They have by-pass caps on both sides and are further
connected to an RF choke which goes to the metering circuit.
You can measure the total tube current of each tube by
measuring the drop across the resistors. The voltage drop is
very small compared to the fixed grid bias so I don't think
they have much effect on it.
I found the same thing in my T4XB, one resistor right on
the nose, the other about twice the marked value. They were
of different manufacture, the good one having a solid body,
the bad one having a mold seam and different surface
texture. I replaced both with metal oxide film resistors.
I did the same in my TR-4.
While all carbon composition resistors tend to increase
in value with age and heat some are much worse than others.
For the most part I've found that Ohmite and Allen-Bradley
resistors tend to stay pretty stable. I don't know who made
the seamed ones and don't see many IRC resistors in the
stuff I've worked on. IRC have a sort of tree bark texture
that was supposed to help dissipate heat and also were
"metalized", I have no idea of what that really was. The
resistors with mold seams going the long way all seem to go
bad.
--
Richard Knoppow
Los Angeles
WB6KBL
[email protected]
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