A look at the schematic indicates that you have hit all
the usual suspects. So, what else could cause the control
grid voltage to go positive? I wonder if the tube socket is
leaky. Even if the tube is on an extender it might be the
socket. It might be something on the surface of the socket
between pins. Pin 6 is the screen grid so leakage from that
could cause the problem. Check the socket for carbon tracks
or anything on the surface. Clean it thoroughly with dry
alcohol. Temperature might make a difference if there is
either a carbon track or something else there.
Perhaps far fetched but easy enough to do and worth a
try. If cleaning makes a difference but does not cure the
problem a new socket might be in order.
--
Richard Knoppow
Los Angeles
WB6KBL
dickb...@ix.netcom.com
----- Original Message -----
From: "Randy WB4SPB" <wb4...@earthlink.net>
To: <drakelist@zerobeat.net>
Sent: Thursday, February 09, 2012 8:53 PM
Subject: [Drakelist] R4B audio distortion
Hi folks,
I've owned my beloved R4B since the mid-70's. From about
1985 until 2000,
it was stored while I got busy with job/family. I used it
again for several
months in 2002 and 2003, and I didn't notice anything
unusual. Now I am
active again and want to use it full time, but I am
hearing audio distortion
after about 5 minutes of operation.
Poking around, I observe the cathode of V7 is at higher
voltage than
expected, and the grid has more than 2 volts, where 0 is
expected. Coupling
cap C185? No, lift one side, and no change. R154 has the
correct value.
Cathode resistor? R44 has the correct value, and I tried
a different one
anyway, in case it was thermally unstable, with no change.
I've replaced
C176 (along with the other small electrolytics in the RX),
no difference.
I haven't tackled replacing the big electrolytic can yet,
but voltages and
ripple seem pretty much OK on all terminals of that unit
(e.g. 7mv at C90A,
and 35mv at C90D, 75mv at C90C).
At power up, the V7 grid is slightly more than 0V, cathode
voltage nominal.
As time goes on, these creep slowly upward. After 5-6
minutes, the grid is
approaching 3V.
30 seconds after power up:
V7 pin 1: 4.8
pin 2,5: .006
pin 6: 134
pin 7: 146
5-6 minutes after power up:
V7 pin 1: 6.2
pin 2,5: 2.9
pin 6: 129
pin 7: 139
I note screen and plate voltages have fallen off a little.
If they were a
little high to begin with, it may be because my line
voltage is sometimes a
bit high: I measured 122VAC today. I presume the drop in
these values
reflects increased current through V7. I get the same
basic behavior when
I have the receiver on a variac and control the line
voltage to something
less than 120VAC.
The audio sounds good for the first several minutes, but
it becomes harsh
sounding at some point. I don't know what the output
waveform looks like.
So I could feel better about V7 running hard while I poked
around, I put a
fan on it, above the chassis ... and lo, the grid/cathode
voltages started
to drop. The grid stabilizes at about 0.3V with a healthy
amount of forced
air on the 6EH5, and the audio sounds good. Conductive
socket material when
hot? I don't think so: I put the tube on a 1.5-inch
extender to reduce
heat at the socket, with no change in behavior. Bad tube?
No, two others
behave the same way.
I've researched other R4B audio distortion discussions,
some on this list.
None seem helpful. I'm stumped... any ideas?
Thanks and 73,
Randy WB4SPB
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