Steve -
Yes, the tube noise only seems to be a problem in the 3rd Mixer, and 'noisy' tubes can often be used
in the IF with no problem. I guess it has something to do with the way the stages are biased, but
I've never really investigated it. I'm lazy, I'll just select a good tube, and it usually outlasts
the radio.
I've had a couple (three?) guys who doubted me on this, and all three replaced every component in
the AF Output stage..... All three eventually ended up selecting a 6EH5! :-) It's a tough sell,
cause 'I put in a NEW tube'...... :-)
It's kinda like the IDLE current in a PA. If the supply voltages are there, the PA tubes WILL draw
current. The only question is how much, and there you have an adjustable bias to take care of minor
differences.
Except in this case you don't have 'easily' adjustable bias. If you have the right cathode
resistor, and the voltages are present, the tube WILL draw ~30 mA. However, (there's always a
however',) when the tube gets _hot_ if the grid starts emitting electrons, or there are gas
particles in the 'vacuum', the 'natural order of things' is screwed up and the tube starts drawing a
little more current. Which gets a little more grid current, which gets the tube a little hotter,
which ....
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>
Steve Wedge wrote:
I just wound up fixing an R-4C - and one of the things I did was to swap V4 and
V6.
That single exchange did more to reduce the noise in the receiver than anyting else I could have
done. I did go on and convert the third mixer to cathode-feed, and that improved things a bit
more, but you're on to something with respect to the audio output.
OTOH, I still sit here and wonder. At audio frequencies, is everything as designed by Drake
really that critical? It might be a good time to replace components to find out...
Yeah - I'm a troublemaker...
Steve Wedge, W1ES/4
"I can't complain, but sometimes I still do."
- Joe Walsh
If the above message appears, it came from Steve's Son of Laptop!
----- Original Message ----- From: "Garey Barrell" <k4...@mindspring.com>
To: "Mike Bryce" <proso...@sssnet.com>
Cc: "Randy WB4SPB" <wb4...@earthlink.net>; <drakelist@zerobeat.net>
Sent: Friday, March 09, 2012 10:21 PM
Subject: Re: [Drakelist] R4B Audio Distortion
Mike -
I think the problem is that 'new' tubes are 50 years old.. Even a few molecules of air a day
adds up after that long. When Drake assembled the radios, the tubes were _really_ new!! The
6EH5 gets 'really' hot in normal operation, and perhaps the heat/cool cycles are harder on the
metal/glass seals at the pins. Some tubes definitely are more prone to 'gas' (grid emission),
the 12BA6 comes to mind. That one shows as a 'drifting' S-Meter as the tubes heat up. The 6EJ7
is prone to noise in 3rd Mixer service, probably for a similar reason. The only answer I have
come up with is to keep trying tubes until you get a 'good' one.
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>
Mike Bryce wrote:
I wonder.....
when drake assembled the radios, did they go through three or fours tubes per radio to get one
that worked correctly?
dosen't seem logical. And although you can't argue with the fact that the subing out the tube
fixed the problem, I wonder if there is something hidden down someplace in the design that
causes the problem
just a thought
Mike, wb8vge
On Mar 9, 2012, at 4:25 PM, Randy WB4SPB wrote:
Altogether, I bought two 6EH5s and one 6CA5. I already had a spare 6EH5, so, considering those
alone, I had to try three to get one. If I count the 6CA5 that I bought, I had to try four to
get one.
The 6CA5 IS very similar. Some sources do not even have the 6EH5 and refer you to 6CA5. The
tube that originally manifested the problem was a 6CA5 that had been in the radio for many
years before these symptoms appeared.
Whether the difference in 6EH5 and 6CA5 is important here, I cannot say. I'll stick with 6EH5s
in the future, while I can get them. For sources I've seen that have both, the price is the same.
73,
Randy WB4SPB
----- Original Message -----
*From:*Al Al <mailto:wenj...@gmail.com>
Out of curiosity, how many 6EH5 tubes did you have to go through to find a
good one?
Al, n7ioh
Payson, Arizona, USA
_______________________________________________
Drakelist mailing list
Drakelist@zerobeat.net
http://mailman.zerobeat.net/mailman/listinfo/drakelist
