Yep - wound up being lucky this time - didn't even have to buy another tube!
Someone else had mentioned the Sherwood audio amp, and this particular R-4C
has that mod. Overall, it sounds good now.
Steve Wedge, W1ES/4
Live free or die: Death is not the worst of evils.
John Stark.
All my computers have my signature with various pearls of wisdom appended
thereto.
--------------------------------------------------
From: "Garey Barrell" <[email protected]>
Sent: Saturday, March 10, 2012 11:20 AM
To: "Steve Wedge" <[email protected]>
Cc: "Mike Bryce" <[email protected]>; "Randy WB4SPB"
<[email protected]>; <[email protected]>
Subject: Re: [Drakelist] R4B Audio Distortion
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" <[email protected]>
To: "Mike Bryce" <[email protected]>
Cc: "Randy WB4SPB" <[email protected]>; <[email protected]>
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:[email protected]>
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
[email protected]
http://mailman.zerobeat.net/mailman/listinfo/drakelist
