[AMRadio] DX-100B on Phone - bad audio distortion / oscillation

[Mike Sawyer]

>From Glen Zook's K9STH document, "The 6146 Family of Tubes":
RCA "claimed" that the 6146B was directly interchangeable with the earlier 
members of the 6146 family. Unfortunately, this did not hold true in most 
cases. Collins, Heath, and probably other companies, at first issued various 
documents saying that the use of the 6146B in their equipment was "fine". 
But, this soon proved otherwise!

For example, when the 6146B was used in the Collins 32S-1, 32S-2, 32S-3, 
32S-3A, KWM-2, and KWM-2A it was discovered that the components in the 
neutralization circuitry "burned up" in a very short amount of time. Thus, 
Collins had to retract the statement that it was "OK" to use the 6146B. 
Then, due to the fact that the United States military establishment wanted 
to "standardize" on the 6146W equivalent of the 6146B, the neutralization 
components had to be redesigned to allow the 6146B to be used. Fortunately, 
these changes did not affect the use of the earlier 6146 and 6146A in those 
transmitters manufactured to use the 6146B. All three types of tubes may be 
used without any problem in these transmitters.

Replacing the 6146 / 6146A tubes with 6146B types often results in spurious 
emissions, parasitic oscillations, etc. This is due to the fact that there 
are different bias requirements, different inter-electrode capacitances, 
etc. of the 6146B versus the other two. It is often difficult to neutralize 
6146B tubes when used in place of the 6146 / 6146A. If neutralization can be 
achieved, often it lasts for just a few minutes before the tube(s) goes into 
oscillation.

found here:
k9sth.com/uploads/The_6146_Family_of_Tubes_1.doc

Mod-U-Lator,
Mike(y)
W3SLK
----- Original Message ----- 
From: "Rick Poole" <wa1rkt at arrl.net>
To: <amradio at mailman.qth.net>
Sent: Saturday, December 03, 2011 5:34 PM
Subject: Re: [AMRadio] DX-100B on Phone - bad audio distortion / oscillation


At 02:56 PM 12/3/2011, ne1s at securespeed.us wrote:

 >>>>>
>Does the RF output frequency change when it goes into oscillation, and you
>kill the RF drive?
<<<<<

The SWR goes off the scale, which is a dead giveaway that the
transmitter is putting out RF on pretty much any frequency but the
one it's supposed to.

 >>>>>
>  I'm thinking perhaps the finals are taking off on their
>own, since they appear to be generating RF with no drive from the 5763. If
>so, you may need to add neutralization. But I can't explain why this would
>happen in AM and not CW.
<<<<<

It's happening in CW now, too.  It happens reliably when I tune a
little off-resonance on the high side, i.e. the Plate pointer points
to a higher number, and the plate tuning capacitor is more
meshed.  Maybe that's why I didn't notice it on CW before.

The PA has a neutralization modification whereby a wire is routed
from the driver capacitor to somewhere near one of the PA tubes, then
you move that wire back and forth to find the sweet spot for
neutralization.  I have tried every position of that wire and nothing works.

I've ordered some 6146A's to replace the 6146B's installed now, since
some report problems with the 6146B's, and I know that some Collins
KWM-2's, including mine, don't play well with -B's.  We'll see what
happens when those arrive.

I'm just trying to figure out whether I should remove the
neutralization mod before installing the 6146A's.

Rick WA1RKT