[Johnson] Not only no but HELL NO!

[Glen Zook]

The 6146B requires different screen voltage and
different operating bias.


--- Gary Schafer <garyschafer@comcast.net> wrote:

How would you "design" a transmitter with a 6146A any
differently than one that would use a 6146B in the HF
bands ? Other than possibly a slightly different size
neutralizing cap.


Depends on the transmitter.  Some are easier to modify
than others.

"It can be easily added to transmitters like the
DX100."


I disagree wholeheartedly with this statement!  There
usually is NOT a problem with a component other than
the fact that the circuit was not designed for the
6146B.  Yes, you can change the circuit to use the
6146B series.  However, since there is no lack of 6146
/ 6146A / 8298 tubes, why do this?  You are destroying
the originality of the transmitter.  If you don't care
about this, then most certainly modify the
transmitter.

"If it "will not neutralize properly" then it should
be fixed. That usually indicates that there is a
problem with a component."


No arguments here!

"There are a lot of gremlins that show up with
improper neutralization. One is that the grid drive
seems to be more than adequate. This can indicate that
the transmitter is on the edge of instability.
Properly neutralized you will usually have much less
grid drive. Then you sometimes have to go and fix the
driver stages to obtain enough drive."


The "B" version having more gain will show up that the
circuit was NOT designed for the 6146B!  Now, back in
the late 1970s Motorola decided not to stock the 8032
which is the 12 volt equivalent of the 6146A / 8298. 
Instead, they started supplying the 8552 which is the
12 volt equivalent of the 6146B / 8298A in boxes that
were marked as 8032.  At the time I owned the Motorola
reconditioned equipment center for the south-central
U.S. and we were shipping between 50 and 100 Motrac
units per week on average.

The HHT version of the Motrac in the lowband and high
band versions used the 8032 in the driver and/or final
stage in the 25 watt highband and 30 and 50 watt
lowband versions up through the "D" model.  The 80
watt highband and 100 watt lowband use them in the
driver only.  Only the highband "E" model was designed
for the 8552.  The LHT and MHT versions were designed
around the 8552.  

We ran out of the "normal" 8032 tubes and had to start
using the 8552 tubes provided by Motorola.  Within a
week we started getting complaints that the units were
arriving with one, or usually, both tubes broken. 
Prior to this we had not received a single complaint
(had shipped thousands of Motrac units).  Since this
was a very new problem, and was occuring only with the
8552 tubes, we did some very serious testing.

What we found was that the 8552 tubes were overheating
so badly due to parasitics in the 1 to 2 minutes that
were normally used to tune up the equipment that the
glass envelope was being anealed.  The Motrac is such
that you cannot properly tune the driver and final
without the heatsink being in place.  When it is in
place, you cannot see the tubes at all.  As the boxes
were subjected to "normal" handling the tubes were
being shattered.  The transmitters could NOT be
modified to use the 8552 tubes because this violated
type acceptance.  

At first Motorola tried to say that we had gotten a
bad batch of tubes.  However, within 2 weeks they got
so many complaints from their service shops that
Motorola had to admit that they had blundered. 
Frankly, it cost Motorola many thousands of dollars to
pay the warranty claims cause by the fact that they
wanted to reduce their tube inventory by one tube
type!

"The B version of the 6146 having more gain than the
regular will show up defects in circuits sooner."


The problems that Heath ran into was with NEW
production and not with units that are decades old! 
The grounding of the shields, etc., had nothing to do
with the problems with the 6146B tubes.

"I have seen many stability problems in the Heath SB
series transmitters that usually turned out to be poor
grounding of the shielding. You could  neutralize them
and a day latter they would be unstable again. Nothing
to do with the tubes or neutralizing, but poor grounds
on the shielding.  Sometimes the pa cage grounds,
usually the shields on the underside where the driver
and grid coils reside. Even the wipers on the band
switch shaft have to be making proper contact to keep
them stable."


Same thing as with the Heath equipment, the 6146B
problems came with brand new equipment and not
equipment that is decades old!  The shield grounds
were "bright and new" and not corroded as they can get
over years and years of use.  Frankly, the Collins
engineers knew what they were doing!  The problems
WERE caused by the 6146B tubes.
 
"Same with the collins S line transmitters. Usually
poor shield grounds on the cans under the chassis that
cause instability."


Not "some", but virtually all of the equipment
suffered from the 6146B problems.  Of course after
they were modified they worked fine!  That was the
purpose of the modification!
 
"True that the components in the neutralizing circuit
had to be changed on some in order to properly
neutralize them when installing B tubes. But once done
properly they worked just fine."


It often takes a lot more than just replacing the
parasitic suppressers to make a transmitter work with
the 6146B tubes if it wasn't designed to use them! 
You have to change the screen voltage in many cases,
the bias (Collins accomplished this by the very nature
of the 516F-2 power supply in which the bias can be
adjusted, Heath the same thing), the neutralization
circuitry, etc.

"Checking for an off value of the resistor in the
parasitic suppresser often works wonders at solving
those problems."


Now, again, I am not saying that some transmitters
don't work using the 6146B tubes without modification.
 Yes, some transmitters do!  I am not saying that the
transmitter cannot be modified to use the 6146B tubes.
 In most cases this can be accomplished with a varying
amount of difficulty.  What I am saying, rather really
asking, is since there is no problem obtaining the
older versions of the 6146 then why modify the
transmitter?  I have no problems with those people who
want to modify their rigs if that is what they want to
do.  However, I am also a proponent of the old saying
"if it ain't broke then don't fix it".

Glen, K9STH

=====
Glen, K9STH

Web sites

http://home.comcast.net/~k9sth
http://home.comcast.net/~zcomco

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