[Johnson] Not only no but HELL NO!

[Gary Schafer]

Glen Zook wrote:
> What is usually not "right" in the circuit is that the
> transmitter was NOT designed for the 6146B!  

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.

> 

> A very good check on whether or not an individual
> transmitter will work OK with the 6146B is to
> neutralize the transmitter (unfortunately, not all of
> the older "boat anchor" transmitters have this
> capability).  

It can be easily added to transmitters like the DX100.

>If the transmitter will not neutralize
> properly, then the 6146B tubes should be immediately
> replaced with the non-"B" versions.  If the
> transmitter does neutralize then it needs to be
> watched for at least several hours of actual
> operation, if not for several days of operation.  If
> the neutralization changes, then again the 6146B tubes
> need to be replaced with the non-"B" versions.

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

If a transmitter is neutralized properly and it does not "hold" then 
there is a problem that needs to be fixed. Bad screen bypass, 
Neutralizing cap that is drifting, grid coupling cap drifting, Driver 
producing trash, shields not grounded properly etc.


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.

Another quick indication of instability is to see if you have maximum 
output at the plate current dip. Also grid current should peak slightly 
at that same point. If these do not occur simultaneously then that is a 
good indication that neutralization is off. Also tuning the plate tune 
will sometimes show wild swings in plate or grid current. another 
indicator that things need to be fixed.

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


> Towards the end of their production, Heath's primary
> supplier of 6146 tubes stopped production of the
> earlier versions and went only to the 6146B.  Heath
> originally said not to use the 6146B in their
> equipment.  

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 with the collins S line transmitters. Usually poor shield grounds 
on the cans under the chassis that cause instability.

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.

> 
> 
> 
> Now, even though the transmitters designed for the
> 6146 / 6146A appear to operate correctly with the
> 6146B I am very hesitant to use the 6146B.  Even if
> the transmitter neutralizes fine, I still use a GDO in
> the wavemeter mode to check for VHF parasitics.  More
> often than not, the 6146B tubes are putting out
> parasitics even though the transmitter "appears" to be
> operating normally.

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

The driver being unstable sometimes causes problems that appear to be 
final problems.


73
Gary  K4FMX


> 

> Glen, K9STH
> 
> 
> --- Gary Schafer <garyschafer@comcast.net> wrote:
> 
> I have used 6146B's in many transmitters that
> originally had 6146s and have never had a problem.
> Problems usually originate from something not right in
> the circuit. Poor ground, bad component etc.
>