[Johnson] Why is the Johnson Viking II final amplifier NOTneutralized??

[Sherrill Watkins]

Dear Glenn: After carefully reading your email, I examined the schematic
diagram of my Viking II Transmitter. When you say "grid", I presume you mean
the CONTROL grid, that is the grid the closest to the cathode of the 6146
final amplifier tubes. I noted that the two control grids of the Valiant II
are tied in parallel. Also, there is a lead from the control grids going
through an RFC choke L6, through a 1600 ohm resistor R24, through a meter
shunt resistor SH4 and then connects to the negative bias side of the power
supply. The bias voltage at the power supply is labeled -75 volts. (The
Viking II does not have a resistor that connects the control grids directly
to chassis ground but has negative bias voltage applied to the control
grids.) I do not know what the exact bias voltage is on the control grids of
the two 6146, but does this have an effect to reduce the gain of the two 6146
tubes? If I assumed that the two control grids are at, let's guess and say
-60vdc. Therefore, if one applied -60 volts of dc to the control grids, would
that have an effect to lower the gain of the 6146 tubes more so than if they
were just connected to chassis ground through a resistor? I will appreciate
your comments very much as I have learned a lot from your kind posts to this
board. Best regards, -Sherrill W. k4own. 

-----Original Message-----
From: Glen Zook [mailto:gzook at yahoo.com] 
Sent: Wednesday, September 29, 2004 2:31 PM
To: Sherrill Watkins; johnson at mailman.qth.net
Subject: Re: [Johnson] Why is the Johnson Viking II final amplifier
NOTneutralized?? 


I believe that the Valiant II also utilizes
neutralization!

Now, as for the Viking II:  The Heath DX-100 also does
not utilize neutralization as well as a number of
other transmitters which use one or two of the 6146
family tubes.  First of all, the 6146, 6146A, 8298,
6293, etc., tubes are much more "stable" than the
6146B, 8298A, and other tubes of the "B" family which
are NOT the same as the earlier model tubes although
RCA originally claimed that they were interchangeable.

It is possible to keep from needing to neutralize the
earlier 6146 tubes by "loading down" the grid
circuitry.  That is, by circuitry requiring more grid
drive than necessary if the final amplifier does not
have as much loading (lower value resistor to ground,
etc.).  The Heath DX-35 and DX-40 are good examples of
this.  Because of the lower resistance to ground from
the grid this lowers the absolute gain of the stage
and helps keep the parasitic emissions from happening.

Also, the parasitic chokes in the final plate leads
also help to keep the stage "stable".

If you haven't read the information on the 6146 family
of tubes (article was in Electric Radio a while back)
you can find it on either of the websites that are
listed at the end of this message.

Glen, K9STH


<Sherrill.Watkins at dgs.virginia.gov> wrote:

Gentlemen: I would like an intelligent explanation of
why the Johnson Viking II transmitter final amplifier
is not neutralized?! It uses two 6146 tubes in
parallel and does not incorporate a neutralization
circuit of any kind. On the other hand, the Viking
Valiant transmitter uses three 6146 tubes in parallel
and DOES have a neutralization capacitor? This seems a
bit strange and illogical?? If the Valiant II is go
great a design as I have been led to believe, why are
not the final amplifier tubes neutralized??

=====
Glen, K9STH

Web sites

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


		
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