I am trying to modify a 1 tube amp for 6 meters using a 6146. I think the
amp was origionally used for CB though it does tune up on 10 meters. It
worked OK before I removed the input circuit and put in one for 6M. The
problem is I cant neutralize the damn thing. It makes a great osc.I was
thinking of scraping the tuned input and replacing it with a 200 ohm
resistor and a 4:1UNUN. I figure this low of an impedance on the grid would
swamp out any feedback that might me causing it to osc. I have about 5 watts
PEP of drive.

Jimmie

"Jimmie D" wrote in message
...
I am trying to modify a 1 tube amp for 6 meters using a 6146. I think the
amp was origionally used for CB though it does tune up on 10 meters. It
worked OK before I removed the input circuit and put in one for 6M. The
problem is I cant neutralize the damn thing. It makes a great osc.I was
thinking of scraping the tuned input and replacing it with a 200 ohm
resistor and a 4:1UNUN. I figure this low of an impedance on the grid would
swamp out any feedback that might me causing it to osc. I have about 5
watts PEP of drive.

Jimmie


It could be the screen bypassing--it is critical.

If you can find an old VHF ARRL Handbook
from the sixties you might find some ideas, tips and tricks regarding using
these tubes on VHF. Another problem is unwanted inductive feedback, such
as when the ground ring lugs on some style sockets were used for the
bypass caps... Those rings had enough inductance at VHF to cause unwanted
coupling... Can you neutralize the tube using the conventional neutralizing
stub method??

Pete k1zjh

The VHF Hanbook from the 1960's used a 6252 -- I believe that this was also
the bube used in various transverters sold by Hallicrafters --- my but no
6146 design -- I still have a bunch of 6360's hanging around from that era.

There is a 6146 transverter design in the 1973 ARRL Handbook --
comprehensive enough to get you done.

It might help to give us some more information (see below):

On Sat, 12 May 2007, Jimmie D wrote:

I am trying to modify a 1 tube amp for 6 meters using a 6146. I think the
amp was origionally used for CB though it does tune up on 10 meters. It
worked OK before I removed the input circuit and put in one for 6M.

A key question I would ask at this point is whether the amp, used
originally for CB (27 mHz), was, itself neutralized? Some ham transmitters
were not neutralized (eg. Heathkit, Ranger) although they were operated up
to 10 meters.

The
problem is I cant neutralize the damn thing.

When you removed the original input circuit, what, exactly, did you
replace it with?

I have found that neutralization of grounded-cathode amplifiers (i.e. high
gain) can be a major pain.

It makes a great osc.I was
thinking of scraping the tuned input and replacing it with a 200 ohm
resistor and a 4:1UNUN. I figure this low of an impedance on the grid would
swamp out any feedback that might me causing it to osc.

Nah, I gonna bet that ain't gonna work.

I have about 5 watts
PEP of drive.

FWIW:
My approach to this would be to build something grounded-grid and that
means any grid tube where you can get all the grids to (real) ground. You
_might_ get 6AG7s or 6CL6s (two might be enough) in parallel and
any decent input matching network might do the job for you and get
about one "S-unit" of power gain. Tube efficiency will be a little lower
at 50 mHz than 27-28 mhz but not that much.

One of the guys said that screen grid bypassing is critical and I can
attest to that, too. Some of the published material even gives separate
neutralization circuits for just the screen grid circuit. Very big pain in
the neck if you have to do this by trial and error.

Good luck!


Jimmie

Jimmie D wrote:
I am trying to modify a 1 tube amp for 6 meters using a 6146. I think the
amp was origionally used for CB though it does tune up on 10 meters. It
worked OK before I removed the input circuit and put in one for 6M. The
problem is I cant neutralize the damn thing. It makes a great osc.I was
thinking of scraping the tuned input and replacing it with a 200 ohm
resistor and a 4:1UNUN. I figure this low of an impedance on the grid would
swamp out any feedback that might me causing it to osc. I have about 5 watts
PEP of drive.

Jimmie


Transmitter in the 1967 ARRL HB used a 6146 in gk on 6m.
The input circuit was 9 turns of #20 spacewound 5/8" dia.
It was center tapped, with the center tap connected to a 7uh
rf choke, other end of the choke was grounded to bias supply
(or 22k 1w resistor for grid leak bias, class C). One end of the
coil goes to the grid of the 6146. The other end of the coil is
connected to ground via a 30pf tuning cap, and also to a probe wire
that serves as one 'plate' of the neutralizing capacitor (the plate
of the tube is the other capacitor plate). This probe wire is simply
moved near the tube till adjustment of the plate tuning capacitor does
not vary the grid current, this indicates the tube is neutralized.

The three cathode connections of the tube are connected together at the
socket by a "Y" shaped piece of copper, this is then grounded at a
single point.

"ken scharf" wrote in message
.. .
Jimmie D wrote:
I am trying to modify a 1 tube amp for 6 meters using a 6146. I think the
amp was origionally used for CB though it does tune up on 10 meters. It
worked OK before I removed the input circuit and put in one for 6M. The
problem is I cant neutralize the damn thing. It makes a great osc.I was
thinking of scraping the tuned input and replacing it with a 200 ohm
resistor and a 4:1UNUN. I figure this low of an impedance on the grid
would
swamp out any feedback that might me causing it to osc. I have about 5
watts
PEP of drive.

Jimmie


Transmitter in the 1967 ARRL HB used a 6146 in gk on 6m.
The input circuit was 9 turns of #20 spacewound 5/8" dia.
It was center tapped, with the center tap connected to a 7uh
rf choke, other end of the choke was grounded to bias supply
(or 22k 1w resistor for grid leak bias, class C). One end of the
coil goes to the grid of the 6146. The other end of the coil is
connected to ground via a 30pf tuning cap, and also to a probe wire
that serves as one 'plate' of the neutralizing capacitor (the plate
of the tube is the other capacitor plate). This probe wire is simply
moved near the tube till adjustment of the plate tuning capacitor does
not vary the grid current, this indicates the tube is neutralized.

The three cathode connections of the tube are connected together at the
socket by a "Y" shaped piece of copper, this is then grounded at a
single point.


Found a schematic where a 50 ohm swamping resitor was used on a 4cx250. This
worked great on a pair of 6146s