Steve, there are two possible ways to do the 12 meter mods to older Alpha
amps, but neither is easy! These ideas are not directed specifically at
any particular model, just general comments.
1) You can try to move the series resonance of the plate RF choke
half way between 12M and 10M. This can be done by two ways
--either by removing a few turns from the choke or by adding a shorted
turn around the choke as done in one published mod. But both techniques
are tricky. The exact resonance of the RF choke is somewhat variable
from amp to amp due to minor variations in lead dress and other nearby
components (and also apparently due to minor changes in the number of
turns on the chokes as supplied by the choke vendor). You need to
determine the RF choke series resonance accurately and move that
resonance to a very narrow window between the bands. Move it too
much and the choke will now blow up on 10M instead of 12M. Fortunately
the resonance is very high Q one and this technique CAN work but there
is a high probability for disaster if not done correctly.
2) The second way of handling the series resonance problem is the
way all current Alpha/Power amps are made. You can use two,
series-connected, chokes and switch in one of them for the high
bands and both of them for the low bands. The problem with this
is that you need to detect what bands you are on from the bandswitch
and this is not an easy task. There are no available contacts on the
bandswitch, nor are there and "sneaky" ground paths on the band
switch that can be used to detect high bands from low bands. So that
means that you would need to add another section to the bandswitch but
that isn't easy due to space and general hassle. Or you can add something
to the shaft at the front that allows a microswitch, or a reed switch, or
a photocell to be activated when in the 15M and 10M positions. Not easy
either. (You could add a small toggle switch to do the choke switching
manually but, if you forget to set it properly for the 12M band just one time,
you will smoke the choke.)
The 78 poses a special problem. The plate TUNE capacitor is not
used on 10M due to the combined capacitance of the three tubes and
the additional capacitance of the pretuned output networks. So on 10M
the 78 is only "broadly" resonated on 10M by strays and the output
is peaked by the LOAD capacitor. Going to 12M (assuming the
plate choke series resonance has been addressed) would normally
involve setting the bandswitch to 10M and adding more capacitiance from
the TUNE cap. But the tune cap is not used on 10M so this won't work.
How about using the 15M position instead? When trying to use the 15M
position for 12M there appears to be too much inductance switched in for
the TUNE cap to properly resonate the amp on 12M with all of the strays
from the tubes and the networks. I am still working on this issue
on the 78s and hope to have some better solutions at some point.
I only mention all of this to provide some insight into the problems
in order to demonstrate that the 12M conversion is not trivial.
17M is easy on all of the old amps. Just set the bandswitch to 15M
and tune the LOAD and TUNE caps as you normally would on
any other band.
When/if I come up with reliable mods for 12M that can be easily duplicated
in the field I will publish the information on the Alpha/Power website.
John W0UN
President and Technical Director
Alpha/Power, Inc.
>But Now Dick, I must pipe in my 1/4 cents worth.
>How in heavens name do I get my (374) not the A 3 holer to work 12 meters
>and not blow up the plate choke.?
>You hint in your statment that 25-27 MHz operation is possible with the
>appropriate "modification"
>Any chance that you can share that modification info with (me at least)
>the group.
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