Gentlemen:
I'd like to hear from W0ID/W4ETO on this as I have a "cherry" ALPHA 77SX
that is just dying to get on 12M!!!! Assuming 17M is not a problem.
Lee
At 07:07 AM 10/20/98 -0600, John Brosnahan wrote:
>
>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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>
>
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