[Amps] Alpha 76A 3cpx800 Conversion Tuning?

[Carl]

Ive done several of those conversions using the Ameritron choke which is 
sold to/by RFP as their RFC-3, its $5 less in the MFJ catalog.

I use a piece of hard Teflon wedged in as a spacer from the blower which may 
or may not be needed if using 3CPX800 tubes and more HV; I wasnt going to 
take a chance.

As far as 10-15M tuning I do the armstrong method (only needed less than 
half  the time) and open up the coil spacing a bit in the front. It doesnt 
take much spreading to reduce the L a bit and allow the Tune cap to go thru 
a complete peak. Looks neater than moving taps IMO.

I use a 76PA ( got it as a basket case as payment for fixing a Titan) at the 
second station and havent had a reason to do the conversion yet. The three 
8874's from an upgraded to SS UHF government site will easily do 1800W with 
100W drive and since I run low drive and 1200W out they should last beyond 
my expiration date (-;

Carl
KM1H

----- Original Message ----- 
From: "Robert Harmon" <k6uj at pacbell.net>
To: <amps at contesting.com>
Sent: Sunday, April 05, 2015 4:47 PM
Subject: Re: [Amps] Alpha 76A 3cpx800 Conversion Tuning?


Hi Rich,

I am in a similar quandary.

I upgraded my 76CA to two 3CPX800’s along with the Peter Dahl transformer 
also.  I didn’t change the
existing RFC though.  It tunes up beautifully on all bands (except 12) (more 
on that later)   More power out
on all bands.  While I was at it I put in new power supply caps and 
equalizing resistors.
I know the 76’s don’t like 12 meters with their existing RFC and I purchased 
the RFC-3 to install later.  I read
that this choke would work fine and provide 12m. However,  later on the Eham 
amplifier forum I asked for info
about using the RFC-3 and read that the RFC-3 is 1” in diameter and would be 
too close for comfort and would
cause tuning issues.  It was suggested to find one that is 3/4” in diameter 
or install the RFC-3 but mount it farther
away from the tank circuit components than the original Alpha RFC  .Also an 
Ameritron RFC is supposed to be
another alternate. That is where I am now, I need to find out more on this. 
Hopefully we can hear from
someone that has done this mod successfully.

 (I would suggest posting this issue on the eham amplifier forum.  The 
amplifier gurus like Carl, KM1H and Lou W1QJ have been repairing
amps for years and usually jump in to help.)

Anyway, thats my two cents.  Please keep me posted on what  you discover.




73,
Bob
K6UJ







On Apr 5, 2015, at 12:36 PM, Rich Hallman - N7TR <rich at n7tr.com> wrote:

> After converting an Alpha 76a to 3CPX800's with a new Peter Dahl upgraded 
> Transformer and installing the new upgraded RFC-3 Plate Choke I am seeing 
> a problem with tuning on 15 and 10 meters.   All bands except 15 and 10 
> seem to tune ok.   On 15 and 10 meters....The tuning indicator is at 0 and 
> it's the best tuning I can get.  For those who have completed a conversion 
> in the past, can you let me know if you have seen this issue before and if 
> so, what was done to fix the problem?  I've seen some info on the net from 
> removing stators from the tuning cap to moving the taps on the tank coil.
>
> Thanks for any help you can give!
>
> Rich
>
> Rich N7TR
> ex KI3V, N3AMK, WB3JOV
> www.n7tr.com<http://www.n7tr.com>
> http://www.qrz.com/db/N7TR
> Telnet: dxc.n7tr.com N7TR DXCluster
>
>
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