For your Alpha 78, you might see what choke Dick Byrd uses in his Alpha 76/78
conversions. I have a 76PA that works fine on all bands. I haven't used it on
30 meters, but did try it into a dummy load, no problem. 30 meters should be
done in the 40m bandswitch position.
As I remember from opening it up for cleaning when I first got it, the choke
appeared to be a stock choke with a few unconnected turns wrapped around the
original to break the resonances. This is a gimmick that I also remember from
an MLA2500 mod article to break the 17M resonance on that amp. I'd pull my amp
off the shelf an look at it again, but I don't want still another hernia
operation.
73
Bill K5FUV
On Friday, April 13, 2018 7:04 AM, Jim Thomson <jim.thom@telus.net> wrote:
Date: Wed, 11 Apr 2018 09:53:53 +0700
From: Martin Sole <hs0zed@gmail.com>
To: Vic Rosenthal <k2vco.vic@gmail.com>
Cc: amps@contesting.com
Subject: Re: [Amps] Alpha 77D anode choke swap?
<Here's a bit of an update.
<I had a change of thinking and decided the use of 12m might be more
beneficial in my Alpha 78. This has a broadband input circuit that's
pretty flat from 1.8 to 30 MHz without any switching. The 78 is a tight
work space and the Ameritron choke is larger than the stock Alpha choke
both in diameter and length. Still I felt it would fit with reasonable
clearance.
Out of circuit and on the bench I used my Heathkit GD1 GDO with a
counter to test the shorted Ameritron choke. I found it to resonate
around 9.1MHz. I was? concerned that's quite close to 30m but knowing it
would likely change in place I continued to fit it. In place and again
shorted I found the dip to have moved to around 8.5MHz. Okay this is
halfway between 40m and 30m pretty much. The choke is close to the tank
coil which is perpendicular and right at one end. It's also close to the
blower housing and the lower part of the choke is close to the tube plenum.
Operation on 40, 20, 15 and 10m seems much as before. If anything it
might be down fractionally and desiring a bit more drive for similar
outputs. It tested fine on 12m with slightly less output than on 10 or
15 presumably due to the less than optimum tank values. On 30m things
got "inneresting". With the amp on 7MHz I found a dip and proceed to
adjust though with not much output. At around 200 watts output a
somewhat stentorian retort signaled the demise of the plate choke.
Clearly this is either not a good choke for 30m in this application or I
need to make other "adjustments". The rest of the amp is stock and I
left the small choke with capacitors at the point where the HT is
connected to the base of the main RFC .
Is this choke known to be limiting at 30m or are there other things I
can do to over come the problem.
Martin, HS)ZED
## Those ameritron chokes all resonate, on the bench at 12.6 and also 27
mhz , using
w8jis method of testing. I think whats happening is either u have a defective
choke,,, pretty rare,
Most likely the 12.6 mhz resonance has dropped down a bit, closer to the
30m band. And its
probably shifted down, due to the stay C from new choke to surrounding
metal or insulation
material. You only need barely a few pf to shift the resonance down quite a
bit..and screw up a
choke.
## This is the w8ji method of testing for resonances on plate chokes.
https://www.w8ji.com/rf_plate_choke.htm
Myself, I also use it, as I cant trust the GDO.....and choke leads temp
shorted. With choke leads temp shorted and
GDO used, what u are really doing is finding the parallel resonance points.
With the temp short across the choke leads
removed, those parallel resonance points now become the series resonance
points. Its sorta close sometimes,
but not very accurate.. vs the w8ji method.. which is dead on.
## Ur choke lower resonance shifted, is too close to 30m band.... and u had
the classic choke fire.
Id use the 7 mhz position on the amp vs the 20m position. You can try both
and see which one is better.
If u use the 20m position, the tank Q will skyrocket. If you use the 40m
position, tank Q will drop quite a bit.
On paper, the 20m position will result in increased circulating current in
the tank coi.. heating it up some more.
Tuning will be sharper, but the 30m band is only 50 khz wide, so it may not be
an issue.
Jim VE7RF
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