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Re: [Amps] Alpha 77D anode choke swap?

Subject: Re: [Amps] Alpha 77D anode choke swap?
From: Martin Sole <>
Date: Tue, 17 Apr 2018 17:00:47 +0700
List-post: <>

You comment about the unconnected turns got me thinking and Jim commented on this as well.

So I wrapped 3 turns of insulated solid wire, taken from some CAT5 twisting the ends together so it's just a shorted but otherwise unconnected coil wound over the top of the plate rf choke.

Testing with the W8JI light bulb method showed an initial bulb brightness peak (no gimmick coil) around 25Mhz, likely fireworks for 12m indeed. I tried a 4 turn coil first and it moved the resonance up to 27.5Mhz, I thought that a bit close to 10m so I removed one of the turns making it a 3 turn shorted coil. Resonance peak, maximum bulb brightness, is now around 26.5Mhz. Is this the magic sauce?

What am I missing? Is there going to be sufficient heat in the choke to melt the pvc insulation causing an almighty mess and associated fireworks?

Martin, HS0ZED

On 13/04/2018 20:41, BILL KENNAMER wrote:
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.
Bill K5FUV

     On Friday, April 13, 2018 7:04 AM, Jim Thomson <> wrote:
  Date: Wed, 11 Apr 2018 09:53:53 +0700
From: Martin Sole <>
To: Vic Rosenthal <>
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

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

##  This is the w8ji  method of testing for resonances on plate chokes.
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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