[Amps] Alpha 77D anode choke swap?

[BILL KENNAMER]

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 at telus.net> wrote:
 

 Date: Wed, 11 Apr 2018 09:53:53 +0700
From: Martin Sole <hs0zed at gmail.com>
To: Vic Rosenthal <k2vco.vic at gmail.com>
Cc: amps at 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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