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[Amps] Alpha 77D anode choke swap?

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Subject: [Amps] Alpha 77D anode choke swap?
From: "Jim Thomson" <>
Date: Thu, 12 Apr 2018 08:23:25 -0700
List-post: <>
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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