[Amps] Alpha 77D anode choke swap?

Martin Sole hs0zed at gmail.com
Tue Apr 10 22:53:53 EDT 2018

Thanks Vic,

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

On 10/04/2018 23:09, Vic Rosenthal wrote:
> It SHOULD work without issues. But it wouldn’t hurt to check for series resonances with the choke in place (check with GDO, power off, choke temporarily shorted).
> Vic 4X6GP
>> On 10 Apr 2018, at 12:03, Martin Sole <hs0zed at gmail.com> wrote:
>> Hi,
>> About to swap the anode choke in my Alpha 77D for the Ameritron RFC in the hope it will work on 12m without issue. Any gotchas or is it pretty much plug n play?
>> Thanks
>> Martin, HS0ZED

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