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

To: Martin Sole <>
Subject: Re: [Amps] Alpha 77D anode choke swap?
From: Vic Rosenthal <>
Date: Wed, 11 Apr 2018 08:28:21 +0300
List-post: <>
For what it’s worth, I’ve found with several amps that did not have a 30 meter 
position on the bandswitch, 30 worked best with the bandswitch set to 20 meters.
I have used this choke successfully on 30 in a SB1000 and a homebrew 4CX1000A 

Vic 4X6GP 

> On 11 Apr 2018, at 5:53, Martin Sole <> wrote:
> 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 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
>> 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 <> 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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