[Amps] Alpha 77D anode choke swap?

[Martin Sole]

Thanks for all the very useful replies guys, lots of stuff to mull over 
and check out before making the next (mis)step :)

@ VE7RF. Jim you mention the L4B as working FB 80-10 and no problem on 
17+12. CAn you confirm that? I have an L4B minus the supply. If that's a 
workimg solution for 12m it would probably be good enough as is and I'll 
just keep it for that but adding 12m  to one of the more complete 
coverage Alphas's or the big Henry 2k Classic X  without upsetting 
anything else might still be a worthwhile option.

The Ameritron choke seems to be exactly the RF parts RFC3, 3 windings, 
approx 86, 54 and 44 turns, looks like 30 awg, 25.5mm diameter former. 
N4UQ mentions the RFC 3 in his FAQ in the Alpha 77 section, quote

"RF Parts" sells the RFC-3 but it is resonant on 30 meters.  While it is 
sold as a high powered choke, the wire is awfully small.

Hmm, I also found a big resonance around 10MHz.

W8JI says:
Starting with a continuous winding core, the fully wound choke had 
resonances at 10 MHz and 20 MHz. Looking at voltage hot spots, sections 
of windings were removed. The lower gap near the choke middle moved the 
10 MHz resonance up to 12.5 MHz or so. This shifted the upper resonance 
from 20 MHz to near the 24.8 MHz band. The upper gap moved the resulting 
second-order series-resonance from 25 MHz up to 27 MHz. Without the 
upper gap, the second overtone resonance is too close to 24.8 MHz. The 
gaps park unwanted series-resonances between 30 and 20 meters, and at 
the lower end of 11 meters. This results in the highest possible 
inductance for 160 meters, while keeping harmful resonances away from 
normal operating frequencies.

I'm going to set up a test using his method and see what I get. I will 
rewind the the choke just replacing the burnt section plus a couple of 
turns either side. Hopefully that will give me some useful numbers. 
There seems to be a lot of variation in numbers so I'm hoping I can fnd 
something a bit more consistent.

I like the trick to add some unconnected turns, presumably the phantom 
coil is shorted end  to end?

Martin, HS0ZED


*
*
On 11/04/2018 14:35, Martin Sole wrote:
> Hi,
>
> An interesting point about the band switch. The "interesting moment" 
> occurred with the band switch in the 40m position. I'd read elsewhere 
> that that was a preferred setting but that was a different choke so...?
>
> I do see with everything hooked up and the band switch on 7MHz a nasty 
> dip around 9.5MHz. Think I had been doing this wrong before. If I then 
> move the band switch to 14MHz the choke resonance moves down to around 
> 6.5MHz. I guess I need to see if the tank has enough range to cover 
> 30m in the 20m position.
>
> But I think my "repair" was not really to the task as it let go again, 
> this time on 20m when probably slightly off resonance. It may just be 
> this choke is not well suited to this design of amp. I'll restore the 
> original to see if it plays as it should then think about what to do 
> next. I do have a couple of other candidates for a 12m mod, Henry 2k 
> Classic X, SB220, L4B, A77D, 30S-1.
>
> Martin, HS0ZED
>
>
> On 11/04/2018 12:28, Vic Rosenthal wrote:
>> 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 amp.
>>
>> Vic 4X6GP
>>
>> On 11 Apr 2018, at 5:53, Martin Sole <hs0zed at gmail.com 
>> <mailto:hs0zed at gmail.com>> 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<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
>>>
>