[Amps] Alpha 77D anode choke swap?

Wed Apr 18 04:43:50 EDT 2018

AT the risk of feeling like I'm talking to myself ;) here's an update.

With the applied shorted link (gimmick) winding at the top of the plate 
choke there have been no issues. There's also no output on 12m to speak 
of which is annoying of course. No fireworks on any band, same output at 
essentially same settings on all bands 160-10. Good output on 30m and 
17m 30m in the 7MHz position and 17m in the 21 Mhz position. I can get a 
bit of output on 12m in the 21MHz position but both tune and load C's 
are essentially at minimum and the efficiency is abysmal, 25% at best. 
On 10m the 78 is "semi-bandpass" it means they disconnect the tune C 
relying on circuit strays and output is just peaked with the load C. 
Worls good on 10m but nothing on 12m.

Are there other mods needed to enable 12m? it would seem the tank 
circuit needs some investigation.

Thanks
Martin, HS0ZED

On 17/04/2018 17:00, Martin Sole wrote:
> Bill,
>
> You comment about the unconnected turns got me thinking and Jim 
> commented on this as well.
>
> So I wrapped 3 turns of insulated solid wire, taken from some CAT5 
> twisting the ends together so it's just a shorted but otherwise 
> unconnected coil wound over the top of the plate rf choke.
>
> Testing with the W8JI light bulb method showed an initial bulb 
> brightness peak (no gimmick coil) around 25Mhz, likely fireworks for 
> 12m indeed. I tried a 4 turn coil first and it moved the resonance up 
> to 27.5Mhz, I thought that a bit close to 10m so I removed one of the 
> turns making it a 3 turn shorted coil. Resonance peak, maximum bulb 
> brightness, is now around 26.5Mhz. Is this the magic sauce?
>
> What am I missing? Is there going to be sufficient heat in the choke 
> to melt the pvc insulation causing an almighty mess and associated 
> fireworks?
>
> Martin, HS0ZED
>
>
>
> On 13/04/2018 20:41, BILL KENNAMER wrote:
>> 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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