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@telus.net> wrote:
Date: Wed, 11 Apr 2018 09:53:53 +0700
From: Martin Sole<hs0zed@gmail.com>
To: Vic Rosenthal<k2vco.vic@gmail.com>
Cc:amps@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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