[Amps] Alpha 77D anode choke swap?
jim.thom at telus.net
Fri Apr 13 10:52:23 EDT 2018
### U still have to be real careful with plate chokes. Wrapping unconnected turns around the oem choke is a new one for me, thats a good trick.
Typ the best you can do is to shift an unwanted resonance up /down a bit. You dont want the 17M band resonance moving too far up..and u dont want it moving too far down.
Another method is to just remove a few turns from top end of choke, to shift a resonance higher in freq.
## Drake used a 154 uh solenoid choke on my L4Bs... which is a 80-10m amp. It works fine on 17 + 12 m. I have not tried it on 30M, even into a dummy load. Manual sez it will work anywhere from
3.2 mhz to 30.0 mhz. The 154 uh oem drake choke will also work on 160M, on L4B amps that have been modified for 160m use.
## Interesting to note on the drake L7 amp ( also a 2 x 3-500Z) amp.... the L7 is a 160-10m amp, including 30/17/12 m. The plate choke they used is a one off setup I have not seen b4. They wired I think a 47 pf cap, from the
center of the choke.... to the bottom of the choke. Then the usual bypass cap at the base of the choke.. to chassis. I still have not figured out how it works, only ideas at this point.
## I dont operate 30m, 12, 10m on my hb amps. I have no interest on those bands. So I factored that in when designing the choke.
## For those that use a switched choke setup, that scheme cooked up by the VE3 fellow is simple..and unique...and one of those concepts that invoke..... why didnt I think of this 30 years ago ?
He used both a big value choke..and a smaller value choke. Bottoms of each choke are bonded together..and bypassed to chassis with the usual bypass cap. SPDT vac relay toggles between the
top of each choke. Common of vac relay goes off to the tube anode. Then its a simple matter to wind 2 x chokes that are optimized for both the lower and upper bands. IE: 50 uh for 40-30-20-17-15-12 m bands.
50 uh will also work just fine on 80-10m. A buddy uses a 50 uh choke on his hb YC-156, which is a 80-10m amp. A 50 uh choke will resonate at 35 mhz, regardless of wire gauge used, or diameter of form its wound on. The big choke is typ 90-135-180-200-225-250 uh , depending on whether you want 160m . If you dont require 10 + 12m, the 50 uh small choke can be replaced with a slightly larger 70-80 uh choke.
## Other schemes will use 2-3 x 50-80 uh chokes in series, and typ all at right angles to each other. I have also seen a 135 uh + 50 uh, in series used on a 160-10m amp. These schemes dont require a relay.
## another popular relay switched choke scheme, used on a lot of commercial higher powered amps uses 2 x chokes in series, typ a 45-50 uh choke..and also a 135 / 180 uh choke. Bypass cap at base of big choke.
A 2nd bypass cap is connected to the junction of the 2 x chokes. Cold end of this 2nd cap is wired to one side of a plane jane spst mech relay. Other side of same spst mech relay is bonded to chassis. 3 x 1 meg, 3 watt series resistors are wired from cold end of 2nd cap..to chassis. On low bands, both chokes are in series. The resistors ensure the cold end of 2nd cap is dc grounded to chassis. When spst mech relay closes, cold end of cap is not only
DC grounded to chassis, it is also RF grounded to chassis..... the larger value choke is now effectively out of the circuit, leaving just the small value choke functioning. The beauty of this scheme is a plane jane spst mech relay is used, no vac relay required. NO B+ appears on the mech relay input contact, since the 2nd cap blocks the B+.
## HB amps typ have the luxury of more space to play with it. You often dont have that in tight confined spaces in commercial table top ham amps.
From: BILL KENNAMER
Sent: Friday, April 13, 2018 6:41 AM
To: Jim Thomson ; amps at contesting.com
Subject: Re: [Amps] Alpha 77D anode choke swap?
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.
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
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.
## 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
## 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.
Amps mailing list
Amps at contesting.com
More information about the Amps