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

To: "BILL KENNAMER" <>, <>
Subject: Re: [Amps] Alpha 77D anode choke swap?
From: "Jim Thomson" <>
Date: Fri, 13 Apr 2018 07:52:23 -0700
List-post: <>
###  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 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 
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. 

Jim   VE7RF    

Sent: Friday, April 13, 2018 6:41 AM
To: Jim Thomson ; 
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.


Bill K5FUV

On Friday, April 13, 2018 7:04 AM, Jim Thomson <> wrote:

Date: Wed, 11 Apr 2018 09:53:53 +0700

From: Martin Sole <>

To: Vic Rosenthal <>


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.

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


##  This is the w8ji  method of testing for resonances on plate chokes. 

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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