[Amps] Alpha 77D anode choke swap?

[Jim Thomson]

Date: Mon, 16 Apr 2018 08:05:46 -0500
From: Rob Atkinson <ranchorobbo at gmail.com>
To: "amps at contesting.com" <amps at contesting.com>
Subject: Re: [Amps] Alpha 77D anode choke swap?

>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.


<I have doubts about 50 microhenries being adequate for 80 m.  Well, it
<might be okay if you used a tremendous amount of bypass capacitance
<but I'd prefer more inductance, less capacitance.   A couple
<questions:  How much bypass capacitance does your buddy use, and how
<much RF voltage is he willing to subject his power supply to?   I'm
<assuming his filter is a bank of electrolytic caps.


<The best way to not have to sweat the h.v. power supply plate choke is
<to use a balanced output network with the choke in series with the
<center of the output network coil where the RF v. is already low.


<73

<Rob
<K5UJ

##  You are not after XL.  You are after Z..and there is a big difference. 
The 50 uh plate choke on the hb  YC-156 amp  is   wound with  20 gauge magnet wire. 
Bypassed at the base with a 4700 pf  @ 10 kv disc ceramic capacitor.  Also a 1500 pf
@  15 KV   HEC  doorknob cap.  The  4700 pf cap  does a better bypassing job on the lower freqs.
The  1500 pf  doorknob does a   better bypassing job on the higher freqs.  

##   HV filter in the 6700 vdc supply consist of  4 x 47 uf @  4.5 kv oil filled caps in series parallel.
That plus a an external box, with 20 x 2000 uf   @  450 vdc lytics in series for an additional 100 uf.  Total
of  150 uf used.   I think additional bypass caps  were used inside the box with the string of lytics. 

##  In any of these 2  piece plate choke setups, like used by creative electronics, bear-electronics in japan,
my hb versions etc, they  typ use a   135 /  180 uh large choke in series with a 45 –50 uh  smaller choke. 
2-3  x  4700 pf bypass disc ceramics  used at base of large choke.  At the  junction of the chokes,  the junction
is fed to the hot end of some  500 pf bypass caps.   Cold end of the  500 pf bypass caps  is wired to one side of 
a  spst mech relay.  Other side of mech relay contact is  bonded to chassis.   3-4  1 meg  @ 3 watt  series resistors
are wired between   cold ends of 500 pf bypass caps..and chassis.    The resistors ensure the  cold end of the 500 pf
bypass caps are  DC grounded, so the 500 pf caps are in B+  blocking mode.   The larger choke is typ would with  22 gauge
magnet wire.  The smaller choke is  typ wound with 18 gauge magnet wire. 

##  with relay  de-energized,  both chokes are effectively in series... and used on 160m  only.   On 80-10m,  relay is
operated and cold ends of the  500 pf caps are now  RF  grounded.  This  effectively switches out the larger 135/180 uh choke.
IF  both chokes are used on 80m band,  you end up with extreme high voltage at the junction point, and all hell breaks loose. 
Ditto if both chokes used on 40m band.   After  seeing the flame thrower show, then I understood why the commercial   makers 
used the smaller choke on 80-10m. 

##  If you check W8JIs  notes, where he uses an analyzer on his Ameritron choke,   you will see that the Z is sky high on several bands,
esp  40M.   RF current is  just a few microamps  flowing through the choke on 40m... and only slightly more on 80m. 
Worse case for that choke is 160m..and also 10M. 

##  Get close to a  series resonance  point..and  Z drops like a rock.  The  B+W  800 choke is 90 uh, but typ resonates either at the top
end of  15M band, or slightly above. The fix for that..back in the day was to remove  4 turns from the top of the choke, to move the resonance 
point up a bit, but back then we did not have a 12M band.    EG:   XL  =  2 X PI   X  FL.     90 uh =   11,869  ohms  of  XL  on 21.0 mhz.  
Well the 11.869 K ohm of  XL does not do any good, when the Z is through the floor, cause the 90 uh choke just happens to resonate on  15M band.

##  A good plate choke will exhibit a Z  of at least 500 k ohms..and higher, this is what you want. 

##  You also  have to factor in the DC plate current flowing through the plate choke.   I tested several hb chokes in the shop, using my 0-60  vdc  @ 5A
current limited lab  supply.   The  27 gauge magnet wire used on the  180 uh large choke, like used in the henry 8 K ultra, was fine up to aprx  1.3  amps.
Above that, it would go into thermal runaway..and fast, and  run extremely hot.   With 22 gauge, I could run 5 A  CCS  for  30 mins and zero temp rise. 

##  On another buddys  YU-148 amp, it too used the 2 piece plate choke   setup,  with the smaller 45 uh  choke mounted at right angles at the top of
the larger 180 uh choke.   We still had  problems  with extreme HV  at the junction point,  even using a vac relay between cold end of  500 pf caps..and chassis. 
The concept was the 500 pf caps  at the junction point would dc block the  8kv B+.... so any  junk box  spst mech relay could be used to ground the the cold ends
to the chassis.   The fix was the inclusion of the  3-4  megohms between  cold end of 500 pf caps...and chassis.  Only then is the 500 pf caps  really in B+  blocking mode. 
We found that out in the alpha  87A  manual, etc.  They used 3 x resistors in series  to DC ground the cold ends of the junction caps.   These days, alpha uses the same gapped choke
as what you see in the ameritron chokes..and also sold by RF parts.  All  3 x chokes are identical.   So no more 2 x piece choke  + relay + resistors. 

##  The  ameriton  / rf parts choke is superb, but its wound with puny  28 gauge wire, so  has its limitations, esp if high plate currents are used... and  / or  higher B+  voltages 
are used.  The higher the B+ voltage, the  higher the RF voltage is on the choke..which will result in higher  RF current  through the choke. 

##  On a side note, while having all the problems with the YU-148 choke setup, the 45 uh smaller choke was used by itself for a few days.   The 45 uh choke  worked just
fine on 160M, albeit, a lot more  tune C  was required.   The B+  supply for the YU-148 amp used  24 x   3900 uf lytics in series.    The Z was still high on 160M..even with just 
45 uh.  The 180 uh large choke was reduced to just 135 uh....  with the same flame thrower show going on.  Later on, with the resistors added, all was well, provided both chokes only used
on 160m.  Since the YU-148 amp only covers  160-15m, I decided to increase the value of the smaller choke  from 45 uh... up to 70 uh.   The 18 gauge magnet wire was removed from the 
small 45 uh choke..and replaced with 22 gauge wire.   With the smaller gauge wire, more turns on the same teflon form  resulted,  so the uh was increased from 45  up to 70 uh.   And that
was using the same form as b4.   We could have gone even higher,  but time constraints   didnt allow for a lot of experimentation.   70 uh resonates  well above the 15M band....and  70 uh works
just fine on 80-15M.   135 +  70  = 205 uh..which is what is currently being used on 160M.   180+ 70 =  250 uh, which will also work good on 160M. 

##  The  CERAMITE brand of  disc ceramics  in the  4700 pf  size, are available in both 10 kv..and also 15 kv size.  Sold by both mouser...and  henry radio.   In the henry 8k ultra, they used
a  180 uh  +  20 uh setup...with the larger choke being completely shorted out end to end on the higher bands.  At the base of the large choke,  3 x 4700  pf,  10 kv disc ceramic caps used...and also 
3 x 500 pf,  15 kv   HEC  doorknobs.     The last production runs of  8K ultra used an entirely different plate choke setup.  The last production 8K ultras also did not use the 1 turn chrome parasitic 
suppressor.   No suppressor was used at all in the last production runs.   The secret is to NOT feed B+ though any parasitic winding.   The plate choke setup sits off to one side of the tube..and any 
parasitic suppressor sits off to the other side of the tube.  If no suppressor used, the RF  still exits the tube  well away from the plate choke to anode connection. 

Jim   VE7RF