[AMPS] Plate choke

[km1h @ juno.com]

On Mon, 30 Mar 1998 09:19:37 -0600 Wes Atchison <wes@Iphase.COM> writes:

>Carl,
>
>I have been following this tread on Plate RFC's.  I have a single 
>3-500Z
>amp made by AEA, now out of business, called a LA-30.  I may have 
>missed
>the design info on the RFC used in the AL-80A/SB-1000 amps.


That was another amp with a very short life span Wes ( a renamed
Vectronics).  

There are any number of chokes that would work but  I would need the
dimensions you have to work with. I converted a few of those over to 6M
but it has been several years since they were dumped on the market  by 
AEA at fire sale prices....dont see them much any more and I never put
one "on the air" on HF. 


  Could you
>expand on this or point me to where I can find the info on that RFC.


The AL80A/B chokes are sold via Ameritron Service Dept. I also gave out
Ameritron winding info plus my own "homebrew" type here last week.


>
>The problem I had with the amp originally was that it would not tune 
>on
>160m.  I found the loading cap needed to be increased. I added 
>additional
>fixed capacitance to the fixed cap in the circuit.  It now tunes.  
>
>Then the RFC fried when tuning up on 10m.


Of course it wasn't a design problem...right!


 I have wound several RFC's 
>and
>used a GDO to find the series resonances.  I now have a RFC that seems 
>to
>work except that a bypass cap at the base fo the RFC is getting HOT 
>and
>smokes on 160m.


Too little inductance.


  The original circuit had a RFC would on a 3/4 inch 
>rod,
>nylon I think.  


Nylon...YUK!  Delrin not much better either. Use ceramic or hard Teflon.



At the HV supply side of the RFC are two .003 uf caps 
>to
>ground.  Then a commerical RFC, I think a Z-50 type, in series with a 
>.001
>uf cap to gound.  The cap at the bottom of this stack up, the .001 uf, 
>is
>the one that smokes.
>
>I assume the Z-50 type RFC is the second one refered to in some of the
>mail.  With this circuit I do not understand why I should see enough 
>RF to
>heat up the cap.  
>
>Any ideas as to what is going on?  Any suggestions as what I should do 
>to
>get the amp on the air?


Throw away the Z-50 and replace with a 20 Ohm 20W enamel wirewound, you
can leave the .001 value but replace with a 6KV rated disc ceramic. The
original probably garbage by now if it got real smoked. The resistor will
also dampen any funny VHF energy floating around...the Z-50 has been
known to enhance them!

Rewind the RFC per my notes, since it is already on a 3/4" form
(188uh).......just looked it up....3 5/8" #28 Soldereze or similar close
wound.   The Ameritron is physically  wider and longer.
I wonder if those dual .003 are doing any good....the total C is decent
but you may wish to go to a single .01 and a .001 in parallel.

Bring the amp up carefully with only 40-50W of drive and look for any
funny readings which would indicate choke problems. Let me know how you
make out....either way.

73  Carl  KM1H



>Wes
>
>CUL, 73's
>
>WA5TKU                              Phone: 214-654-5296
>Wes Atchison                        FAX: 214-654-5586
>Interphase Corporation
>QTH EM13ii
>
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