[AMPS] AL 811 Parasitic Choke Problem

[Rich Measures]

>
>  Hi,
>  I have a friend who has problems with his AL-811 and the anode
>parasitic chokes on 28 mhz.  He doesn't have Internet access so I'll
>have to mail any answers to England.  I sold my AL-811 to fund a bigger
>amp purchase so I can't even help him with the resistor  values.  His
>letter:
>
>" All works very well on all bands except on 28 MHz (10m).  On this band
>alone I find that if I  tune up to produce even around 300 watts out,
>the resistors in the anode parasitic chokes overheat.  I burnt out the
>first set, George.  I have installed a second set made by a guy over
>here who is marketing a linear amp somewhat similar to the Ameritron
>linear using four 811 tubes.  However even with this guy's parasitic
>chokes the resistors get to hot and I smell burning unless I limit power
>out to around 200w on 28 MHz.  

VHF parasite suppressors that do not get hot at 28MHz are probably not 
very resistive at VHF.  

>His chokes consist of three 220 ohm 1
>watt resistors in parallel with about 5 or 6 turns of say 16swg wire
>around them.

When the ohms of the VHF suppressor R is about the same number of ohms of 
reactance that which the suppressor inductor exhibits at the anode's VHF 
resonance, the desired stagger-tuning effect of the VHF suppressor is 
optimal.  .  
 To achieve this result:
1.  Use a dipmeter to measure the amplifier's VHF anode-circuit resonance 
on the lead from either end of the DC blocking capacitor.  Typical values 
are in the 100MHz area.  
2.  Measure the inductance of the suppressor-L  and calculate its 
reactance in ohms at the anode-circuit's resonance.  
3.  The resistor's ohms should be similar to the reactive ohms of the 
suppressor-L.  

note:  Film-resistors are much more hardy than carbon-comp. resistors.  
However, there are two caviats -- a film-resistor should not be wound 
with wire because the resistive film is under the paint.  Better to wind 
an air-wound L and solder it in parallel with the suppressor R, allowing 
an insulating air-gap of 2 of so mm.  .  The 2nd caveat is that some 
film-resistors are too-inductive to be used in VHF suppressor service.  
It is my opinion that a suppressor R should ideally have no more than 20% 
of the inductance of the suppressor-L.  .  For those who do not own a 
Z-analyzer, a fair estimate can be made by scraping the paint off a 
film-resistor and having a look.  If the resistive film is cut into a 
one-turn or less spiral from end to end, the inductance is probably 
acceptable.  

>  I wonder if you have had this problem on 28 MHz with your Ameritron,
>George? (I never had any problem with mine, George) Perhaps you know of
>a solution.  One old friend over here who runs a home-brew linear with a
>pair of 813 tubes became so fed up with the parsitic choke resisotrs
>burning out (similarily only on 28 MHz) that he removed them altogether
>and simply makes the connection with lengths of coax shield wire.
>  Apart from this one frustaration in having to keep the power down to
>about 200w on 28 MHz only, I am very pleased with the Ameritron. I do
>find the manual very basdic and would welcome on with more information.
>For instance what value are these resistors in the anode parasitic
>chokes?  My original ones were so beaten up that I cannot be sure but
>thy look like 200 ohm 2 watt.  If so, then 200 ohm seems quite a high
>value to me for this job.  Perhaps you would let me know the value of
>these resistors if you can, George.
>PS I had no such problem with my previous linear running two 3-5000z
>tubes."
>
>Thanks in advance for any help, I won't be able to check for any answers
>until Sunday, so won't be able to supply any additional information
>until then.
>73, George / KF9YR



Rich...

R. L. Measures, 805-386-3734, AG6K, www.vcnet.com/measures  


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