>
> 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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