First of all, you can't trust the dip meter because the frequency will change
when you
couple it to the tank circuit. When I use one I wrap a turn or two of wire
around it and
connect it to a frequency counter. Then when I get a dip I look at the counter.
The best way to adjust a tank circuit is to compute the plate load impedance,
and then
temporarily place a non-inductive resistor of that value from plate to ground
(power OFF,
of course!). Then connect an antenna analyzer to the output and adjust for a
1:1 SWR. That
way you are establishing that the tank can transform the load impedance to 50
ohms.
You will need also to estimate or measure either the inductance or capacity in
the
circuit, including strays, to make sure the Q is in the ballpark. I use a AADE
LC meter to
do this, but you have to isolate the component (L or C) that you are measuring
from the
rest of the network.
If the parasitic suppressors are getting hot, then perhaps the inductance of
the U is too
high, but also I'm guessing that if the network is not set up properly the RF
current
could be higher than expected.
I don't think there's an advantage from making the U out of Nichrome.
On 1/21/2011 8:13 PM, Gary Myers wrote:
> Hello....
>
> I've just completed converting a Heathkit HL2200 amp, stripping all the tank
> out as
> well as the input ckt. I left the existing plate tuning cap in and am using
> it as the
> load cap. The plate tuning cap, now where the load cap was, is a 100pf
> variable. This
> was all down according to an April '08 article on the same. The input is a T
> network
> and it by the way works perfectly with a 1:1 SWR.
>
> The suppressors are as the article says - nichrome ribbon, 1/4" wide formed
> into a U
> with 3 2W carbon resistors in parallel at its base. All soldered using silver
> solder
> for the nichrome. All seemed to go fine.
>
> After installing the tank components I used a dip meter to try and see if the
> tank coil
> was set right. I first confirmed the meter setting by checking it on a radio
> set to
> 50.1Mhz... and then tried to insert its coil into the limited area of the
> tank coil...
> it measured 60Mhz! So I squeezed the turns together - it dropped to 54Mhz...
> I decided
> to leave it as is and wait until I could power it up.
>
> When I first tried to run it I was only able to get about 60W out with about
> 20W drive
> and the plate tune cap was at its minimum setting... I believe this indicates
> that
> indeed the coil is too much inductance (does it not?) and so I pulled the
> turns apart
> as best I could. This allowed me to get about 350W out (with about 840W in)
> with maybe
> 35W drive (I didn't measure this by itself - just going by where I had the
> power set on
> the 746Pro). But very quickly, before I could increase the input, it started
> to smell!
> The carbon resistors were getting VERY hot.
>
> I quickly removed the cover (obviously made sure the HV was 0) and with a
> thermal gun
> checked the temperature of the suppression resistors - one set was at about
> 160 deg and
> the other set about 125 degrees. One resistor in the one set appears to be
> higher than
> the others - but this might just be the difficulty in checking with a thermal
> gun.
>
> So my question is: WHY are these getting hot?! I would have thought, if the
> article
> said use these resistors, that it would work fine. The U nichrome wire is
> almost
> identical to the one they have pictured (mechanical picture for scale) in the
> article.
> Note I couldn't find the 150 ohm resistors they said - I used 3 180 ohm
> resistors - but
> I find it impossible to believe that it would work with 50 ohms eqv and not
> 60 ohms eqv
> nor that the 60 ohm eqv should dissipate MORE power, not less.
>
> I'm also a bit troubled by the efficiency... but until I get this resolved
> I'm not
> going to worry too much about this - however if someone things it is related
> I'd love
> to hear it.
>
> Thanks for your time.
>
> Gary K9RX
>
> [the amp was bought from someone that said it had been converted to 6 meters
> replacing
> the 10 position and the other bands still there... it didn't work on 6 which
> didn't
> make me happy so I decided to rip it all out and just make it a 6 only amp...
> note
> however it would put out about 1150W on 20 meters before the change.]
>
> ps: Also note that when I remove the cover (the HV short is disabled) I get
> 550W out
> with about 800W in but now the load cap is at its maximum value!
>
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--
Vic, K2VCO
Fresno CA
http://www.qsl.net/k2vco/
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