Hi Gary,
Dick K5AND is exactly right in his analysis that the amp has too much Tune
capacitance. After we wrote the QST article (using an unmarked junkbox
capacitor for the Tune cap) we realized that the capacitor should have been
spec'd as a 5-35 pf unit. That later appeared as a correction in QST, but we
should have caught that in the original article. You don't need very much
C here.
As to the parasitic suppressors, you might try shortening up the U a little
bit, but first get the tank circuit into resonance.
Good luck!
Steve
Steve Gilbert
K1SG
K1SG@AOL.com
In a message dated 1/22/2011 6:21:35 A.M. Eastern Standard Time,
amps-request@contesting.com writes:
Message: 3
Date: Sat, 22 Jan 2011 04:17:41 -0600
From: "Dick Hanson" <dick@dkhanson.com>
Subject: Re: [Amps] Help with HL2200 6 meter conversion
To: "'Gary Myers'" <garymyers@powerc.net>
Cc: amps@contesting.com
Message-ID: <000a01cbba1d$9a604750$cf20d5f0$@com>
Content-Type: text/plain; charset="us-ascii"
Morning, Gary.....
First of all, congrats on your project!
Glad to see more folks out there willing to do conversions etc to get more
db on six.
>From what you've said, and by the choice of your plate tuning cap, you
still
have too much minimum C.
A 100pf variable is likely to have a minimum C of around 10-15pf, which is
just too much.
If you have a means to check the cap, do so, and I'll wager that you'll
find
your lowest reading (installed in the amp) is simply too much.
There are a number of physically smaller caps available that are made by
the
same mfger of your caps now that would be a much better choice.
For example, the Cardwell 154-11-1 is a good choice. It has a max C of
about
45pf.
Remember, your tubes contribute 10-12pf each to the plate C.
I would think that your plate tune C, at resonance, will be only 7-10pf
assuming a loaded Q of 12 or so.
The Cardwell unit can be made to go lower on minimum C by removing a couple
of rotor plates.
I have used this cap in numerous six meter amps with excellent results.
These caps sell at Dayton for $20 on average.
My suggestion is to get the plate tank resonant first and then fiddle with
the parasitic chokes.
You may well find that what you have presently is fine once you get your
tank resonant and the efficiency up.
You haven't mentioned any changes to the filament choke; have you removed
some of the turns on this choke?
Also, have you changed the plate RFC?
I'm sure Carl and some others who do these conversions for a living will
chime in after a bit, so you will get some more help on this.
I've got a number of those Cardwell caps should you decide to pursue a
physically smaller and electrically smaller plate tune cap.
Good luck,
Dick, K5AND
Austin, TX
> -----Original Message-----
> From: amps-bounces@contesting.com [mailto:amps-bounces@contesting.com]
> On Behalf Of Gary Myers
> Sent: Friday, January 21, 2011 10:14 PM
> To: Amps
> Subject: Re: [Amps] Help with HL2200 6 meter conversion
>
> 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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