[Heathkit] foldback

Sun, 12 Oct 2003 13:24:51 -0400

Actually,
The problems encountered with the input circuit on "Consumer Amplifiers" of
the SB-220
variety is space and heat, in a nut shell! Now before everyone goes "OFF" I
will try to
explain.

The alignment procedure in the manual is correct to a point, however the
space inside
the cabinet is NOT conducive to good air flow across the input circuit
components, and
they are physically small. When in operation the circuit components heat up
the coils
migrate and change the tuned freq. This leads to the high reflected power
back to the
driver source. Paper tubes are NOT good for coil forms that handle any
appreciable
power, the recommended type is still ceramic forms with good point surface
contact for
heat-sink conduction away from the coil. The caps used in the circuit are
mica caps or
were, some have been replaced with higher voltage disk ceramics, in any case
the best
replacement cap is the CRL 800 series of transmitting caps in the 1000 volt
rating. This
solution would correct the problem, however the owners of the SB-220 units
don't like
to modify the unit in this way.

The "Bottom Line" is that the SB-220 configuration is a compromise due to
the Table
Top packaging of the unit. Bear in mind that the military and government
agencies use
the same configuration 2 X 3-500Z class "B" amplifiers, however they also
spec that it
WILL operate over a certain temperature range, not require depot level
maintenance
to maintain proper operation of the unit, and pretty much run 24-7 with no
problems.
When they say the source will see 50 ohms, that usually means whether it is
in the desert
or the Artic, and they get this "Feature" by using large components with
known temperature rating, adequate cooling arrangements (read air flow) and
over rated items.

You can run the pants off an SB-220 in SSB mode but only for a little while,
after things
heat up and move around, the signals get wide, and trashy, and spurious.

For what it is worth, go here http://eshop1.chem.buffalo.edu/SR-120.html
read the
description and checkout the Eimac spec sheet for the 3-500Z then look at
the components used in the circuitry and note that these rigs run 24/7 at
full power output,
most importantly note the fan and the air flow of the plenum!

My two cents!
Jim
WB2FCN
http://eshop1.chem.buffalo.edu

----- Original Message -----
From: "Jim Brannigan" <[email protected]>
To: "Heathkit Reflector" <[email protected]>; "Mike Hyder --N4NT--"
<[email protected]>
Sent: Friday, October 10, 2003 5:26 PM
Subject: Re: [Heathkit] foldback

> From "Some Notes on the Design and Construction of Grounded-Grid Linear
> Amplifiers"
> QST, Dec. 1970, Page 22
>
> For a "cold" and safe adjust:
> The authors suggest using an Dip meter and impedance bridge (now use an
MFJ
> bridge) Terminate the output of the network in the characteristic
impedance
> of the tube. (120 ohms for a pair of 3-500's) and adjust or substitute
> components as necessary.
>
> The values shown on the Rich Measures site are a good starting point.
>
> Jim
>
> > Carl, if you refer to the following response you gave:
> >
> > ------------------
> > "There were several changes to the input circuit to improve the VSWR.
> These
> > were published in various contest and DX magazines once SS xcvrs became
> > popular.
> > If no one else has something handy let me know later in the week and
I'll
> > try and dig them up."
> > ------------------
> >
> > I did not find that your post contained any helpful information, just
> vague
> > reference to vague publications.  If you made any suggestions to help
the
> > man with his input VSWR problems, they have not shown up here.
> >
> > Because you obviously know more than Measures and because I am in the
> > process of reworking an SB-220, I would like to know your ideas on
> improving
> > the input VSWR of my amplifier.
> >
> > 73, Mike N4NT
> >
> > ----- Original Message -----
> > From: "Carl" <[email protected]>
> > To: "Mike Hyder --N4NT--" <[email protected]>; "Heathkit
> > Reflector" <[email protected]>
> > Sent: Friday, October 10, 2003 11:49 AM
> > Subject: Re: [Heathkit] foldback
> >
> >
> > >
> > >
> > > > Perhaps you could shed some light on the SB-220 input impedance
> problem
> > >
> > > I already did but I guess you didnt bother reading that post.
> > >
> > > > instead of just bashing someone else.
> > >
> > > Trying to present a balanced discussion is now called bashing? Gee and
I
> > > thought the purpose here was to help people and possibly not make
> > expensive
> > > mistakes. Sorry, go on your merry way ostrich style.
> > >
> > > I have found Rich Measures to be of
> > > > much more help than QST.
> > >
> > > He has some good advice.
> > >
> > > >
> > > > And considering most of the stuff in QST, it is probably a high
honor
> > NOT
> > > to
> > > > be published there.
> > >
> > > Obviously you are totally clueless what transpired and are not
> interested
> > in
> > > anything but QST bashing and your own beliefs.
> > >
> > > Enjoy yourself
> > >
> > > Carl
> > >
> > >
> > > >
> > > > 73, Mike N4NT
> > > >
> > > > ----- Original Message -----
> > > > From: "Carl" <[email protected]>
> > > > To: <[email protected]>
> > > > Sent: Tuesday, October 07, 2003 10:37 PM
> > > > Subject: Re: [Heathkit] foldback
> > > >
> > > >
> > > > > You may want to read the series of rebuttals by real engineers in
> QST
> > > > about
> > > > > the Measures articles. And that he has been banned from future
> > > publication
> > > > > there.
> > > > > I consider most of his parasitic theory pure garbage and have
proven
> > it
> > > > time
> > > > > and again to skeptics. This is old news as it took place on the
AMPS
> > > > > reflector several years ago.
> > > > >
> > > > >
> > > > > Carl
> > > > > KM1H
> > > > >
> > > > >
> > > > >
> > > > > ----- Original Message -----
> > > > > From: "David Cook" <[email protected]>
> > > > > To: "'Jim Brannigan'" <[email protected]>;
> > > <[email protected]>
> > > > > Sent: Tuesday, October 07, 2003 9:48 PM
> > > > > Subject: RE: [Heathkit] foldback
> > > > >
> > > > >
> > > > > > Wow, what a great paper! I recently acquired the first amplifier
> > I've
> > > > ever
> > > > > > owned, an SB-200. I put in all the Harbach mods, but still
didn't
> > > > > understand
> > > > > > much about it. Many of Rich's explanations of the design
decisions
> > and
> > > > > > performance considerations for the SB-220 apply to the SB-200.
> > Thanks
> > > > for
> > > > > > posting this link. I'm going to fire off an e-mail of
appreciation
> > to
> > > > Rich
> > > > > > as well.
> > > > > >
> > > > > > 73, Dave, WA�TTN
> > > > > >
> > > > > > > -----Original Message-----
> > > > > > > From: [email protected]
> > > > > > > [mailto:[email protected]]On Behalf Of Jim
> Brannigan
> > > > > > > Sent: Monday, October 06, 2003 2:38 PM
> > > > > > > To: Hotmail; [email protected]
> > > > > > > Subject: Re: [Heathkit] foldback
> > > > > > >
> > > > > > >
> > > > > > > This is Rich Measures' site.  There is a wealth of information
> on
> > > the
> > > > > > > SB-220.
> > > > > > > http://www.somis.org/sb220ci.html
> > > > > > >
> > > > > > > The SB-220 was designed when most exciters were tube based
> > > > > > > with a pi-network
> > > > > > > output.  The pi net output is able to match a wide range of
> > > > > > > loads and the
> > > > > > > input impedance of the amplifier was not a serious design
> > > > > > > consideration.
> > > > > > > Many Solid state exciters are not as forgiving and some
> > > > > > > attention to the
> > > > > > > amplifier input impedance is necessary.
> > > > > > >
> > > > > > > Jim
> > > > > > >
> > > > > > >
> > > > > > > ----- Original Message -----
> > > > > > > From: "Hotmail" <[email protected]>
> > > > > > > To: <[email protected]>
> > > > > > > Sent: Sunday, October 05, 2003 12:10
> > > > > > > Subject: [Heathkit] foldback
> > > > > > >
> > > > > > >
> > > > > > > > I have a great SB220 with the latest Harbauch mods that I
> > > installed.
> > > > > > > >
> > > > > > > > Question:
> > > > > > > >
> > > > > > > > My exciter (TS140S) folds back to 50 watts out on 10
> > > > > > > meters. How do I go
> > > > > > > about tuning the amplifier (input circuit I guess) to
> > > > > > > eliminate the foldback
> > > > > > > issue or is this normal?
> > > > > > > >
> > > > > > > > Thank you,
> > > > > > > >
> > > > > > > > Bruce
> >
> > _______________________________________________
> > List Administrator: Duane Fischer, W8DBF
> > ** For Assistance: [email protected] **
> > $$ See the vintage area on the HCI web site - http://www.w9wze.org $$
> > Heathkit mailing list
> > [email protected]
> > http://mailman.qth.net/mailman/listinfo/heathkit
>
> _______________________________________________
> List Administrator: Duane Fischer, W8DBF
> ** For Assistance: [email protected] **
> $$ See the vintage area on the HCI web site - http://www.w9wze.org $$
> Heathkit mailing list
> [email protected]
> http://mailman.qth.net/mailman/listinfo/heathkit
>
>