[Heathkit] sb200/220 fan

Sat, 30 Nov 2002 11:10:56 -0500

Rich and I have a serious disagreement with what causes some events.
I, for one, do not believe in ghosts nor magic parasitics that manifest
themselves particularly when the tubes are in full cut-off.

A. Tubes like the 3-500Z WILL arc internally from outgassing, this has been
known for decades. Heath decided to eliminate HV surge suppression for
whatever reason. At least the grid chokes offer some protection.

B. SB220 bandswitches arc primarily because of the shorted turn effect.
During tune up it is possible to generate VERY high voltages in the tank
circuit, the Tune cap and switch arc as a result.
See your NCL-2000 schematic for the proper way to mitigate that effect.

C. You will find VHF resonances in any amp. The internal tube structure is
the biggest one and the parasitic suppressor is designed for that purpose.
The L4B uses a monster resistor that will survive. Amps that use just a
single 2W carbon resistor are asking for eventual problems as the resistor
changes value.
Other VHF resonances are found in the tank circuit, primary plate choke and
antenna safety choke. The latter 2 are harmless. Its when you have 2 chokes
in the plate circuit connected by a length of wire that problems begin.
Funny looking magic parasitic cure-alls do nothing except generate revenue.;
they address the symptom and not the cause.

Carl
National Radio 1963-69
Part of NCL2000 Design Team

I currently offer repair and modification services to any amp, HF thru
Microwave

----- Original Message -----
From: "Jim Shorney" <[email protected]>
To: <[email protected]>
Sent: Friday, November 29, 2002 9:12 PM
Subject: Re: [Heathkit] sb200/220 fan

> -----BEGIN PGP SIGNED MESSAGE-----
>
> On Fri, 29 Nov 2002 20:35:45 -0500, Carl Huether wrote:
>
> >There are all kinds of mods I recommend for the 220 series, I wrote them
up
> >some years ago and they made the rounds of several reflectors.
Unfortunately
> >i didnt save a copy; something to do on a cold winter night.
> >
> >One easy mod is to replace the VHF choke, RFC2, with a 10-12 Ohm 20W
> >wirewound resistor; NOT cement.
> >This will limit arc current to a safe value. That choke is series
resonant
> >VERY close to the natural parasitic frequency of the tube and contributes
to
> >problems.
> >It is perfectly natural for those tubes to outgas and arc occassionally;
if
> >its controlled it wont cause damage.
>
>
> Rich Measures has an impressive amount of info on his website about the
> SB220, and parasitics in amplifiers in general.  For the record, I
> don't own an SB220; I've got an L4B and an NCL2000 in the house.  I
> read his stuff on the SB220 with great interest, though, for
> cross-comparison with the L4B.  I did follow his suggestion of checking
> for VHF resonances in the L4B anode circuit with a dip meter, and found
> a couple; one was quite strong.
>
> - --
> Jim Shorney      -->.<--Put complaints in this box
> [email protected]
> [email protected]
> Ham Radio NU0C
> Lincoln, NE, USA
> EN10PT
> http://incolor.inetnebr.com/jshorney/
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