Re: [Amps] SB-220

[Pete Smith <n4zr@contesting.com>]

Just a couple of thoughts:

Not all SB-220s are prone to parasitics.  I suspect that a lot depends on 
how well they were built in the first place.  I have one with Eimac tubes 
in it that has been unconditionally stable since I got it 7 years ago.

I took out the Measures/Harbach parasitic mods and reinstalled stock 
suppressors and picked up ~75 watts on 10 meters.  not a decisive amount, 
but not trivial either.

For what it's worth W8JI -- Rich's nemesis -- recommends getting rid of the 
RF chokes altogether and strapping the grids directly to chassis ground at 
all three pins.  He says that this marginally increases stability and gain.

73, Pete N4ZR

At 11:01 AM 11/3/2004, kwasny wrote:

> > On Nov 1, 2004, at 4:50 PM, kwasny wrote:
> >
> > > I smoked one of 1mH chokes on my sb-220.
> >
> > Darryl -- This is caused by a parasitic oscillation ar c. 110MHz.
> > Replacing the choke with resistors will not prevent vhf oscillations.
> > There's an article about parasites in the 220 on my web site that
> > explains this phenomenon and a method of reducing vhf gain in the 220.
> > .
> >
> > > Also saw some arcing around the meter area. I am going to replace the
> > > chokes with resisters. Have to replace one of the 220pF caps that got
> > > smoked up. Does anyone know what may have arced near the meter area?
> > > Fact is, what made the rf choke smoke?
> >
> > The high grid-I during a vhf parasite creates a lateral EMF on the hot
> > filament helix.  This occasionally bends the filament far enough
> > sideways to short against the grid -- which is grounded through the
> > choke.  Since the filament xfmr is connected to the +110vdc power
> > supply on Rx, it shorts the supply to ground, causing a large current
> > flow-- which smokes the choke.
> >
> > > What other damage might I have done. What is a good source for the
> > > 220pF caps and any other parts that I may need?
> > >
> > > Thanks to all, Darryl
>
> Thanks to all that replied. I am being very cautious before applying power
> to the amp again. I have looked all over the meter area, the band switch
> area and everything else in that area can cannot find any evidence of any
> arcing. I checked the zener diode and it seems OK. I checked the meter board
> and all of the diodes and resisters test OK also. I am still trying to find
> out what might have arced in that area, any ideas? I would like to get it
> working again before making any other changes to improve the parasitic
> oscillation problem. After I get it working I will make the modifications. I
> have been wanting to get the paracitic kit from Harbach but he is not
> accepting any orders at this time, perhaps in a few weeks. I have checked
> the tubes for any shorts from one element to another and both tubes show no
> shorts. I realize that when cold that they may not show up. Any futher
> information or help would be appreciated.
>
> Thanks, Darryl K5WAS
>
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