Re: [Amps] SB-220

[R.Measures <r@somis.org>]

On Nov 3, 2004, at 8:01 AM, 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?

Some have reported that the arc is between an anode connector (usually 
V-2's) and the perforated cover over the output compartment.  My guess 
is that this why V-2 is the tube that typically shorts in a SB-220 
during a big-bang.  Since the arc distance from the anode connectors to 
the metal cover is > triple of what would normally be possible with a 
3000v PS, this is a clue that either some yet-unknown species of tiny 
spider can spin conductive webs -- or that something extraordinary is 
causing the arcs.



> I would like to get it
> working again before making any other changes to improve the parasitic
> oscillation problem.

Doing so on the CW/TUNE tap would be safer since more V = more gain.

good luck, Darryl

> 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

Richard L. Measures, AG6K, 805.386.3734.  www.somis.org

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