Re: [Amps] SB-220

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

On Nov 3, 2004, at 9:52 AM, Pete Smith wrote:

> 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.
-  Count yourself fortunate as having 3-500Zs with less than average 
gain.  A friend of mine's TL-922 had a pair of 3-500Zs that had so much 
gain that they were a real pain.  The toasted 922 bandswitch picture in 
the QST article "Parasitics Revisited", was the work of these frisky 
tubes.

> 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

Tom is hardly anybody's nemesis -- except perhaps his own.  His 
persistent insistence on having an unmoderated Newsgroup discussion 
with me on parasites led to a debate which cleared up some of the fuzzy 
aspects of parasitic oscillation that I had when I wrote the magazine 
articles about the subject.  Another helpful person was Wes D. Stewart, 
Jr., N7WS, who made pro-bono measurements
http://www.somis.org/Rp-comp.html.
of various parasitic suppressors' parallel-equivalent resistance (Rp) 
at 50MHz, 100MHz, 150MHz, et cetera with a Hewlett-Packard Model 4191A 
RF Impedance Analyzer.
-  There was also a helpful person in New York City who had previously 
observed the tactics of Mr. Rauch during debates as well as the 
synergistic tactics of his groupies.  The New Yorker's advice to me 
proved to be a valuable asset during the debate.

> -- 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.

This seemed like a pretty logical conclusion until I measured the 
grid-resonance of a SB-220 with stock, grid-bypassing capacitors and 
again with direct grid-grounding.   Stock resonance was c. 88MHz, and 
with direct-grounding it was about 1MHz Lower!  This surprise was 
apparently due to the XC of the grid-byass caps canceling some of the 
XL in the grid structure and the grid header.  In my opinion, 
grid-resonance would have to be well above the extant anode-resonance 
of 110MHz to affect a noticeable  improvement in VHF stability.

cheers, Pete
> 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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Richard L. Measures, AG6K, 805.386.3734.  www.somis.org

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