Will, that was absolutely NOT what I was saying. Of course, parasitics do
happen, and amplifier designers do install parasitic suppression if their
design proves to be prone to them, The stock SB-220, of course, has
them. My point was that not all SB-220s (with the stock suppressors) will
have parasitics. Mine doesn't. YMMV.
Actually, I don't think the AL-1500 has a parasitic suppressor in the
anode; this is one of Rich's favorite topics, so I'm sure we'll hear about
it now.
As for pops and bangs, the only way I've found that I can make my SB-220 do
those things is to have a fault in an antenna or inadvertently go to full
power with the loading set much too light. That is reproducible any time I
want to, but for obvious reasons I try not to prove the point.
I apologize to everyone for re-triggering this thread, but did feel that it
was important that Darryl be aware that there is another whole school of
thought and that Rich's attribution of everything noisy and theatrical in
amps to parasitic oscillations is controversial.
73, Pete N4ZR
To all,
A quick question here, and may open up a big can of worms, if there is no
such things as parasitics in amplifiers, why do they all come with
parasitic suppressors on the anode leads? Now if I recall, a certain
somebody used to work for Ameritron I do think. So, don't Ameritron amps
all have parasitic suppressors on all their anode leads? If I recall, and
I'm not sure how far back, parasitic suppression has been used in amps for
60+ years that I know of (1940's). I think it was that far back or
further, I can't remember. If I'm wrong on this let me know. Also, I've
seen some amps using two triodes, running in G-G, which have anode
voltage, and a cathode connection, run by their self without parasitic
suppression. After the suppression was installed, this self oscillation
ceased. Maybe it was a one time thing, I'm not sure, I didn't see it tried
again after that. Anyhow, if any or all could answer any of the above, it
would be welcome.
Will Matney
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.
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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