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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