On Nov 3, 2004, at 10:50 AM, Pete Smith wrote:
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.
It certainly is controversial, Pete, because I did not say
"everything", I have had amplifier arcing from antenna tuner caps
flashing over due to too much suds, from dirt falling in through the
holes in the perforated top, and from mysterious arcing from something
that simultaneously incinerated the VHF suppressor resistor that was
paralleled with a few inches of #16 Cu wire.
cheerz
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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Richard L. Measures, AG6K, 805.386.3734. www.somis.org
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