[Amps] SB-220

[Will Matney]

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
>>
>> _______________________________________________
>> Amps mailing list
>> Amps at contesting.com
>> http://lists.contesting.com/mailman/listinfo/amps
>
>
> _______________________________________________
> Amps mailing list
> Amps at contesting.com
> http://lists.contesting.com/mailman/listinfo/amps
>
>
> __________ NOD32 1.880 (20040928) Information __________
>
> This message was checked by NOD32 antivirus system.
> http://www.nod32.com
>
>
>