[Amps] SB-220

Wed Nov 3 13:50:53 EST 2004

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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>>
>>
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