[Amps] SB-220

R.Measures r at somis.org
Wed Nov 3 16:28:13 EST 2004 

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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>>
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Richard L. Measures, AG6K, 805.386.3734.  www.somis.org

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