[Amps] SB-220

[R.Measures]

On Nov 3, 2004, at 10:58 AM, Will Matney wrote:

> Steve,
>
> That's the way I always looked at it too. But, and here's a big but, I 
> read a few webpages that says VHF oscillations are a bunch of bologna. 
> These very authors was or still might be high up in the ARRL, I'm not 
> sure, and did work for, or still do, the amp manufacturers. They say 
> that it's impossible and that it couldn't cause arcing in components. 
> Now to me, if the lead(s) length is correct, and the gain high enough, 
> the internal capacitance of the tube along with the inductance of the 
> lead(s) could make up a VHF oscillator. Another thing is that most 
> tank circuits in production amps are hand wired. The lead lengths 
> aren't strictly controlled so they cant tell from one to another if 
> they hit the sweet spot or not!
>
The sticky wicket is that, in a HF amplifier, there is always a VHF 
resonance formed by the anode C, the anode lead, and the Tune C.   This 
would not be a problem if the (grounded) grid resonance was higher in 
frequency than the anode resonance.  Unfortunately, in a 3-500Z, the 
grid resonance is c. 88MHz and the anode resonance is c. 20MHz higher.  
Even though the grid can do a fair job of shielding the cathode/input 
from the anode/output below its resonance, it can not do so above its 
resonance.

> It would look to me that over this many years of using parasitic 
> suppressors, and having found direct formulas for them from a lot of 
> experimentation, anything that says VHF oscillation don't exist would 
> have been caught. As far as I know, the ones saying it doesn't happen 
> has only been in the past few years. I would think that 60+ years is a 
> loooong time to go before catching it? From this, I'd like to hear a 
> good discussion and or debate about it.

A month or so after "Parasitics Revisited" appeared in *QST*, I 
received a telephone call from Charles Thomas Rauch, Jr., W8JI -- 
a.k.a., Tom Rauch.  He wanted to discuss this article.  In regards to 
the SB-220, he stated that he had repaired about 400 SB-220s and that 
many of them exhibited parasitic-oscillation damage. When I mentioned 
that the AL-80 used a parasitic oscillation suppressor design that was 
virtually identical to that used in the SB-220, he began speaking into 
the telephone so loudly that my wife Susan, N6FEY could understand what 
he was saying about 6-feet away from the telephone.
>
> Best & 73's
>
> Will Matney
>
>
>
> Steve Thompson wrote:
>
>> On Wednesday 03 November 2004 18:23, Will Matney wrote:
>>
>>> 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?
>> There certainly are parasitics (unwanted vhf oscillation) - 
>> especially with tubes with long grid leads - but not every amplifier 
>> will have the gain/feedback/load to allow oscillation if suppressors 
>> aren't fitted.
>>
>> It's likely that few amplifier builders have the equipment to 
>> properly assess whether their amplifier needs a supressor, and/or 
>> whether the suppressor they fit actually does the job of reducing the 
>> gain by the right amount at the right frequency.
>>
>> Steve
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>
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Richard L. Measures, AG6K, 805.386.3734.  www.somis.org