[AMPS] Blown TL922A... What to do?

Tom Rauch W8JI@contesting.com
Fri, 8 Sep 2000 16:28:32 -0400

From:           	"Steve Thompson" <rfamps@ic24.net>
To:             	<W8JI@contesting.com>, "amps" <amps@contesting.com>
Subject:        	Re: [AMPS] Blown TL922A... What to do?
Date sent:      	Fri, 8 Sep 2000 15:15:55 -0700

> >Tell me how a GDO can be used to measure the mode of
> >resonance? How do you know the impedance is high, instead of
> >low? How do you know what you are measuring in a complex
> >system, since it has no directional ports?
> In order - it can't, you don't, you don't. On the other hand, sometimes
> when I look at a cap which could do service as a tune C, I see more than
> one resonance. If the tune C in a particular amp has similar

I can send you a sample of the most typical capacitor used in HF 
PA's, like SB-220's up through current models.

You can bolt it to a metal sheet that represents the chassis, and 
measure impedance characteristics.

It's very enlightening. It will show you why, especially with proper 
lead connections to the capacitor, it is impossible to have mid to 
upper upper VHF voltages that cause a tank to arc.

> characteristics then dip frequencies from a GDO can be a piece of a jigsaw
> puzzle if it's not possible to do proper measurements

If I was in the business of helping people with advice, especially for 
a number of years and especially if I wrote articles,  I'd have made 
proper measurements.

"Getting by with a guess" is ok for a one-off event, like 
homebrewing an amp....it is a bad idea for verifying theories.

 Proper measurements aren't that difficult to do, I'm amazed so few 
have done them.

 > >I can dip my
> amplifier tank on 1.8 MHz. Does that mean it >oscillates there also? >
> >These are serious questions, don't take them as flip remarks. I'd >like
> to know those answers.
> I've tried to make the replies concise.

I just want to assure you I am picking at a theory, not a person. 
For me this is a technical issue, not personal.

My amplifier tank has a strong resonance at the operating 
frequency does this mean it will oscillate?

We all know resonance is not a condition that indicates a circuit 
will oscillate, or at least I hope we do. With that in mind, it also 
isn't evidence of a "parasitic problem". 

> >OK, so you can't measure a PA...and haven't measured one.
> >Without doing that, on a device that measures impedance of
> >coupling between ports, how do you know all this stability
> >handwaving is true?
> I don't - I'm just gathering data that might help me understand things in
> my own way a little better. Sometimes along the way, the questions might
> help someone else do the same, in the way that you and others here have
> done for me.

Great. I respect that. That's how I learn also.

A person with all the answers who only dictates his way or no way 
is quickly left in the dust...or is it mud?

73, Tom W8JI

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