[Amps] R. MEASURES PRAYERS ANSWERED

[Will Matney]

Colin,

When I was in the service work, and built those with both sweep tubes, large glass bottles, or steel, I seen them act differently from one to another even though they were the same design each time. Sweep tubes especially as they first don't have as tight of QC as broadcasting tubes, plus they have high interelectrode capacitances, etc with high gain. The capacitance and high gain have a lot to do with the oscillation problems. Keep in mind most of the ones I built were grid driven also where the circuit gain is higher yet. Every now and then I would get a stinker though and I would observe how they acted and why. I even found that if tubes were mounted too close together using sweep tubes it contributed to this. Another manufacturer tried to make a 4 tube version like mine using M-2057 tubes which they never could get to operate correctly. It would come into self-oscillation on and off no matter what they did. I used the same design, but spaced the tubes out farther by about 1 inch, and the beast was as stable as it could be. I mentioned this to the owner of the company down in Memphis and he about dropped over what the cure was. With them, they tried to use pre-punched chassis used for several different models. Because of this, and where thy tried to cram as many tubes in a small space, the tube spacing was about 1/2-3/4" apart. I moved them out a good 1 to 1-1/2" and that corrected it. I also found problems in the large glass and steel tubes too. Especially when using multiple tubes. The reason I used parasitic supressors on all was over one thing. What if you had a stinker out of say every 10 that wasn't? The customer would get the amp and you get a bad rap over a squirelly amp. The manufacturer figures that it's better to install them in every amp and not take a chance than to let a stinker go out and cost business.

Concerning the SB-220, I always did think part of the cause was where they mounted the input tuned circuit at the front of the amp close to the output tank (even thought they tried to shield it). Also, they used a lot of coax to connect the antenna relay. To me, the relay should have been right at the coax connectors in the rear. The input tuned circuits should have been between this and the tubes, and be mounted as close as possible. That would have left only one coax run from Load C to the relay. They did the input this way over cost so they didn't have to have a long shaft selector switch going to the input circuits. They could have still put the antenna relay at the coax connectors though, but they mounted the thing in the middle of the amp. Why they did this is beyond me. I was always taught to keep all leads as short as possible, and I think this kind of looked past that rule.

Best,

Will


*********** REPLY SEPARATOR  ***********

On 6/2/06 at 5:47 AM k7fm wrote:

>Rich said:
>
>"The carbon-comp R changes value by an amount not adequately explained
>by simple aging.  For example, a 300% to 500% increase in R in one
>day is not what I would call normal aging.  Also, such resistors show
>no outward indication of extreme age."
>
>Rich's statement implies that the changing of value is due to the dreaded 
>parasitic.  30 years ago, I wanted to use some 2 watt carbon composition 
>resistors in a commercial circuit.  I needed to be able to dissipate 2
>watts 
>on occasion for brief periods of time.  I built a test fixture and thought
>I 
>could cycle the resistor a few times and age it so that the resistor would 
>then be more stable.  What I learned was that if the 2 watt resistor was 
>heated to 1.5 watts, then cooled, it changed value - permanently.  What
>was 
>more surprising was that cycling the resistor again would change the value 
>again - but not necessarily the same direction.  I drew a graph of the 
>changes in value and it was almost circular.  When I terminated the test, 
>the resistor that started out at 300 ohms was at 475 ohms - and still 
>changing.
>
>I suspect the resistor was acting as a hygrometer and probably other 
>things - but I did not really care as the results showed that the 2 watt 
>carbon composition resistor was not suitable for what I needed.  I then 
>tried a Corning Glass film resistor that was repeatedly stable and worked 
>fine at 450 MHz.
>
>I used dc to conduct the original tests.  I thought the results were due
>to 
>the nature of the carbon composition resistor - but it now appears that
>the 
>results may have been due to parasitics in the power supply, which caused 
>the change in value of the resistor.  Boy, how stupid I was when
>conducting 
>those tests.  Never even dawned on me that it could have been parasitics.
>
>Colin  K7FM 
>
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