>
> Recommendation:
>
>
> My experience with several commercial amps has driven home one
>serious design weakness common to most brands. This is severe PA tank
>circuit resonance drift caused by heat instability in the fixed padder
>caps used to tune the tanks down to the low bands (most often 80 and 160
>meters).
? Amen to that, Roy.
>
> Transmitting ceramics simply do not have the stability to be
>incorporated in this demanding application.
? Some transmitting ceramic capacitors are temp. stable. Some are not.
Values over 200pF are usually not temp. stable.
>During "heavy duty" operating,
>the heat given off by the PA tubes in the tank compartment causes violent
>drift in the ceramic caps.
? A quick look at the mfg. specs and some calculations typically
reveals that the RF current rating is grossly being exceeded. The reason
why is semi obvious. A commonly needed Load-C padder for 160m is 1000pF,
so the designer parks his brain and selects a 1000pF, 5000vdc doorknob.
However, there are two gotchas. At 1.8MHz, the current rating of the
cap. is c. 2a, which is typically about 20% of the current encountered in
a 1500w tank. - so naturally the capacitor gets very hot, very fast.
And, the temp. coefficient is "X5U" which roughly translates as only a
bozo uses this sucker in a tuned circuit.
>This in turn detunes the tank, causing more
>plate dissipation to occur which further heats the caps,
? semi-true.
>and so on. In
>other words, a vicious, destructive cycle exists which is absolutely
>objectionable. I believe some heating is also internal to the caps
>themselves, generated by the dielectric in this high RF current service.
>
? agreed
> I have had to replace all such caps in my amps with their stable
>counterparts: namely, xmitting micas. I realize they are hard to find and
>are expensive and take up more space. But they are absolutely
>necessary
? We gotta pay attention to mfg. current ratings on All transmitting
capacitors, whether they be mica or ceramic.
>--unless the variables themselves are "big" enough to tune all
>the way to 160 meters. (Also not cheap, I know).
>
> We are willing to pay more; please refrain from using ceramics in the
>Titan II. And make known this good feature in your advertisements.
>
> Thanks & 73, Roy K6XK
>
? This is seemingly The sticky wicket, Roy. The same amplifier
"experts" who failed to perform the necessary AC circuit analyses (for
whatever reason), who ignored capacitor current ratings, - and didn't
know that K5U means crapola temp. stability - are typically prepared to
argue that they made an "exactly correct" decision, for years if
necessary. . At the risk of boring those who read "The Nearly Perfect
Amplifier",(Jan. 1994 *QST*), I believe that a mfg. who took care of the
laundry-list mentioned in the article - and said so in his advertising -
would quite probably gain a reputation as "the Toyota of amplifier
manufacturers". . . .// Yo, editor Mark and publisher Dave.
When was the last time a *QST* Product Review measured the
filament-voltage and filament inrush current in an amplfier? Has there
even been a first time? . I'll bet a large pizza with five toppings and
salad bar that when *QST* reviews the new 'improved' MFJ .amps. with
RF-actuated Tx/Rx bias switching, that the IMD test is performed with a
2-tone instead of with human speech. .
- I think I'm gonna puke.
Rich...
R. L. Measures, 805-386-3734, AG6K, www.vcnet.com/measures
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