> From: Rich Measures <measures@vc.net>
To: <amps@contesting.com>
> Date: Sun, 3 Aug 97 18:09:58 +0000
> >> - In your opinion, Mr. Rauch, do you feel that a Centralab 5kv 1000pF
> >> Series 58 is adequate for the 77's load C 160m padder?
> >the impedance at the point where the cap is
> >located, ...
>
> The circulating current through the load C and tune C are virtually
> identical and roughly equal to tank-Q*I-anode.
Tank current relates only the the fundamental frequency RF current
driving the tank and the Q. If the operating anode impedance (done by
Chaffe analysis) is 2000 ohms, and RF fundamental power is 2kW,
circulating tank current would be one ampere multiplied by about the
value of tank Q.
But that would be true if anode current was 200 dc amperes, or 1 dc
ampere. It is an impedance and RF power problem, NOT anode current.
Using your figures of anode impedance (which I never checked) a
single 8877 with a tank Q of 12 and anode operating impedance of 2000
ohms at 2000 watts would have somewhere around (but not
exactly) 12 amperes circulating current in the loading cap of the
pi-L. We really need to look at the distributed voltages and
reactance values in the tank to get an exact value.
> >1000 pf 58 series caps are not
> >generally available in low thermal drift,
>
> large value 58-Series capacitors are not available in low thermal drift
> characteristic.
They are available as specials with better temperature
characteristics, but in my opinion not good enough. They should be
NPO, or a very low TC.
> >so they would not be my
> >personal choice for that reason alone.
>
> This issue hardly seems to be a matter of personal choice.
> - There is about 20A of tank RF circulating current in an amplifier
> whose anode current is 2A and whose tank Q is 10.
That would be in the ballpark if anode impedance was 1000 ohms,
RF power was 4 kW, and Q was ten at 4 kW. But just a while ago you
said the Q was 6 at that power. That reduces the tank
circulating current to about 12 amperes.
> The 58-series
> capacitor in the amplifier is rated at 1.4A at 1MHz.
Is that a 500 pF or a 1000 pF? The 1000 pF is 1.6 amperes at 1 MHz,
the 500 is 1.4.
It appears you are using the rating incorrectly. The rated current is
specified for a 30 degree C rise with minimal heat-sinking and no
airflow on the cap, and that is a CCS rating. The maximum
operating temp is 85 degrees C.
The text describing ratings clearly indicates the current can be
increased with shorter duty cycles and better heat dissipation, so
long as the voltage rating is not exceeded.
Current causes heat, so cooling and duty cycle are important and you
ignored both. (Besides getting the currents wrong.)
>Since roughly half
> of the tank circulating current flows in this load-C padder at 1.9MHz
On 1.9 Mhz the 30 degree C rise rating in a non-force air cooled
non-heatsinked application is about 2 amperes for this component.
The following factors improve things:
1.) The allowable operating temp is 85 C, the rated current is
spec'ed at 30 degree rise. The chassis and air around the cap isn't
likely starting at 55 C (131 F), and the chassis heatsinks the cap.
2.) The short term duty cycle is not 100% at full power of 4000
watts. If the short term duty cycle is 50% twice the current would be
available (depending on thermal resistances).
3.) The capacitor current is less than you "estimated".
It seems to me the capacitor could easily be well within its rating.
The limiting factor would be tuning drift, and it would appear LONG
before the rating was exceeded.
>it appears that the amplifier's designer made an engineering
>error.
It's tough to tell since your long drawn-out condemnations
are based on incorrect, misinterpreted, or missing data.
Rich, it seems really silly to spend so much time publishing
incorrect information just to find fault with one single little
$5 doorknob in a $3000 amplifier.
Have there been a rash of field failures in this component? As I
said, my friend's two tube 8877 ETO ran just fine on 160, and I used
it hard for several weeks. He's used it for YEARS and he mainly works
160 and 75. I asked him if he had any problems and he said no, none
at all (except for a metal support arcing to a resistor, that I
repaired).
73, Tom W8JI
(My opinions are my own. I am an independent consultant for several
manufacturers and government agencies. I have no vested interest or control
beyond supplying data and making suggestions. I do receive royalties tied to
the sales or use of certain paten
ted RF
medical devices.)
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