[Amps] 8877 dissp

[John T. M. Lyles]

Yes, as pointed out by others, plate dissipation is only that 
electrode dissipation. And for 70% efficient amp in class B or C for 
FM service, it is only 1100 watts or so. But that leaves little 
margin for error, mistuning, VSWR effects. I would not be running a 
single 8877/3CX1500A7 at 2600 watts output anywhere except maybe 
pulsed or SSB. The transmitter I worked on the design of with Bill 
Harness years ago was 1750 output max, at 88-108.

The 8877 I am running in our lab at this minute is dissipating 600 
watts, and we are sweeping it completely off resonance, about +/- 150 
kHZ around 5 MHz, driving a load with higher Q than the amp. It takes 
the beating no problem, but you can feel a lot of heat in the 
exhaust, about 50 deg C when it is idling and varying as we sweep it. 
I am building an ALC loop right now, to lock incident power at 1 kW 
or whatever we dial up, irregardless of VSWR. I would never attempt 
this with an 8877 running closer to its maxium rating.

BTW, the filament power in 8877 is miniscule compared to anode 
dissipation. So it can be dealt with strictly as a blower issue, 
cooling the base of the tube when lit. When you get to very large 
tubes, like 4CW250,000B tetrodes, you have to do things like leaving 
water running in the anode, even while filament is lit. The 7835 
grounded grid triode that we have in sockets running (4), has 30 kW 
filament power. We do account for this in our efficiency 
measurements. We see it in the calorimeter on the plate water course 
even.

ABSOLUTE MAX is exactly what it says. The 7835 is ABSOLUTE MAX 300 kW 
plate dissipation. I have cracked the tubes open - ceramic rings - at 
270 kW plate dissipation. Now i interlock them at 250 kW through the 
calorimeter setup. Maybe CPI-Eimac is more conservative with its 
ratings..?

73
John
K5PRO


>
>Subject: [Amps] EIMAC Confusion
>
>In the EIMAC Technical Data for the 8877/3cx1500A7
>is is stated in the ABSOLUTE Ratings the Plate Dissipation
>is 1500W. I take this as an ABSOLUTE MAX.
>
>However in the TYPICAL Operation at 88-108MHz USEFUL POWER OUTPUT
>is rated at 2600W.
>
>Is this meaning that the tube is providing 2600W to the load while plate
>dissipation
>is only 1500W? Is this possible ? If so What is then the TOTAL TUBE
>DISSIPATION
>and how is the filament power related to it ?
>
>Mark

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