> > There will be some who claim that the amplifier is unstable -
> > far from it. If Ameritron have not changed the design and it
> > is built with good, low impedance ground on the screen ,
>
> 8877 screen?
Slip of the fingers ...
> > A significant problem with the 8877 is the ability to easily
> > overdrive the tube. An 8877 takes about 60 watts of drive to
> > reach 1500 watts out. Your FT-1000MP/MKV will drive the pants
> > off the tube! Be very careful about drive levels and tuning
> > to keep the grid current below about 60 mA ... otherwise the
> > tube is in danger (Eimac recommends 100 mA max for the grid).
>
> Call for reference.
> - In its early 8877 spec sheet, Eimac recommends not
> exceeding 25W of grid dissipation. At a cathode-grid drive
> level of 70V-rms, this represents about 350mA of DC grid-I.
> In other spec sheets, the grid-P is rated at 20W max, which
> represents c. 285mA of grid-I.
If the grid is capable of safely handling 350 mA, why does Eimac
say 74 mA single tone and 25 mA two tone grid current at 3500 Vp?
Why do they say 10 mA with 2500 Vp at 220 MHz? Why do Eimac
recommend a grid trip circuit with a 100 mA trip point?
By the way, Eimac lists a peak driving power of 64 watts for
for 2 KW out. Under those conditions the single tone DC grid
current is 74 mA and the peak RF drive is 81 volts. Assuming
the RF drive is sinusoidal the peak current will be 1.4 times
the average value or 100 mA. At lower levels of drive the
grid current will be less ... so figure a 3:1 or even 4:1
peak to average.
According to the constant current charts, with 81 volts of RF
(-81 V on the cathode) and 3500 volts on the plate, the grid
current should not exceed 50 mA.
Your grid current recommendations would result in a very "wide"
signal and premature tube failure ...
73,
... Joe, W4TV
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