[Amps] FW: AMERITRON AL-1500 ????

[R.Measures]

On Aug 7, 2005, at 12:31 PM, Joe Subich, W4TV wrote:

>
>
>>> 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 ...

Chortle.  A finger slip off of the G key might produce an F or  T or Y 
or H or B or a V, but an S seems like a bit of a Stretch.
>
>>> 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?

Because that the grid-I under "Typical Operation", Joe.
.
> 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?

Please identify and quote the document.
>
> 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.

The grid dissipation rating is not in peak watts.
>
> 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.

Maximum ratings are not found on the constant current graphs.
>
> Your grid current recommendations would result in a very "wide"
> signal

True

> and premature tube failure ...

Not quite, Joe, just lots of rotten splatter.
Few of us appreciate the current carrying ability of the 8877 grid.  
For example, at it's rated f-max of 250MHz, with a 4kV PS, the 8877's 
grid must be able to carry over 40 Amperes of current from the 10pF of 
C to the anode.
(2670V-rms across 10pF @ 250MHz = 2670v / 65-ohms = 41.07A-rms)
>
> 73,
>
>    ... Joe, W4TV
>
>
>
>

Richard L. Measures, AG6K, 805.386.3734.  www.somis.org