[Amps] Alpha 8410 using 4CX1500B - why?

[Jim Thomson]

Date: Wed, 29 Oct 2014 18:45:29 -0400
From: "Carl" <km1h at jeremy.qozzy.com>
To: "Manfred Mornhinweg" <manfred at ludens.cl>, <amps at contesting.com>
Subject: Re: [Amps] Alpha 8410 using 4CX1500B - why?

Welcome to tetrodes Manfred. Very few are designed for very low IMD and 
excessive, IMO, idle current is a way to appease the masses in the ham 
world.

The 600W dissipation Russian GU-74B was fraudulently changed to 800W as the 
4CX800A by the Svetlana management as a way to market it to US hams and amp 
manufacturers.
Original Russian IMD tests from Jan 1993 came up with -15/-24 dB 3rd and 5th 
order respectively in a standard grounded cathode AB1 circuit with the 
control grid swamped with 50 Ohms. Ep was 2200V, idle Ip 150 ma.

After all said and done they settled on a 24 Ohm cathode resistance as NFB, 
a 250 ma idle current, -33.5/-36 dB IMD 3rd/5th and an efficiency of 56%. 
That is 550W of idle power heat which helps explain the revised plate 
dissipation spec.

Various ham manufacturers pushed it even harder with 2500-2600V Ep under 
load and 625W idle power...per tube. No NFB. No wonder they died at an early 
age.

Carl
KM1H

## agreed.  Most commercial ham amps will use 225 ma idle PER TUBE.
That’s a whopping 675 ma for the three hole amps.... which is looney tunes
imo.    Its done to get reasonable imd.   You folks can keep the 4CX-800As. 

## The L4B runs just fine with 100 ma idle @ 2500 vdc.....and 40 ma idle,
when on the 1900 vdc position.   2 x 3-500Zs.   For CW use, you can rune the
idle down to 10-20ma on any GG tube. 

##  dunno why alpha continues to use these stupid tube line ups over the last 40 years.
They already use a single chinese 8877 on their latest auto tune amp.  They shoulda
just made a manual tune version of it, and be done with it. 

Jim   VE7RF