[Amps] SB220 running plate current when no drive; Problem

[Jim Thomson]

Date: Tue, 6 Oct 2020 06:11:38 -0500
From: Rob Atkinson <ranchorobbo at gmail.com>
To: "amps at contesting.com" <amps at contesting.com>
Subject: Re: [Amps] SB220 running plate current when no drive; Problem
found

<3-500s may still be in production but the cost is high compared to
<years ago and quality may be questionable.

<Good tubes in ham service should last for decades.  There are SB220s
<out there with the original Emacs going strong.

<A tube failure is due to one or more of the following:  Poorly made
<tube, improper operation, or poor amplifier design.

<Ameritron has a single 3-500 amplifier they advertise as a 1 KW output
<amp.  Anyone who drives that single tube to that level all the time
<will have to get a new 3-500 every few years.


<Rob
<K5UJ

##  The  ameritron  AL-80B...running at 1000 pep output  with a  single  3-500Z
is  pure fantasy....esp when it only has  aprx  2700-2750  volts loaded   at  550 ma.

##  apparently,  the  550 ma  rating is..the  ICAS   tube rating.   Thats a new one for me.
No mention of any  icas  ratings on any tubes in the  eimac catalog....unless its for  pulse  service,
with extremely low duty cycle, and  typ  10  microsecond long pulses.   Apparently  Eimac approved
of the  icas  ssb  rating,  for ssb use.  On  CW mode, the AL-80B  is rated for  850w  out....running  500 ma.

##  W8JI tested  every brand of  3-500Z...and only one of em  would do 1000 w pep out  with   -35 db   IMD.
## chinese  3-500Z’s, when run at  1 kw pep out  for a single  3-500Z,  typ last  5-6 years max.   Same  chinese
tubes in a SB-220,  L4B  etc,  will last longer. 

##  eimac catalog  depicts  400 ma  for a 3-500Z.   A better way to run the 3-500Z, would be to use more B+.
Back in the 70s  I had a hb  single  3-500Z..and used well over  4 kv  no load.  Then  plate current could be reduced
a  bunch..to get 600-700 watts out.


##  I know  several  folks  who are still using the original  eimac  3-500Zs in their  80s built  SB-220s. 
##  another option is to stuff a 4-400A /  B  /C   into the socket. But they have to be run with zero bias.
Also the grid pins  have to be directly bonded to  chassis.  Grid caps and grid chokes have to be  removed.

Jim   VE7RF