Date: Sun, 11 Apr 2010 21:45:35 -0700
From: "Bill, W6WRT" <dezrat1242@yahoo.com>
Subject: Re: [Amps] A tale of two IMs What happens?
ORIGINAL MESSAGE:
On Sun, 11 Apr 2010 14:45:24 -0400, Roger <sub1@rogerhalstead.com>
wrote:
>
>No one has touched the question on how reducing power without returning
>affects IM I now have another one.
REPLY:
Let me take a guess on this one. I have not measured the change in IMD
with reduced power, but I think this makes sense:
Most amps that we use are biased for class AB1 or AB2. With full drive
they have a certain IMD. As you reduce the drive you move closer to
class A. At a certain low level, you actually are operating class A.
It seems to me therefore, that the answer to your question is that
reducing drive does indeed reduce IMD. Hopefully someone here has the
equipment to make accurate measurements and can provide a conclusive
answer.
73, Bill W6WRT
## I'd agree with this too Bill. But actual tests would need to be run, to
verify it.
A stock Drake L4B runs zero bias... and with Eimac 3-500z's [ Mu=130].. and with
2650 vdc no load... idle current is 230 ma.. which is a bunch. When amp is
tuned for
normal operation.. it runs at 2500 vdc @ 800 ma. 230/800 ma = 28.75%
Now if the amp is only driven to say... 600 ma..... then it becomes 230/600ma=
38.34%
If you really wanted to reduce power, reducing drive level is not the way to do
it. The plate
load Z goes way up.. and tank eff goes down. You are better off to reduce
plate voltage
a bit.. and also reduce drive /plate current.... such that the plate load Z
remains constant.
I also noticed that the gain of the tube is really high, with very low drive
levels... and gain keeps
dropping down.. as drive is further increased. [ anp ret-tuned in each case]
. A friend tried his
5 watt qrp rig, driving his 4-1000 amp back in the 80's.. and remarked how
he got an easy
175w from the 4-1000 [ 15.44 db gain... tuned for max out, with 5w pep drive]
Looking at new and old Eimac notes... the 3-500Z seems to have best imd, when
run at
400ma [per tube].. and at reduced B+ vs really high B+. I'd take that
with a grain of salt though.
Later specs depict zero bias [240 ma] @ 2500 vdc... then -10vdc bias @ 3
kv...... then -15vdc bias
@ 3.5 kv....... and all 3 x had good imd. In no specs, do I see the tube
being run at low plate current
levels, for a given B+ [ high plate load Z] .
Apparently, 1 x tube running at say 600w out.... vs 2 x tubes @ 1200 w
out..... with both configs
using same B+ value.... and the 1 x tube amp running 400 ma..... vs 800ma for
the 2 x tube amp,
the 2 x tube amp will [on paper] have slightly worse imd. From the tech notes,
and theory, multiple
tube amps will have progressively slightly higher imd. To test that theory,
one would have to compare
1-2-3-4 x 8877's. My guess is, not much diff, [if any] maybe 1-2 db..
between a 1 and a 2 x tube amp.
if you require more than 2 x tubes to do the job, you really need a bigger
tube. Nobody puts 2-4 x engines
under the hood of a car.
Later... Jim VE7RF
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
|