[Amps] input matching question on the YC156?

[Leigh S. Jones]

Neil

I can't imagine that this has gone on all day and no one has told you.
I may get a little heavy handed here.  Sorry in advance.  Applying
drive power without pre-heating the filaments is not allowed for this
tube.  I'm not an Eimac employee, but I have read the 3CPX5000A7
data sheet.

Two simple rules:

rule 1: never apply plate voltage to the tube nor apply drive power
until after the prescribed filament warm-up period has elapsed

rule 2: never apply drive power until plate voltage has been applied

details:

1) apply forced air cooling
2) apply filament voltage
3) delay according to tube manufacturers instructions
4) apply plate voltage
5) connect antenna switch
6) allow cathode current (bias to operating level)
7) apply drive power to transmit
8) stop applying drive power
9) bias to cut-off cathode current
10) disconnect antenna switch
11) repeat steps 5-10 in order as many times as needed
12) remove plate voltage
13) remove filament voltage
14) delay according to tube manufacturers instructions
15) remove forced air cooling

Sorry if this sounds like me preaching to you, but someone has
to tell you.

KR6X

----- Original Message ----- 
From: "Neil Carr G0JHC" <g0jhc at blueyonder.co.uk>
To: <amps at contesting.com>
Sent: Thursday, September 16, 2004 8:16 AM
Subject: [Amps] input matching question on the YC156?


I'm currently playing around with a YC156 and have and have discovered
an
interesting phenomena which I'm having difficultly explaining.

On previous amps  I have never really bothered tuning the  i/p
matching
until I've hit it with RF (I build single band amps, so its quite easy
to
guess).


 Now I have a MFJ I'm able to "play a little" and do a cold test setup
first.


I set my input matching up for 1:0 SWR with the MFJ, at this time the
heaters are not on. Soon as I switch them on SWR shoots up to 2:1,
then over
a 6 minute period gradually to 4:1.

So I retuned for 1:0 SWR once things had settled down, after 6
minutes, and
it stays rock solid all day long.

Of course in the real world I'm not going to ever engage the i/p
circuit
until minute 6 of warm up with this tube, at which time the i/p SWR is
always now 1:0, but I'm puzzled in what may be happening?

Has anyone else ever experienced  this with either the YC156 or any
other
tube?

This is how its set up now.


Heaters off   swr = 4:1 on i/p match

Switch heaters on, swr immediately goes to 3:1

After min 1 swr = 3:0

After min 2 swr = 2.9

After min 3 swr = 2.7

After  min 4 swr = 1.8

After min 5 swr 1.3

After min 6 swr 1.0 ..and stays there until the heaters go off.



This to me looks like a thermal "thing"  due to the large size of the
heaters on a YC156 and heat expansion.


What do others think?


Neil G0JHC







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