[Amps] 3-500Z filament voltage and current

Thu Apr 10 20:42:33 EDT 2008

Larry:

I recently built an amp using two of the 3-400Z Eimac tubes.  While in the
process, I realized the new old stock Stancor 5 VAC 30 amp filament
transformer provided 4.66 VAC under load, with a 118VAC supply line.   So, I
started hunting down answers to the same questions you appear to be dealing
with.  

Right or wrong, I measured the voltage at the socket contacts, not the
actual tube pin.  I found a series of e-mails in amps history files on
filament voltage, with some interesting comments.  As my memory recalls,
several were on commercially sold linear's that had high filament voltage,
one found to be 5.7 VAC on 3-500Z's.  That will ruin tube life.

I would suggest reading the Eimac Bulletin AP-18.  You can get your own from
their web site, try http://www.cpii.com/docs/related/22/AB18.pdf

Simply getting to the point quickly, I highly recommend giving the tube the
recommended filament voltage.  Dial it in to the 5 volt area, nothing too
high or too low.  I put a boost buck transformer in my primary circuit, then
also added a 25 watt 15 ohm ceramic pot.  I adjusted the pot to 5.0 volts on
the socket contacts, and checked the voltage over a two hour period and
found little if any change.  That is how my amp runs now.  

Just compare the cost of a little boost buck transformer and a Hamfest pot
to the replacement cost of the tubes, I think I would get the voltage right
as fast as possible.  Look at the graphs showing tube life vs. filament
voltage included in the Eimac AP-18 Bulletin, that may push you real fast
towards getting the voltage right on the money.  AP-18 turned out to be
quite a help for me.

I also use a step start circuit on the filament primary,  I feel that is
another very simple and helpful item to increase tube life.  I did not make
any accurate measurements, I simply added the resistance, I don't remember
how much, I think it was about 40-50 ohms to the primary and got a nice slow
warm-up for about 4-6 seconds, then the relay picks and the tubes go to full
voltage.  It gave about 2.9-3.1 volts to the tubes.

Good Luck,

Chuck  WA9POU

-----Original Message-----
From: amps-bounces at contesting.com [mailto:amps-bounces at contesting.com] On
Behalf Of Larry
Sent: Thursday, April 10, 2008 4:54 PM
To: AMPS
Subject: [Amps] 3-500Z filament voltage and current

Question for those with more experience in this particular area than I have:

The Eimac spec sheet sez 5.0 +/- 0.25 volts measured at the socket. Do 
they really mean the socket contacts or the tube pins? I see about 0.1 
volt total drop across the pin to socket interfaces using the Eimac 
SK-410 recommended sockets. How critical is the lower voltage limit? I 
have always managed to get 5.0 at the tube pins in the past but this 
time around I'm having fits just getting to 4.8 on the tube pins. If 
this were a real tube like a 3CX6000 or the like, I'd have more control 
by design but it really seems like more than it's worth to put in a 
boost transformer at this point in the 3-500Z project. Comments?

Eimac cautions against inrush currents exceeding 2X the normal filament 
current. I have a box full of tubes with grid to filament shorts that 
attests to the accuracy of that statement. In the past I have always 
used the typical resistor in series with the filament transformer with a 
time delayed relay to short it out after a couple seconds. I used a 
resistor value suggested by others and never actually checked the inrush 
current. I have never had a tube failure after installing such inrush 
limiting in all my other amps. This time around I made the resistor 
adjustable and measured the current. I use a clamp on ammeter with an 
analog meter and am under no illusions about it's ability to accurately 
capture the peak current. My goal was to set up the resistor to minimize 
the inrush current at turn on while also keeping it as low as possible 
when the resistor was shorted out.

Turns out that the best I could do was 45 amps (two tubes) shown on the 
ammeter at both turn on and when the resistor shorts out 2 seconds 
later. I'd guess that the 45 amps I see on the meter is really at least 
60. If I increase the resistor value, the initial turn on current is 
lower, but the peak current is higher when it is shorted out. Like wise, 
I can decrease the resistor value and increase the initial turn on 
current and the shorted out peak current will drop. If I had known it 
was this close, I would have made the step start a three step process 
instead of a two step. Too late now, as the controller board is already 
built and installed. Experience would suggest that this is good enough 
and I guess I could verify if I had a current transformer in order to 
capture the actual peak on the scope, but I don't, so can't. Any comments?

73, Larry
-- 
Larry - W7IUV
DN07dg
http://w7iuv.com
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