[Amps] PSU design question

[alexeban]

Actually it's simple: you measure the voltage drop of the plate current
through that resistor.
If the current is in the hundreds of mA range all you need is a couple of
ohms to get a few hundreds of millivolts for the meter. A kiloohm will cause
a voltage drop of a few hundred volts, that substracts from the high
voltage. You wouldn't love that!
Alex	4Z5KS

-----Original Message-----
From: amps-bounces at contesting.com [mailto:amps-bounces at contesting.com] On
Behalf Of Ulf Tjerneld
Sent: Friday, November 06, 2009 8:48 PM
To: amps at contesting.com
Subject: [Amps] PSU design question

Hi all,

There is one thing I still haven't been able to understand; It is the  
question about having B- a little bit above ground. I have now read  
tons of info on the subject and I do get the basic idea about creating  
a point where you can measure plate current in a safe way. But what I  
don't understand is exactly how you determine that potential, i.e. the  
actual value of the resistor. I have seen a lot of designs on the  
Internet and in the literature where this resistor ranges from a few  
couple of ohms up to about a kilo ohm, but I have still not found a  
satisfactory explanation to why this resistor sometimes is only 5 ohms  
and sometimes 1 K? Maybe it is not important to know... why not just  
copy a design from the Internet? Well, I could, but I'm determined to  
learn everything about amplifiers and I also have a profound respect  
for the HV! That is why I want to know exactly what I'm doing!  This  
also leads to the following question: Since my design is a cathode  
grounded tetrode design; do I really need to create the independent B-  
for the sake of metering? I also intend to build the amplifier and the  
PSu in the same chassis, so as I understand it; I don't need B- return  
for the sake of safely connecting HV to the amplifier.

I'd be happy if someone could enlighten me further on this!

73's

Ulf /SM0NOR
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