[Amps] PSU design question

[pc5m, Carel]

Ulf,

If you use a small valued resistor for the sake of measuring grid current in
a triode grounded grid configuration and as such make sure the voltage drop
over that resistor is smaller as 0.5V under normal operation, you can put
some really BIG diodes parallel to that. Those diodes will make sure that
you NEVER get a floating B-  with respect to chassis ground. Keeping the
voltage drop low is key, otherwise your grid meter is reading
incorrect....and of course floating B- is very lethal :-)

Gl, Carel. 

> -----Original Message-----
> From: amps-bounces at contesting.com [mailto:amps-bounces at contesting.com] On
> Behalf Of Ulf Tjerneld
> Sent: Friday, November 06, 2009 7:48 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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