ORIGINAL MESSAGE:
On Fri, 6 Nov 2009 10:55:08 -0800, "Steve Katz" <stevek@jmr.com> wrote:
>
>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
REPLY:
I'll give it a try.
The value of the resistor in the B- supply depends entirely on the type of meter
you use for the cathode current. There are several types.
Probably the most common meter is one that measures the current directly and has
a very low internal resistance. In other words, if you are measuring one amp,
the entire amount of current flows through the meter and there is no external
shunt. With this type of meter, you can connect the B- lead directly to the
negative terminal of the meter and the positive terminal to chassis ground or to
the tube's cathode, whichever design you have.
One caution however: there is no protection for the meter if there is an arc and
the current rises to a large value. The meter could be destroyed. The best way
to protect the meter is to use a pair of heavy duty diodes such as the 6A10,
connected in anti-parallel (back to back with reverse polarity) across the
meter. BUT!! Most meters of this type have a very low internal resistance,
typically less than .1 ohms. and the diodes will not begin to conduct in time to
protect the meter. The solution is to connect a resistor in series with the
meter and connect the diodes across both meter and resistor. The value of the
resistor is chosen so that it and the meter will show a drop of about .5 volts
when the current is about 120-150% of full scale. For example, if the meter is 1
amp full scale and has a resistance of .1 ohm, you would use a ..317 ohm
resistor in series for a total of .417 ohms. .417 ohms will develop a drop of .5
volts at 1.2 amps and the diodes will conduct, shunting any further current
around the meter. Of course, it is not likely you will fine a resistor of
exactly .317 ohms, but you should get as close as you can.
The other type of meter is actually a voltmeter, even though the scale is
calibrated in amps. This type requires an external shunt resistor and the value
depends on the ohms per volt rating of the meter. Simple ohms law will figure
out the value of the shunt, and it should also be diode-protected as above.
One further comment. HV metering should always be done in the negative lead,
never the positive one. Metering the positive lead works, but places the entire
B+ voltage on the meter and this is not good. Some older meters actually have a
metal zero-adjust screw and that screw would have the entire B+ on it, ready for
you to touch! Don't meter positive - negative only.
Hope this helps. More questions?
73, Bill W6WRT
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