This one is interesting to me for a number of reasons, not the least of
which I spent a lot of time mulling over the different designs. In the end
I do as Bill wrote below, using an ammeter with the appropriate value in
series with the B- lead. The positive pole of the meter is connected to the
B- return from the bias circuit and the negative pole to the negative side
of the HV supply. I use a pair of 5408s antiparallel across the meter as
well as a cap, and it already saved the plate current meter during a
flashover a few years back. Dead diodes but meter was fine.
I float B- above ground with roughly 125 ohms of R, in one case
(separate PSU & RF deck) with 2 separate 250 ohm 25w Rs, one in each
chassis, as well as a 1N5408 & .01 3kv cap on each side as well a la Lunar
Link. The other HB amp has a single 50w 100 ohm R as well as the "float"
diode / cap arrangement (and of course meter protection). Of course using a
series meter of the correct scale makes the float resistor value irrelevant
in that respect. That said, I "thought" floating B- even in this situation
was a matter of safety. Am I missing something?
----- Original Message -----
From: "Bill, W6WRT" <dezrat1242@yahoo.com>
To: <amps@contesting.com>
Sent: Friday, November 06, 2009 10:09 PM
Subject: Re: [Amps] PSU design question
> 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
> _______________________________________________
> Amps mailing list
> Amps@contesting.com
> http://lists.contesting.com/mailman/listinfo/amps
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
|