[Amps] B- questions

[Bill, W6WRT]

On Sun, 8 Nov 2009 00:31:32 -0600, Rob Atkinson <ranchorobbo at gmail.com> wrote:

>I'm not sure of the meaning of "anti-parallel."  do you mean two
>diodes anode to cahode to anode to cathode from one end of the
>resistor to the + side of the meter, or by "back to back" do you mean
>diode cathode to anode to anode to cathode?

Anti-parallel means the two diodes are connected in parallel but with reversed
polarities. The cathode of each is connected to the anode of the other. A simple
parallel connection would have them connected cathode to cathode and anode to
anode. 

>
>Also, what about the diodes must be known in order to choose a drop of
>1/2 volt for the meter/resistor combination?  It seems like there is
>some information there that you left out.  A conduction threshold
>voltage?  it looks like if you reverse the diodes you are dealing with
>PIV which on a 6A10 is 1 kv isn't it?  So I'm obviously confused about
>some details.

All conventional silicon diodes have a conduction threshold of about .5 volts.
Their conduction is not linear however, and as you apply increasing voltage the
current goes up rapidly. Most conventional silicon diodes reach a limit at
around .7 volts and will not go any higher. If you try to apply higher voltages
the current will rise to extreme values and the diode will be destroyed, almost
invariable by becoming a dead short. When I say "conventional" I mean the basic
silicon diode, not Schottky or hot-carrier or other exotic types. Just plain
vanilla silicon.

When two diodes are connected in anti-parallel, the reverse voltage rating is of
no importance. Regardless which polarity of forward voltage is applied to the
pair, one will begin conducting long before the other reaches its reverse
voltage rating. 

>
> I don't think there is any wonderfully safe way, as all three
>power supplies - HV, grid and screen  - are only connected to chassis
>through the G2+ connection. In particular it needs a fully floating HV
>supply with a very well insulated negative side.

I'm not sure what the author is talking about on this. I'd have to see a
schematic.
>
>What is a G2+ connection?

G2 is the screen grid and the + refers to the positive side of the screen grid
supply voltage. 


>Isn't B- far removed from ground potential anyway, usually around -3kv
> +- a few 100 v. in typical ham amp power supplies?  there is clearly
>something I'm missing here also.

No, in nearly all amplifi8ers, the B- line is either connected directly to
ground (in a grid-driven amplifier) or connected to the cathode (in a
grounded-grid amplifier) and the only connection from B- to ground is through
the grid current meter. A few minutes with each schematic should make this
clear. The B- line is normally always within a few volts of chassis ground. I
have seen some specialized instrumentation amplifiers where B+ was grounded and
B- was indeed far from ground, but I have never seen this in ham radio
amplifiers. 

73, Bill W6WRT