B- on RF Amp
 

I was wondering what the advantges are of having B- on RF amplifer
instead of just grounding it? I think it may be that the plate meter
doesn't have a huge voltage across it, that can arc to ground. But what
if you are using a triode and just have a cathode meter, no plate meter
is the B- necessary? Any comments are appreciated.

thanks-Chuck (W3FJJ)

On Mon, 14 Mar 2005 18:37:26 +0000 (UTC),
(Chuck Hanavin) wrote:



I was wondering what the advantges are of having B- on RF amplifer
instead of just grounding it? I think it may be that the plate meter
doesn't have a huge voltage across it, that can arc to ground. But what
if you are using a triode and just have a cathode meter, no plate meter
is the B- necessary? Any comments are appreciated.

thanks-Chuck (W3FJJ)

What kind of circuit? Is the B- applied to the grid? What class of
operation? You mention meters so this is the final stage of a
transmitter?

In article ,
default wrote:
On Mon, 14 Mar 2005 18:37:26 +0000 (UTC),
(Chuck Hanavin) wrote:



I was wondering what the advantges are of having B- on RF amplifer
instead of just grounding it? I think it may be that the plate meter
doesn't have a huge voltage across it, that can arc to ground. But what
if you are using a triode and just have a cathode meter, no plate meter
is the B- necessary? Any comments are appreciated.

thanks-Chuck (W3FJJ)

What kind of circuit? Is the B- applied to the grid? What class of
operation? You mention meters so this is the final stage of a
transmitter?


Class B linear, 3cx3000A7, Can be a final stage or used as
an exciter for 3cx1000000, hi hi, the B- doesn't not go to the grid but to
the centertap of the filimnet tranformer. well throught a cathode DC amp
meter...

On most modern amplifiers schematics they all seem to
use a b- lead which is basically the negative lead of the high voltage
lifted abouve ground by low ohm resister (10 ohms, 25w ). Im sure this is
done for a reason. Im looking for input on way this matters,
thanks-Chuck

Chuck:

Placing the plate current meter in the B+ lead is risky unless the meter is
well insulated from ground and mounted behind a glass or plastic panel so
the operator can't accidently come in contact with high voltage.

Isolating the B- from chassis ground allows the plate current meter to be
placed between the cathode and the power supply's negative terminal. The
meter will then read plate current but won't have the HV B+ on it. A low
value resistor is used between B- and the chassis to keep the voltage
difference between chassis and B- small.

Is your triode amp configured for grounded grid operation? You can also
place a second meter from cathode to chassis ground to read grid current.

If the B- from the power supply is grounded to the chassis and you have a
single meter between cathode and ground, it will read cathode current which
is the sum of plate and grid currents.

73,

Roger K6XQ

On Tue, 15 Mar 2005 15:11:40 +0000 (UTC),
(Chuck Hanavin) wrote:

In article ,
default wrote:
On Mon, 14 Mar 2005 18:37:26 +0000 (UTC),
(Chuck Hanavin) wrote:



I was wondering what the advantges are of having B- on RF amplifer
instead of just grounding it? I think it may be that the plate meter
doesn't have a huge voltage across it, that can arc to ground. But what
if you are using a triode and just have a cathode meter, no plate meter
is the B- necessary? Any comments are appreciated.

thanks-Chuck (W3FJJ)

What kind of circuit? Is the B- applied to the grid? What class of
operation? You mention meters so this is the final stage of a
transmitter?


Class B linear, 3cx3000A7, Can be a final stage or used as
an exciter for 3cx1000000, hi hi, the B- doesn't not go to the grid but to
the centertap of the filimnet tranformer. well throught a cathode DC amp
meter...

On most modern amplifiers schematics they all seem to
use a b- lead which is basically the negative lead of the high voltage
lifted abouve ground by low ohm resister (10 ohms, 25w ). Im sure this is
done for a reason. Im looking for input on way this matters,
thanks-Chuck

I think Roger has your answer. I thought you were seeing a B- power
supply - which is sometimes used in Class C finals for CW ops. What
you are dealing with is just a sampling point to sample current
without the high voltage insulation problem you would see with a B+
inserted current meter.