Vic Rosenthal wrote:
>
>In the common circuit used for GG triode amplifiers that separates the B- from
>ground in order to allow for convenient plate and grid current metering, there
>is a 'safety resistor' from B- to ground.
>
>Most designs seem to use a value like 10 or 20 ohms. As G3sek
><http://www.ifwtech.demon.co.uk/g3sek/> points out, if you use the 'voltmeter
>method' to measure grid current, the high value of the shunt resistor requires
>a
>higher value for the safety resistor (he suggests 1000 ohms) to keep the grid
>metering accurate.
>
>My question is: What malfunction is this 'safety resistor' protecting us
>against? What could happen if it were omitted or opened in service?
The resistor is there to keep the B-minus rail somewhere close to ground
(chassis) potential, even if the high voltage supply is operated
separately from the rest of the amplifier.
Without it, the whole rectifier/capacitor stack would be completely
floating with respect to chassis, at voltages determined by unknown
factors such as transformer leakage and inter-winding capacitances. It's
then pure chance whether the insulation around the negative end of the
circuit will hold up. Good design says that you take control of that
situation - hence the resistor.
In an integrated desktop amp, there's probably no significant advantage
in using the resistor, because the B-minus rail is permanently tied down
to the metering resistors and protection diodes.
The article on my web site that Vic mentioned discusses the differences
between the two options for measuring grid current: (a) using a low-
resistance shunt directly in parallel with the grid current meter; or
(b) using a higher-value shunt resistor that will develop about 0.5V at
full-scale, with the meter wired across it as a 0-0.5V "voltmeter".
73 from Ian G3SEK Editor, 'The VHF/UHF DX Book'
'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.demon.co.uk/g3sek
--
FAQ on WWW: http://www.contesting.com/ampsfaq.html
Submissions: amps@contesting.com
Administrative requests: amps-REQUEST@contesting.com
Problems: owner-amps@contesting.com
Search: http://www.contesting.com/km9p/search.htm
|