[hamboy]

I would like someone to explain the following:

In many grounded grid linear Class AB HF amplifiers using triodes the
designers tie a 47k ohm resistor to ground connected to the center tap
of the filament transformer. The rest of the circuit is usually a 5 or
7 volt 10 watt zener diode activiated through a relay on "transmit"
that grounds out the center tap and 47k resistor through the zener
terminating with a low ohm resistor.

I understand the action of the zener diode which drops the filaments to
a lower potential relative to ground - 5 or 7 volts difference - and
allows the tubes to draw idle current. What I don't understand is how
you can get a positive dc reading on the center tap in either standby
or transmit status. In most of these types of circuits there is no
positive connecton with a DC source, only the 47k ohm differential to
ground. ??

hamboy

[robert casey]

hamboy wrote:

I would like someone to explain the following:

In many grounded grid linear Class AB HF amplifiers using triodes the
designers tie a 47k ohm resistor to ground connected to the center tap
of the filament transformer. The rest of the circuit is usually a 5 or
7 volt 10 watt zener diode activiated through a relay on "transmit"
that grounds out the center tap and 47k resistor through the zener
terminating with a low ohm resistor.

I understand the action of the zener diode which drops the filaments to
a lower potential relative to ground - 5 or 7 volts difference - and
allows the tubes to draw idle current. What I don't understand is how
you can get a positive dc reading on the center tap in either standby
or transmit status. In most of these types of circuits there is no
positive connecton with a DC source, only the 47k ohm differential to
ground. ??


If I got it right, idle mode is when the zener is not switched in, So
all the tube's current must pass thru the 47K resistor. That would put
the tube's cathode (here the filament) just enough positive compared to
the grid (which here is grounded) to keep the tube nearly turned off.
When you want to transmit, the zener is switched in, effectively
bypassing (in terms of current) the 47K resistor, making the cathode
only 5 to 7 volts above ground. Which makes the tube "think" that the
grid is biased -5 to -7 volts below the cathode. Which, given a
reasonable B+ voltage, will put the tube in a mode that it draws
significant current and will amplify. Basically, you have switchable
cathode bias.

[Skipp says hello]

Let's see if I can shed some light on your questions. If you put a
resistor across a battery it will draw x-value of current based on the
resistance value of the resistor.

The converse is placing the same value of current through the same
resistor with the same developed voltage (as measured with a common
volt meter).

When you place the resistor in series with the filament center tap
the developed voltage with a low value of cathode current (receive
mode - no signal) is like a battery in series in between the transformer
ct and the ground or negative dc return path (depends on how you wire
the main supply negative return - direct to ground/chassis or through
a metering resistor to ground)

The transformer ct connection is "lifted" above ground/dc-return... the
amount being the voltage developed across the series resistor path to
ground/dc return (with no zero signal idle current through the tube).

I can sort of claim fame to the 47K resistor value often seen in many
of the Dentron, Amp-Supply and home brew construction projects. It was
a value I used in an ARRL dual 811a amplifier project built many decades
back. Without it, the amplifier had a diode hash noise in receive mode
that was hosing my receiver(s). Adding the 47K series resistor developed
enough voltage to shut down the amplifier diode hash. Lower values didn't
develope enough voltage to cut off the tubes.

Some people just break the ct connection with relay contact... a practice
I'm not fond of. I'd rather bring the tube(s) out of a known cut-off
state vs the cathodes floating to some unknown value that will probably
move around a bit.

A lot of people reposted the Dentron cathode current cut-off information I
placed in the public domain. The fly in the soup is how they hacked the
modification to show the relay breaking the cathode path. If you look
at my Dentron GLA-1000 Amplifier Upgrade information at

www.radiowrench.com/sonic

... you'll clearly see the relay contacts in the cathode lead path
simply remove the series resistor by shorting a path across it. A much
better way to go.

cheers,
skipp

: robert casey wrote:
: hamboy wrote:

: I would like someone to explain the following:
:
: In many grounded grid linear Class AB HF amplifiers using triodes the
: designers tie a 47k ohm resistor to ground connected to the center tap
: of the filament transformer. The rest of the circuit is usually a 5 or
: 7 volt 10 watt zener diode activiated through a relay on "transmit"
: that grounds out the center tap and 47k resistor through the zener
: terminating with a low ohm resistor.
:
: I understand the action of the zener diode which drops the filaments to
: a lower potential relative to ground - 5 or 7 volts difference - and
: allows the tubes to draw idle current. What I don't understand is how
: you can get a positive dc reading on the center tap in either standby
: or transmit status. In most of these types of circuits there is no
: positive connecton with a DC source, only the 47k ohm differential to
: ground. ??
:

: If I got it right, idle mode is when the zener is not switched in, So
: all the tube's current must pass thru the 47K resistor. That would put
: the tube's cathode (here the filament) just enough positive compared to
: the grid (which here is grounded) to keep the tube nearly turned off.
: When you want to transmit, the zener is switched in, effectively
: bypassing (in terms of current) the 47K resistor, making the cathode
: only 5 to 7 volts above ground. Which makes the tube "think" that the
: grid is biased -5 to -7 volts below the cathode. Which, given a
: reasonable B+ voltage, will put the tube in a mode that it draws
: significant current and will amplify. Basically, you have switchable
: cathode bias.
: