Grid Bias for Plate modulated Class C amplifiers

[Gito]

Hi


Reading the old TT3 RCA transmitting tube manual. I found out that A plate modulated  Class C amplifier needs/desires  a poor voltage regulation  min Bias.
That's why the old broadcast transmitter used only automatic bias ( using grid leak resistor} with no fixed bias.

It's because the bias must change with modulation in the plate circuit,if linear operation over the entire audio-freqency cycle is to be obtained.

Gito.n

[k4kyv]

Or the driver stage can be modulated about 20-40%, simultaneously and in phase with the final.

[KE6DF]

I wonder how this issue would work out if you used a zero bias tube for the RF final.

You could use, for example, an 805 as a class C final amplifier.

You wouldn't need to use a fixed bias to protect the tube if the zero bias current was low enough to prevent damage in case drive was lost.

You could use grid leak bias to drive it into class C.

[ke7trp]

I found this out recently.  My transmitter runs this way.  The only drawback is that if you loose the exciter Grid, Your plate current will jump.  There are several ways to put safety features in such as RF sensing relay on plate supply. No Drive, No Plate HV.

C

[Gito]

HI

I think all class C operated Tubes, including the Zero bias tube can be operated without fixed bias,only using automatic bias like the Grid Resistor.

The problem is  ,when the exitation is lost ,it's true that zero bias Tubes  draws lest current than the other tubes,that draws a lot of current  can damaged the tubes.

So with this tube we used a combination of fixed  bias and grid resistor bias to provide the desired variation in bias voltage.

The fixed bias is used only to limit the current when there's no drive.in other word tubes that takes little current at zero bias did not need a fixed bias to protect the tube.
Like 805 in class C operation it needs - 170 v grid voltage and 45 ma grid current ,so it needs a  3777 ohm grid leak resistor.
it is different when we used a fixed well regulated min bias at -170 v,without grid leak resistor. than the bias will not vary with plate modulation,and the  linear operation of the entire audio frequency is "lost" with plate modulation

The point is the bias must change with modulation in the plate circuit,the easy  way is to used a grid -leak resistor to developed the bias voltage.


Gito.N

[WD5JKO]

Gito,

Remember this thread?

http://amfone.net/Amforum/index.php?topic=23963.25

My reply at post 29 showed 100% grid leak circuit along with protective bias in case excitation is low, or non existent. Many other ideas from lots of good folks were also presented.

Jim
WD5JKO

[Gito]

Hi

jim You are right,
but truth fully I just know that the grid leak resistor have an important part in the lnearitty of a plate modulated Class C amplifier.

The point is we don't need a well regulated min bias for this Class C amplifier,Contrary of what we think , that the bias must be stiff and well regulated

Yes we need at least the minimum bias ,to make the Class C took  small current when the drive is gone, The smaller the bias and the bigger grid-leak resistor is what We aim(combination of fixed bias and grid leak resistor)

If we read the page of RCA transmitting Tube Manual  that I attached,
It stated  Grid-bias voltage in class C service is not critical.

It also stated A suitable design bias rectifier may also be employed to give a bias with poor regulation desired.

Gito.N