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 GROUNDING THE GRIDS IN A CATHODE DRIVEN AMPLIFIER

Here are some things to think about before you decide what grid configuration to use for your cathode driven amplifier using 811A or 572B tubes. 

 

Option1:  Directly ground the grids.

 

Directly grounding the grids in an 811A does one thing with certainty.  The control grid connection to ground will be inductive  (.060uH) regardless of how well it is connected to ground, you simply can’t change it.  Using the shortest widest strap available will add only few nh of inductance and a wire slightly more but the inductance will never be less than .060uH.  It is simply fixed at .060uH because of the internal grid lead length.  Calculate it for yourself and you will find the 2 inch lead in the tube from pin to grid connection plus 1 inch to rid center .020uH will be approximately .060uH --- .020uH per inch.  You’re trapped with a parasitic magnet!

  

A DIRECT GROUND IS AN INDUCTIVELY FLOATED GROUND --- THINK!

 

Option 2:  Float the grids with a capacitor.

 

A bit of ingenuity can fix the inductive grid lead problem.  A 220pf capacitor was selected to connect the grid to ground for three reasons:  To change the grid connection from inductive to capacitive, provide a measure of negative feedback, and to neutralize the tube.

 

Calculate the series resonant point of a 220pf capacitor (With 1/2" leads .030uH plus .060nH inductor for grid lead.  It will be somewhere around 34MHz --- what is the benefit?  The connection will progressively increase in capacity until it reaches 34MHz where it becomes a perfect zero j ohm ground, then it gradually becomes inductive; but never as inductive as when directly grounded.

 

A common mistake is using more capacity i.e. a .01uf cap because you think it will be a better shunt to ground; this is not well thought out.  The series resonant point of a .01uf capacitor and .090nH inductance is around 5.3MHz.  It is a great shunt to ground at 5.3MHz; better than a directly grounded grid BUT, it becomes inductive at frequencies above 5.3MHz. 

  

The connection using a 220pf cap will remain capacitive up to 34MHz where it is series resonant providing a perfect ground again better than a directly grounded grid. 

 

Using a class 3 disc ceramic capacitor in that circuit is a poor design practice.  A suitable capacitor would be a class 1 ceramic, Mica, or Polypropylene which would be fine because they are stable caps.  The class3 ceramics i.e. the Z5U are incredibly unstable with capacity varying wildly with voltage demonstrating significant Hysteresis effects.

 

The 220 pf cap that you find in many amps was not randomly selected.  It was specifically selected by Bruene for the 30S1 4CX1000A to provide an approximately 40/1 ratio of negative feedback.  The voltage divider bridge in the 30S1 is composed of C103, a 5pf cap, connecting the plate to the control grid and C104, a 200pf capacitor, connecting the grid to ground.  The added 5pf cap connecting the plate to CG in the 30S1 is necessary because the screen is RF grounded leaving only the internal .015pf capacity between the plate and control grid to generate negative feedback.

 

A 220pf capacitor was used in the 30L1 and a plethora of other 811A amplifiers for the grid to ground half of the voltage divider and the 5pf to 6pf inter electrode plate to grid capacity for the plate to grid half; a convenient solution. 

 

Negative feedback:  

 

The feedback ratio was selected to improve linearity and to increase the drive requirement to a more natural level for use with the 32S and KWM series radios as well as other exciters.

 

The correct feedback voltage divider bridge design is from plate to grid to ground which is a metric sense of the tubes linearity, NOT from cathode to grid to ground which has been distributed for decades.

 

The following is a quote from the 30S1 design notes written by Warren.

 

“A fixed amount of r-f negative feedback, from the output circuit of the power amplifier to the input of the power amplifier, produces a high degree of linearity of the amplified signal. This feedback is accomplished by capacitor 0103, which couples some of the plate energy back to the grid circuit. Although there is no phase inversion between the cathode and the plate circuits of a cathode-driven amplifier, there is a phase inversion between the cathode and the grid circuit, providing the grid is not bypassed completely at the r-f frequency. Therefore, the feedback voltage is out of phase with the grid voltage. Capacitors 0103 and 0104 form a voltage divider circuit to maintain the proper amount of feedback voltage.”

 

There has been an ongoing argument that grid current disturbs the cathode to grid to ground bridge circuit in the 811A amplifiers. The problem with that theory is the bridge is not cathode to grid to ground.

  

The author of that theory simply misinterpreted the original design.  He apparently thought that because the screen in the 30S1 was at RF ground, the plate could not participate in the feedback bridge.  What he overlooked is, the 30S1 has a 5pf capacitor connecting the control grid to plate shunting around the RF grounded screen. The same error was used to condemn the circuit for the 811A amplifier.

 

In the 811A amps the 5pf plate to grid capacity is used in place of the 5pf cap added in the 30S1.        

There’s more detail to this design regarding the effects that negative feedback has on neutralizing and parasitics in cathode driven amplifiers.

     

I am an advocate of neutralizing any RF amplifier, not just for stability but linearity.  The lack of neutralization in cathode driven amplifiers has been linked to instability and oscillation.  Think about this.  In order for a grounded grid amplifier to oscillate the feedback fraction from plate to cathode multiplied by the amplification factor will have to be unity or better.  The use of a 220pf capacitor to ground the grid reduces the amplification factor significantly.  It is easy to notice that it requires notably more drive to reach the same power level when the capacitor to ground is used rather than direct grounding.  Is it not common sense to conclude the capacitor will reduce the risk of instability?  Some will argue that the capacitor reduces the effectiveness of the grid shielding effect.   A directly grounded grid is an inductive connection.  Which do you prefer capacitive or inductive?

  

Last but imperative thought:  Get your calculator out and calculate the feedback from plate to grid using the 1/40 ratio of the voltage divider.  You will find it to be approximately 22 volts of negative feedback.

Now calculate the voltage feedback from plate to cathode using the same plate P to P level of 1300V, (.7pf + .1pf for grid to cathode coupling), and across an approximately 100 ohm load (The cathode).  It will be approximately 21 volts.  A little bit of logic.  If you apply the positive feedback to the cathode and the SAME voltage to the grid, does that not equalize the grid cathode effect???  THE VOLTAGE DIVIDER CONSISTING OF THE 220pf CAPACITOR FROM GRID TO GROUND AND 5pf PLATE TO GRID INTER ELECTRODE CAPACITY NEUTRALIZED THE TUBE. 

  

Oscillation in a grounded grid amplifier will occur at the frequency where the plate and cathode circuits are both resonant, not just below the resonant frequency where grid driven amplifiers oscillate.

  

There is no recorded evidence that suggests any grounded grid amp has been found to oscillate at the frequency where the input and output both were tuned to the same frequency.

 

I hope this information will help you to decide for yourself with confidence, in a common sense way, exactly what grid configuration to select for your amplifier.  I’m certain that most tube failures in 811A amps using more than one tube are not connected to neutralization. It is obvious that the flagrant dismissal of the parameters defined in the RCA 811A data sheets is at work here.  The data sheets specifically state when more than one tube is used they should be matched and the RCA handbook suggests using per tube bias adjustment.  Also noted is the range of values for plate current for a given grid voltage can vary at a ratio of over 2/1 and grid over 3/1.

  

  

A configuration with four unmatched tubes is an invitation to a disaster --- simply think thought the effects the 2/1 and 3/1 ratios will have on the combinations of idle and max current on the strongest tube if three are not contributing. 

  

Some think most failures in the four tube designs are related to the combined feedback compounding the risk of instability due to the lack of neutralization.

  

Think for a moment, assume there is 4 times the plate to cathode feedback.  There will also be 1/4 the impedance in the cathode circuit, that is, if the input circuit was designed properly.  Is neutralization the problem or is it the disregard for parameters called out in the data sheets for multi-tube amplifier design? 

  

I advocate the neutralization of any amplifier  and believe it to be important for every band not just the high bands because it provides a measure of linearity as well as stability.

  

 

Regards Jim K9AXN 

 

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