Miller Capacitance Calculation for 6550 and KT88 Tubes

[Jay D]

Hello Forum:
How does one calculate the grid stop resistor value for a new amp, using either 6550 tubes or KT88 Tubes?
From the RCA Receiving tube manual, RC28, for the 6550 tube, Direct inter-electrode capacitances:
1.) Grid 1 to plate = 0.85 pF
2.) Grid 1 to cathode and grid 3, grid 2 to base seeve and heater = 14 pF
3.) Plate to cathode & grid 3, grid 2 base sleeve and heater = 12 pF

Also to make things more confusing, the Maximum grid 1 circuit resistance values for fixed bias operation = 50K ohms, for cathode bias operation = 250K ohms.

Can the forum help me with - or direct me to books that have the methods to calculate the grid stop values.

Thank You,

Jay D

[Wilder Amplification]

Hello Forum:
How does one calculate the grid stop resistor value for a new amp, using either 6550 tubes or KT88 Tubes?
From the RCA Receiving tube manual, RC28, for the 6550 tube, Direct inter-electrode capacitances:
1.) Grid 1 to plate = 0.85 pF
2.) Grid 1 to cathode and grid 3, grid 2 to base seeve and heater = 14 pF
3.) Plate to cathode & grid 3, grid 2 base sleeve and heater = 12 pF

Also to make things more confusing, the Maximum grid 1 circuit resistance values for fixed bias operation = 50K ohms, for cathode bias operation = 250K ohms.

Can the forum help me with - or direct me to books that have the methods to calculate the grid stop values.

Thank You,

Jay D

The value of the grid swamper isn't that critical so long as it's there. Values between 1.5K and 10K do the job. You shouldn't hear any audible difference between these values since the HPF corner frequency is well above the audible human hearing range. The point of it is just to keep the grid from oscillating.

As far as the grid 1 circuit resistance = max 50K, this refers to the sum of the grid swamper and the grid leak resistor. In fixed bias amps, this resistor is the one that comes right off of the bias supply whereas in cathode bias amps the grid leak resistor goes to ground. The data sheet does state no higher than 50K combined resistance, however I think this refers to super high voltage / dual rail applications (i.e B+ = 700V/Screen = 350V) where they would be biased right at 70% (or 100% for Class A applications) as I've seen many Marshalls @ B+ = 450V use 150Ks for grid leak resistors with zero issues, and in a Marshall running at 450V, you don't need to bias them anywhere near 70% dissipation.

[Jay D]

Miller Capacitance Calculation for Grid Stop resistors

Hello John:
Thank You for the information.
Once the power stage is constructed, I will have to select a value, as not to have the output stage break into oscillation.

Again, Thank you for the reply.

Jay D

[kg]

miller capacitance is a largely non-issue in multigrid tubes run as such.

miller comes into play as a mutiplicative factor when elements inside the tube with associated capacitances are both swinging in or out of phase.

if they are in phase, the miller effect will reduce the effective capacitance. if they are out of phase, it will increase the effective capacitance.

the idea of multi grid tubes is that additional grids "screen" one another from the effects of the next. if you have a BT such as a kt88, and you run it in pentode/tetrode mode, g1 only "sees" the electrical fields of elements that are sitting at DC... so there is no "miller" effect going on. all you need to do is add up the various capacitances (g1 to k and g1 to g2).

like jon said, don't sweat it too much. a surprisingly wide range of values for series rg1 "stoppers" will be sufficient to keep oscillations down (assuming that the rest of the amp and lead dress is reasonably clean).