Tetrodes in Grounded Grid mode.

[Ed/KB1HYS]

Is there a simple rule of thumb type equation to predict the output of a tetrode like a 4-250 or 4-400 if run as a grounded grid amp? I know they are less efficient in that mode, but I'm just curious as to how bad they really are.

[K1JJ]

Is there a simple rule of thumb type equation to predict the output of a tetrode like a 4-250 or 4-400 if run as a grounded grid amp? I know they are less efficient in that mode, but I'm just curious as to how bad they really are.

Ed,

Here's a few general guidelines....

Figure the GG LINEAR tetrode amp will have about 11-13 db of gain or so. 
For example, about 100W drive in gives about 1500W out or so.  These are rough figures. Compare this to a grid driven tetrode amplifier OR a GG triode that is designed for linear service that will do gains of 20db or more.  But gain isn't everything.

Figure the power output pep will be about twice the plate dissipation, maximum, to be reasonably clean. So, a pair of 4-400's GG (800W diss) will do about 1600w out max.  A GG linear 4-1000A with reasonable voltage and drive will do about 2KW pep out, etc.    Of course, with real high voltages and high drive (abuse) you can almost double that power out, but the signal will be very dirty and splatter like heck.

Use forward biased diodes in the fil transformer  CT to set the plate current idle to about 1/3-1/2 the tube's total plate dissipation for the cleanest signal.


BTW, the tube's plate efficiency is roughly the same whether run grid driven or GG (cathode driven).  You are still pulling screen and grid current either way, and the plate structure and tank don't know the difference. In fact, since some the input drive power at the GG cathode is ADDED in series to the tubes's output power, GG is probably a little more efficient.  (minus the drive power lost in the grid and screen)

The built-in negative feedback of GG operation is a plus too.

Hope this helps.

T




     

[Opcom]

Diode voltage drop is a function of current, and this will change the bias over the audio cycle causing a small amount of distortion. A reduction in this distortion can be obtained by putting a bypass capacitor across the bias diode(s). The reactance of the capacitor at the lowest audio frequency should be less than a tenth of (Vbias/Ik), so maybe 1000uF/16V is good, bypassed with a 0.01uF cap forany RF that slips in.  It is only a small improvement, but it is also only a small cost.

[K1JJ]

Diode voltage drop is a function of current, and this will change the bias over the audio cycle causing a small amount of distortion. A reduction in this distortion can be obtained by putting a bypass capacitor across the bias diode(s). The reactance of the capacitor at the lowest audio frequency should be less than a tenth of (Vbias/Ik), so maybe 1000uF/16V is good, bypassed with a 0.01uF cap forany RF that slips in.  It is only a small improvement, but it is also only a small cost.

Thanks for the suggestion, Patrick.   I added a 10,000 ufd  50V cap and a .01  across the diode bias stack today.

Later -

T