[Amps] Tetrodes in GG Configuration

[Colin Lamb]

"As long as the bias and screen voltages are clean, the IMD will be low as it should be."


This statement is not correct.  Look at the Eimac imd curves for a 4CX250 or a 4CX1000A.  The charts are made with absolutely stable bias and screen voltages.  I do not have access right now to the charts but they are about -22 db 3rd order distortion, which I consider unacceptable except for cb use.  If the screen voltage is not regulated, the distortion will increase.  A number of hams are running 4CX250 or 4CX1000A tubes on 6 meters, grid driven and with no feedback and you can hear the splatter when the band is open.   

Now, compare the 4CX1500, which is designed for sideband service.  In exactly the same configuration, it will yield something like -45 db 3rd order distortion.  It is extremely clean.  I remember a ham running a the 4CX1500 tube in his amplifier and the difference was night and day.  The first time I tuned through his signal, I noted there was no sideband splatter.   

Something else can also be learned by looking at the curves - that distortion does not necessarily increase as the tube is pushed harder.  There seems to be an assumption that if you take a high power tube and run it at low power that the imd will improve.  That may not be so.  Look at the 4CX250, for example.  If you drop the power down to 50 watts, the distortion products actually increase (this from memory).  

All this means that it pays to read all the information you can get from the manufacturer and other critical tests, and then do not assume that results you get will be similar unless you are running under the same conditions.   Although a spectrum analyzer can be useful, a reasonably good receiver and knowledge can tell you a lot.  Rich probably has the procedure, using a receiver, mentioned on his website.   

It was probably a good thing that I never did examine the distortion products of my 4 x 6AG7 amplifier I built many years ago.  I ran a couple of hundred watts but never burned out any of the finals, and never could get the metal case of the 6AG7 to glow.

Colin  K7FM