[AMRadio] 3-500Z vs 4-400A

Bob Bruhns bbruhns at erols.com
Sun Dec 12 12:04:08 EST 2004

Hi Pat,

The 20K resistor idea was published in an old article on getting 120 watts
from a pair of 807s, but I have been told it really didn't produce that much
power, it did some really odd things at low audio levels, and it really
didn't sound good.

The idea about tying the control grid and screen of a 4-400 together and
using it as a triode might be better, but I suspect that the 3-500 will be a
better triode than a kludged 4-400.

Tying the screen of a 4-400 to the plate will *drastically* reduce plate
voltage limits, and plate current will be much lower at acceptable grid
potentials (instantaneous E-grid *below* instantaneous E-plate) - so it
would be impractical, because the power output capability would be massively
reduced.  To do ultra-linear right would require a separate, center-tapped
screen winding on the mod transformer.

I am in favor of the 4-400, but with active screen drive to improve
linearity.  Some kind of circuit could take instantaneous control-grid drive
and produce a corrective pre-distortion to the screen voltage, aimed at
making the grid-plate transfer as linear as possible, and making overload

With lower voltage tubes such as the 6L6, 807 and 8417, it is actually
possible to accomplish this with simple resistors in series with the
screens.  But the big, higher voltage tubes have so much secondary screen
emission that such resistance can not be tolerated.  So with higher voltage
tubes, a more complex active screen control is required.

But when the smoke clears... you will have the output and gain of a tetrode,
but with the linearity of a triode (or better).  With decently designed
feedback, you can have the source resistance of a triode as well.

I've done it with 6L6s and 8417s.  Some day maybe I'll try it on 4-400s or

  Bacon, WA3WDR

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