[AMRadio] (no subject)

wa5bxo at pctechref.com wa5bxo at pctechref.com
Sun Dec 12 14:04:54 EST 2004

Hi Bob:
	I thought the 20K resistor trick was invented by RCA.  I seem to remember (of course these days I forget a lot) an article in a RCA manual about the experiments done by RCA prior to the 6L6, where the 20K resistor was to get rid of the distortion in Class B Zero bias triode operation.  I have gotten a lot of power from 1625s doing this.  For intermittent amateur service I have run 1200 Volts on them till the goats came home, and 1800 volts for a few moments of "FOURRRRRRRS".  Of course at 750 volts, or was it 800 volts, I am not sure of the exact output but I believe with the perfect match and new tubes that they would do 120 watts as RCA said.  As for distortion, at the rated output, weeeell, I believe in inverse feed back in all cases, if possible, HIHI. 

	I did the bit on 813s and found the best was full drive to the screens and reduce drive the grid via another XFMR.  But I placed full screen voltage and grid bias on both while driving both screen and grid.  It was a configuration I had never seen before and I don't know why I did it. (Because I could?) but it produced a lot of output at very low distortion with great damping.

I love this experimental talk!!!!!!!!!!!!!

John, WA5BXO 

-----Original Message-----
From: amradio-bounces at mailman.qth.net [mailto:amradio-bounces at mailman.qth.net] On Behalf Of Bob Bruhns
Sent: Sunday, December 12, 2004 11:04 AM
To: Discussion of AM Radio
Subject: Re: [AMRadio] 3-500Z vs 4-400A

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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