[AMRadio] Improving the linearity of multi-grid power tubes

Bob Bruhns bbruhns at erols.com
Sun Dec 12 16:05:46 EST 2004

I like the idea of full drive to screens and reduced drive to the control
grid.  It's something like the deal with 55% screen modulation with 100%
plate modulation.  And the bias idea... hmm, why not?  Makes me think of
trying a family of bias arrangements, to see which one is best, or to see if
I can finish the tests, or even get the same results twice!

My favorite arrangement now is class AB1 grounded-cathode beam tetrodes with
control grid drive and essentially normal control and screen grid DC
voltages, but slight *negative* audio applied to the screens.  If series
resistors are used to produce the negative screen audio, then damping is
slightly higher, but the damping of this arrangement is not great unless
feedback is used.

  Bacon, WA3WDR

----- Original Message ----- 
From: <wa5bxo at pctechref.com>
To: <amradio at mailman.qth.net>
Sent: Sunday, December 12, 2004 2:04 PM
Subject: [AMRadio] (no subject)

> 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
> 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
> 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
> 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
> and produce a corrective pre-distortion to the screen voltage, aimed at
> making the grid-plate transfer as linear as possible, and making overload
> graceful.
> 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
> tubes, a more complex active screen control is required.
> But when the smoke clears... you will have the output and gain of a
> 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
> something.
>   Bacon, WA3WDR
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