[AMPS] 30L1 Grid circuit

[km1h@juno.com]

On Wed, 14 May 1997 15:24:20 +0000 w8jitom@postoffice.worldnet.att.net
writes:
>Hey Guys,
>
>Look at the grid circuit of the Collins carefully, and look at a few 
>things.


LOTS of Snips

Tom and others, I was always under the impression that the 30L1 grid
circuit was developed primarily to obtain decent efficiency on the higher
frequencies. The internal construction of the 811A is such that the grid
and its lead to the socket  has a fair amount of   XL at say 28MHz. 
Everything that I was taught and read is that those 220pf grid capacitors
were strictly to tune out the reactance and permit true RF grounding and
thereby improve efficiency and stability. Nowhere was there ever any
mention of some "voltage divider network" to support this latest theory
from Tom. 
In fact, those original Bill Orr articles about "Super Cathode Driven"
amps was always referring to tetrodes and most specifically the 4-400A
and 4-1000A. I fail to see any relationship to the 30L1.

IMO, the Collins ENGINEERS did a damn fine job and I wonder if what I
just read here was not some form of revisionist history & engineering. 
Collins rates the 30L1 at -30dB IMD worse case and I prefer to give them
credit for actually running the tests. Those numbers are quite
respectable for a tube introduced in the 1930's. 

The 30L1 is still on the air in huge numbers, has a great reputation for
reliability AND clean signal ( I cant say that about some recent wannabe
clones). Not bad for something that is almost 40 years old.   

LOTS more Snips

As for arc protection, putting arc protection in the grid is not very 
>smart. The grid to chassis resistance allows the grid to pull up to 
>anode voltage during an arc. The next thing in line is the cathode, 
>and the exciter is connected right to the cathode through a few low 
>voltage capacitors. An anode to grid fault (the normal discharge 
>path) can be passed right along to the cathode system, and straight 
>out to the $$$ exciter.
>Arc protection belongs in the anode's dc supply line, not in the 
>grid.


The 30L1 uses arc suppression/current limiting in the plate DC line. 


>I'm not saying the "Super Cathode Driven" circuits always cause 
>problems, but in PA's I've measured gain flatness and IMD on I've 
>found that system to INCREASE IMD unless the PA was very carefully 
>tuned and drive properly adjusted. Removing the "Super Cathode 
>Driven" system and grounding the grids produced much better gain 
>flatness and generally better IMD performance, while INCREASING 
>stability and improving damage protection.
>
>Draw it out on paper and plug in some values, and remember the GK 
>impedance is time-varying with drive. You'll see the SKD system does 
>almost exactly what we don't want it to do.

> If the 220 Heath continued, we were planning on pulling the grid chokes
and small > mica caps and grounding the grids.

I dont have the review of the SB-220 handy but the TL-922 is almost a
clone and uses similar grid circuitry. The 3rd order IMD is reviewed as
-36dB; not shabby performance for a "poorly designed" circuit. 
I have experimented with the SB-220 and directly grounding the grid pins.
Power changes very little ~30W or so but the incidence of Tune cap
spitting seemed to increase. I never carried the tests thru to a real
conclusion and felt the Heath ENGINEERS knew more than I did anyway.  The
SB-220 started off with 200pf grid capacitors to tune out the XL but
later changed to 130pf in the SB-221 and HL-2200 as a supposedly better
value. I do note that those amps seem to have less bandswitch problems
than prior versions. 
 Maybe Rich would care to comment also in order to give a more balanced
viewpoint.  Rich, please no references to ur web pages, etc. ...just a
purely technical opinion if you will. 


73....Carl   KM1H
 
>
>If you have one working, fine. But unless it's in a class A or  AB1 
>tetrode that feedback system probably isn't helping things. 
>
>73, Tom W8JI 
>
>--
>FAQ on WWW:               http://www.contesting.com/ampfaq.html
>Submissions:              amps@contesting.com
>Administrative requests:  amps-REQUEST@contesting.com
>Problems:                 owner-amps@contesting.com
>

--
FAQ on WWW:               http://www.contesting.com/ampfaq.html
Submissions:              amps@contesting.com
Administrative requests:  amps-REQUEST@contesting.com
Problems:                 owner-amps@contesting.com