[AMRadio] low u triodes modulate better (was 813 tubes)


Bernie Doran qedconsultants at embarqmail.com
Fri Jul 16 16:44:22 EDT 2010


Hi Nigel:   well the issue with tetrodes is that the screen voltage has far 
more effect on plate current that the plate voltage.  They do seem to do OK 
when the screen is fed through a dropping resister from the plate supply. or 
through a choke. On  the triodes, I have  Gates BC1 G here on 80 meters and 
have wondered why Gates modulated the pair of 807 drivers ( pair 833A final) 
so I started looking at tube plate curves ETC and it is interesting that 
almost none of the tubes manufactured by RCA ( and if you look at them most 
if not all have  rectangular plate) even come close to the " square law" 
double the plate voltage- double the plate current, as is required for 100 % 
plate modulation.    I believe that Gates did that to drive the tubes harder 
under modulation and thus clean them up, push them into closer to the square 
law issue. Eimac tubes tend to all have concentric elements and seem better 
on the transfer charactericts. the 304 TL is even better ( double the plate 
V more than doubles the plate I)
The only reference to this that I found is on the data sheet for the eimac 
3-1000A  It states that hi mu tubes must have the driver modulated. I have 
searched for the reason to no avail. I am guessing, but I suspect that the 
high mu tubes have a thick electron cloud around the filament and grid that 
starts to act like a screen grid.    looking at the 3-1000 curves, that 
critter looks almost like a tetrode.    That said I am probably too fussy, I 
do hear tetrodes all the time and they do sound good. I also have an 
unconverted 20V2 here, pair of 4-400As, the manual for it shows a lower max 
percent modulation capability and also higher distortion that the Gates. The 
Collins cost more that twice what the Gates sold for and I believe that 
Collins engineering was at least as good, probably better than Gates.


another interesting thing I have been pondering is this.  assume a fixed 
output driver. under modulation in the up direction the plate voltage will 
double, the final is class C, when the voltage doubles, the bias remains the 
same and the tube gets closer to class B, so the plate eff falls somewhat. 
on the neg side the plate V goes down and the tube goes into a deeper class 
C mode.    If the driver is modulated, under pos peaks the drive goes up, 
bias goes up,  maintaining the final in class C. under the neg side,  the 
drive falls, the bias falls, and the final slides toward lower eff. it looks 
to me like this becomes a sort of neg peak limiter. perhaps this is why 
Gates says something like 120 % pos modulation.

I am going to post this on the AM site for comment, been wanting to do a 
little more searching before I did, but this might stir the pot a bit.   I 
am also somewhat biased  ( pun) toward triodes, but do try to be as 
objective as possible.  Bernie W8RPW      PS one of my favorites is the 
813!!! so do not hesitate to try them.
----- Original Message ----- 
From: "Nigel Holmes" <Holmes.Nigel at abc.net.au>
To: "Bernie Doran" <qedconsultants at embarqmail.com>
Sent: Monday, July 12, 2010 6:20 AM
Subject: RE: low u triodes modulate better (was 813 tubes)


Hello Bernie,

Can you recommend any good text or articles regarding why low mu triodes 
modulate better. Is it because they have a more linear transfer 
characteristic?

I have a couple of NOS 45's which I was considering putting into a "classic" 
120's cw tx as well as some 6AS7 double triodes which I was going to use as 
a cathode-coupled driver for 2x807 in class B. I might have a play with one 
of the 45 or 6AS7 as a plate modulated amp on 160m instead. The 6AS7 has a 
mu of 2 ...

I have a series modulator that can supply upto 400 volts/150 mA essentially 
"100%" modulated that is reasonably linear from 30-8,000 Hz

Regards & 73

Nigel VK3DZ


-----Original Message-----
From: amradio-bounces at mailman.qth.net 
[mailto:amradio-bounces at mailman.qth.net] On Behalf Of Bernie Doran
Sent: Saturday, 10 July 2010 8:56 AM
To: Discussion of AM Radio in the Amateur Service
Subject: low u triodes modulate better (was 813 tubes)

you can usually find them on ebay, neat older tube, I used them in the 50s 
and still have some, keep thinking  I will put them back on.
you have modulate the screen or let it " self modulate" through a choke. if 
you have significant drive power, consider triodes, especially for a 
monoband amp, plus they just modulate better, especially the low mu tubes.
and the lower the mu the better.   Bernie W8RPW

----- Original Message -----
From: "RICHARD GEORGE" <k6kwq at msn.com>
To: "amlist" <amradio at mailman.qth.net>
Sent: Friday, July 09, 2010 4:11 PM
Subject: [AMRadio] 813 tubes


> I'm gathering parts for an 813 monoband amp. Probably not the best choice
> of tubes but looking at the nostalgia amp for my AM station.
> I see RF parts has Taylor brand (probably from china} tubes and also see
> there are some Russian tubes available. Are they any good ? Are there
> better sources for 813 tubes I will need 2.
> Thanks
> K6KWQ Dick
> Amps by "MORE POWER"
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