[AMRadio] 4-1000A Grounded Grid Class-C


Bry Carling bcarling at cfl.rr.com
Fri Oct 4 06:33:45 EDT 2013


It's great to have the benefit of a forum with so much experience and knowledge.
Thank you guys.

A Question please:

What would be the best type of linear amplifier to use with a DX60B please? I really like the  
reduced carrier scheme, (even though I know a few AM ops prefer to hear a fat carrier  
blanking the noise...)

I would be interested to explore the possibilities of both the homebrew route and also the 
commercial route, which is a more likely scenario for me. I was thinking about something like 
an AL80B or the LK500ZC which you can sometimes find at reasonable prices. 

Any thoughts on AM linear mode for suppressed carrier in general would be greatly 
appreciated.

73 - Bry, AF4K

On 4 Oct 2013 at 2:28, Bernie  Doran wrote:

From:           	"Bernie  Doran" <qedconsultants at embarqmail.com>

> Hi Rob: The problem with modulation is that a large percentage of
> the 
> cathode driving power passes through and appears as output and thus
> unmodulated.  It has nothing to do with the voltages.
> 
> Certainly using the tube way under its ratings reduces the driving
> power, 
> but what is the point? much better choices than a 4-1000 if the only
> desired 
> power is the so called 375 watt or whatever it is for A.M.
> 
> For the same output, class C is always going to take significantly
> more 
> power than class B. Makes no difference if the is grid driven or
> cathode 
> driven.   Class B has a conduction angle of 180 degrees or more,
> class C 
> usually is around 120 degrees and the tube must be driven at all
> times to 
> near saturation so that the the plate current will be double during
> the 
> peaks of modulation.  That brings up the problem common to all multi
> element 
> tubes and high MU triodes, they do not modulate well with out
> playing some 
> games, I E screen modulation and or driver modulation.   As an
> example look 
> at the tube curves for a tetrode, take any point and double the
> voltage, the 
> plate current hardly changes, that is why the screen voltage must
> also be 
> modulated. In the case of a 4-1000 the screen current is rather high
> and the 
> "best" way to modulate the screen is by a separate winding on the
> mod 
> transformer or a dropping resistor from the plate supply. That
> becomes a big 
> fat resistor  and some means must be provided to make sure the
> screen 
> voltage never rises uncontrolled.  Under some conditions the screen
> current 
> can be come negative and with a simple dropping resistor the screen
> rises to 
> the plate voltage and goodbye tube.    The screen can not ever "
> float" for 
> the same reason, one safe way is for another resistor placed
> directly from 
> screen to ground.  The amount of current flowing through that
> resistor is 
> subject to some variation but probably should be at least 1 1/2
> times the 
> max screen current. Running at the Eimac rating with 3kv on the
> plate, they 
> do not like voltage below 3kv,  screen current is around 140 MA thus
> the 
> resistors should carry at least about 210 MA plus. Near 300 Ma would
> be 
> safer.  600 to 900 watts of heat!!   I think 4-1000a are really neat
> tubes, 
> I have seven, but a bit difficult to work with.
> 
> 
> Some triodes, Hi Mu, have issues with modulation.  "Worst" are the
> 3-XXX 
> series, their curves resemble multi element tubes and even Eimac
> states that 
> the 3-1000 should have driver modulation. ( that in grid driven 
> configuration)  Look at the curves for low Mu triodes and one can
> easily see 
> that the plate current doubles as the plate voltage doubles. The
> 304TL being 
> my favorite.
> 
> Low Mu triodes simply make the best amplifier tube choice for AM
> service, 
> regretfully the perceived need for rapid band switching has pushed
> them 
> aside like open wire has been replaced by the inferior coax for feed
> lines. 
> Bernie W8RPW
> 
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