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


Paul Baldock paul at paulbaldock.com
Fri Oct 4 12:13:30 EDT 2013


I use a pair of parallel 813s, grid driven and get about 800W PEP 
when driven with my DX-60B. The grids have a swamping resistor (200 
Ohms) so that the drive voltage is correct for the 813's.

FYI my DX60B is set for about 3dB carrier control.

- Paul



At 03:33 AM 10/4/2013, Bry Carling wrote:
>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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