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


Bry Carling bcarling at cfl.rr.com
Sat Oct 5 08:09:49 EDT 2013


Thanks Paul. Does the 3dB range give you something approaching normal full carrier AM?

I prefer the DSB reduced carrier effect on the DX60 but that's just me. It sound slike normal 
AM in many ways, but had a lot more audio punch.

On 4 Oct 2013 at 9:13, Paul Baldock wrote:

> 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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