[Amps] 4CX250F Tubes

Tue, 24 Sep 2002 09:11:24 -0700

Hi Bill, see below...

> Thanks Steve.  I thought I might have a new top cover
> made for the Mark 1 that is tall enough.  A friend of
> mine owns a metal working shop and told me he could
> get to it in a day or two depending on how elaborate I
> wanted it.  May be the way to go.  New 3-500Z tubes
> would outlast me if I take care of them.
> 
> I like the new CTI PLA800 amp that uses two 4CX250B
> tubes.  Heard anything about it?
> 
	[Steve Katz]  Yes, I have, but I've never seen one.  The claims made
seem inappropriate for the design indicated, at least to me.  I wonder how
they get "1200W PEP" output power from a pair of 4CX250s running at 2000V
and 600mApk-?  Seems to me that's 1200W input power.  And the use of two
little axial fans blowing against the backpressure of the 4CX250s and its
enclosure/output grille, etc, doesn't seem sufficient, regardless of claims.
I've always used centrifugal blowers for these tubes, the backpressure is
very high, much higher for the 250B design than for the old 150A.  But,
again, I've never seen one up close, and would love to.  -WB2WIK/6

> 73,
> Bill
> 
> 
> --- Steve Katz <stevek@jmr.com> wrote:
> > Hi Bill,
> > 
> > Lots of 4CX250B amps out there, all you need to do
> > with the "F" is change
> > the filament transformer.
> > 
> > I modified a few Mark I (and other) amps using
> > 3-400Zs to 3-500Zs without
> > making the chassis covers any taller...just sunk the
> > sockets down below the
> > main chassis level, so the 3-500Zs sat lower in the
> > amp.  That's not
> > difficult to do with the Mark I, but if you do so,
> > you need to make sure
> > there's airflow across the filament pins.  If I
> > recall correctly (it's been
> > about 20 years), I just added another fan under the
> > chassis, blowing on the
> > tube sockets.
> > 
> > 73,
> > 
> > Steve WB2WIK/6
> > 
> > "If everything seems under control, you're just not
> > going fast enough." -
> > Mario Andretti
> > 
> > > -----Original Message-----
> > > From:	Bill Smith [SMTP:ko4nrbs@yahoo.com]
> > > Sent:	Tuesday, September 24, 2002 8:35 AM
> > > To:	Steve Katz
> > > Subject:	RE: [Amps] 4CX250F Tubes
> > > 
> > > Thanks Steve.  I may forgo the idea and just use a
> > > couple of 3-500Z tubes by cutting a hole in the
> > top of
> > > the enclosure and install a nice screen mesh.
> > > 
> > > I think I will look for an amp that uses 4CX250B
> > > tubes.  They are cheap and easy to find!!
> > > 73,
> > > Bill
> > > --- Steve Katz <stevek@jmr.com> wrote:
> > > > 
> > > > 
> > > > 
> > > > 
> > > > > >Just came in to possession of a box full of
> > new
> > > > > >4CX250F tubes.  Thought they may be a very
> > low
> > > > cost
> > > > > >alternative to the 3-400Z tubes in my Swan
> > Mark 1
> > > > amp.
> > > > > > I know it would take quite a bit of work to
> > do
> > > > but I
> > > > > >would be in tubes for the rest of my life!!
> > > > > >
> > > > > >Can these tubes handle the 2500vdc the Swan
> > puts
> > > > out? 
> > > > > 
> > > > > On SSB/CW, yes.  
> > > > > 
> > > > > >I know they are rated at around 2000vdc plate
> > > > voltage.
> > > > > 
> > > > 	[Steve Katz]  I've used 4CX250F's in VHF PAs
> > with
> > > > great success.
> > > > Because they have full gain/output up to 400
> > MHz,
> > > > they might make great
> > > > oscillators when used in an HF amp unless new
> > > > precautions are taken.  And,
> > > > obviously, two of them (one each to replace
> > 3-400Z)
> > > > won't run nearly the
> > > > power of the original tubes...it would take four
> > to
> > > > do that.  I've never
> > > > used 4CX250's in grounded grid.  They require
> > > > "bottom to top" cooling,
> > > > pressurized, with air system sockets and
> > chimneys. 
> > > > Guess you could make a
> > > > big metal plate to cover the original socket
> > holes
> > > > and punch that for the
> > > > SK600 series sockets.....-WB2WIK/6
> > > > 
> > > > 
> > > > > >73,
> > > > > >Bill
> > > > > 
> > > > > 
> > > > > -  R. L. Measures, a.k.a. Rich...,
> > > > 805.386.3734,AG6K, 
> > > > > www.vcnet.com/measures.  
> > > > > end
> > > > > 
> > > > >
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> > > =====
> > > Bill Smith KO4NR
> > > 
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> 
> =====
> Bill Smith KO4NR
> 
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