[Boatanchors] P&H LA-400 linear

[MICHAEL BITTNER]

Interesting that the Radio Museum web site shows the pin diagram for the 1625 with cathode and grid 3 
connected at pin six, but with them separated in an accompanying schematic for a parallel-1625 amp.
Mike, W6MAB

> On August 16, 2019 at 1:36 PM Gary Schafer <garyschafer at largeriver.net> wrote:
> 
> 
> The 837 has a separate beam forming lead on one pin so it can be grounded.
> They worked pretty good in gg.
> 
> Some 1625's the beam forming plates came thru the glass with a separate wire
> and both the cathode wire and beam forming wire went into a single pin on
> the base. Some guys used to use a solvent to loosen the glue between the
> phenolic base and the glass envelope. You then had to desolder all of the
> pins and remove the base. You could then separate the beam forming lead from
> the cathode and put it into an unused pin in the base. 
> Another way (easier to me) was to cut a slot part way around the base near
> the bottom and you could fish out the beam forming lead and place it into an
> unused base pin. Like doing micro surgery.
> I have done many of those lots of years ago.
> 
> None of the RCA brand tubes could be modified as the beam forming and
> cathode were tied together inside the glass.
> National Union was one brand that I remember could always be modified. Some
> of the other brands could and some could not be modified.
> 
> If you look carefully at the bottom of the glass where the leads go thru you
> can visually trace the beam forming leads and the cathode leads thru the
> glass. If you see two separate leads then the tube can probably be modified.
> Most often on tubes that couldn't be modified you could see a strap welded
> between the leads before they went to the bottom of the glass and there
> would only be one wire going thru the glass for both elements.
> 
> None of the 807 tubes were ever made with separate leads on the beam forming
> plates. They were always connected to the cathode internally so they would
> not work in gg.
> 
> I think the OP was looking for an amp for his 20A. The 1625's will work but
> the 20A is short on drive for them. If you get 100 watts out of 4 of them
> you are doing good and that requires near 1000 volts on the plates.
> 
> I used to use a pair of 6ag7's in gg driving some 1625's with my 20A.
> Simple circuit and stable.
> 
> 73
> Gary  K4FMX
> 
> 
> 
> > -----Original Message-----
> > From: boatanchors-bounces at mailman.qth.net [mailto:boatanchors-
> > bounces at mailman.qth.net] On Behalf Of Robert Downs via Boatanchors
> > Sent: Friday, August 16, 2019 2:19 AM
> > To: boatanchors at mailman.qth.net
> > Subject: Re: [Boatanchors] P&H LA-400 linear
> > 
> > One subject that I don't recall being mentioned in this thread but that
> > has
> > been in almost every similar thread in the past is that some if not most
> > of
> > these 200 watt nominal linears using four 807's or four 1625's are wired
> > as
> > grounded grid amps.  And that the beam forming plates inside the tubes
> > that
> > must be grounded are electrically connected to the cathode, not brought
> > out
> > to a separate pin.  So stock tubes apparently won't work but must be
> > modified.  I never paid that much attention to the threads but seems
> > like at
> > this point most threads split off into which brands of tubes could be
> > modified and which couldn't.  Some 1625's apparently had the cathode and
> > beam forming plates electrically connected inside of the tube envelope
> > and
> > cannot be modified.  Some had the connection made inside of the tube
> > bases
> > and can be.  I've long ago forgotten which brands could and which
> > couldn't.
> > And why the grounded grid configuration was preferred over the
> > conventional
> > grounded cathode.
> > 
> > Robert Downs
> > 
> > 
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