[Boatanchors] P&H LA-400 linear

[Gary Schafer]

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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