[Amps] Amps Digest, Vol 54, Issue 3

[Bill L. Fuqua]

	There are a lot of reasons not to replace 3-500z tubes with 4-400  tubes but it is not because they don’t work well in grounded grid zero bias configuration. They in fact do as well documented over the year by reputable engineers including Bill Orr at EIMAC. In fact the 3-400z and 3-500z evolved from the grounded grid operation of the 4-400 . They were made as a less expensive tube for that operation hence the lack of base and base shield. 
 	The reasons are as follows
1.  Physical differences. The 4-400  is taller and does not work with the 3-500z chimneys. The taller tube reduces the gap between the plate cap and the shield in the amplifier cabinet. Also they can’t be cooled by side blown air as the 3-500z tubes because the metallic base shield block air necessary to cool the pins.

2.  Electrical differences. The 4-400 has different output z than the 3-500z when operated in grounded grid. Even though the pi or pi-l network will compensate for this it means. Higher voltages are required to get the same amount of output power. Thus if you replace a 3-500Z with a 4-400 you will get less output power unless you make changes in your power supply and perhaps also increase the voltage rating of plate tuning capacitor, bypass capacitors etc.  And the obvious that the 4-400 has less plate dissipation. Internal element capacitances are different as well.  The 4-400z has not been fine tuned for zero-bias grounded grid operation but it does work well. 

3. Most 4-400 tube that are purchased are used and may have somewhat reduced emission. Why  buy a new 4-400 when you can get a  new3-500 for much less. 

    If you are designing a your own amplifier you may consider 4-400 or 4-1000 tubes if you have a good source of the tubes. However,  for the above reasons you perhaps should not replace a 
3-500z with a 4-400 in an existing amplifier.

73
Bill wa4lav

-----Original Message-----
From: "Tyrone Burns Sr." <tyke1954 at yahoo.com>
To: amps at contesting.com
Date: Sun, 3 Jun 2007 09:21:10 -0700 (PDT)
Subject: Re: [Amps] Amps Digest, Vol 54, Issue 3

mark, gongrats on the acquisition!!!  as far as the
4-400s' . don't try replacing the 3-500s' with them.
the 4-400s'a are screen grid tubes and the 3-500s' are
grounded. the 4-400s' don't like grounded grid
configuration.  check out BAMA and mods.dk web pages 
for manuals and schematics . they both have a lot of
stuff.if you use mods.dk join up by sending him a
msnual or schematic that he don't have. you can do a
single download , only if you do not view it first.
vistors are allowed one viewing or download per day,
so be sure of what you are looking for. the
BAMA(boatanchor manual archive is a good site also.
downloads are bit iff at the regular site, but if you
go to the edirisste or link that is listed ,, it flies
much faster and is easier to navigate through. just
remeber to look in all directories kisted on the page.
some will slip by if you are looking close.   

cul  tyrone  n5xes



       
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