There are a lot of reasons not to replace 3-500z tubes with 4-400
tubes but it is not because they donât 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 canât 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.
From: "Tyrone Burns Sr." <firstname.lastname@example.org>
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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