THE 7189, 7189A, &
EL84/6BQ5
POWER PENTODES
7189 versus EL84/6BQ5
The 7189 industrial tube is similar to the common
EL84 (European) or 6BQ5 (American) 9-pin power pentode. But the 7189 is designed for
circuits operating in the 400 volt range. The EL84/6BQ5 is designed for
the 300 volt range.
You can safely substitute a 7189 tube for a 6BQ5 (sound issues aside) but you
might think twice about substituting an EL84 for the beefier 7189.
7189A
The 7189A power pentode is the same as the 7189 but has two pinout
differences, as follows:
-
The 7189, the EL84, and the 6BQ5 tubes all reserve pin #1 for "Internal Connection" by the
tube manufacturer. A user can't assume that pin #1 connects to any
particular tube element, or to none. Pin #1 of a 7189A tube,
however, always connects internally
to pin #2 (the Control Grid). In a 6BQ5 or 7189 tube, pin #1 might or might not
connect to the control grid.
I've seen old GE 6BQ5's with pins 1 and 2 connected and I've seen current EL84's
with them unconnected.
Some vintage Magnatone amps sported 7189A tubes powered by over 400 volts. They
wired pin #1 of the socket as the control grid, assuming it would connect
to pin #2 inside the tube. If
someone installed a plain vanilla 7189, EL84 or 6BQ5 tube, the amp wouldn't work because the control
grid wouldn't get
signal. This was a friendly reminder to install the higher rated 7189A tube.
-
The 7189, EL84, and 6BQ5 tubes also reserve pin #6 for internal connection. But inside
the 7189A tube,
pin #6 always connects to pin #9, the screen grid. I've
never seen a 6BQ5, an EL84, or a plain 7189 tube with pin #6 and pin #9 connected.
Some vintage Magnatone
amps actually have the screen grid voltage wired to pin #9 on one power tube socket and to pin #6 on
the other.
In a pinch, you can rewire 7189A-specific tube sockets to accept any of the
tubes under discussion. Just move any wires going to pin #1 over to pin
#2. Then move any wires going to pin #6 over to pin #9. Do this on each of the power tube sockets.
Click here for tube pinout diagrams.
If you like, have a professional measure the plate voltages in your amp,
especially if you're blowing out non-industrial tubes.
