ASK AMP MAN
JEFF BOBER
Ampeg B-15-N
Hi Jeff,
I recently acquired a ‘60s Ampeg bass amp.
It says B-15-N on the top of the chassis. It’s
in really nice shape, but the problem I have
is the plug for the speaker cabinet is missing and I don’t know how to wire on a new
plug. Can you help? Thanks.
John
Over the years, the amplifier to speaker con- nection on Portaflex amps was accomplished in a few different ways. Early Portaflex mod- els used an octal plug and socket pair, similar to the base of an 8-pin tube (like a 6L6) and
the socket it plugs into. I’ve even seen a very
early model where the speaker connection
was accomplished with the bolts and clasps
used to secure the flip-top chassis to the
speaker cabinet. While I can’t remember if I
believed this to be a stock factory configuration at the time, I don’t think it’s a very reliable method. The later models, which seem
to be the majority of the Portaflexes in existence today, use a 4-pin XLR plug and socket.
This is the style connector I’ll be referring
to, and should be easily identified by a 4-pin
male XLR connector attached to the side of
the speaker enclosure. You will, of course,
need to source a 4-pin female XLR connector
for the end of your speaker cable.
Amps) and Denny Kager (Sundown Amps).
But I digress. On to your question.
Hi John,
Very cool amp you have there. Those amps
definitely do sound great and make a great
recording tool in the studio. They’re known
as Portaflex bass amps and countless bass
tracks in the ‘60s and ‘70s were recorded
with them. If you’re a fan of the Motown
sound, you’re listening to the bass grooves
of a Portaflex. Besides being a Motown fan,
the other cool factor that these amps have
for me is that this generation of Ampeg
amps was manufactured in my home town
of Linden, NJ. The Ampeg factory was also
stomping grounds for some other notables in
our industry, such as Ken Fischer (Trainwreck
as the connection for the negative side of
the speaker. Connect the white wire to pin
3. This is connected to the circuit ground
of the amplifier. Lastly, connect the red
wire to pin 2. This wire is actually part of a
safety feature designed into the amp—and
is the reason that pins 2 and 3 are shorted
together on the cabinet connector. One of
the most damaging things you can subject
a tube amp to is attempting to play it without a load (no speaker connected to it).
In order to keep this from happening, the
Ampeg Portaflex amps (as well as some others, including the early B25s and SVTs) had
a built-in safety feature that would prevent
the amp from having any output unless the
speaker cable was plugged into its proper
speaker cabinet. This was accomplished in a
couple of different ways.
To verify that your cabinet is
wired properly, look inside at
the XLR connector mounted
to the side of the enclosure.
You should see the two
speaker wires connected to
pins 1 and 4 of the connector. Pins 2 and 3 should be
shorted together (more on
the need for this later). If
this looks okay, next look at
the cable coming from the
chassis. There should be four
wires inside the cable. If this
is all good, you’re ready to
attach the connector.
In your amp, when the plug was inserted,
the shorted pins would connect the cathode
resistor of the phase inverter to ground,
enabling the phase inverter, which was then
able to drive the output tubes. This was
probably deemed a bit safer than its design
predecessor, in which the connector furnished a ground connection to the center
tap of the high-voltage winding of the mains
transformer. I guess it just seemed safer to be
switching the low voltage connection in the
phase inverter than the high voltage connection in the power supply—but where is the
adventure in that?
Strip and tin all four conductors of the cable. Looking
at the rear of the cable
mounted XLR connector you
are about attach, you should
see pins marked 1, 2, 3 and
4. Connect the green wire
to pin 1. This is the positive lead from the output
transformer which connects
to the positive side of the
speaker. Connect the black
wire to pin 4. This will serve
Anyway, now you know how to get your
Portaflex up and running. With any luck,
the speaker is in good shape, and all it may
require is a bit of maintenance and possibly
a new set of tubes. Hopefully, it still has the
cool Plexiglas light-up Ampeg logo. You
could find someone to engrave your initials
in it as players used to do “back in the day.”
Now go lay down some low down.
‘Till next time…
Jeff Bober
Co-Founder and Senior Design Engineer
Budda Amplification
jeffb@budda.com or www.budda.com
©2008 Jeff Bober
