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Bassmen and Bulgin Plugs
by Jeff Bober
Hey Jeff,
I have a huge problem with my ‘68
Bassman head and I’m praying you can
shed some light on it. It blows fuses
instantly after I take it off standby.
I’m using the right fuse. I isolated the
transformer by disconnecting the red
leads and it didn’t blow, so it’s not
that. I also checked every diode (I’m
using 1N4007, is that right?), all the
caps, all the tubes and nothing has
helped. What’s going on here?
I’m desparate!
- Sam
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Hi Desperate Sam,
Nice initial troubleshooting you’ve done
there. You’re obviously knowledgeable in
the ways of tube amps, so let’s sum up
what you’ve already done and what’s been
missed. Note: The following troubleshooting
tips are based around the power supply
circuitry of a tube amplifier. This area
contains lethal voltages and the following
procedures should only be performed by
an experienced repair technician specializing
in tube amplifiers.
Since you mention that the fuse blows
when you take the amp off standby, we
can assume that the problem is somewhere
in the high voltage area, since placing
the amp in “play” mode energizes the
high voltage. You mentioned using 1N4007
diodes and they’re fine.
You also said that you checked the caps.
Unless you have a capacitor tester that can
place the caps under actual high-voltage
conditions, they could still be suspect as
they can break down when the high-voltage
is applied. The only true test would be
to individually lift each capacitor from the
circuit, one at a time. In this particular amp,
the first two caps in the power supply
have high-voltage applied to them as soon
as the power switch is turned on, so you
can rule those out.
You mention checking the tubes, which
is great, but again, unless you have a
dynamic tester that applies real-world voltages
and loads, it may not be a real representation
of the tubes’ condition. Chances
are that if they test okay, they don’t have
a dead short, but the best check is to pull
them out and see if the fuse still blows.
If you’ve pulled the tubes, lifted the caps
and the amp is still blowing fuses, we
need to look at other possible causes. The
most probable would be an output transformer
with a short to ground. This could
be a primary winding shorted to ground,
or possibly shorted to the secondary windings
which are virtually at ground potential.
It could even be one of the primary leads
shorted to the case. The easiest way to
assess whether the output transformer is
shorted would be to disconnect the center
tap from the high voltage.
To do this, locate the red lead of the primary.
The primary leads are the blue, red
and brown set. Disconnect the red lead
from its connection on the circuit board
and turn the amp on. If you can then take
the amp off standby and the fuse doesn’t
blow, you’ve more than likely found the
cause of the problem – a shorted output
transformer.
If at that point the Bassman is still blowing
fuses, the next probable cause would
be a shorted choke. The choke is a small
transformer-looking device with two black
leads. Disconnect the two choke leads
from the circuit; one should be connected
directly to the standby switch and the
other connected to the circuit board. Using
a digital multimeter, measure the resistance
from each lead to the chassis; there
should be no resistance or continuity of
any kind, or what’s known as a completely
open circuit. If there is continuity, the
choke has a short to the case and needs
to be replaced. Barring some sort of
bizarre wiring short, that should get your
Bassman up and running and you should
no longer be known as Desperate Sam.
Jeff,
I recently purchased a Marshall Major
and it’s missing the AC power cup on
the back of the chassis. It’s the part
where it looks like a round hole with
three prongs. I actually need the whole
setup – power cup, male and female.
Can you suggest where I may find this?
Thanks,
Randy
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Hey Randy,
Nice amp, if you can keep power tubes in
it! I actually owned one in the ‘80s and the
only output tubes that the amp wouldn’t
fry were GE 6550s. At the time, no KT88,
or even KT90, would hold up. Hopefully
things have improved since then. There is,
however, one other problem that you may
encounter after you get it up and running:
where to play it! But hey, let me do my
part to help you rock the dBs.
The parts you are looking for are manufactured
by a company called Bulgin. They are
generally referred to as a Bulgin plug and
Bulgin receptacle and are available from
numerous amp parts suppliers – try my
friend Steve at angela.com.
Now you can take down a few walls!
Jeff Bober
Co-Founder and Senior Design Engineer – Budda Amplification
jeffb@budda.com
www.budda.com
©2007 Jeff Bober
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