ASK AMP MAN
JEFF BOBER
Speaker Loads and Overheating Amps
Hey there!
I just bought a Sears Twin Twelve 1484
head, and I am using the speakers/wiring
and cabinet of my Fender Red Knob Evil
Twin combo amp. The speakers are 8 ohms
100 watts each. Is this cool? It works, but
I’m not sure if the ohms are matched right,
and I don’t wanna mess up anything.
Tom
Hey Jeff,
I have a Crate amp. It will stop playing—
like it’s kicking out a breaker—then reset
itself. As long as I don’t play it loud, it’s ok.
Thanks, T Moore
Hi Tom,
While two twelves is the correct speaker
configuration for that Silvertone amp and
most typical Fender Twin speaker configurations would be a perfect match, your
Red Knob Evil Twin is not. Most standard
Twins, both blackface and silverface, are
typically loaded with two 8-ohm speakers wired in series (plus to plus, minus
to minus on both speakers), providing a
total load of 4 ohms. This, according to
the schematic, is the proper load for the
1484 head. Your Red Knob Twin, however, utilizes two 8-ohm speakers wired in
series (plus of one speaker to minus of the
other), totaling a 16-ohm load.
Hello T,
Thanks for your question. Since I don’t
know the exact model of your amp, or
have a complete description of the failure,
I can’t be as specific as I’d like to be, but
I’ll attempt to provide you with possible
causes. My instinct here is that your problem is possibly heat related.
dirty or oxidized, the signal will become
intermittent. This can also happen from a
temperature rise in the amp causing the
metal contacts in the jack to expand, possibly just enough to make a compromised
contact intermittent. Cleaning or possibly
replacing the jack(s) will cure the symptom, if it is indeed the cause.
You can rewire the speakers in your Twin
for proper operation by connecting them in
parallel. Remove one of the wires coming
from the amp on only one of the speakers. Connect it to the opposite speaker so
that you have both speaker wires from the
amp connected to two terminals (plus and
minus) on one speaker. You should now
be left with one additional wire connected
between the speakers. You’ll need to connect this wire to the same terminal on both
speakers (pick either the + or -). You will
need an additional wire to connect the
opposite terminals on both speakers. You
should now have the two speakers connected by two wires, one from + to + and
the other from – to – , and the wires coming from the amp should be connected to
the + and – on one speaker. You can now
use your Twin speakers with the Silvertone
head with a proper impedance match and
maximum power transfer. Should you want
to reconnect the speakers to the Twin
chassis, simply select the 4-ohm setting
and rock on!
Because you say it seems like it’s kicking
out a breaker, then resets itself, I assume
that the unit actually loses all power: no
sound, filaments in the tubes are not lit,
and possibly the pilot light extinguishes. If
this is the case, and the unit actually needs
to cool down a bit before it will come back
to life, the first thing I would suspect is
the presence of a resettable thermal circuit
breaker in the mains transformer. Playing
the amp at a relatively low volume may
draw less current from the transformer
than playing loudly. The more current is
pulled from the transformer, the warmer
it can get. Once it reaches the temperature of the thermal breaker, the breaker
will open, disconnecting the AC mains
input to the transformer windings. If this
is indeed the cause, the problem is not
that the breaker is doing its job, but why.
What is causing the unit to overheat? Does
it lack proper ventilation? Most amps can
get pretty hot if they’re not given enough
ventilation space. If it is a relatively closed
unit with a fan, maybe the fan has stopped
working, or the unit is extremely dirty and
unable to ventilate. Whether this is the
cause of the failure or not, it’s certainly
worth checking out.
Yet another possibility is the speaker, which
could manifest a problem due to heat.
Playing at lower volumes, the voice coil in
the speaker will not generate much heat.
Once higher power is applied to the speaker, the voice coil dissipates that power in
the form of heat. This is especially true with
overdriven or distorted sounds. The distorted or overdriven signal to the speaker,
instead of being a pure sine wave, contains
many forms of square wave. The speaker
sees parts of these square waves as a form
of DC voltage, which causes the speaker
voice coil to generate more heat. The heat
could then cause a winding or connection
to open, causing loss of sound. If it’s possible, disconnect the current speaker from
your amp and connect the amp to another
speaker. If the symptom disappears, a
speaker replacement is in order.
There you have some starting points. I hope
it helps bring your Crate back to life.
Another possible cause of a loss-of-sound
failure may be an intermittent effects loop
jack, if the unit has an effects loop. In
most amps that have a passive series loop,
the full signal passes through the switching contacts in at least one of the effects
loop jacks. If this set of contacts is either
Jeff Bober
Co-Founder and Senior Design Engineer
Budda Amplification
jeffb@budda.com or www.budda.com
©2008 Jeff Bober
