Magnatone Giveawya

September 2014
more... Ask Amp ManGearAmpsHow-TosGear BlogTube ComboRepairJune 2009

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


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.

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!

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

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.

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.

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 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.

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.

Jeff Bober
Co-Founder and Senior Design Engineer Budda Amplification jeffb@budda.com or budda.com.