Wattage and Power

April 1, 2010

gp0410_gearseriesparaI HAVE A QUESTION ABOUT SPEAKER power-handling capability when connecting two speakers in series. I know that if I connect two 8Ω 30-watt speakers in parallel, I get a total impedance of 4Ω and a power handling capacity of 60 watts. If I connected the same speakers in series the impedance would be 16Ω, but what would the power handling capacity be? —Dave Ellis

The power-handling capability when using a pair of identical impedance speakers will be the same in series as it would be in parallel, which in this case is 60 watts. When multiple speakers of the same impedance are used, the wattage from the amp is divided between the numbers of speakers. The wattage handling capability goes up as more speakers are added. But just because a set of speakers is rated at 60 watts does not mean you can safely use them with a 60- watt amp. Amplifier wattage is usually rated at the onset of clipping, and guitar players usually turn their amps up past that point and into an overdrive condition. So if you were to use a speaker configuration rated to handle 60 watts with a 60-watt amp, you would most likely blow the speakers. A good rule of thumb is to allow at least two watts of speaker for every one watt of amplifier. That is to say, a 30-watt amplifier will not blow 60 watts worth of speakers. —Gerald Weber, Kendrick Amplifiers

I have a Matchless Clubman 30 with a 2x12 cabinet and I am running it at 4Ω. The amp sounded fine when it was new, but the tone rapidly deteriorated. I have changed the tubes but that didn’t help. What should I do? —Randy Anderson

The Matchless Clubman amp has a design flaw that needs to be addressed. It uses 450-volt filter capacitors as the main filter capacitor and the screen supply capacitor. The idle plate voltage is 425 volts. The problem is not when it is idling, but when you are playing the amp. The plate voltage of any amp will fluctuate depending on how much current is going through the power supply at any particular moment. As the instantaneous current goes down, the plate voltage goes up. You can remove both output tubes to simulate a ”no current” condition, and the plate voltage will go to 512 volts, which is 62 volts more than what the main filter cap is rated! So 450-volt caps will work for a while, but when the filter caps go bad, the amp won’t be as loud and won’t sound in-tune when playing in the key of B or Bb—especially with the amp cranked.

The solution is to replace the main filter capacitor and the screen supply filter capacitor each with two 350-volt capacitors in series. The microfarad value must be at least double the stock values. Solder a 220kΩ 1-watt resistor across each of these caps to keep the voltage divided evenly. Observe correct polarity such that the minus (-) lead of one cap goes to the plus (+) lead of the other. The remaining unused minus lead goes to ground and the unused plus lead from the other cap goes to the circuit. This will replace the main filter cap. Now you will do the same for the screen supply filter cap. By having two 350-volt capacitors in series, you are in effect making a 700-volt rated cap. However, putting them in series will divide the microfarad value in half, which explains why the value of the new 350- volt caps must be at least double the stock value. [Caution! As with any amp, you must drain the electricity from the power supply before servicing it. The easy way to do this is to put the amp’s standby switch in the “play” mode, unplug the AC cord, and remove the amp chassis from the cabinet. Connect a jumper wire from pin 1 of any preamp tube socket to the chassis ground. Leave this connected for about one minute and allow all the electricity to drain from the power supply.] —Gerald Weber, Kendrick Amplifiers

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