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May 2014
more... GearAmpsAsk Amp ManGear BlogJuly 2010

Turning Your Amp On

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Hey Jeff,
I’m a big fan of PG and I very much enjoy reading your column. I’m in need of a little advice. About six or seven years ago I purchased a Fender Hot Rod Deville 2x12. I played this amp, on and off, for maybe a year, bought a new house and packed the amp away. I have not used this amp in four or five years and now want to use it again.

I’ve spoken to some friends and have been told I shouldn’t just “turn it on,” but that there is a procedure I should follow. So I’m writing this letter—and feeling quite stupid—to ask exactly how should I approach turning this amp on. Please bear in mind I’m not a technical person at all, so I ask you to explain this as if you were speaking to a four-year-old.

As stupid as this question may seem, I thank you for shedding any light you can on my dilemma.

Thanks,
Mike R.


Hi Mike,

Thanks for reading and for writing. As I tell everyone, there are no stupid questions, so I thank you for asking yours. Let me see if I can shed a little light on this while trying to remain as non-technical as possible.

The reason that your friends told you not to just turn the amp on after it has been in storage so long has to do with the condition of the electrolytic capacitors in the power supply circuit of the amplifier. I know that may already be sounding complex, but I will do my best to explain it as simply as possible.

The power supply in the amplifier converts AC voltage from the wall (120VAC in the US) to higher DC voltages which are needed to power the internal circuitry of the amp. The capacitors in the power supply are needed to “smooth out” these voltages. The “smoother” the voltages, the quieter the operation of the amplifier (know that this is over-simplified, as there are many other factors that determine the noise level of an amplifier). If these electrolytic capacitors sit dormant for too long, the insulation between the internal components can become minimized and the capacitor can begin to draw a considerable amount of current. The result can manifest itself in a couple of different ways.

The first, and I must say the less spectacular, would be that they would cause the fuse to blow. If you install a new fuse and try again, you’ll get the same result. This would more than likely cause you to either take the amp to an experienced amp tech in your area, or to put it back in the closet for another four or five years, realizing that you didn’t really need it anyway! The other and more colorful result would be to turn the amp on, turn the standby on…crack, hiss, smoke! This is because the capacitors, because of excessive current draw, overheated and “vented.” Most capacitors nowadays are manufactured with a “vent” allowing them to discharge the internal pressure. If there was no vent, you would simply substitute the “crack” with a “boom.”

Anyway, this is what can theoretically happen when old, inactive capacitors are not properly re-formed. The truth is though, I’ve seen many newer amps that sat unused for quite a few years be plugged in and turned on with no adverse effects. That said, if you’re still looking to play it safe I will give you a better way to go about it, should you have the ability to so.

Obtain (beg, borrow, or buy) a piece of equipment known as a Variac (also known as a variable transformer or an autotransformer). This is a variable transformer that plugs into a wall outlet and will allow you to slowly increase the AC line voltage applied to your amplifier. [Ed. note: If you are not qualified to work on amps, be sure to seek the supervision of a trained electrician—there are lethal voltages involved here.] Start by removing all the tubes in your amplifier (mark them so you can reinstall them in the proper locations). Plug the Variac into the wall outlet and plug your amplifier into the outlet on the Variac. Set the Variac to zero volts. Turn on the Variac, as well as the power and standby switches on your amplifier. Raise the knob on the Variac to approximately 10 volts. Wait 10 minutes and raise it another 10 volts. Do this every 10-15 minutes or so until you reach approximately 100 volts (note: you may not see the pilot lamp begin to illuminate until you reach the 50-60 volt range).

Once you reach 100 volts for 10 minutes, return the Variac to zero volts and turn it off. After a few minutes, unplug the amplifier from the Variac and turn the power and standby switches off. If during this procedure the fuse has not blown, your amp is more than likely fine. Reinstall the tubes, plug it in, turn the power switch on, wait a minute or so, turn the standby on and you should be good to go.

Good luck. Hopefully that will keep you from having a Hot Rod Destruct.


Jeff Bober, one of the godfathers of the low-wattage amp revolution, co-founded and was the principal designer for Budda Amplification. Jeff has just launched EAST Amplification. He can be reached at pgampman@gmail.com

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