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Re-forming Tube Preamps from Storage


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Should a tube premamp that has not been used, for say 6 months or more, be started back up the same way tube power amps are? ie: re-form the power supply caps by first removing the tubes, then slowly powering the unit up with a variac, or is this just a power amp thing?

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On 7/25/2004 10:13:08 PM seti wrote:

What is this re-form of which you speak? I learn something new everytime I am on this forum.



Thanks guys.

Seti, 're-forming' of the power supply capacitors needs to be done on vacuum tube type equipment that has not been turned on for some time. The capacitors loose their charge over time. If the amplifier is turned on after the capaictors have lost all their storage charge, there is an excessive rush of incoming current when the amp is turned back on until the capacitors 're-form' enough to hold their charge. The large current draw can damage internal components including the capacitors themselves as well as the tubes. Apparently from the insight provided above by Craig and Mike this applies more so to power amps than preamps, probably because of the large power supplies involved.

There are basically two methods that are used to re-form the power supply caps. One is to use a variac. A variac is basically a variable transformer and can vary the voltage power supplied from the wall electrical outlet. When you turn the amp on, the variac is at its lowest setting (0%). You gradually increase the voltage until you reach 100% which will be equal to 100% of the voltage from the outlet.

Another method uses a homemade device. You take a regular incandescent light bulb socket and wire it in SERIES to ONE SIDE of regular lamp chord wire. One end is plugged in to the wall outlet, the other end goes to the amplifier's power cord. Use a low wattage light bulb, 15-25 watts is fine, no more than that though. If the amp hasn't been used in a long time, when it's turned on, the bulb will glow more brightly as the power supply capacitors draw more current while they "re-form". As the become formed, the bulb's glow will grow more dim, almost to the point where it is almost out. That is when the capacitors have re-formed. One benefit of this particular procedure that the low wattage light bulb also acts a fuse when wired in series. Should anything catastrophic occur (such as a power supply cap blowing) the light bulb will burn out like a fuse and prevent excessive current draw.

I use both a variac and the light bulb method because the variac allows the caps to re-form slower, more gradually, with less 'shock'. The light bulb provides additional protection. When using a variac the light bulb will not start glowing brightly until you reach near 100% voltage, or maybe not at all if this is done slowly enough. With either method the tubes should be removed from the amps. Make sure you put the same tubes back in the same positions (with some equipment may be important, others not, but its just good practice)

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You could put an inrush current limiter in your power supply to help out. Also helps protect 12AX7 from flashing when the current hits. They are about the physical size of a large ceramic disk capacitor.


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