Tube Handling and Installation
Although tubes should be handled carefully, gloves are not necessary. Any fingerprints can be removed with a clean cloth. Dirty tubes can be cleaned with soap and water and thoroughly dried with a clean cloth.
To insert them in their sockets, make sure the pins are lined up properly and gently rock them into place. To remove them gently rock them as you pull them from the socket. This will lower the stress on the sockets.
Bent pins can be straightened gently with your fingernail or needle nose pliers, but not against the glass.
Setting the Bias
It’s a good idea to lower the bias on the tubes you are replacing before removal, as the new tubes will most likely require less bias. We are speaking of bias as standing current, as is the present practice. Old timers often speak the opposite in that bias is a negative voltage that is increased to decrease the standing current).
That said, we are encouraging you to reduce the current by some 30% while the old tubes are in place to prepare for the new tubes. Turn off the amplifier and wait a few minutes. Insert the new tubes per the above instructions and turn the amplifier on. Allow it to warm up for a few minutes then set the bias 20% below the amp manufacturer’s specifications initially. Look for any plate color (red glow) in a dark room. Reduce bias current if glow is noted. After 10 hours of use raise the bias to10% below the manufacturer’s specifications. After 20 hours of use, raise the bias to the full specification. Check at 100 hours, then every 1000 afterwards for Music Reference amps, more often for other amps.
Tube pins can be gently brushed with a small (toothbrush sized) soft wire brush (stainless or brass). The dull brown color on most miniature type tube pins is natural and will not ever be bright like plated pins. It is a character of the metal used and is normal and makes a good connection. Octal pins are normally plated and will remain bright. If they are corroded they may be cleaned.
Gently rocking a tube in the socket while the unit is on will reveal any noisy connections. When both the socket and pins are clean enough, there will be no crackling. Sockets can be cleaned with spray tuner cleaner available from Radio Shack. Be careful to limit power to speaker or use a speaker of little value as large levels of noise may be encountered. Good reliable sound can only be achieved when all connections are solid.
Simple Emission testers (B&K DYNA-JET series) are an adequate and more reliable way to determine the condition of a tube than complex Transconductance (Hickcok 539 etc) testers for two reasons. 1) The transconductance testers do not run the tube at high enough voltages and currents. 2) They are not stable or precise enough for matching. Our tester is a precise measuring device that was designed to make these measurements accurately and at real amplifier conditions. The measured transconductance will be stable as long as emission is adequate (above 75%). When Emission falls below 60% (middle of the yellow "?" range, the tube should be replaced.
Another good measure of remaining life is getter condition. This getter is the silver coating on the inside of the glass. It erodes to a brown tinted glass as the tube ages. When all the silvering is gone, the tube will get gassy, as there is no more getter to absorb gases. The tube is then in danger of bias run-away and potential damage to the amplifier.
Be not afraid to tap on your tubes, chassis or connections, but do it with speakers of little value or use a resistive divider to limit the power. There are many of you using equipment where the slightest tap will send it into noisy oblivion. This is far from proper condition and will never give good sound. Tapping is the only way to know the condition. If you can't tap it without a normal level of microphonics, you shouldn't be listening to it.