I have an early 1960 fisher x-100 amp. I have replaced all the tubes and replaced all caps in the signal path and filter. It worked great. After 2 weeks the left channel would work for about 40 sec. and then just shut off. at that point, I notice that one output tube (7868) is glowing brighter than all others. But the tube test good on my tester. so I am thinking bias? maybe?

Greets, Shaun.

I had this happen with an import receiver some years ago.

Check the voltage at your bias resistors while the unit is acting up. You may have a bad resistor.
Hope it helps.

If the tube is glowing brighter, it won't necessarily test poorly, but may have a short that doesn't normally show up with the usual test procedure. You need to perform an element to element test. This test varies from tester to tester. With my tester, the grid and plate levers go to the up position for normal testing, and the cathode levers go down. To perform an element to element test, all levers are left down except for the element in question. You throw that lever up. If there is a short between that element and any of the other elements, the shorts light will light up. If that element uses more than one pin, the shorts light will light up. If you are testing the heater, you will get a short unless both (or more if the heater uses more than 2 pins) pin levers are thrown up. If all heater levers are thrown up and you still get a short, this means that the heater is shorting to something else--most likely the cathode.

You cannot do an accurate shorts test, though, by simply testing the tube. You must perform an element to element shorts test to get accurate results. An intermittant short could be occurring if the channel sometimes works and sometimes doesn't. Be sure to allow the tube to warm up well while performing the element to element shorts test. Jar the tube with your finger to vibrate the elements....do this for each element test. Tedious, but necessary!

Regarding other problems, sudden failure could be caused by a connection to a tube element opening up either within the tube or at the socket. Most likely this would be either a cathode, control grid, or plate connection.

Condensers can suddenly and intermittantly short out. Any condenser in the audio section can cause a dead channel. It could be an audio bypass condenser. It could be a tone condenser. It could be a screen grid filter condenser (since screen grids are usually fed by resistors except perhaps in the output section....for pre-amp sections usually .1 MFD condensers or there-abouts are used).

As was also suggested, resistors should be checked. Resistors don't usually open up, though, unless cracked. Wire wound ones can open up, though, if corrosion is present. If a wire wound resistor is used for cathode bias, its opening would cease all current flow to the cathode, and would leave the channel dead.

Finally, you could be having a selector switch issue, if a selector switch is used. Jiggling the switch may reveal the problem.

Lots of possibilities, though.

Also, if one tube glows brighter (heater), and yet it tests out well in every respect, it could simply mean that either that tube's heater is of a different design (especially if of a different brand), or that the heater has a possible short to itself, causing it to glow more brightly. This may or may not affect tube performance otherwise. As long as the heater doesn't short to the cathode, it shouldn't affect performance. Sometimes a bright heather can overheat other elements, though, causing them to warp and short. If the bright tube doesn't test out poorly in any way (absolutely no shorts of any kind), you can try it in the working channel to see if the problem moves to the working channel.

Thomas

Also, if you replaced all of the tubes, were the original tubes necessarily bad? You may want to try them to see if they work better...then, if so, one of the new tubes is faulty. Tubes shouldn't be replaced unless they test bad or otherwise perform poorly. There are a lot of people out there who simply replace all of the old tubes without a real reason. I see this a lot on eBay--"replaced all of the tubes, FULL RESTORATION! Should work great!" It seems to almost be an encouraged practice. This practice doesn't solve anything and is wasteful. Replacing all of the condensers if the originals are paper is a better idea. Condensers are more likely to fail than tubes. Tubes should simply be tested for proper performance and shorts, and should be replaced based upon performance. A new tube is just as likely to have a short as an old one, so replacing all of the tubes isn't going to prevent shorts, especially if the new tubes came a long way in a truck (though good packaging will eliminate much trouble here).

Thomas

Agreed.

Replacing tubes simply for the sake of replacing 'em isn't necessarily, well, necessary. :) If you replace tubes with used ones of questionable quality this practice may cause more issues than it solves.

Thomas is right on the money. If swapping the output tubes doesn't cure your problem some troubleshooting is in order.

The issue I referred to previously involved a Japanese-made tube set of early '60's vintage. It had those light grey "dogbone" wirewound resistors. From a cold start the set would play fine for 20 minutes or thereabouts then quit. I found that one of the bias resistors was opening when it got warm. The defective resistor didn't look any different from the rest.

Hope it helps.