Deang Posted October 20, 2002 Share Posted October 20, 2002 Does the bias need to be readjusted if replacing tubes OTHER than the power/output tubes? Quote Link to comment Share on other sites More sharing options...
justin_tx_16 Posted October 20, 2002 Share Posted October 20, 2002 Does this help?... There are relatively few causes of output tube bias problems. All of them involve the grid not being held at a negative enough voltage with respect to the cathode. In any amplifier: If the output tubes have just been replaced with new ones, "infant mortality"/early dying of the new tubes. Leaky/shorted coupling capacitor from the driver tube plate to the power tube grid. Note that the chances are that only one of the multiple output tubes will have this problem. Dirty, corroded or just old tube socket not making good contact to the tube grid pin Note that the chances are that only one of the multiple output tubes will have this problem. Eagle-eye the socket hole contacts with a magnifier and try to see if they are corroded or dirty looking or maybe the contacts have lost their springiness, and don't make good contact. Try removing the tube, squirting a little tuner cleaner into each socket hole, then wriggling the tubes back in. You can also very gently pry the contacts out so they have more pressure on the tube's pins. The socket could be broken. Rarely, a very poor or inexpert rebiasing attempt by an unskilled technician. Rarely, an output tube itself that is shorted grid to cathode Note that the chances are that only one of the multiple output tubes will have this problem. Rarely, the resistance from the tube grid to its bias supply will be open. This can lead to "runaway" in power grid tubes. Measure the resistance from grid socket pin to bias supply point in fixed bias amps or to ground in cathode biased amps and make sure that both (all) output tubes have the same nominal resistance - usually 100K to 220K, never more than 470K. If one grid has a lot higher resistance to ground or to the bias supply, there is a problem with the grid leak resistor on that tube. Even more rarely, a wiring flaw in the wires getting the bias voltage to the grid. I have seen an amplifier that came from the factory with all of the wires inserted into the socket lugs and wrapped around the contacts, but not soldered. It worked that way for 20 years, and had intermittent troubles the whole time. In fixed bias amplifiers: If the tubes have just been replaced with new ones, failure of the owner to get rebiasing done, or "infant mortality"/early dying of the new tubes. Failure of the bias section of the power supply, especially the adjustment pot or the bias supply filter cap In cathode biased amplifiers: If the output tubes have just been replaced with new ones, "infant mortality"/early dying of the new tubes. Leaky/shorted cathode resistor bypass capacitor Rarely, a noninductively wound cathode resistor with an internal short Rarely, a broken or incorrect wire. Quote Link to comment Share on other sites More sharing options...
Jim Cornell Posted October 20, 2002 Share Posted October 20, 2002 Justin that you? LOL When replacing the output tubes with new ones youll need to adjust the bias, my conrad johnson, has a light on, light off screw adjustment ! You turn the screw untill the light is off, then do it again 6 hours after use! Although the light flashes when in use, this is normal! Hope this also helps, mobile where are you LOL! Jim Justin you just blew me away, way to go kid. Im glad to see you into this at your age im very impressed! Jim Quote Link to comment Share on other sites More sharing options...
NOSValves Posted October 20, 2002 Share Posted October 20, 2002 Deang, What amp are you using ?? In general you can roll any tube in the amp except the Output tubes without do any bias adjust. So roll away with out worry !!! Craig Quote Link to comment Share on other sites More sharing options...
Deang Posted October 20, 2002 Author Share Posted October 20, 2002 Craig, My AE-25 push-pull. I just replaced the EL-84 current source tubes with some RCA's. I was here at work and just got to thinking about it. Thanks Craig. Quote Link to comment Share on other sites More sharing options...
NOSValves Posted October 20, 2002 Share Posted October 20, 2002 Well aren't those your output tubes ?? EL84/6BQ5s are output tubes ! Craig Quote Link to comment Share on other sites More sharing options...
mobile homeless Posted October 20, 2002 Share Posted October 20, 2002 No, those are in the input/driver section. Actually, they act as a current source for the 6922 tubes. These are in the Signature and DJH edition. The Superamp uses a variety of output tubes such as KT-88, EL-34, KT-90, KT-66, 6L6s, or 6V6. The bias is fully user adjustable (the base model is fully auto bias). But Dean, you dont have to adjust bias except when you change the outputs...or as tubes start to wear. STill, I would go ahead and take bias readings while you are messing with the amp. On my amps, I usually just test them once a month or less. 98% of the time, there is no drift but I actually enjoy testing it. Also, you can experiment with different bias for more sweet sound....as long as you dont go overboard. Higher bias usually means a slight sweetening with some loss in low end definition (this is just in general..not always). Less bias means more control but a little less sweet... kh Quote Link to comment Share on other sites More sharing options...
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