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No messing about with "I'll get new tubes" because on a 700$ used amp, another 100$ for a quarted is a big deal and the poor sound may come from busted caps or a shorted tranny.īackup power tubes are almost compulsary when gigging anyway as are some extra fuses. When shopping for a used one I would really only get one if it sounded right straight away. In mine, the OEM tubes (Svetlana?) lasted a long time in mine, replacement tubes have never sounded as great (Ruby) and have never lasted as long (Sovtek). The E元4 is considerd to be the typical Marshall tube, but they have used others.įor example in the early Clapton/Hendrix era Marshall didn't use the E元4 penthode at all, but the KT66 beam tetrode. You could have it rebiased if you want it to accept E元4 tubes again. So I wouldn't call the JCM900 a 6l6 ampīut if it has those it was either from that particular time period or the previous owner had it rebiased for those. It was originally designed and sold with those, then E元4 supply dried up in the early 90s and they switched to 5881 for a while. UNLESS you want to change tube types (from E元4 -> 6L6 or E元4 -> KT88 for example)īut there are other issues with such a switch.īTW I've owned a Marshall for 20years and have serviced it with my dad, who holds a MoEE and who was lucky enough to attend classes by the professor who invented the penthode. So if you really think you need biasing to get that perfect tone then go ahead, but I wouldn't worry about it. Some people get so anal about that they absolutely want their amp biased in a "sweet spot" (disregarding tube drift), but there is no guarantee youwill get the amp back into that same sweet spot with a change of tubes and a change of bias. Improper biasing is mostly felt in tone and response, sometimes causing a transformer to heat up a bit more than expected,īut the act of biasing itself (via trim pots or resistor replacement) also changes the tone and response, as it changes the impedance of those circuits. Lastly tubes are built to take a lot of beating, but they also will change reponse over time.
#How to bias guitar amp tubes full
If they charge you more for it, they are ripping you off.Īlso consider that a full complement of powertubes (four tubes for approx 100W) will cost you 100$ anyway. If an amp is designed to be biased via trim pots, a qualified tech could bias your amp in five minutes.
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Sometimes tube classes are defined (within tolerances of ~5%) relieving you of the task to bias as long as you replace tubes like for like. Many amp designs don't allow biasing (Fender, Mesa) and are designed to operate with tubes spanning large tolerances. The importance of biasing is often overstated.