Gashole Posted October 1, 2008 Share Posted October 1, 2008 Can I safely swap out 6L6's for EL34's in the BC30? I'm wanting to get closer to a Voxlike sound, but don't want to damage anything. Is this what "self biasing" refers to, the ability to use various tubes and have the plate somehow compensate? Link to comment Share on other sites More sharing options...
cGil Posted October 1, 2008 Share Posted October 1, 2008 Well, the "self biasing" concept is misleading. A "properly" cathode biased tube will settle in, to some degree, to the conditions presented to it. For example, if an amp had dual cathode bias resistors and bypass caps (yours doesn't) you wouldn't need to use closely matched pairs of tubes. Your amp has a shared cathode bias resistor and bypass cap, and needs fairly closely matched pairs of power tubes to sound its best. Because the amp is cathode biased, as opposed to a fixed grid bias popular in larger amps for the extra power it can help generate, you don't need to have your amp rebiased every time you throw in a new set of matched power tubes. The "self biasing" nature of the cathode biased circuit will take them the rest of the way as they break in. But there is a wide range of bias settings within which the tube will work, although it may not sound its best. However, to generate the most power or the warmest tone or whatever, the cathode bias needs to be set specifically for the particular tube you want to use. Manufactures take advantage of the bias range for marketing purposes, and set the bias for the lowest common denominator tube that'll be used. Then they can give a list of tubes that'll function within the range created by that bias setting. Your BC30 was originally biased for 5881's, with a higher value bias resistor than is probably needed for best performance with a 6L6GC or EL34. So a 6L6 will be biased a little cooler than the 5881, and an EL34 will be biased even cooler than the 6L6GC. You could even try JJ 6V6S's, but those would be biased kinda hot and may not last too long. Sovtek or EH 6V6 would probably burn out real quick. Same with any 6L6G, GA, GB or GT. Stay away from those because they just aren't rated to handle your plate voltage like the JJ 6V6S, or any manufacturer's 6L6GC can. Gil... Link to comment Share on other sites More sharing options...
lostindesert Posted October 2, 2008 Share Posted October 2, 2008 You can use different types of 6L6/5881 but NEVER any other output tube like EL34, EL84, 6V6 etc. Peter Link to comment Share on other sites More sharing options...
layboomo Posted October 2, 2008 Share Posted October 2, 2008 You could try a set of THD yellowjackets with el-84's for a more ac-30 ish tone! Link to comment Share on other sites More sharing options...
cGil Posted October 2, 2008 Share Posted October 2, 2008 You can use different types of 6L6/5881 but NEVER any other output tube like EL34' date=' EL84, 6V6 etc. Peter[/quote'] Yup, you're right. Too hot for 6V6, too wrong (without yellowjacket) for el84, and not enough heater current for the EL34! I was just looking some tube data for the EL34, and two of those will require 1.2Amps more heater current than a pair of 6L6 or 5881. KT66 only requires 0.74A more, but I'd be concerned enough not to run those either. http://tdsl.duncanamps.com/show.php?des=5881 http://tdsl.duncanamps.com/show.php?des=6L6GC http://tdsl.duncanamps.com/show.php?des=EL34 http://tdsl.duncanamps.com/show.php?des=KT66 Gil... Link to comment Share on other sites More sharing options...
tunghaichuan Posted October 3, 2008 Share Posted October 3, 2008 Your BC30 was originally biased for 5881's' date=' with a higher value bias resistor than is probably needed for best performance with a 6L6GC or EL34. So a 6L6 will be biased a little cooler than the 5881, and an EL34 will be biased even cooler than the 6L6GC. You could even try JJ 6V6S's, but those would be biased kinda hot and may not last too long. Sovtek or EH 6V6 would probably burn out real quick. Same with any 6L6G, GA, GB or GT. Stay away from those because they just aren't rated to handle your plate voltage like the JJ 6V6S, or any manufacturer's 6L6GC can. Gil...[/quote'] Unless the BC30 comes with the new Tung-sol reissue, I would guess that there are Sovtek 5881 tubes in it. Not to mention that a Sovtek 5881 isn't a real 5881. It is not even in the 6L6 family. It is a Russian tube used in Mig fighter planes. They are very tough tubes. I biased up a couple in Class A to 40 watts an only one of the two had a hint of red plating. They are kind of sterile and flat sounding for guitar amp use, IMHO. tung Link to comment Share on other sites More sharing options...
Gashole Posted October 3, 2008 Author Share Posted October 3, 2008 You could try a set of THD yellowjackets with el-84's for a more ac-30 ish tone! The yellowjackets are a "Class A" style adapter, right? How long are they? I wonder if with valves attached if the whole set up would fit in the BC30. Thanks for everybody's input! Link to comment Share on other sites More sharing options...
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