smurph1 Posted June 2, 2011 Share Posted June 2, 2011 My Classic 30 is getting new tubes here shortly..It runs 3-12ax7 pre-amp tubes and 4 EL84s in the power section..Can I just swap the new ones for the old ones, or do I have to get it re-biased? I will need the amp next weekend, and would probably have to send it out of town to get it biased..HELP!! Link to comment Share on other sites More sharing options...
H Posted June 2, 2011 Share Posted June 2, 2011 Hi Smurph, I believe these are cathode-biased amps so, unless you've had it modified with a bias pot, it should just be a case of plugging the new tubes in. Link to comment Share on other sites More sharing options...
Guest HRB853370 Posted June 3, 2011 Share Posted June 3, 2011 My Classic 30 is getting new tubes here shortly..It runs 3-12ax7 pre-amp tubes and 4 EL84s in the power section..Can I just swap the new ones for the old ones, or do I have to get it re-biased? I will need the amp next weekend, and would probably have to send it out of town to get it biased..HELP!! My amp guy told me it's self biased, so plug and play!! Link to comment Share on other sites More sharing options...
blackjack Posted June 3, 2011 Share Posted June 3, 2011 Peavey Classic 30? The schematic I found is fixed bias. Either way, the possibilities are... FIXED BIAS: You would have at some point put in a matched quad of EL84, rated by the seller with a number or color that is relevant to bias current. After that (in theory) one simply replaces with the same rated, or close to it, and bias will (if no other components in that circuit have drifted) be close enough. If you don't know the bias point, and can't check it, there is a real possibility that you'll get over or under bias condition. Under bias (hot bias) will sound OK maybe great for a short while and then fail. Over bias (cold bias) will just sound awful. You might get lucky, but there's size-able risk. CATHODE BIAS: Still a chance of under bias, not as bad. If the designer used power tubes with a native current draw that is medium, then set the bias point hot, all is well until someone later installs tubes that natively draw higher current. Then with hot tubes and a smallish cathode resistance (which now is a too-low bias), even a cathode bias amp can fry tubes. Strictly speaking, you need to know the bias point, and check it periodically, even for cathode bias amps. It doesn't take as much luck in this case to avoid trouble, compared to fixed. Power it up in a dark room and watch, anywhere from 10 seconds to 10 minutes I would say, for the sides of the tubes to get a menacing red glow and slightly tinny sound. It sounds hot and thin. Then power down immediately and curse yourself for stressing your tubes. Link to comment Share on other sites More sharing options...
blackjack Posted June 3, 2011 Share Posted June 3, 2011 And I wasn't trying to discourage the instinct to plug in tubes and go, thus avoiding the cost of bias. I just couldn't advise someone to do it, that's all. On some forums you'll see where someone was simply advised to get medium rated tubes and play on... I would still power it up in the dark, and observe. Link to comment Share on other sites More sharing options...
kbp810 Posted June 3, 2011 Share Posted June 3, 2011 I would still power it up in the dark, and observe. Most likely you can plug 'em in and play and be just fine; but the above suggestion is a really good idea. If they are cold, you might not quite get the sound you want, but the tubes will survive. If it's too hot, powering them up in the dark will show you right away if the plates start glowing red. So: If after putting the new tubes in the amp doesn't seem as "alive" as it used to, take it to a tech If after putting the new tubes in you see red/orange glow in the plates, turn it off right immediately and take it to a tech If after putting the new tubes in everything sounds and looks just fine - you are good to go! Here are a couple of examples of what red plates look like: (The first tube is red plating) (The second tube is red plating) Link to comment Share on other sites More sharing options...
DetroitBlues Posted June 3, 2011 Share Posted June 3, 2011 Never bothered before to see what a redplate tube is.... cool pics (or should it I say hot?), thanks for the examples! Link to comment Share on other sites More sharing options...
tulk1 Posted June 4, 2011 Share Posted June 4, 2011 Peavey Classic 30? The schematic I found is fixed bias. Either way, the possibilities are... FIXED BIAS: You would have at some point put in a matched quad of EL84, rated by the seller with a number or color that is relevant to bias current. After that (in theory) one simply replaces with the same rated, or close to it, and bias will (if no other components in that circuit have drifted) be close enough. If you don't know the bias point, and can't check it, there is a real possibility that you'll get over or under bias condition. Under bias (hot bias) will sound OK maybe great for a short while and then fail. Over bias (cold bias) will just sound awful. You might get lucky, but there's size-able risk. This is what my tech told me on my Delta Blues, which is basically the same amp. Most likely you can plug 'em in and play and be just fine; but the above suggestion is a really good idea. If they are cold, you might not quite get the sound you want, but the tubes will survive. If it's too hot, powering them up in the dark will show you right away if the plates start glowing red. So: If after putting the new tubes in the amp doesn't seem as "alive" as it used to, take it to a tech If after putting the new tubes in you see red/orange glow in the plates, turn it off right immediately and take it to a tech If after putting the new tubes in everything sounds and looks just fine - you are good to go! Here are a couple of examples of what red plates look like: (The first tube is red plating) (The second tube is red plating) Awesome pics!! Never actually seen that before. According to the guy that built my 5E3, this is what blew the tranny. The tubes tested great on the ... uh, tester. But red plated and caused all sorts of nastiness in the amp. Link to comment Share on other sites More sharing options...
smurph1 Posted June 6, 2011 Author Share Posted June 6, 2011 My amp guy told me it's self biased, so plug and play!! Yep..I just found that out!! Cool!! Link to comment Share on other sites More sharing options...
Steiner Posted June 6, 2011 Share Posted June 6, 2011 Most likely you can plug 'em in and play and be just fine; but the above suggestion is a really good idea. If they are cold, you might not quite get the sound you want, but the tubes will survive. If it's too hot, powering them up in the dark will show you right away if the plates start glowing red. So: If after putting the new tubes in the amp doesn't seem as "alive" as it used to, take it to a tech If after putting the new tubes in you see red/orange glow in the plates, turn it off right immediately and take it to a tech If after putting the new tubes in everything sounds and looks just fine - you are good to go! Here are a couple of examples of what red plates look like: Excellent post KBP810 I've had the horror of seeing this happen. It is audible and frightening at the same time. Link to comment Share on other sites More sharing options...
DetroitBlues Posted June 7, 2011 Share Posted June 7, 2011 I'm hoping I never have the problem. Is it caused from just a tube going bad or something worse? Link to comment Share on other sites More sharing options...
kbp810 Posted June 7, 2011 Share Posted June 7, 2011 Excellent post KBP810 I've had the horror of seeing this happen. It is audible and frightening at the same time. Indeed it is, 'tis also one of the amp builders worst nightmare! (though usually easily rectified) Same reason why I keep a rag tag group of cheap used tubes around for some of those first fire ups Link to comment Share on other sites More sharing options...
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