zeno
Junior Member
Posts: 63
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Post by zeno on Nov 12, 2015 19:24:22 GMT -7
I put in a couple of NOS 6Bq5(El84) tubes in my MAZ 18 & after a few minutes I heard this hissing noise through the speaker. When I looked t the tubes (in the back) , one of the EL 84's was glowing bright red & hot! I immediately turned off the amp & let them cool down. Then after a couple of minutes I turned the Maz back on (in stand by). A few minutes later I switched off the stand by & all seems to be well......Actually sounds great & both EL84's look & sound fine. My question is, what happened? Is this what self-biasing is? What should I have done differently when replacing power tubes?
Thanks
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Deleted
Deleted Member
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Post by Deleted on Nov 12, 2015 20:06:21 GMT -7
Not a matched set???
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Post by BritInvasion on Nov 12, 2015 20:06:52 GMT -7
It sounds like one of the EL84's started to red plate. Pretty much can guarantee if it did it once it will do it again. It's a bad tube and needs to be replaced.
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Post by smolder on Nov 12, 2015 20:44:11 GMT -7
There are no "self basing" amps. Fixed bias typically have a potentiometer that allows you to optimize the bias. Cathod bias amps (such as yours) still have a limited range and it is possible to have tubes that run hotter or colder than the amp is set up for. While most cathode bias amps have a fairly wide range, they typically aren't one size fits all. I've adjusted the bias circuit resistor in plent of cathode bias amps for better tube life and better tone.
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zeno
Junior Member
Posts: 63
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Post by zeno on Nov 13, 2015 10:10:32 GMT -7
Yes, forgot to mention they are a matched pair. So, if it is probably going to do it again (as suggested), is it safe for the amp to continue running them & just replace when one goes permanently bad? I played two sets yesterday at near full volume and the amp sounded great. Hate to change them yet if I don't have to.
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Post by zpilot on Nov 13, 2015 13:02:03 GMT -7
You are rolling the dice there. Most likely that tube will redplate again but there is always a chance it won't. My experience is that it will happen at the most inopportune time and if it does it could damage the amp. As Dirty Harry said, "You've gotta ask yourself one question. Do I feel lucky"?
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Post by BritInvasion on Nov 13, 2015 13:40:54 GMT -7
^^ This
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zeno
Junior Member
Posts: 63
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Post by zeno on Nov 15, 2015 16:00:43 GMT -7
You are rolling the dice there. Most likely that tube will redplate again but there is always a chance it won't. My experience is that it will happen at the most inopportune time and if it does it could damage the amp. As Dirty Harry said, "You've gotta ask yourself one question. Do I feel lucky"? Good point. I guess I'll carry a spare set of JJ's to gigs in case the Amperex's give up the ghost mid-set. Thanks
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