ol'bosey
42 posts
Feb 17, 2011
2:50 PM
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Howdy Campers,
What's the scoop with re-biasing a Pro Jr? A friend of mine is sending Me a Pro Jr(for free!) that I sold to him about 5 years ago. It seems to have been in storage ever since. I'm planning on doing a full retube. I've replaced the power tubes in the past with out rebiasing with no problems (being a class A circut I never even thought about it).
I was poking around in the archives and it seems a few people think it's in the best interest of the amp to re bias.
Any input would be appreciated.
Thanks!
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tmf714
497 posts
Feb 17, 2011
3:03 PM
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Unless the amp has been modified,or upgraded to include an adjustable bias pot,there is no bias adjustment. These amps left the factory fixed bias,non-adjustable.
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ol'bosey
43 posts
Feb 17, 2011
3:12 PM
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That's My understanding as well.I browsed through alot of info on several sites and guess I got a little turned around.
None the less I'm stoked to be getting that amp back. I've missed it ever since I sold it.The size/weight/volume ratio on those amps is amazing.
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5F6H
539 posts
Feb 18, 2011
1:57 AM
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The bias current can be read with bias probes inserted between the tube & tube socket...but these are expensive, unless tube rolling is something yuo expect to do a lot of, or you have a few amps, you might not think it worth the investment. In reality, Blues Jrs & Pro Jrs are usually underbiased (too much current) but EL84 are typically sturdy little tubes and take it well enough.
Reflector/Sovtek do an E84L/EL84M which is a tougher military grade EL84 that takes 13.5W plate dissipation, as opposed to 12W for a bit of leeway if you're going to slap in tubes without checking & good tube vendors often supply "retube kits" for popular amps, where power tubes are selected to run at appropriate bias in the average amp.
I'd perhaps steer clear of NOS types that draw a lot of current compared to the modern tubes the amp was designed to run (unless you get a bias pot installed).
Short answer, if it doesn't sound like you have a problem & the amp isn't overheating & failing, then you probably don't have a problem.
Pro Jr (Blues Jr, Laney LC 15) are not class A amps, they are class AB, like nearly all amps with 2 or more power tubes in push-pull.
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ol'bosey
46 posts
Feb 18, 2011
2:30 PM
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Thanks for the info. If You haven't figured it out by now, I'm not much of a gear head. I will most likely retube it with JJ's (decent bang for the buck), and it's a backup amp.
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