Biasing a Blues Junior?

Discussion in 'Amps and Cabs' started by trumpus, Oct 15, 2005.

  1. trumpus

    trumpus Supporting Member

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    Hey,

    I have 2 amps, a Mesa DC-3 and a Fender Blues Junior. I know on the Mesa it is self biasing, so when I change the tubes, i don't need to bring it in. What about the Fender? I have been thinking of changing out the tubes but wanted to know if i needed to bring this in to be biased.

    Brian
     
  2. calieng

    calieng Member

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  3. leofenderbender

    leofenderbender Supporting Member

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    You can apply a mod to the Blues Junior that allows the bias to be adjustable in an attempt to squeeze more tone out of the tubes.

    Bill M's BJ mods
     
  4. Blue Strat

    Blue Strat Member

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    Neither the Blues Jr nor (I'm almost positive) the Mesa are self biasing.

    They're both fixed biased, non adjustable. Both can have bias pots installed.
     
  5. Jon Silberman

    Jon Silberman 10Q Jerry & Dickey Gold Supporting Member

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  6. calieng

    calieng Member

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    Sorry I didn't read the previous thread closely but in any case whether it is fixed or cathode, you can just replace the tubes as long as the new set are not really mis-matched or out of spec. Mismatched ones will cause some hum and out of spec tubes can run too hot or too cold.

    I would only recommend installing the bias pot if you have experience working on amps.

    To answer your original question, you do not need to take it to a tech to change the tubes.
     
  7. trumpus

    trumpus Supporting Member

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    So....yeah, i'm still confused.

    I was told that on my mesa, at least, i didn't need to have it biased. I've changed the tubes on it before and never had it biased, and never had a problem.

    So, whats the concensus? Do i need to have either amp re-biased when i swap the tubes out or no? Can i just pull 'em out and stick new ones in?

    Brian
     
  8. calieng

    calieng Member

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    You can just change the tubes. If it sounds good and they don't red plate you are ok.

    It is handy to get a bias tool to check the tubes are in spec if you want to invest in one. You need a 9 pin unit for EL84s.

    https://weberspeakerscom.secure.powweb.com/biasrite/br_page.htm

    BR-AH (nine pin) $30 - if you already have a multimeter.


    The difference between a fixed bias amp like you have and an adjustable bias amp is that you may come across a set of tubes to far out of spec and you will not be able to use them. They may run too hot and red plate or too cold and sound brittle.

    If you do the bias adjustment mod, you can usually adjust the current to make any set of tubes work. As it is right now for you and 99.9% of the people out there using these amps - you cannot adjust the bias. You simply replace the tubes.
     
  9. Blue Strat

    Blue Strat Member

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    Final word:

    You can replace the tubes in either amp without thinking about it and hope for the best. Worst case, you'll have very short tube life and possibly take out internal components in the amp when the power tube(s) fry. There's probably about a 1 in 6 chance of this happening.

    Neither amp has a bias pot but both are FIXED BIAS (which means that the bias must at least be checked). This does not relieve the owner of checking bias, installing a bias pot, or buying tubes from a dealer who knows which particular tubes (not brand, which particular tubes of ANY brand) that will not self destruct when installed in the amp.

    Redplating: This only occurs when tubes are being run in excess of 200% of their maximum spec. Since tubes in fixed bias amps should be run no higher than 70%, red plating is no indication of anything more than imminent destruction. Hence, it's use as an indicator of safe bias range is completely fallacious.
     
  10. ButchR

    ButchR Komet Player Gold Supporting Member

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    Oh Mike............what do you know about tubes anyway?;)
     
  11. calieng

    calieng Member

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    Gosh...I'm afraid to even turn my amp on now. It's a wonder anyone has ever been able to change tubes in their amp and survived to tell about it.

    :)

    If you want to look at it from an economic point of view. Your pair of tubes cost $15 to $20. An amp tech would want $50 to $65 minimum charge to look at the amp. Your amp is worth $300 used. The amp is designed to just plug new tubes in and go. If you make sure the tubes are not red plating - meaning you don't have a defective tube or you haven't bent a pin putting them in or they are not seriously out of spec then IMHO you are good to go. Next time you buy a pair you can get the pretested ones from Groove Tubes and pick a set in the low rating since the amp tends to run a little hot.

    http://www.groovetubes.com/product.cfm?Product_ID=1175

    There are lots of other suppliers who will sell you a matched set and give you a relative rating for the tube.

    http://www.thetubestore.com/el846bq5types.html

    https://ssl.eurotubes.com/cart/index.php?page=view_products&category_id=3&sub_category_id=18

    http://www.tubedepot.com/el846bq5.html

    I still recommend buying a bias probe just so you can feel certain that the tubes are in spec. That is a good investment. Taking the amp to a tech everytime will end up costing more than the amp is worth. I'm sure all the amp techs reading this will freak out but in the real world thousands of people have bought that amp and swapped the tubes with no problems. The only thing the tech can tell you for that amp is whether the tubes are too hot or too cold in which case you have to buy another set and try them etc etc etc. or pay the cost of modifying the amp to adjustable bias.

    I certainly do not agree with trying to modify the amp yourself for the average user. Fender circuit boards tend to be cheaply made. You can end up cracking it or damaging the solder traces if you are not careful not to mention electricuting yourself if you are not experienced in amp repair. You also will void any remaining warranty. That amp mods page is a nice resource for the experienced amp builder or tech.
     
  12. Curly

    Curly Member

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    uh ... this thread got a little complicated

    just call or email Bob at eurotubes and tell him what style of play you want, and he'll hook you up. while you're at it, get a spare duo for replacement


    on amps with a bias pot, you can set the bias point, regardless of the tube rating

    on fixed bias amps, like the B Jr, Mesas, and certain Fenders like the Princeton, you want to get tubes rated in a range that fits your playing style, i.e. - "loose" or "tight"

    Fender does this with a red, white, or blue color, GT does it with their 1-10 rating, and Eurotubes rates them on a tube tester, with a specific reading.

    I think my JJs are "36s". I've logged plenty of hours on my B Jr, and it sounds terrific. With a little tube tweaking and a decent speaker, they're a very worthy little amp, IMHO.

    Peace
     
  13. jreardon

    jreardon Member

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    For the Blues Junior, the answer is Yes.... been doing this with my BJ ever since I got it with absolutely no issues.
     
  14. Blue Strat

    Blue Strat Member

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    Good point! :p
     
  15. Blue Strat

    Blue Strat Member

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    Economics have nothing to do with maintaining a guitar amp properly.

    If you found a vintage Marshall worth $2000 in a garage sale for $20, would this justify spending no more than $10 to maintain it properly?

    I've stated the relevent facts. Feel free to believe whatever you like.:cool:
     
  16. Blue Strat

    Blue Strat Member

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    Oh, one last thing. BJs bias settings are VERY hot right from the factory. You can either have a pot installed or it's best to ask for a "low idle current" pair of tubes from your tube dealer to prevent untimely death of your EL84s.
     
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  17. calieng

    calieng Member

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    Mike - you make no sense at all with that statement. I said the amp is worth $300 used and you have to decide if you want to pay $50 or more each time just to get a tech to check the bias on a fixed bias amp??? Not to mention maybe waiting a week or more for the tech to look at it. As well the tubes only cost $15 to $20 a pair so who cares if you get 6 months from them or 7 months. For the average user it's fine just to swap the tubes.

    A Marshall amp worth $2000 is worth $2000 no matter what you paid for it and as a more valueable piece would warrant more care.

    Your statement about 1 in 6 tubes being at risk of burning up your amp is also complete BS. Get real. How many people on here actually lost an OT from a warm bias setting? Out of about 40 pair of EL84s I have purchased I had one bad one and it red plated immediately. The rest were all fine. And the one that red plated did not hurt the amp at all.

    As I continue to say. I recommend getting a bias probe just to be safe. Everyone who owns a tube amp and wants to change the tubes themselves should get one. It's a good investment. I am not disputing that you should not take care of your amps. It's just a matter of what is reasonable in this case.

    Feel free to flame me all you want but suggesting a tech has to look at it or that he should go to the trouble and expense to modify the amp is more than what is really necessary.
     
  18. Blue Strat

    Blue Strat Member

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    What does how much biasing costs or how long you have to wait have to do with maintaining your amp, no matter what the cost?

    Also, if you had read my earlier posts, you'd notice that I recommended buying tubes from a dealer who can select the right tubes for your amp.

    Once again, you haven't read my posting. I never said anything about OTs, though they DO get fried from power tubes running too hot...I've replaced many which blew due to impropper bias.

    I was thinking of things like screen resistors. 50 cent items which will knock your amp out of commission, require an amp tech's attention and cost the owner at least the minimum bench charge to replace (maybe more if they're on the PC board).

    Sure, not in the first 5 minutes. I'm sure your experience with 80 EL84s trumps my experience with over 5000.
    :rolleyes:
    Read my posts. I don't see any flaming. I just feel that it's irresponsible to downplay this aspect of basic amp maintenance.
     
  19. ButchR

    ButchR Komet Player Gold Supporting Member

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    Anyone want to buy a blue's jr?:D
     
  20. luwiluwi

    luwiluwi Member

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    I like to use gear page and other forums for research, and ran across this post while looking for information I needed. I found the misinformation disturbing and want to clear up a couple of incorrect things that were said here.
    A “Fixed Bias” circuit is one in which we adjust the bias when we change out power tubes via a trimmer pot, that then remains “fixed,” until we change tubes the next time and readjust it.
    A “Cathode Bias” circuit is one that is dynamic and does not need to be adjusted when we change power tubes.
    Louis Camblor
    USN EE 1977
    Working with pro-audio for forty year.
     

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