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Does my amp actually need biasing?

Discussion in 'Glowing Bottle Tube Amp Forum' started by GermanGuitarNoob, Nov 22, 2018.

  1. GermanGuitarNoob

    GermanGuitarNoob TDPRI Member

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    Hello folks,

    I´m new to this forum so I hope I don´t step on anyones shoes by opening this thread.
    Thing is, I bought new matched 6L6 Powertubes for my PCL Vintage Amp - Sa2 Tube Amp (build in Germany in the eigthies, I heard it´s somewhat similar to the Mesa Boogie Mark I, but I´m not too sure about that).
    Since I´m sort of new with tube amplifiers and short on cash, I watched a few tutorials on what to pay attention to when swapping tubes. One thing that came up was biasing.
    So I opened the amp and had a look inside. I´m not sure if there is an actual bias-potentiometer inside, I also couldn´t find a testing-point where I could get a contact with the voltmeter.
    I also read that there are cathode and fixed bias amps, and that one of those doesn´t actually need rebiasing when swapping tubes. Could this amp be one of those who don´t need biasing? I really don´t want to break the tubes by just trying...
    I´m happy for every tipp and opinion.[​IMG]

    Image of the Amp´s Insides: https://imgur.com/a/7sOV8sY
     
  2. brogh

    brogh Moderator Staff Member

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    Ja hallo und Wilkommen !

    well somebody will need to follow some circuit lines there to tell, seems very nicely built, i suppose you don't have a schematic at hand right ? similar to one model of amp could mean a lot of things, the guys here will probably help you out.

    but it think the answer is usually a tube amp does... what tubes do you have on board actually ?

    Mesa usually has a fixed cathode bias, not sure about the mark1, mesa advises replacing the tubes with mesa marked ones because they will be in the realm where the amp works best.

    looks like a cool amp, can we have some other pics ?

    on a side note, you need to pay attention when opening up a tube amp, it ca really kill you if you 're not sure about what you're doing, be safe


    have fun !
     
    Last edited: Nov 22, 2018
  3. Old Tele man

    Old Tele man Friend of Leo's

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    IF cathode-bias, answer is no.

    IF fixed-bias, answer is yes.

    HOWEVER, it is ALWAYS a good idea to MEASURE/CHECK bias whenever output tubes are replaced because today's tubes are rarely 'close enough' for plug-n-play operation.
     
    GermanGuitarNoob likes this.
  4. clintj

    clintj Friend of Leo's

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    I see what looks like a fixed bias circuit, but no adjustment potentiometer.

    A few options for you:

    A. Install the tubes, let the amp stay on for a few minutes while you play and watch the tubes, and see if you notice any redplating. This is the probably the worst approach from a tech standpoint, but it does work most of the time. You're risking damaging a set of tubes to find out, just so you know.

    B. Buy a bias probe and learn how to take measurements to calculate bias. This is a good method for an owner to know, but if you're uncomfortable with high voltage this isn't a great way to learn. Also, if the bias is too hot or too cold you would need to adjust it which is not the easiest thing on a circuit board amp, but is possible if someone experienced can guide you. You have to swap a resistor out to adjust bias which involves soldering.

    C. Amp tech. It'll cost you a little money, but he'll check and adjust bias for you and has the skills and tools to do it quickly and easily.

    Hope this helps.
     
  5. GermanGuitarNoob

    GermanGuitarNoob TDPRI Member

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    Thanks for the many quick responses and tipps!

    @brogh It runs on two 6L6, three ECC 83 and one ECC 81. Unfortunately that´s all the information I got. I had a chat with the manufacturer a few days ago, and they completely swapped to producing (really well sounding) solid state amps, so he couldn´t really provide much more information about the older tube-powered models the company had built in the 80s.
    I made a few more pictures though:

    https://imgur.com/ufkoMtv
    https://imgur.com/hyRNcbQ
    https://imgur.com/RogOhu4
    https://imgur.com/I1mcZUc
    https://imgur.com/yo7lL9H

    Could the circular component in Pic N° 5 be the biasing-potentiometer?


    @Old Tele man Thanks for the tipp!

    @clintj Thanks, this helps a lot! I guess I will probably go for Option A and C if it fails. I really wouldn´t feel comfortable soldering around in an amp without any knowledge of electrical engineering.. I´d probably break the whole amplifier 20 seconds in.
     
  6. clintj

    clintj Friend of Leo's

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    That close up picture is much better. That's definitely an adjustment potentiometer, now that I can see it clearly and see the screwdriver slot and connections.
     
  7. brogh

    brogh Moderator Staff Member

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    oh wow !

    looks really cool !!!! Ev speaker, a MASSIVE power transformer, looks really nice !!!

    You found a pot i see, i don't know where you live in Germany but i'd second the advice of clintj about the tech, here it costs 40 bucks to have it checked and biased by a proper tech, probably you find some tech around there for that price, I'd invest that money to be sure my amp comes out as new :)
     
  8. GermanGuitarNoob

    GermanGuitarNoob TDPRI Member

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    Alright, guess I got my answer then :) Tech it is!
    I actually called my local musicstore (which also happens to have a workshop for guitars and amps) earlier today. I was told that they charge 50 bucks solely for opening up the amp and having a first look. I hope the biasing won´t be too expensive there. (And that they don´t charge another 50 to close the amp in the end)

    Thank all of you so much! You really helped me out here!
     
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  9. Wally

    Wally Telefied Ad Free Member

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    That amp is of an age that it needs a tech evaluation anyway, imho. I equate playing on a newly acquired amp without a good technical assessment of what is there to be the same as buying a used motor vehicle and taking off on a thousand mile trip without checking fluids, belts, hoses, brakes, etc.
     
    brogh likes this.
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