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Should I bias if it sounds good?

Discussion in 'Glowing Bottle Tube Amp Forum' started by A13X, Jun 15, 2020.

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  1. A13X

    A13X TDPRI Member

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    I did a complete tube replacement in my amp. (That cheap monoprice/stage right 15w). Before the retube at bedroom level the thing sounded great ,but lately I've been playing it out 3/4 of the way cranked and it honestly just sounded bad. After the retube, WOW!!! I fell in love! It's hard to believe this is a cheap little amp (with about $100 in the tubes)

    So given that it sounds amazing, and I really don't want it to change, should I check and change the bias?

    For those interested here is what I used:
    Position 1 - Genalex Gold Lion 12AX7
    Position 2 - Sovtek 12ax7LPS
    Position 3- EH 12ax7 balanced triodes
    Power tubes - Matched JJ el84s
     
  2. Dacious

    Dacious Poster Extraordinaire

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    I'd suggest your amp may be cathode biased - provided you use matched tubes you should be OK.

    Whenever you swap new tubes in if you hear splutter, crackle or other weird sounds they are probably bad and won't get better.

    Also keep an eye open for redplating within the first hour of playing.

    images (1).jpeg

    Here's a guy tube rolling in a Monoprice. Not a bad summation really. The fun would be rolling some old used AX7s through, 5751, 12AY7.

     
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  3. A13X

    A13X TDPRI Member

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    While I didn't look out for redplating like I should have, it's been played through a few consecutive hours so far. Thing just sounds too good not to play..

    I did recently come across some old Fisher 12ax7s I have 5 or 6 of them I want to run through it, but I didn't know if I should test them first somehow before putting them in.
     
  4. Dacious

    Dacious Poster Extraordinaire

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    If the tubes are white inside they're no good, it means air leaked in. Otherwise if there's no burning or scorching evident they're probably physically OK - they still could be microphonic or sound horrible - bin them.

    If you have a multimeter you can check that there's no elements shorted inside that shouldn't be. On 12AX7s pin 9, 4 & 5 which are the heaters should have connections, but nothing else should connect to any others. A cheap DMM will do for that

    That way you can be sure the tube won't take out your heater windings.

    images (41).jpeg

     
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  5. Milspec

    Milspec Poster Extraordinaire

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    I am in the camp of believing if the amp is quiet after tube replacement...no need to re-bias. I am sure others would disagree, but it is sort of like the old days when we did tune-ups on cars. If you did a full plug replacement along with wires, cap, rotor, and it ran well...the timing was likely close enough to be fine. Same for an amp, if the bias was way off, that amp would be noisey at idle and sound bad playing, so if it doesn't act like that...play away.
     
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  6. telex76

    telex76 Doctor of Teleocity Ad Free Member

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    It may sound fine, but it could go to sounding glorious with a little change in bias. I couldn't stand not knowing the amps full potential.
     
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  7. A13X

    A13X TDPRI Member

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    Those were my thoughts, but I was personally battling with it because I know that "technically" it would be 'proper' to bias. I guess it's not a bad idea to just check it for my own knowledge.
     
  8. A13X

    A13X TDPRI Member

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    Well damn.... Just burst my bubble then. Lol

    So to be clear, it wouldn't be bad for the amp if the bias isn't technically correct, right?
    If that's correct, why don't we just always bias by ear? Provided it's Pot controlled....

    Or can it be bad?
     
  9. Dacious

    Dacious Poster Extraordinaire

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    If your amp is cathode biased you can try it - but cathode biasing tends to be self-regulating. If you go smaller resistor to pull up bias, it can just dissipate more heat through the resistor.

    With grid biasing, small changes can make a big difference.

    People don't play much with 5E3s or Voxes because you can just find headroom drops or you heat up the transformer a lot.
     
    Last edited: Jun 15, 2020
  10. drmmrr55

    drmmrr55 Tele-Holic

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    If it sounds good, don't fix it!
     
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  11. Milspec

    Milspec Poster Extraordinaire

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    ....and that is exactly how i approach it. Just like the car's timing has a factory number, that is always a compromise number. You should adjust it up or down to meet your desired tone. It takes patience as you have to set a base line and then play it for awhile before making another adjustment, but you will end up with the best setting for you in the end. As long as the bias isn't too far extreme hot or cold, you will not damage it. If it already sounds great to you...you are likely already there, play on.
     
  12. Paul G.

    Paul G. Friend of Leo's

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    There is no such thing as "correct" bias. There is a safe operating range, if you're in there and your amp sounds good, you are good.
     
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  13. VintageSG

    VintageSG Friend of Leo's Ad Free Member

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    It's a fixed bias amp and it may benefit you to check the bias.

    Check the attached service manual for the Laney Cub 12r, which is the Monoprice in original form. It gives you the basic check for the bias. 'Better' results may be obtained using probes and meters, but if it sounds good and works fine, enjoy the thing and don't fret.
     

    Attached Files:

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  14. Silverface

    Silverface Poster Extraordinaire Platinum Supporter

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    On a fixed bias amp I always check the bias - as @telex76 says, it may be quiet, but you may not be getting the full potential. I'll adjust it if I get a tonal improvement doing so.

    But on cathode biased amps I simply run voltage tests and if those are OK and it sounds good there's no need to mess with the bias setting.

    On a cheap amp like a Monoprice I probably wouldn't even look at the voltages - if the tubes weren't cherry red and it sounded OK, I'd leave it alone. This is the one type of situation where the "cathode bias=self bias" rule is good enough.
     
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  15. Geo

    Geo Poster Extraordinaire

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    The Monoprice 15 does have a bias adjustment. There are photos of it buried deep in a
    long Monoprice thread on this or the Amp Central. However if doing well then probably fine
    as is without adjustment. One other way to get a fair indication is if you have a kitchen
    temperature probe touch it to each power tube for a minute or two after the amp is
    been on for 5 minutes or so. If the temperatures are within 5 degrees (F) of each other
    that should be fine.
     
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  16. MuddyWolf

    MuddyWolf Tele-Meister

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    Its probably ok. You should learn to find the bias point on the amp though. Why be ignorant?
     
  17. ocduff

    ocduff Tele-Meister

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    I’m not sure my amp tone has been any better since I learned to test for matched tubes in my amp and adjust bias correctly in my amps.

    So - for guys who don’t tech for fun - if it sounds good and isn’t red plating, just play it.

    BUT - having learned to tech at the very least I can make an amp that sounds weak sound good on a technical level. Although I probably could’ve done that just by swapping tubes until one set sounded good. :)

    So...mess with it if you want to, or don’t. This is my existential approach to anything these days...
     
  18. Jack Clayton

    Jack Clayton Tele-Holic

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    You're obviously getting a lot of input from smarter people than me, but my two cents is that you answered your own question with the phrase "if it sounds good". Don't jump at the reflection in a puddle. If you like what you have, I'd stick with it.
     
  19. ricardo1912

    ricardo1912 Friend of Leo's

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    Coincidentally I'm just about to put new tubes in a well used Laney Cub 12r. Being new to doing anything on an amp I checked their service manual and it just talks about putting the tubes in but nothing else.

    Many of the replies above have both increased my knowledge and reassured me. As far as I know the Monoprice is basically the same amp, altho not readily available in the UK.
     
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