D.I.Y. valve biasing .

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GuitarseGuitarse Frets: 165
edited March 2017 in Amps
One of my amps needs a new set of valves, and, being quite handy, I feel like having a go at biasing them myself. I have no tools for the job yet, so any recommendations welcome, and any hints/tips?

I know it involves a multimeter and a probe. Thought I'd ask here first before going on a Google

Cheers

Al
Never ever bloody anything, ever!
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Comments

  • mli3mli3 Frets: 206
    Carr mercury has a bias check and potentiometer. Check out the user guide on the website it explains it. 
    No sure if that helps…
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  • ecc83ecc83 Frets: 1589

    I have read some bad things about some bias probes. If the amplifier is of a certain design (cough!) bias can be checked in complete safety without one if you follow some simple rules. We need the make and model.

    Yes you need a basic digital multimeter but also some insulated croc to croc leads. Your local Maplins should have both.

    Dave.

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  • GuitarseGuitarse Frets: 165
    OK, it's a Marshall 4210, a JCM800 combo. 
    Never ever bloody anything, ever!
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  • martinwmartinw Frets: 2149
    tFB Trader
    Guitarse said:
    OK, it's a Marshall 4210, a JCM800 combo. 


    As you're handy, instead of buying a probe, desolder the earth connection at pins 1&8 on the output valves and solder a high quality 1 ohm resistor in there instead.

    Then measure away, do the maths etc. Usual safety rules apply.

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  • DJH83004DJH83004 Frets: 196
    It's worth remembering if you are measuring off the cathode, that the current measured is the sum of the Anode current and the screen grid current. As a rule of thumb the screen current is approx. 5% of the total, so this needs to be factored into your calculation. But don't whatever you do be tempted to measure off the anode for safety reasons.
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  • crunchmancrunchman Frets: 11413
    As others have said, use leads with croc clips on the meter.  The most basic rule is never put two hands inside a live amp.  If you want to be really safe, attach both leads to the relevant points before turning the amp on.  Just make sure the croc clip isn't shorting between two terminals of the valve.
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  • ICBMICBM Frets: 71959
    crunchman said:

    The most basic rule is never put two hands inside a live amp.
    And never touch the outside of the chassis with the other hand either. That's why you will often see old-school techs with one hand in their pocket if they have to poke around inside something they're not sure is safe.

    DJH83004 said:
    It's worth remembering if you are measuring off the cathode, that the current measured is the sum of the Anode current and the screen grid current. As a rule of thumb the screen current is approx. 5% of the total, so this needs to be factored into your calculation. But don't whatever you do be tempted to measure off the anode for safety reasons.
    At least with the cathode current method the error is in the 'safe' direction - the valve is running cooler than you think.

    I do use the anode method - it's safe if you use insulated clip leads and check that everything is tight and can't jump off, before you turn on the power. It's the most accurate method.

    NB - by measuring voltage drop across the measured resistance of the OT primary, not the 'shunt' method which is extremely dangerous and also inaccurate, in the 'unsafe' direction.

    "Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski

    "Just because I don't care, doesn't mean I don't understand." - Homer Simpson

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  • DJH83004DJH83004 Frets: 196
    I wasn't suggesting measuring  off the anode is a no go for everyone IC, just anyone who is not that experienced and may be tempted to break into the anode circuit and put their multimeter in series on the mA setting, which in my opinion is dangerous if you aren't a 100% sure what you are doing. 
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  • ICBMICBM Frets: 71959
    edited March 2017
    DJH83004 said:
    I wasn't suggesting measuring  off the anode is a no go for everyone IC, just anyone who is not that experienced and may be tempted to break into the anode circuit and put their multimeter in series on the mA setting, which in my opinion is dangerous if you aren't a 100% sure what you are doing. 
    Yes, agreed - I certainly wouldn't break into the anode circuit, that's likely to cause instability and at best spurious readings even if it's not as bad as the shunt method for that - and is equally dangerous.

    Basically I would never feed current outside the amp, even at the cathode - the right method is always to measure voltage using a resistance inside the amp, either a resistor or the transformer primary.

    (Counting valve-socket bias probes with a built-in resistor as 'inside the amp'.)

    "Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski

    "Just because I don't care, doesn't mean I don't understand." - Homer Simpson

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  • BoromedicBoromedic Frets: 4700
    Bizarrely I was about to ask ICBM and anyone else a question about bias settings and lo and behold this thread pops up! Sorry to hijack it, and also this is a pretty non specific question but wouldn't mind an answer.

    EVH5150 MKIII 100w rebiased by a tech 18 months ago, down from a hot 120mA to a pretty cold 80mA, it's now developed a small fuzzy sound on all channels but loudest on the clean and is headed back to the tech. A quick chat with the main engineer at Fender for this amp reveals a factory bias of 110mA and he says 80 is too cold. 

    Therefore 1) What difference does too cold or too hot make? 2) Would 95mA literally be a good middle ground 3) Any known issues on these from you guys in the know?

    Cheers all and apologies again.

    My head said brake, but my heart cried never.


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  • jpfampsjpfamps Frets: 2723
    Boromedic said:
    Bizarrely I was about to ask ICBM and anyone else a question about bias settings and lo and behold this thread pops up! Sorry to hijack it, and also this is a pretty non specific question but wouldn't mind an answer.

    EVH5150 MKIII 100w rebiased by a tech 18 months ago, down from a hot 120mA to a pretty cold 80mA, it's now developed a small fuzzy sound on all channels but loudest on the clean and is headed back to the tech. A quick chat with the main engineer at Fender for this amp reveals a factory bias of 110mA and he says 80 is too cold. 

    Therefore 1) What difference does too cold or too hot make? 2) Would 95mA literally be a good middle ground 3) Any known issues on these from you guys in the know?

    Cheers all and apologies again.

    The "correct" biasing depends on a number of factors including valve type, HT voltage, class of operation and loading.

    Generally in a 100 W amp using 4  6L6s or EL34s in class A/B push pull a la 5150 the HT will be around 460-500, and the bias will be adjusted so that the valves are drawing between 25 mA and 35 mA which will result in around 50-60% of maximum recommended dissipation at idle.

    If the bias is set too cold, then one side of the push pull output arrangement will be driven into cut off before the other side can take up the slack so you end up with distortion in signal. Harmonics generated by crossover distortion are generally regarded as not pleasant.

    Biasing the amp too hot can imperil the output valves as they will dissipate too much heat at idle.

    Essentially any biasing value below the notional maximum permitted that sounds good is fine, so if you like the sound of your amp biased at 95 mA, then that's perfectly fine as this will not be over dissipating the valves.


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  • BoromedicBoromedic Frets: 4700
    jpfamps said:
    The "correct" biasing depends on a number of factors.......


    Cheers guys, makes sense. 

    My head said brake, but my heart cried never.


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  • ICBMICBM Frets: 71959
    Yes, that slight buzzy sound most noticeable on the clean channel is very likely to be crossover distortion, which sounds nasty and not like normal (clipping) distortion - caused by the amp being biased slightly too cold.

    "Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski

    "Just because I don't care, doesn't mean I don't understand." - Homer Simpson

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  • BoromedicBoromedic Frets: 4700
    Thanks mate, will see what happens!! Cheers 

    My head said brake, but my heart cried never.


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  • DJH83004DJH83004 Frets: 196
    Just had a quick scan at the schematic and the bias is set by measuring the voltage drop across a 1 ohm cathode resistor common to all four 6L6GCs. They recommend 80mV e.g. 80mA, which at 485Va is way too cold, but from my experience, Fenders are always biased cold out of the factory, I was told once by a rep that this was to get them safely through the valve warranty period - but who knows. I certainly think 95-110 mA would be fine.    
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  • ecc83ecc83 Frets: 1589

    All B's fixed biased amps were biased to 25mA cathode current per valve and I never had a punter ask for it to be made hotter.

    The "fashion" for biasing very close to the limit seems to have passed? Not only did it put the valves on the edge but HEAT is the enemy of all things electronic so traffs and caps suffer as well, needlessly IMHO. Then, an amp could be biased to 95% of max Pa but with mains in at 230-240V and it CAN get to 253 and still be legal!

    Dave.

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  • BoromedicBoromedic Frets: 4700
    edited March 2017
    DJH83004 said:
    Just had a quick scan at the schematic and the bias is set by measuring the voltage drop across a 1 ohm cathode resistor common to all four 6L6GCs. They recommend 80mV e.g. 80mA, which at 485Va is way too cold, but from my experience, Fenders are always biased cold out of the factory, I was told once by a rep that this was to get them safely through the valve warranty period - but who knows. I certainly think 95-110 mA would be fine.    
    Thanks Dave, will see what the tech comes back with my mate is concerned there may be something else going on. Or maybe it's just eaten the power valves after 18 months, he plays in a hard rock metal cover band and average about 1.5hrs a week at performance volumes so thats over 100hrs. Designer at Fender reckons a 40hr life span!!

    My head said brake, but my heart cried never.


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  • ICBMICBM Frets: 71959
    Boromedic said:

    Designer at Fender reckons a 40hr life span!!
    That's got to be wrong if they're biased fairly cold. Even if they were biased hot it would be...

    "Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski

    "Just because I don't care, doesn't mean I don't understand." - Homer Simpson

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  • BoromedicBoromedic Frets: 4700
    Direct quote from the message he sent: 

    "I recommend a new set of power tubes if they have 40 or more playing hours on them."

    I don't want to embarrass the guy as he's been really helpful, but even I in my limited knowledge had to scoff at that............

    My head said brake, but my heart cried never.


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  • jpfampsjpfamps Frets: 2723
    ecc83 said:

    All B's fixed biased amps were biased to 25mA cathode current per valve and I never had a punter ask for it to be made hotter.

    The "fashion" for biasing very close to the limit seems to have passed? Not only did it put the valves on the edge but HEAT is the enemy of all things electronic so traffs and caps suffer as well, needlessly IMHO. Then, an amp could be biased to 95% of max Pa but with mains in at 230-240V and it CAN get to 253 and still be legal!

    Dave.


    Of course Blackstar are the last word in valve amplification, so of course that's the way to do it..............
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