Bias, is this the cause of the difference in my 2 JH heads?

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jboyjams
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Bias, is this the cause of the difference in my 2 JH heads?

Postby jboyjams » 30 Sep 2011, 06:02

I added similar info (and pics) to my other thread regarding my new (used) full stack purchase on-line from GC in East Brunswick, NJ - which worked out great and packaging was second to none.

However, it led me into another topic regarding Bias, and I did do some research on it but still have questions....

I am selling one of the awesome big bottom 1982BJH cabs and I originally planned on selling one of these heads (I bought the stack to get the slant cab - I already owned a mint JH head and a 1982BJH) but I can't decide which head I like best - they are very similar and mint - however the amount of breakup and headroom is noticeably different. My new one is not quite as loud and breaks up easier vs. my original head is cleaner and louder. Also, my newer oner has more gain when maxed than my original head - both are excellent to say the least.
I tested them extensively - they both appear stock, even the tubes. (both appear to be as brand new)
This makes me wonder if it's due to bias...?

If biased lower, will this mean more breakup and less headroom? Does higher bias produce more headroom typically? And which one is harder on the amp - low or high? Or would this only affect the power tubes?
Ex: Tube Depot tubes display Bias Point on the tube sticker - 27 is said to be more distortion vs 33 which is more headroom. Does this also relate to the mA or whatever you set the amp bias to?

Also does biased colder mean biased lower? I asked the guy on the phone if biased colder/lower produced more distortion and less headroom but he said that would mean it's biased hotter instead - I wondered if it was opposite...? I'm a little confused here about this subject, obviously - and thought I read the hotter bias produced more headroom, but I may be mistaken. Also - after further review possibly these (lower mA and "Colder") could be 2 dfferent things.... or are they the same?

Is it just 1 internal trimpot adjustment that handles the Bias on this type of amp?

Thanks for your time!
Amps - Marshall Super 100JH, 1987x, 2204 Master Volume, 4502, Bassman 59 LTD, Fender Champ (79), Fender 75
Guitars - Les Paul Standard, ES-335, 62 Strat RI, 85 MIJ Squier Tele, Guild D55
Effects - Picture Wah, Octafuzz, Sunface NKT, KOT, KR Boost, MDV-2, DM-2

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Re: Bias, is this the cause of the difference in my 2 JH hea

Postby surfnorthwest » 30 Sep 2011, 06:12

I am sure Steve will chime in but I would suspect it is things like the tubes installed, and that particular amps transformers making the difference you hear. Of course if the bias of both amps are within range there should not be a big difference at all giving everything else is equal.
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Re: Bias, is this the cause of the difference in my 2 JH hea

Postby captaincrunch » 01 Oct 2011, 06:33

Hey man congrats on the second jimi :Cheers , under biased amp( hotter) is idiling too high so tubes are running hot, causing them to burn out faster and possibly short out. the tubes plates will glow red from heat and the amp will lack punch and might hum.

over biased amp ( cooler ) is idiling too low so tubes are running cool. the amp will sound dirty at any level and will sound low on power. this type of distortion is called crossover distortion. crossover distortion is a non musical type of distortion and is not pleasing to the ear as harmonic distortion.


Taken from the tube amp book from groove tube, i highly recommend this book i got it for £20 from amazon and for £20 you cant afford to be without it.

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Re: Bias, is this the cause of the difference in my 2 JH hea

Postby jboyjams » 01 Oct 2011, 12:28

Thanks Man - much appreciated.

I figure they're both probably within acceptable ranges as they both sound killer and the tubes look to be working good - cool to see that push/pull stuff going on.

I need to start biasing my own amps!
Amps - Marshall Super 100JH, 1987x, 2204 Master Volume, 4502, Bassman 59 LTD, Fender Champ (79), Fender 75
Guitars - Les Paul Standard, ES-335, 62 Strat RI, 85 MIJ Squier Tele, Guild D55
Effects - Picture Wah, Octafuzz, Sunface NKT, KOT, KR Boost, MDV-2, DM-2

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Re: Bias, is this the cause of the difference in my 2 JH hea

Postby captaincrunch » 02 Oct 2011, 02:54

yes invest in a bias probe save's a lotta time and money. need to retube my jimi :cowbell soon, currently been using TAD KT66 and they have been great.

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Re: Bias, is this the cause of the difference in my 2 JH hea

Postby SteveD » 02 Oct 2011, 05:34

If you want the same performance from both sets of output valves then they must be matched for transconductance (gain). This has an impact on clean headroom and distortion. If the transconductances are substantially different (and they can be), they will deliver a different sound characteristic even in the same amp if the settings are left the same for both sets of valves. Both will be great sounds but different as you've already noticed.

Bias has nothing to do with a valve's transconductance except for setting the optimum idle point in the first place due to that transconductance. This is why bias is usually made adjustable.
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Re: Bias, is this the cause of the difference in my 2 JH hea

Postby jboyjams » 02 Oct 2011, 18:06

Thanks a lot Steve, as always - great info.
Amps - Marshall Super 100JH, 1987x, 2204 Master Volume, 4502, Bassman 59 LTD, Fender Champ (79), Fender 75
Guitars - Les Paul Standard, ES-335, 62 Strat RI, 85 MIJ Squier Tele, Guild D55
Effects - Picture Wah, Octafuzz, Sunface NKT, KOT, KR Boost, MDV-2, DM-2

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