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Right bias for JCM 800?


 
1/21/1998 1:52 AM
Markus Torvinen
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Right bias for JCM 800?
Hi guys!  
What is right bias voltage and current for Marshall JCM 800?  
Markus
 
1/23/1998 7:30 AM
J Fletcher
Not really a Marshall guy myself,but I repaired one of these a while back.Biased the output tubes to about 35ma per tube.Bias voltage will depend on the tubes to reach this point.
 
1/23/1998 11:41 PM
GVB
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The bias current you mentioned would be approximately right (not that there is an exact proper bias point for the tubes) for a 100W with EL34's installed. But the 800 series of Marshalls used several different plate voltages/transformers and two different types of output tubes, 6550's and EL-34's. This gives various "proper" bias points from 24ma up to 44ma.  
 
The "proper" current could be calculated as follows.  
 
Ik = PA/(2 X V+)  
 
where Ik is the bias current, Pa is the maximum plate dissipation rating for the tube in W, and V+ is the measured plate voltage.  
 
The PA for the EL34 is 25W and the 6550 is 35W.  
 
Although this method sets the bias current to about 1/2 of PA you could also set it between 1/3 and 2/3 of PA safely. However higher currents will age the tubes more quickly and obviously will require replacing more often.  
 
GVB
 
1/26/1998 11:54 AM
J Fletcher
You are right...I was wrong...I made an assumption that we were talking about a 50 watt EL34 equipped amp.I would suggest that the minimum bias point is where the crossover notch disappears,but I've never been too sure where the max point was.Obviously you can't exceed max plate dissipation,but where does the 2/3 value come from?
 
1/26/1998 12:44 PM
GVB
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I use a "generic" method of setting the bias in an amp because the scope method is not reliable on it's own. By this I mean that you may set the tubes to hot if you only look at the notch, and not measure the current. If for instance you are not using matched tubes, the notch may not disappear until one tube is running to hot etc... Setting the bias point to 1/3 to 2/3 of the dissipation rating is strictly empirical. I have read a few articles over the years, and these suggested that beyond 1/2 there was not much of an improvement in sound, but there was shortened tube life. You can check if it sounds OK at 1/2, and adjust from there to suit your needs.  
 
I forgot to mention though, that these settings should be done with no input, all the volumes at zero, and should supposedly be done with no load (i.e.. no speaker). I really don't see this last point, because there will be no power/signal transfer in the output transformer because there is no signal, only DC, and therefore a load should make no difference to the tube or OT for that matter.  
 
In most of the amps I've biased, I find that the bias does need to be just a little hotter than 1/2 for my tastes, but not that much more. Beyond that, the article is right - I did not notice an appreciable improvement in tone, and I'm REALLY picky about tone. I've found that most of the articles that say to set your bias point to _ current at _ voltage with _ type of tube, do indeed go just slighly hotter than 1/2 PA.  
 
GVB
 
1/26/1998 2:13 PM
Doc
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".....supposedly done with no load (i.e.,no speaker)"  
 
This should be done not using a speaker, yes, but there MUST be a resistive load connected, corresponding to the nominal impedence required by the output winding. (For an 8ohm speaker, use an 8ohm power resistor.)  
 
What you are setting is quiescent idling current (zero signal operating point) of the power stage. you need a correct size load connected with resistance but no reactance. That's why the speaker gets disconnected, but it gets temporarily replaced with a resistive load. You can possibly fry an output transformer primary under certain conditions with an open secondary. Don't take the chance.  
 
Doc
 
1/26/1998 3:02 PM
GVB
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Exactly, this is what I was taught, but it doesn't make sense to me as far a theory is concerned. At 0Hz/DC regardless of a resistive or reactive load in the secondary, there should be no power transfer, no stress on the primary or secondary, and the impedance "reflected" by the secondary will be infinite and make no difference on the primary. I suppose that if you change the direction of the current (i.e. raising the bias voltage and then lowering it) would cause the power to be transferred to the secondary.  
 
The only thing I'm not sure of, is the point when having too high an impedance on the secondary becomes a problem. For instance with speakers, it is not a problem to use a higher impedance speaker than specified by the OT taps. But at what point will "damage" occur to the OT? Can anyone enlighten me on this?  
 
GVB
 
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