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Re: Setting Bias


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7/14/1998 11:06 AM
Steve Jones
Re: Setting Bias
Hi,  
 
I have a question from the Tube Amp FAQ. It talks about the Output Transformer Shunt Method where you hook a current meter from the plate (anode) across "half of the primary  
of the output transformer". My question is, what does this mean? In my circuit, the plate of each 6L6 is connected to one end of the OT primary. Do I hook one meter connector to the plate pin of the 6L6 and the other connect to some spot further down the OT primary wire connected to that same pin?  
 
Also, my circuit does not have a bias pot (this is a 1950's monoblock tube power amp that I am turning into a guitar amp). How/where do I install a bias pot? I haven't seen instructions on that anywhere yet. Could someone point me to some?  
 
Any help appreciated. Thanks!  
 
Steve
 
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7/14/1998 4:13 PM
R.G.
On push-pull amplifiers with two output tubes, each tube plate connects to one end of the output transformer,and the center tap connects to the B+ power supply. "Half the primary of the output transformer" means the positive lead on the B+/Center tap, and the negative lead on a tube plate.  
 
You need to check the wiring to be sure that you have one tube plate each on opposite ends of the primary and the B+ attached to the CT in the middle, but this is almost certainly what you have. Check to be sure - and with the power off while you check!!  
 
While you're checking, see if you have the output tube cathodes tied to ground, or to a 100 to 300 ohm resistor. If the cathodes are tied to ground, you have a "fixed bias" output stage, and this advice applies. If they are connected to ground through a large power resistor, it's a cathode biased output stage, and although you can adjust the bias point, you can't easily do it with a to-be-installed bias pot.  
 
The idea is that with the meter set to a current range, the internal resistance of the meter is less than the resistance of the half of the output transformer primary it is in parallel with, and almost all the current flows through the meter to be measured. It's not perfect and it certainly is dangerous (with probing the live B+), but it sure is quick. The errors with this method are just that some of the current still flows through the transformer unmeasured, but it's not too bad.  
 
You haven't seen advice on wiring in a bias pot because it is tricky to write down how to do it in general, for all cases.  
 
To install a bias pot:  
 
(1)trace the wiring to find out how the negative voltage that must exist on the output tube grids is generated. If they are each tied to ground through a 100K to 2M resistor, then your amp is most likely cathode biased, and the following does not apply.  
(2)Once you have traced the wiring and found that the grids tie to a source of negative voltage through the same large-value resistors, trace the wiring of that negative voltage. It is probably made from either the B+ winding on the power transformer or a separate tap that makes -80 to -100Vdc through a half wave rectifier and filter cap, possibly some resistors. In either case, there is a resistor divider that the two output tube grid resistors connect to.  
(3)Find that resistor divider, and replace it with a divider that has a potentiometer in the "middle", voltage-wise, that allows you to get, say, -30 to -60V on the wiper. Reconnect the grid resistors to the wiper. As a "fail safe" in case the wiper opens, tie the wiper to the -60V end of the pot with a 100K to 500K resistor.  
 
Which amp are you modding? That would help.
 
7/13/1998 9:53 AM
Jim S.

It's interesting to note that, in the tube audio hi-fi world, power amps have always been biased by measuring the current going through the tube, and not by using a signal generator and a scope. A fair number of hi-fi tube power amp models have been manufactured over the years that have test probe access points on the outside of the chassis just for performing this type of adjustment. (And in the MI world, the famous Ampeg SVT bass amp was built with these access points).  
 
The problem with using ONLY a signal generator and a scope is that you are not measuring anything! Also, observing a waveform and deciding when there is no visible cross-over distortion can be rather subjective and not easily repeatable.
 
7/14/1998 4:28 PM
anonymous
Here's my personal way of biasing that will no doubt disgust and horrify you guys: I seem to like the sound of the amp with a lot of juice on the tubes. The sound seems big and fat with huge amounts of bass. I turn the bias pot until the plates just turn barely red, then I ease off a bit to where I get the best sound and the plates are their normal gray.(btw, it'a 5f6a)  
 
I never understood about lower plate voltage, and a browner sound. Sure it seems to distort easier, but at higher voltages you thump on that E string and boy, you feel it! Comments welcome.
 
7/16/1998 9:08 AM
R.G.
If it works for you, use it.  
 
What you outline is the procedure for getting the output tubes as deeply into class A bias as you can without melting them. This means that they're always running at about their dissipation limit; if you are happy with the tube life you get that way, great.  
 
The classical problem with biasing by ear or by oscilloscope is that people LIKE the sound biased as far into class A as they can get away with. It reduces the crossover distortions that make the sound cold or grating. The Tube Amp FAQ goes into this in more detail. Biasing is usually a process of trading off available output power and higher distortion versus lower crossover distortion and higher power dissipation in the output tubes. You're avoiding the problem of inadvertently getting them too far into class A by putting them into thermal overload deliberately and then backing them out.  
 
I might suggest a slowed-down DC fan in the chassis to keep them running longer, as well as periodically checking them to ensure that they haven't drifted back in to the red-plate zone.  
 
If it works, use it.
 
7/16/1998 1:04 PM
anonymous Re: Setting Bias/Question for R.G.
Thanks R.G., I've always felt I was doing something wrong although I loved the way it sounded.  
Here's one more question: Assuming I continue to set the bias as I do, are there any particular tubes that are known to sound especially good at these higher settings?  
(and can take the heat)
 
7/16/1998 1:44 PM
R.G.
It's not wrong, just a different means to an end. The important thing is you do identify the danger limits and have a repeatable procedure to stay inside (outside??) them.  
 
I don't know that there is any particular tube that sounds good for the condition. The common wisdom is that all tubes sound best just in the seconds before they melt down and the molten glass vents the atmosphere in - I have a tube like this, by the way - but I don't know that you could pick one that's best at high bias. I personally would pick a tube I could get ready replacements for, like say a russian 5881 or a EL84, and diddle with it until I liked the sound, but that's me.  
 
In either case, you can make most tubes take the heat by the fan trick. Full wave rectified 6.3VAC gets you about 8-9Vdc, and this runs a 12VDC rated muffin fan slowly. I've seen this reduce the glass temperature on 6L6's from hot enough to actually sizzle a wet fingertip down to warm to the touch but not too hot to pull, same electrical conditions.
 

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