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Bias a Marshall 2204


 
3/2/2000 2:41 PM
Stefan
Bias a Marshall 2204
Help needed!! What I have to do, to bias my amp proper?  
I have heard, that you can messure the current between the  
output tube and the middle of the x-former. What is the correct current (Tubes are EL 34).  
Is there any other methode for biasing?  
Thank,  
Stefan
 
3/2/2000 5:51 PM
PTJohn

There is the time-tested method of putting a 1 ohm resistor in between the cathode of the power tube and ground. This is the principle behind the "bias probe." I built one, and it works great. As far as correct current, that depends on your plate voltage.  
You could also put the amp into a load, crank it up clean, and use an oscilloscope to notch out the crossover distortion.  
And, there's always "by ear", but be VERY careful.
 
3/2/2000 6:08 PM
Liam

quote:
"You could also put the amp into a load, crank it up clean, and use an oscilloscope to notch out the crossover distortion.  
And, there's always "by ear", but be VERY careful."
 
 
I'd avoid either of these. The crossover notch never quite disappears on some Marshalls, and by ear can leave you with very short tube life, I know 'cos I tried!  
 
Bias probe is safest and possibly the best way of doing this, but I actually check the voltage across each side of the OT at idle, then turn off, pull the tubes, and check the DC resistance of each side of the OT (it does differ). It's an amazingly accurate and easy technique, but your DVM is floating at B+ SO BE CAREFUL.  
 
In my experience most 2204s don't seem to need much more than 32 mA per tube, but I know a lot of people bias them a bit hotter. You can work out the theoretical maximum from the plate voltage and dissipation, but 32 mA is in the ball park.  
 
Hope that helps  
 
Liam
 
3/2/2000 8:13 PM
PTJohn

You're definitely right about the o-scope stuff as well as the "by ear"; that's why I built a bias probe. The current can vary by so much on a Marshall, though. My 2 are the same type (4010), and there is a 100v difference in plate voltages. I recommend setting it by current.
 
3/3/2000 4:06 PM
Liam

quote:
"The current can vary by so much on a Marshall, though. My 2 are the same type (4010), and there is a 100v difference in plate voltages. I recommend setting it by current. "
 
 
I know what you mean about the different plate voltages, but my method does actually (indirectly) measure the current. Basically if you've got say 0.56 volts across a 16 ohm output transformer (or actually half of it) at idle, the bias current is V/R = 35 mA.  
 
Most people don't seem to use this method, but it's quick and easy, more accurate than the traditional OT shunt, and less hassle than a bias probe IMHO. The bias probe has the advantage that you can check tubes individually, but I've never been too worried about precisely matched sets.
 
3/4/2000 4:24 PM
PTJohn

You do have a point, I just like the Bias Probe method because I built my own box to test it, and you know how fiddlers are; they'll go through hell to use their own stuff. But seriously, the Plate Voltages make a huge difference in the Bias current, which is why I've never trusted the "schematic" voltages and recommended currents. If I hadn't checked the model #'s on the xfmrs, and found that they are the same part #, I would have sworn that they put a 100W PT in mine, since the voltages on that amp line up almost exactly like the 100 W version. I DO know that the grid voltage method is completely unreliable, which is what most Marshall techs in my area use, since the voltage is on the schematic. That's why I had to teach myself tubes.  
Also keep in mind that this page, Dan Torres (no flames, please), Pittman's book, and the great Mitch from Antique Electronics have taught me most of what I know. If I'm off base on a posting, please correct me. I got a lot to learn.  
Later.
 
3/2/2000 11:59 PM
Tonefactor

The way you describe is called the transformer shunt method, and is by far the quickest and easiest way to bias a Marshall. It is also the most dangerous, so be careful.  
 
The current should be right around 38-40 mA.  
 
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