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Re: Biasing Marshalls


 :
3/13/1998 9:03 PM
Bruce
Re: Biasing Marshalls
I think Jim also ment to imply that is probably a good idea to take the power tubes out of their respective sockets when you try that bias voltage sweep test.  
It would probably not hurt anyting going up ( closer to 0 is up from negative) to -25 to -30 volts for a short time while mesuring, but what a drag if the pot broke or came undone or you lost all the bias voltage while the circuit was being tested! I don;t know how long a pair of EL34s can go with zero bias. Better safe then sorry.  
And although I have a very nice scope too.. I NEVER set power tube bias with it..... I think it is a COMPLETE waste of time. I don;t like the way they sound like that.  
As a matter of fact, I don't know anyone who really likes the way they sound when biased cold like that.  
Save the scope for trouble shooting or roughing in the bias in a new tube amp design.  
Jim is right about wanting to know the current too.  
The pretty picture won't tell you much about that.  
 
Bruce
 
3/16/1998 4:44 AM
liam
What bias current
Thanks for the info. It's been most helpful. Just a couple of questions. What cathode resistor power rating would you suggest? Also, is 35mA a good bias current for all EL34 valves? I'm currently using Svetlanas, and they sound good. Possibly a bit cold, but really punchy.
 
3/19/1998 11:25 PM
Bruce

The bias current you mentioned is good.  
You could go up or down a little more from there depending how you like the way it sounds. Personaly, I like the way a pair of PP EL34s sound at around 32ma to 36ma with about 425 volts on the plates.  
 
If I used a cathode resistor as a current meter  
shunt , ( I don't ), I would use 1% 1ohm resistors  
at 1/2 watt. If you developed a dead short in a power tube, the tiny 1/2 watt resistor would blow apart and act like a fuse. That could save you some cash later!  
I'll probably get flames for this but...you should be able to get away with 1/4 watt resistors here in a 50 watt amp.  
1/4 watt precision resistors are easy to find.  
A 1% 10 ohm resistor would be more accurate but you have to take that extra resistance into consideration when reading the MV scale of your meter.  
Look at it like this:  
A big fat 200ma flow of current comes up to the cathode through that 1ohm 1/4 watt resistor.  
Ohms law:  
power = E x I  
You measured the voltage across the 1ohm shunt at 200mv and derived that it also means 200ma of current too..  
Well, 200mv x 200ma = .04watts.  
Or, I sqd x R = watts or .2 x .2 x 1= .04watts  
That's not even close to 1/10 of a watt.  
Not much loss to heat there.  
 
I like the Svetlanas. They sound great at 36ma.  
 
Bruce
 
3/20/1998 7:51 AM
liam

So which method do you use, Bruce? I've tried using the output transformer as a shunt, but my Fluke seems a bit too high resistance, and only reads to 10mA. It's got quite a good mV range though.
 
3/21/1998 12:18 AM
Bruce

Oh.. I do use the OT shunt method.  
If I think of it in time I will install the 1ohm or 10ohm resistor. But, I find that I can whip through a bias adjustment much faster with the shunt method and it is a lot easier...but more dangerous too.  
I have blown quite a few fuses in my DMM by getting lazy and using the probe instead of the clips leads. The probe slips... pffft!  
Your Fluke meter should be able to read the current with no trouble. I can only imagine that you might not be using the right shunt on the meter to tap the current.  
Look in your owners manual and see if there is anything the jumps out at you that will help.  
Have you tried reading the current with the max scale first and working your way down to the hundreds of miliamp setting?  
 
Bruce
 
3/21/1998 6:32 AM
liam

It's an auto-ranging meter, so I don't get a choice of shunt, it's preset. I blew 2 fuses when I was trying to do this last time! Quite exciting, isn't it?
 
3/21/1998 8:56 PM
Bruce

Hmmm....I think there might be something wrong with the meter then.  
I have had plenty of auto ranging meters including Fluke and none have blown a fuse when checking current,,,,If I hooked them up right that is! : )  
Bruce
 

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