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previous: Tarrie Thanks, Jason. Yes, I did not comm... -- 2/12/2000 7:17 PM View Thread

Re: Please help me understand HR50 bias

2/13/2000 3:01 AM
Re: Please help me understand HR50 bias
Tarrie , use this as a base rule of thumb.  
5881 are supposed to be a 25 watt tube and so is an EL34.  
A 6L6GC is a 30 watt tube.  
The idle current can be what ever you like as long as those power rating are not exceeded at idle, the power suply can keep up and the tube is not glowing reddish with high volume playing.  
The idle ratings is a DC rating, not AC... and audio is AC.  
A proper power tube can run all day long at about 160% of it's rating if it has half a chance to stay cool.  
That means a 30 watt 6L6GC can really put out some horsepower with the right power supply and drive voltages.  
Now, from my view point, they all sound the best to me when the idle current is enough that when multiplied times the actual plate voltage (measured while at that idle current), the DC idle power value equals about 60% to 65% of the tube's dissipation rating.  
5881 @ 25 watts.  
65% of 25 watts is +16 watts.  
16 watts divided by 500vdc on the plate (if it really is), is 32ma! Da da!  
Now that amp might sound fabulous with the idle current set so the tube is actually idling at 12-13 watts too.  
This is a sliding scale and there is a fudge factor there where those values can go up and down depending on your ears and the amp you are messing with.  
Now, with a pair of new Svetlana 6l6GCs, I like them around +18 watts but some guys I retube amps for like them much better at 14w to 16w.  
Hard core Fender Black face owners come to mind.  
If I know what the plate voltage is while I'm measuring all this, I can bounce the figures back and forth against themselves until I get the plate voltage and the idle current to come out right where the tube is idling at about 18 watts.  
A little hot, but I like the way it sounds.  
Hope that was helpfull in some way and forgive the typos!  

Tarrie OK, bias in general.... -- 2/16/2000 3:18 AM