Bias Question regarding 2204 Build using 6550's

griff10672

griff10672

Well-known member
In the equation to figure out what the Bias should be set at .... because this is a 50 watt build ( 50 watt OT and PT 's used ) should I consider the 6550's as 25 watts each or 35 ???

my Plate Voltage is about 460 ..... so I used 25 watts in the equation to be safe .... came out to about 38mA .... amp sounds and feels great ... but I just generally want to know at this point ...
 
If those are new production 6550's then you can use 35 watts of dissipation and that is reasonable for new production and not excessive. An NOS GE6550A will dissipate 42 watts.

My experience with 6550's is they do not like a hot bias unlike an EL34. Anything above 60% dissipation a 6550 tends to lose its definition and low end detail. These days I have no problem using 50-55% dissipation. You get longevity and the 6550 tubes run cooler plus keep their composure. You can experiment with 70% dissipation and see if you like it as everyone has different tastes and it really is subjective.
 
harddriver said:
If those are new production 6550's then you can use 35 watts of dissipation and that is reasonable for new production and not excessive. An NOS GE6550A will dissipate 42 watts.

My experience with 6550's is they do not like a hot bias unlike an EL34. Anything above 60% dissipation a 6550 tends to lose its definition and low end detail. These days I have no problem using 50-55% dissipation. You get longevity and the 6550 tubes run cooler plus keep their composure. You can experiment with 70% dissipation and see if you like it as everyone has different tastes and it really is subjective.
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Hey man ... Thanks for the info ... much appreciated !
 
My experience after two 2204 with EH6550 builds: Continue to bias the amp as if it was equipped with the 25-watt/EL34 tubes. 70% to 75% dissipation is where I like it. With the amp’s master volume around 6-ish and getting a good workout for many hours, I’ve yet to notice any problems with the power tubes, and the trannies remain on the cool side.

FWIW, bias current adjusted for 75% dissipation of a 25w/EL34 is still just a bit under 50% dissipation for a 42w/6550… sort of!
 
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460V plate voltage, 70% for EL34 is 38mA. This corresponds to 50% of 35W 6550. If you consider 42W 6550, it's just 42%.
 
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