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Post by prowler on Mar 28, 2006 22:31:50 GMT -7
I currently use an SRZ-65 with GTEL34LS tubes rated #6. I decided to find out what my SRZ's bias was set for. I checked my plate voltage & the bias setting. The readings were as follows.
Voltage 435 Bias 34.5 mA
I took the GTEL34LS tubes rating which is 30 watts and used the following equation to calculate my bias.
30*600/435 = 41.37 mA
That being said why is the recommended bias setting for my amp 34 -37 mA directly from Z & not 41.37 mA which is 60% of idle ?
34.5 mA would mean my amp was running at 50% of idle.
30*500/435 = 34.48
In order for me to get 60% of idle & have my bias reading be 34.5 mA I would need to use the following equation-
25*600/435 = 34.48 mA
So what gives here? Why is the recommended bias setting so different from what the equation says my bias setting should be? Is the EL34-LS a 25 watt or a 30 watt tube?
Anyone....... Myles?
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Post by myles on Mar 29, 2006 9:59:46 GMT -7
I currently use an SRZ-65 with GTEL34LS tubes rated #6. I decided to find out what my SRZ's bias was set for. I checked my plate voltage & the bias setting. The readings were as follows. Voltage 435 Bias 34.5 mA I took the GTEL34LS tubes rating which is 30 watts and used the following equation to calculate my bias. 30*600/435 = 41.37 mA That being said why is the recommended bias setting for my amp 34 -37 mA directly from Z & not 41.37 mA which is 60% of idle ? 34.5 mA would mean my amp was running at 50% of idle. 30*500/435 = 34.48 In order for me to get 60% of idle & have my bias reading be 34.5 mA I would need to use the following equation- 25*600/435 = 34.48 mA So what gives here? Why is the recommended bias setting so different from what the equation says my bias setting should be? Is the EL34-LS a 25 watt or a 30 watt tube? Anyone....... Myles? A lot of folks just use around 60%ID and more as a rule of thumb but this is really a bit of misinformation from the radio days and transmitters etc. In a Marshall type amp as an example, some folks bias them up at 40mA and more .... but this makes the amp sound harsh and gets rid of too much of the crossover notch distortion to the point of loosing the characteristic Marshall crunch. You also have amps when underbiased and running too hot where the mid level settings have no life at all. Sort of like setting the idle on your car at 4000 RPM ... runs great down the freeway but will be less than satisfactory at other speeds. The 30 watt LS rating just means that the tube can easily produce and hold up to that rating, but has little to do with the tonal aspects of the tube. I run my SRZ at 37mA and Carl's is at about 34mA at the moment.
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Post by prowler on Mar 29, 2006 10:28:06 GMT -7
Myles...
That being said, how do I go about figuring the correct bias for other peoples amps? Now that I have the tools & knowledge to bias amps I plan on biasing amps for my friends so I can save them money & time.
Should I use the 60% of idle formula or should I be using something else?
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Post by myles on Mar 29, 2006 14:14:52 GMT -7
Myles... That being said, how do I go about figuring the correct bias for other peoples amps? Now that I have the tools & knowledge to bias amps I plan on biasing amps for my friends so I can save them money & time. Should I use the 60% of idle formula or should I be using something else? Prowler ... It really varies amp to amp. 50-60% is a good rule of thumb but some amps such as Fender 6L6 amps all seem to love about 30mA per tube no matter what the plate voltage! I genrally do Marshall Plexis, metal face, and even up to the DSL and TSL amps when they have EL-34's at about 36-38mA per tube. On cathode biased amps I run at 100%-120% In any case .... on my website at www.guitaramplifierblueprinting.com/index2.html in the uppermost right hand corner there is a secret little red button that has no label. This will download an excel sheet to your PC where you plug in the tube rating in watts (anode dissipation) and plate voltage. There is a list of some of the more common tubes as there are differences in some EL34 tubes and 6550 and KT88 types. This is also what I show to folks when they want specs on some preamp tubes. Since I carry no laptop or cell phone anymore after carrying them for ten years at Infonet ... but am always around folks with PCs and they always have a wireless network up, having this little button here makes it easy for me to pass on info to techs without having to write it all down. Once it is on their own PC if the save it, we can make notes so they can support their own touring folks etc. 50% ID is a good starting point for most grid biased amps.
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