Bias question !

[Phoenix1]

I just got done with the JTM45+ from Tube Depit. Did great and worked great and still does.
OK I just finished a Mojotone 1959 super Lead/bass. Did the kit for lead. Anyway I was perfectly ready to hook up my bias meter. But watched a you tube on biasing 100 watt 1959 super lead amp and the guy said for these the bias was set at 90 to 100% for four EL34 tubes ???? Sounded kind of much to me and thought I would ask you guys ? I know after all the dust settled on my JTM45 it landed on about 70% and 37mA. Any ideas there ??

 

[Pete Farrington]

watched a you tube on biasing 100 watt 1959 super lead amp and the guy said for these the bias was set at 90 to 100% for four EL34 tubes ???? Sounded kind of much to me

Yes, on the face of it that sounds too high.
Link to video?

 

[Gutch220]

90-100% seems way too high

the only way I could see a person wanting to do this is if they were recording with it for only a short period of time and wanted a super-hot amp tone.

 

[paul-e-mann]

I just got done with the JTM45+ from Tube Depit. Did great and worked great and still does.
OK I just finished a Mojotone 1959 super Lead/bass. Did the kit for lead. Anyway I was perfectly ready to hook up my bias meter. But watched a you tube on biasing 100 watt 1959 super lead amp and the guy said for these the bias was set at 90 to 100% for four EL34 tubes ???? Sounded kind of much to me and thought I would ask you guys ? I know after all the dust settled on my JTM45 it landed on about 70% and 37mA. Any ideas there ??

The Mojo instructions should explain biasing the amp, no? A quad of tubes are bias'd in pairs, do each pair at 37mA.

 

[Phoenix1]

Yes, on the face of it that sounds too high.
Link to video?

Of course i couldnt find the video I was referring to but the one I basically followed and liked is this one. Also it has a good bias calculator he offers up !

 

[Phoenix1]

The Mojo instructions should explain biasing the amp, no? A quad of tubes are bias'd in pairs, do each pair at 37mA.

Mojo does not provide instructions ! They provide a schematic with no specs and a layout sheet for Super Lead and one for Super Bass. They will on asking provide you with a link to Robinette's "biasing a tube amp"

 

[Phoenix1]

The Mojo instructions should explain biasing the amp, no? A quad of tubes are bias'd in pairs, do each pair at 37mA.

Yes I know the range of mA for EL34 valves and did that with my JTM45+ from Tube Depot. I do follow and recommend this video !

 

[Phoenix1]

90-100% seems way too high

the only way I could see a person wanting to do this is if they were recording with it for only a short period of time and wanted a super-hot amp tone.

Seemed WAY too high to me also and the reason I posted this question ! 36 or 37mA is good

 

[Blake F]

In practice for EL34's, my experience is that the range is 32-38ma. 32 is usually pretty cold and 38 pretty hot. There is a difference in tone and feel and you should audition the tone and see what you like. There are exceptions to the milliampere range I've suggested but it's pretty accurate for most situations with EL34's.

 

[Pete Farrington]

i couldnt find the video I was referring to but the one I basically followed and liked is this one

He's setting the bias to idle at about 18 W. Which is much more sensible than the 22-25W mentioned in post 1.

 

[playloud]

I know most/all of you already know this, but for the benefit of others who might be reading, I think it's worth stressing that (idle plate) current alone is insufficient to determine a suitable bias point for power tubes; you also need to know the plate voltage (which you multiply with current to determine the level relative to max dissipation).

I have EL34-equipped amps with plate voltages from 400V to 750V, and a given current - say 35mA - could verge on too cold in one amp (400*0.035=14W) and result in meltdown (750*0.035 = 26.25W!) in another.

 

[neikeel]

Playloud beat me to it.
I can’t emphasise how quoting mA figures in isolation is utterly meaningless.
You need to know the anode (pin 3, plate) voltage to make the wattage calculation to be no more than 70% of maximum rated output of valve. EL34s in push-pull are 25w. So max is 70/100 x 25 is 17.5w.
There will always need to be some shimmying back and forth measuring and repeating as the voltage will drop when the valve draws more current. The method of actually measuring the current itself is relevant as 1ohm cathode resistor method includes screen current (a few mA). Also if you have these installed inside your amp you cannot use most bias probes.