biasing an amp by OT viltage drop.

[Rotorcraft230]

So I'm trying to measure the bias on a Mesa Triple rectifier amp with 6 6L6GC power tubes. I like using the OT method, Vd/OT ohms X Pv and that gives you watts. I have done this with 2 power tube amps but not with multiple tubes per each half of the OT. The Meas has 3 tubes per side.
Here are the numbers I get. Ill do only one side for simplicity as both are just about the same.

OT ohms per side of CT 22 ohms

Vd 1.3
ma .059
Pv 464

watts 27 watts.

all three tubes are the exact same number which seems odd to me as they cannot be that closely matched. 70% of 30w is 21 watts. I'm sure Mesa didn't bias the amp that hot, what am I doing wrong? The one thing I didn't measure was the -dc in pin 5, I can do that when I get home and post it, but as far as measuring tube current, am I doing it right?

thanks

 

[FleshOnGear]

@Rotorcraft230, wouldn’t that be dissipation of 9 watts per tube if there are three per side? Just making sure I’m not missing something.

 

[Matthews Guitars]

I wouldn't trust that biasing method. I'd use a bias probe or insert 1 ohm resistors on the tube sockets and measure across them.

 

[Rotorcraft230]

@Rotorcraft230, wouldn’t that be dissipation of 9 watts per tube if there are three per side? Just making sure I’m not missing something.

I doubt it, it’s a Mesa Triple rec, it’s gotta be loud than that.

I wouldn't trust that biasing method. I'd use a bias probe or insert 1 ohm resistors on the tube sockets and measure across them.

Not my amp to add the resistors, I do it this way all the time and it’s very accurate, just haven’t done it with multiple tubes per side, looking for advise if it’s right or wrong.
Like I said earlier, I doubt Mesa would bias them that hot.

 

[FleshOnGear]

I doubt it, it’s a Mesa Triple rec, it’s gotta be loud than that.

Biasing doesn’t have that much to do with how loud it is. I’m just trying to make sure we’re on the same page - if you’re measuring a 1.3 volts across 22 ohms, there’s 464 volts and 3 tubes on that side, then your 27 watt deduction is divided between 3 tubes at 9 watts per tube, assuming the tubes are all drawing the same current. That is cold, but it wouldn’t make the amp quieter.

 

[Rotorcraft230]

Biasing doesn’t have that much to do with how loud it is. I’m just trying to make sure we’re on the same page - if you’re measuring a 1.3 volts across 22 ohms, there’s 464 volts and 3 tubes on that side, then your 27 watt deduction is divided between 3 tubes at 9 watts per tube, assuming the tubes are all drawing the same current. That is cold, but it wouldn’t make the amp quieter.

Thanks for the explanation, I figured I was missing something, 59 ma would be the total current of the 3 tubes combined. As far as how loud the amp is we haven’t done that yet, we have a full stack with G12T75s we are gonna fun it through, that will tell us how loud it is.

Thanks

 

[Matthews Guitars]

Mesa biases the amps to run with their pre-tested, pre-selected tubes across all the ranges they classify and sell them as.
They reject tubes that would bias too cold or too hot in their amps. So it's fair to guess that their factory bias setting is going to be right in the middle of the typical range for the tube type.

Some people disagree with Mesa's notion of not giving you adjustable bias but I see the sense in it. A tube might fail during a gig, so you'd put in your spare pair you bought from Mesa, and no bias adjustments required. Or you could put in tubes from another vendor and end up with "edge case" tubes in your amp, causing harsh crossover distortion or, worse yet, meltdown.

I have had excellent results with Mesa branded tubes. They have been consistently reliable and not gone out of limits on bias, so as far as I'm concerned, their system as they mean for it to be applied works well and I recommend it.

Buy a Mesa amp, buy Mesa matched tubes for it, don't sweat about setting the bias. Find the bias color range you like best, and just keep buying matched pairs in that color. Easy.

 

[Rotorcraft230]

Mesa biases the amps to run with their pre-tested, pre-selected tubes across all the ranges they classify and sell them as.
They reject tubes that would bias too cold or too hot in their amps. So it's fair to guess that their factory bias setting is going to be right in the middle of the typical range for the tube type.

Some people disagree with Mesa's notion of not giving you adjustable bias but I see the sense in it. A tube might fail during a gig, so you'd put in your spare pair you bought from Mesa, and no bias adjustments required. Or you could put in tubes from another vendor and end up with "edge case" tubes in your amp, causing harsh crossover distortion or, worse yet, meltdown.

I have had excellent results with Mesa branded tubes. They have been consistently reliable and not gone out of limits on bias, so as far as I'm concerned, their system as they mean for it to be applied works well and I recommend it.

Buy a Mesa amp, buy Mesa matched tubes for it, don't sweat about setting the bias. Find the bias color range you like best, and just keep buying matched pairs in that color. Easy.

I’d buy that, thanks Matthew. I wasn’t or wanted to change the bias I just wanted to know where it was.

Mark

 

[Pete Farrington]

it’s very accurate, just haven’t done it with multiple tubes per side

With parallel output valves, ‘very accurate’ becomes a guesstimate. How do you know all the valves are actually even conducting, nevermind the true current drawn by each of them?

 

[Rotorcraft230]

What do you mean “conducting “

 

[Matthews Guitars]

Conducting, current is travelling between the elements of the tube as they are supposed to. No conduction = no sound. Too much conduction = melted tube and damaged transformer.

 

[Rotorcraft230]

Just playing with the image resized

 

[FleshOnGear]

Just playing with the image resized
View attachment 143457

Man, I’m glad I don’t work at the Mesa factory!

 

[Rotorcraft230]

Right, it does have nice stuff in it
Orange drop
Carling
1w resistors
Classic Tone transformers
Belton sockets
Chassis mounted pots
Very good quality overall
Cliff jacks from what I can tell

 

[Pete Farrington]

OT ohms per side of CT 22 ohms

Vd 1.3
ma .059

What do you mean “conducting “

Your measurements above indicates the OT primary is conducting 59mA to each triple parallel 6L6.
But what if one of those valves is bad, worn out, passing 0mA?
We have no idea how the 59mA is being split between the 3 6L6 per side.

 

[playloud]

With parallel output valves, ‘very accurate’ becomes a guesstimate. How do you know all the valves are actually even conducting, nevermind the true current drawn by each of them?

You can always measure them one-per-side also, to be sure. As long as they're reasonably matched, the total idle plate current draw seems as good a metric to bias to as any.

 

[Pete Farrington]

the total idle plate current draw seems as good a metric to bias to as any

I find it valuable to know what each valve is doing. Not just at idle, but also at full output; other than assessing peak cathode current via a current sensing resistor on the scope, how else would it be possible to check that each valve was pulling its weight?

 

[Rotorcraft230]

I have a tube tester and ran them all through and they were all pretty much the same. It doesn’t do conductance but good enough to let me know they are good anyway. I’m happy with that.
Already passed it ti the owner.

Thanks for all the input I appreciate it.

Mark