Can I check my bias with this fluke

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Dyllheaven88

Well-known member
I own one of these from working on coffee machines . It doesn’t have a MA setting . Was wondering if could still check my amp bias ? Thanks
 

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What amp? Does it have 1 ohm resistors from cathode to ground for the power tubes? Class AB fixed bias?

If no 1 ohm resistors, do you have any octal bias tube adapters?
 
glpg80 said:
What amp? Does it have 1 ohm resistors from cathode to ground for the power tubes? Class AB fixed bias?

If no 1 ohm resistors, do you have any octal bias tube adapters?
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Not sure it has external bias points and trim pots just not sure if I need a different multi meter or if the fluke will work .
 
I have never used bias points, but what i would do, is there are probably three points. I would guess center is a ground. One lead used here, if you put wrong lead, the voltage will read negative, but it will be same. no big deal. test each side and see what reading you get. I think bias points work off of negative voltage or something, so they aren't completely accurate way to bias, but fortin made them, so they are going to be good enough to get you in the ball park.
 
Fortin uses 1 ohm resistors to measure cathode current as a voltage on the output of the external bias points.

Put the negative lead in the black probe point in the center, and measure millivolts one at a time for each respective tube. The trim pots adjust each tube for the side you're measuring.
 
glpg80 said:
Fortin uses 1 ohm resistors to measure cathode current as a voltage on the output of the external bias points.

Put the negative lead in the black probe point in the center, and measure millivolts one at a time for each respective tube. The trim pots adjust each tube for the side you're measuring.
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What that be on the dc setting of that particular fluke pictured or do I need a different multimeter ? Thanks !
 
Dyllheaven88 said:
What that be on the dc setting of that particular fluke pictured or do I need a different multimeter ? Thanks !
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Volts

It would get you in the ballpark to check them, but if you want increased accuracy you're going to need a multimeter that can measure millivolts specifically and not just volts.
 
You don't need a mA setting nor a bias probe. You could measure the resistance across output transformer primary (from each end to center point) and then measure voltage drop across each side of the primary. First, with the amp off, clip one probe to pin 3 of power tube then probe center point of primary to measure resistance. Should be between 10 and 20 ohms roughly. make that for each pair (or single) of power tubes.

Then with the amp on, clip one fluke probe to ground then with the other probe, measure plate voltage (power tube pin 3). Then measure output tranny primary center point voltage. Subtract value from plate voltage, it gives you voltage drop. If your wall voltage fluctuates too much, you could clip one probe to pin 3 and the other to center tap to measure voltage drop directly. Be careful with your hands in the running amp. Then with ohm's law (U=R*I or I=U/R), divide voltage drop with measured primary resistance, it gives you mA for that side of the output tranny, either a single tube or a pair of tubes. If it's a pair of tubes, divide by 2. Same value as measured with bias probe.

I did that no later than last week on my SLO and one of my 2203 that was giving me trouble. Put new tubes in and biased them that way. Worked fine, was quick, everything measure as it should and it sounds great. I used 35mA for EH EL34 (70mA for a pair of tubes) in the 2203 and 30mA for Sovtek 5881 wafer base (60mA for a pair of tubes) in the SLO.
 
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Dyllheaven88 said:
Someone want to recommend a affordable multimeter . I guess a 150$ fluke can’t do the job of an 8 $ multimeter 😑
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anything with mv will be fine. Got to a pawnshop and buy used
i have had cheap and expensive ones for work.
For what you are doing, you just want the regular dc mv option. surprised that any fluke wouldn't have one.
 
Smashedguitarist said:
anything with mv will be fine. Got to a pawnshop and buy used
i have had cheap and expensive ones for work.
For what you are doing, you just want the regular dc mv option. surprised that any fluke wouldn't have one.
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Right ? I was surprised too . Probably run to Home Depot for a 20$ one
 
Dyllheaven88 said:
Right ? I was surprised too . Probably run to Home Depot for a 20$ one
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if you do that, look on the packaging. It should tell you how accurate it is. The reason i say a pawn shop is you can get a better one for the same money, and avoid being in the same situation where you have it only accurate to one volt. You want one focused on dc if you do get one at homedepot. That will make sure that you are in the right ball park. I bought a really badass one from fluke for doing everything in the amp just incase i wanted to go down that rabbithole, but I still haven't jumped in.
But remember, when you bias his amp, he goes for 32, you can turn that pot a bit more or less and listen to how it sounds after you get there. and make the decision there more than the number.
 
Smashedguitarist said:
if you do that, look on the packaging. It should tell you how accurate it is. The reason i say a pawn shop is you can get a better one for the same money, and avoid being in the same situation where you have it only accurate to one volt. You want one focused on dc if you do get one at homedepot. That will make sure that you are in the right ball park. I bought a really badass one from fluke for doing everything in the amp just incase i wanted to go down that rabbithole, but I still haven't jumped in.
But remember, when you bias his amp, he goes for 32, you can turn that pot a bit more or less and listen to how it sounds after you get there. and make the decision there more than the number.
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Yeah that’s part of the reason I’m wanting to experiment with it . It sounds good but I’m curious to roll some nos el 34s and bias it a little hotter . I don’t know if current draw has anything to do with actual temperature? But compared to my old wizards and my Matamp. The fortin is “warm” after about an hour of playing at moderate to low volume . After about 30 mins on the Matamp you could start a fire 😂
 
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