New DSL100HR. Bias question!

[twothemax]

Can anyone help me on this.
I am checking with a multimeter only and testing at the bias points located on the chassis.
Should my meter be set to 200mv or 200ma?
The reason I ask is I read some say MA and some say MV.
If I set my meter to MV meter reads 74 if I switch to MA it reads 20 and the trim pots won't go to 74

Amp sounds amazing, so why check the bias right. I always check the bias when I purchase a new amp. Had a few issues with some other amps
Thanks

 

[twothemax]

Got a reply from Marshall stating 70mv for the new DSL100HR. Should also mention the new DSL100HR comes with TAD EL34 RED BASE power tubes and 4 Marshall red logo ecc83.

 

[KraftyBob]

As you now know is mV. Voltage readings are taken across the circuit (positive and negative) so when your measuring bias you’re measuring across the circuit. Current readings are taken in circuit so you actually have to break the circuit and insert your DMM in the circuit to get a reading. That’s not how the bias points are wired.

The reason you can use mV when measuring bias is because there’s a 1%, 1 ohm resistor between the cathode and ground, and per Ohm’s law, if the idle current, for example, is 35mA, there will be a 35mV drop across a 1 ohm resistor. You are measuring this voltage drop with your DMM.

 

[twothemax]

Thanks for the great information.
So I set my meter to 200mV and turn the trim pots to 70mV per side?
What i noticed is when my dvm is set at 200mV and my readings at 70mV if I switch to mA it reads 20mA. Does that sound correct ?

Going to take it for a spin tomorrow.
Thanks again

 

[KraftyBob]

Thanks for the great information.
So I set my meter to 200mV and turn the trim pots to 70mV per side?
What i noticed is when my dvm is set at 200mV and my readings at 70mV if I switch to mA it reads 20mA. Does that sound correct ?

Going to take it for a spin tomorrow.
Thanks again

Stick with the mV setting as you are measuring voltage across the bias points, not current.

I don’t know why you’re getting the 20mA reading on your DMM but disregard that because you’re not measuring current there. Again, to measure current your DMM needs to be inline with what you’re measuring, not across the + & - like how you measure voltage.

This pic shows measuring voltage. The DMM leads are across the circuit and measuring the voltage drop across the resistor. This is essentially how measuring bias works.


This pic shows measuring current. Notice how the circuit is broken and the DMM is inserted into the circuit. If thats how bias points were wired and you were measuring current, as soon as you took your leads off the circuit would be open and no current would flow. That would also pose a danger because if someone was able to somehow touch those two points the current would flow through them - and 70mA can be lethal. 70mV not so much.

 

[KraftyBob]

Can anyone help me on this.
I am checking with a multimeter only and testing at the bias points located on the chassis.
Should my meter be set to 200mv or 200ma?
The reason I ask is I read some say MA and some say MV.
If I set my meter to MV meter reads 74 if I switch to MA it reads 20 and the trim pots won't go to 74

Amp sounds amazing, so why check the bias right. I always check the bias when I purchase a new amp. Had a few issues with some other amps
Thanks

Also, just to clarify your original question and maybe why you’re getting conflicting responses: bias “current” is what’s flowing through the tube and that’s in mA. But when measuring it, as stated above using a 1 ohm resistor, you are measuring the voltage so you use the mV setting on your DMM.

Edited to add: There is a way to measure the bias current directly, but that method is typically not recommended because of the high risk involved in measuring current. Also, most DMM's are typically more accurate when measuring mV vs mA. Bottom line: set you DMM to DC mV and you're good to go!

 

[twothemax]

Thanks. Great pic.
I will check it out