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JeffYoung 13th November 2019 10:45 PM

How do you measure bias with no source resistors?
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How do you measure the bias of an undegenerated output stage when trying to set it?

Say, for the XA-25, or for an HPA-1-inspired lateral output power amp such as the attached....


2 picoDumbs 13th November 2019 10:47 PM

Assuming you’re using CRC powersupply, then measure across R on the supply.
Bob’s your uncle.

2 picoDumbs 13th November 2019 11:03 PM

Just adding another cat skin to the list.
If you’re making your own pcb then add a place for a 0.1R resistor at the drain of the output mosfets to take the measurement.
Then once satisfied with your bias setting either short them or remove them.
Some people even leave them in but I personally hate the idea of leaving them in.

JeffYoung 13th November 2019 11:07 PM

Thanks, Pico! I can't believe I didn't think of the CRC resistor.

(I did consider using the current meter on my variac, but I have no idea how accurate it is.)


2 picoDumbs 13th November 2019 11:14 PM

I usually bias up all my amps using a lab power supply with current limiting for safety and use the current meter on the lab supply to bias the amp up safely, before connecting to amp power supply, so I guess that’s another option too.

Blues 13th November 2019 11:21 PM

Either because the mosfet is wired or Drain pin is unsoldered, place your ammeter in series between supply and Drain...:warped:

chede 13th November 2019 11:28 PM

I have in my shopping cart a clamp-on ammeter multimeter, Uni-T UT 210E. Will get one with the next order exactly for this purpose.

Recommendation from Zen Mod ...

You just have to look for ones that can read DC current; the inexpensive ones are AC-only ...

JeffYoung 13th November 2019 11:51 PM

I do have a clamp-on, but yeah, it's AC-only. I like the look of that Uni-T....

Mark Johnson 14th November 2019 12:59 AM

Consider the Hantek CC65 from eBay. I have one and I am pleased with the results I get from it.

Eric 14th November 2019 01:33 AM

I've been thinking of making a nice little panel for myself that includes my variac, an integrated dim-bulb tester (bypassable with a switch), followed by an ammeter and voltmeter for just such purposes. This would put everything in one place and eliminate my typical Rube Goldberg approach to this type of task.

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