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JM-1

Joined: 30/09/07
Posts: 604

Multimeter Accuracy / Drift and biasing new

#965651 - 22/01/12 09:45 PM

When I go, I want to go with a 'bang' and so I've decided to bias my own amplifier.

In any case, It'd cost me £50 to get it done by an amp tech, and the kit will cost me only slightly more than that...

One question though - I have a digital multimeter that I bought in Aldi a couple of years ago, and I've heard about calibration drifting over time. Is that a real issue or for our purposes is it negligible?

Regards
Jay

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Folderol

Joined: 15/11/08
Posts: 2542
Loc: Rochester, UK

Re: Multimeter Accuracy / Drift and biasing new

[Re: JM-1]
#965658 - 22/01/12 10:47 PM

Don't worry about it. Considering the kit we used to have to use in the 'bad old days' you'll probably be far more accurate!

--------------------
It wasn't me!
(Well, actually, it probably was)

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JM-1

Joined: 30/09/07
Posts: 604

Re: Multimeter Accuracy / Drift and biasing new

[Re: JM-1]
#965692 - 23/01/12 07:43 AM

Thanks Folderol...for a very useful and (once you made it) obvious point

I always find it amazing when people point out something simple that I've overlooked / not considered

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Folderol

Joined: 15/11/08
Posts: 2542
Loc: Rochester, UK

Re: Multimeter Accuracy / Drift and biasing new

[Re: JM-1]
#965872 - 23/01/12 06:40 PM

If it makes you feel any better...
My workshop meter has 5 digits (supposedly +- 1 on the last digit), and I have caught myself trying to set a voltage on a PSU so it alternately flicks between 4.9999 and 5.0000

--------------------
It wasn't me!
(Well, actually, it probably was)

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JM-1

Joined: 30/09/07
Posts: 604

Re: Multimeter Accuracy / Drift and biasing new

[Re: JM-1]
#965965 - 24/01/12 08:34 AM

I think I'm probably as obsessive-compulsive too - still it's nice to kow I'm not the only weirdo around here

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ef37a

Joined: 29/05/06
Posts: 5622
Loc: northampton uk

Re: Multimeter Accuracy / Drift and biasing [Re: Folderol]
#966089 - 24/01/12 01:44 PM

Quote Folderol:

Don't worry about it. Considering the kit we used to have to use in the 'bad old days' you'll probably be far more accurate!

Ah! But Avos did not drift Will!

Jay, no worries. If you want to do the best, bang up job you can, read on..
I am assuming that the amp has a current sense resistor, usually 1R, in the cathode circuit? If you are lucky enough to have two meters, crock one onto HT at the resevoir capacitor, amp cold of course! Croc the digital meter on the sense R and switch on. Set the current as per the book and if you don't have a figure go for 25mV per valve (EL34,6L6,KT88/66/77 or any "big" octal). Check the HT volts and note.
Let things cook for 10minutes and check. Current will have surely risen but check HT, if that has risen more than 5 V or so and the current is say 27mA per, leave it another 10. All this is because valve current drifts with valve heating and mains variation so you can be chasing your tail all day if you don't check HT! NB. I do NOT suggest you measure incoming mains unless you have a very safe way to do so.
All the above is fun if you don't do it for a living! In practice one gets to know the amps and I used to set them for a mV or 2 under cold knowing that they would move to 50mV or so when cooking.

Dave.

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