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Carlp 29th October 2006 04:51 PM

P-P resistance oddity
 
In testing a simple P-P amp, I am not getting the correct values for resistance specified by the designer. In the schematic posted at the link below, pin 6 is supposed to read about 820 ohms to ground but I get 820 on V1 (as I'd expect) but something else on V2 (I've gotten a variety of readings, but mostly around 800K or so if I remember correctly, or maybe it was ~8K?). I don't understand why V2 pin 6 would read 820 when it seems it should read at least around 8.3 to 10K based on the other resistors in the paths to ground. Am I missing something here? The designer died a few years back so I can't contact him...

Thanks,
Carl

http://www.dogstar.dantimax.dk/tubestuf/mnibl2-2.htm

dhaen 29th October 2006 05:00 PM

Ohm meters are easily fooled by small voltages, and can only be used on "dead" circuits. Maybe a small amount od emission through V1 or some charge on C8 will make measurements wrong. Wait a few minutes and make the measurement again. Also, reverse the leads of your test meter and try again. Thirdly, replace the battery in your meter;) Many of us have been fooled by that one...:bawling:

Fred's projects are simple yet elegant. You'll find plenty who can understand them and can help.

Tube_Dude 29th October 2006 05:15 PM

Re: P-P resistance oddity
 
Quote:

Originally posted by Carlp
In testing a simple P-P amp, I am not getting the correct values for resistance specified by the designer. In the schematic posted at the link below, pin 6 is supposed to read about 820 ohms to ground but I get 820 on V1 (as I'd expect) but something else on V2 (I've gotten a variety of readings, but mostly around 800K or so if I remember correctly, or maybe it was ~8K?). I don't understand why V2 pin 6 would read 820 when it seems it should read at least around 8.3 to 10K based on the other resistors in the paths to ground. Am I missing something here?
Hi Carlp

The difference is that in V1, pin6 is connected directly to ground via R3 a 820 Ohms resistor.:)

In V2 pin 6 the resistence is ~ 8,3 k because R10 , 820 ohms and R11 , 7,5k are in series , but also with a zener to ground.

Short the zener ( provisory ) with the amp disconnected and measure from pin 6 to ground...

Now you must find 8,3k ohms

Measuring with zener active , will give diferent results , depending from the polarity of the measuring probes...

Carlp 29th October 2006 06:40 PM

Thanks, Jorge and John,

I had discharged the caps and read no more than a few 1/10ths of a volt across any caps, yet when I measured the resistance on V2 (on a Fluke 12B multimeter) it switched to DC voltage measurement and wouldn't check resistance. At some point I was able to discharge the caps enough to get the resistance reading, but it seems like the caps get charged by whatever current the meter puts out, so it switched back and forth some.

Note the 12B is designed to switch to voltage measurement if it detects more than 4.5V (ac or dc) on the circuit being measured.

Anyway, I thought V2 should read 8.3K (going to ground through the zener) but hadn't thought to short it to avoid it's effects. So in short, was Fred wrong when he spec'ed 820 ohms on V2 pin 6?

Thanks again,
Carl

PS - BTW, I build his SET first, now the PP and have found them GREAT first DIY projects because he was so thorough in explaining the circuits, giving test parameters and thorough testing instructions, etc. If anyone wants advice for a first project, these are good options.

Tube_Dude 29th October 2006 09:55 PM

Quote:

Originally posted by Carlp
Anyway, I thought V2 should read 8.3K (going to ground through the zener) but hadn't thought to short it to avoid it's effects. So in short, was Fred wrong when he spec'ed 820 ohms on V2 pin 6?


Obviously!:)

Carlp 30th October 2006 02:42 AM

Thanks. It's not obvious to me. I'm new enough to this that I don't tend to think I'm right and the designer is wrong, even if it SEEMS straightforward to me. Particularly when he was so careful about so much else.

Carl

Tube_Dude 30th October 2006 02:49 AM

Quote:

Originally posted by Carlp
Thanks. It's not obvious to me.
I understand you , it take some time to achieve some self confidence... ;)


Cheers:drink:


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