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finding plate voltage drop


mopardave

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6 minutes ago, NOSValves said:

Dave,

 

     Do you have a decent digital camera? If so take a close up shot of one of those meters. I wouldn't be surprised your measurement is more off than the meters. I again agree with Maynard (I think the red sea just parted) this is much to do about nothing . 

I'll get some pics this weekend. The new Klein meter measures more inline with the onboarding meters.  

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On 2/5/2019 at 12:49 PM, NOSValves said:

What are you setting the Klein to measure voltage or current?

Heres some more pics I just took of the guts of the amp and where i'm measuring bias from.   Would this be the correct measure point?  My measures are agreeing with the on board DC meters. Last pic shows where i'm taking measures from on the board connections. I get more consistant numbers there than I do directly each side of that resistor. Pic 2 and 3 shows resistor that connects pin 8 and the on board meters and looks to connect to the large blue caps in the center of the board.

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1 hour ago, tube fanatic said:

Dave, if you are measuring across any component with the meter set to read current the value is not correct.  

 

Maynard

Ok, where should my test points be and what setting should I use on the meter?  The readings I get are inline with the on board DC meter readings, so I thought I had everything correct.

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The meters I'm concerns with and wanted close up pictures of are the meters built into the amps. I want to know if they are reading current or voltage. Again Maynard is right your can not use the current reading function of your digital meter to accurately read the current across the cathode resistor. That resistor is place there sop you can read voltage and use ohm law to figure out the current just like you do on your other amplifier (or should be doing), I personally do not like to use a 1 ohm resistor because the voltage developed will be very small and subject the error especially with inexpensive meters. I'd much rather use a 10 ohm resistor

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13 hours ago, NOSValves said:

The meters I'm concerns with and wanted close up pictures of are the meters built into the amps. I want to know if they are reading current or voltage. Again Maynard is right your can not use the current reading function of your digital meter to accurately read the current across the cathode resistor. That resistor is place there sop you can read voltage and use ohm law to figure out the current just like you do on your other amplifier (or should be doing), I personally do not like to use a 1 ohm resistor because the voltage developed will be very small and subject the error especially with inexpensive meters. I'd much rather use a 10 ohm resistor

Ok, so with function should I be using on the meter to measure plate voltage. I thought using the volt setting would be fine when measuring plate voltage between  pins 8 and 3?   So, I can't measure bias off that resistor taking reading directly from meter?  Half the to do the math on ohms law?      

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On 2/14/2019 at 6:53 PM, NOSValves said:

See those are purpose built analog DC mA current meters which should have little or no internal resistance.. Can you trace back to the tube socket what pins they are attached to? 

 

     

Yeah. They are on pin 8.   The black wire on pin 8 runs to meter and another black wire runs back from meter  to the resistor I was taking readings from.

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22 minutes ago, NOSValves said:

 But when you measure the resistance of the resistor on pin 8 of the output tubes it measures 1.2 ohm? I'm scratching my head big time here.... something is screwy. 

 

Couldn't that depend on what is built into the meters themselves?

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What is the color bands on those resistors? It looks like its Brown/Black/Black which = 10 ohms not 1 ohm. I think your reading 1 ohm because its shunted off by the built in meter which has to have pretty low internal resistance. I'm shocked it has 1 ohm or more.... I've never seen one that wasn't at or real near zero. 

 

    In the end as long as that built in meter is connected to the cathode you can not measure with any meter. When you measured .650 or whatever is was that was .650 of 1 single mA you can't pile meter on top of meter on top of a resistor. All 3 devices were splitting the current running through the tubes cathode.  I believe the 10 ohm resistor is there to allow the tube to still work if the meter fails shorting open. The built in meter is shunting the 10 ohm resistor down to the single ohm so most of the current is running through the meter. As the Italians would say use the onboard meters and forgetaboutit. 

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6 minutes ago, Marvel said:

 

Couldn't that depend on what is built into the meters themselves?

 

 

          Yea but the screwy part is I've seen lots of these meter over the years and own a bunch of them myself both US and from the other side of the pond...I've never seen one have even close the 1 ohm of resistance. 

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My guess is that if the meters are really across the 1 ohm resistors they are actually volt meters which are shunted to allow measurement of current instead.  Back in the day we often had to make do with what we had on hand and did stuff like that.  There were many articles in electronics magazines on how to make new meter scales.  Now, of course, it’s extremely easy.  

 

Maynard

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