Hi! Does anyone know where to find calibration instructions for a Precision 10-20 tube tester? Thank you in advance for any info. BOB

Hi Bob,Unfortunately the calibration information was Precision tube testers was never published.I have a model 912,which is an older model,but uses the same circuit.The two adjustments are as you probably know,filament voltage and meter adjustment on the emission test.However,I found a paragraph,titled"tube brand variations", in the manual for the models 910,912,915 that pretty much sums- up where on the meter Precision expected the needle to rest for a good tube on the emission test.They say this point is the center of the letter "G" of the word good.This point is 2/3 scale on every Precision tube tester meter that I have seen.It may not be true for all of them,I don't know.My testing seem to confirm my deduction,at least on my machine.All the resistors are within spec and the calibration controls are still sealed.The filament voltages are right where they should be.Good tubes(and NOS) on this tester generally(on average) test at 2/3 scale.Another person on the internet who provided information for the 612,states that 3/4 scale is correct.How he came up with this information,I haven't been able to determine.If you would like a PDF of the manual to check my deductions,post your e-mail.I think the Precision testers are excellent and are usually quite cheap,I got my 912 for 35 dollars including the G-140 and the 9-pin adapters.Can't hate that.Good luck.Mike

:Hi Bob,Unfortunately the calibration information was Precision tube testers was never published.I have a model 912,which is an older model,but uses the same circuit.The two adjustments are as you probably know,filament voltage and meter adjustment on the emission test.However,I found a paragraph,titled"tube brand variations", in the manual for the models 910,912,915 that pretty much sums- up where on the meter Precision expected the needle to rest for a good tube on the emission test.They say this point is the center of the letter "G" of the word good.This point is 2/3 scale on every Precision tube tester meter that I have seen.It may not be true for all of them,I don't know.My testing seem to confirm my deduction,at least on my machine.All the resistors are within spec and the calibration controls are still sealed.The filament voltages are right where they should be.Good tubes(and NOS) on this tester generally(on average) test at 2/3 scale.Another person on the internet who provided information for the 612,states that 3/4 scale is correct.How he came up with this information,I haven't been able to determine.If you would like a PDF of the manual to check my deductions,post your e-mail.I think the Precision testers are excellent and are usually quite cheap,I got my 912 for 35 dollars including the G-140 and the 9-pin adapters.Can't hate that.Good luck.Mike Thanks Mike You're right about the price and quality of these Precision tube testers. I got the 10-20 for $30.00 And all the fixed resistors and capacitors were right on. I just had to clean up the variable resistors and contacts.I have calibration instructions for a model #612 that states it should work on other precision tube testers and it probably will. I just was having trouble finding the transformer secondary 300v contact points. I'll try again tomorrow. It is also a good time to record readings off the meter connections using a DVM. I use the readings to make a chart so if the meter ever goes bad I can substitute a digital panel meter.I also record the resistance of the meter for that purpose. O/O BOB

::Hi Bob,Unfortunately the calibration information was Precision tube testers was never published.I have a model 912,which is an older model,but uses the same circuit.The two adjustments are as you probably know,filament voltage and meter adjustment on the emission test.However,I found a paragraph,titled"tube brand variations", in the manual for the models 910,912,915 that pretty much sums- up where on the meter Precision expected the needle to rest for a good tube on the emission test.They say this point is the center of the letter "G" of the word good.This point is 2/3 scale on every Precision tube tester meter that I have seen.It may not be true for all of them,I don't know.My testing seem to confirm my deduction,at least on my machine.All the resistors are within spec and the calibration controls are still sealed.The filament voltages are right where they should be.Good tubes(and NOS) on this tester generally(on average) test at 2/3 scale.Another person on the internet who provided information for the 612,states that 3/4 scale is correct.How he came up with this information,I haven't been able to determine.If you would like a PDF of the manual to check my deductions,post your e-mail.I think the Precision testers are excellent and are usually quite cheap,I got my 912 for 35 dollars including the G-140 and the 9-pin adapters.Can't hate that.Good luck.Mike Thanks Mike You're right about the price and quality of these Precision tube testers. I got the 10-20 for $30.00 And all the fixed resistors and capacitors were right on. I just had to clean up the variable resistors and contacts.I have calibration instructions for a model #612 that states it should work on other precision tube testers and it probably will. I just was having trouble finding the transformer secondary 300v contact points. I'll try again tomorrow. It is also a good time to record readings off the meter connections using a DVM. I use the readings to make a chart so if the meter ever goes bad I can substitute a digital panel meter.I also record the resistance of the meter for that purpose. O/O BOB sorry for the screw up ,I'm new at this.

Hi Bob,It is those very calibration instructions I was referring to;those for the 612.The author states to set the emission calibration to read 3/4 scale.However,if my deductions are correct,the true setting should be for a reading of 2/3 scale,and my tester seems to support my reasoning quite well.I have gotten several e-mails from other Precision tester owners who also believe my info to be correct.Just something to consider.See-ya.

:Hi Bob,It is those very calibration instructions I was referring to;those for the 612.The author states to set the emission calibration to read 3/4 scale.However,if my deductions are correct,the true setting should be for a reading of 2/3 scale,and my tester seems to support my reasoning quite well.I have gotten several e-mails from other Precision tester owners who also believe my info to be correct.Just something to consider.See-ya. ------------------------------------------------------------------------------------------------------------------------------Reply:HI! I tried adjusting the 10-20 using the 612 instructions. I thought it worked great until a good 35Z5 and a 50L6 read bad. Before cleaning the adjustable resistors, I recorded their out of circuit readings so I was able to put them back to original settings.They were still factory sealed before I cleaned them. The tester seems to work best set that way over a range of filament voltages. I'm having trouble setting my Precision 612 using the same instructions. This one will need tweaking since I changed the carbon comp resistors and capacitor. I think I'll put it back to factory setting on the adjustable resistors and then follow your suggestion to set to 2/3 reading.Thanks again. bob

::Hi Bob,It is those very calibration instructions I was referring to;those for the 612.The author states to set the emission calibration to read 3/4 scale.However,if my deductions are correct,the true setting should be for a reading of 2/3 scale,and my tester seems to support my reasoning quite well.I have gotten several e-mails from other Precision tester owners who also believe my info to be correct.Just something to consider.See-ya. ------------------------------------------------------------------------------------------------------------------------------Reply:HI! I tried adjusting the 10-20 using the 612 instructions. I thought it worked great until a good 35Z5 and a 50L6 read bad. Before cleaning the adjustable resistors, I recorded their out of circuit readings so I was able to put them back to original settings.They were still factory sealed before I cleaned them. The tester seems to work best set that way over a range of filament voltages. I'm having trouble setting my Precision 612 using the same instructions. This one will need tweaking since I changed the carbon comp resistors and capacitor. I think I'll put it back to factory setting on the adjustable resistors and then follow your suggestion to set to 2/3 reading.Thanks again. bob

:::Hi Bob,It is those very calibration instructions I was referring to;those for the 612.The author states to set the emission calibration to read 3/4 scale.However,if my deductions are correct,the true setting should be for a reading of 2/3 scale,and my tester seems to support my reasoning quite well.I have gotten several e-mails from other Precision tester owners who also believe my info to be correct.Just something to consider.See-ya. ------------------------------------------------------------------------------------------------------------------------------Reply:HI! I tried adjusting the 10-20 using the 612 instructions. I thought it worked great until a good 35Z5 and a 50L6 read bad. Before cleaning the adjustable resistors, I recorded their out of circuit readings so I was able to put them back to original settings.They were still factory sealed before I cleaned them. The tester seems to work best set that way over a range of filament voltages. I'm having trouble setting my Precision 612 using the same instructions. This one will need tweaking since I changed the carbon comp resistors and capacitor. I think I'll put it back to factory setting on the adjustable resistors and then follow your suggestion to set to 2/3 reading.Thanks again. bob .............................................. :Cont'd: I had to compromise setting the 612. I noticed the line voltage was 90v and most tube testers seem to work best above 96v. So I centered the meter at 97v. line voltage using R5.The filament voltages tested within 10% at this setting. The meter is 160mv rated,so I hooked up a DMM to the meter terminals and got a reading of exactly 80mv when the meter was centered.I came to the conclusion that this is a good way to test the meter for accuracy.So I set up a 6L6 tube for testing, and while holding down the test button,I rotated the load control for a full meter sweep and got a 160mv reading on the DMM. Again a perfect reading.Then I adjusted R1, while testing the 6L6 tube at normal settings, until I got 106.6mv which is exactly 2/3 of the meter sweep.The compromise was that at the above settings the 300v transformer tap read 312v. Not the 300v called for in the calibration instructions but within 5% approx. The tester now works well on tubes with a variety of filament voltages and is within 10% reading of my other testers on most tubes. So thank you Mike for the info. bob

Hi Bob,You are very welcome.Your findings seem to support my theory.The secondary voltage on my 912 also reads slightly above 300 volts across the whole winding.Yet,the filament voltages are almost all dead on when I click up through the different settings.When under load,they seem to hold quite well.Of course,when I pop in something like a 5z3,the filament voltage drops,but that is to be expected on these testers.However this doesn't seem to compromise the accuracy of the tests as other people have also pointed out.Considering the build quality of these Precision testers and the fact they generally work very well with little fuss,make them quite a bargain at less than 50 bucks on average,in my opinion.Have fun.