voltage reading question

[howardnair]

i finished my 2nd stereomour all is well thank you doc and paul it is up and running-the mundorf rxf oil seem fine --tosoon to comment-ok all my voltage readings are very close to spec except 1 pair--on terminals
1]  354 should be 350
2] 202 should be 230
14] 202 should be 230
15] 354 should be 350
active load board both OA and OB are 202 and should be 230
this is within 10 to 15% but as all the other readings are very close it struck me as a little odd
i included 1 and 15 only to show how close to spec they happen to be-every reading is very close except 2/14 and OA an OB  any thoughts??---howie

 

[Grainger49]

All those measurements are within +/-15%.  Terminal 14 and OA/OB are the farthest off but, still just barely under 15% off.

What is your incoming voltage?  

(sorry I don't have the information on the Stereomour so I don't know where any of this is in the circuit)

 

[Paul Joppa]

The C4S outputs go to the driver plates, which will vary from one tube to another since the bias (2.5v) is fixed by the 431 chip. Looks like your 12AT7 is well matched between sections, with a bit more transconductance than the average. This is not uncommon early in the life of the tube.

 

[InfernoSTi]

202 is great!  Take a look at the numbers from my build and Paul's comment on the theoretic value being 200....

John

So my only worry is that a four of my readings are right on the edge of acceptable (the vast majority of reading are either spot on or within 6-7%).  However, I have a handful that are 10-15%, so I figure I'd ask folk's opinions to see if I should worry (or just grab an adult beverage and listen to some music and stop nit picking!).   I need to mention that my mains VAC measures 123 VAC (not the test 119VAC).

Here are the handful of high readings (just the big ones, not the 6-7% readings):

Terminal     Expected     Actual   Variance %

T2             230            207        -10%
T14           230            194        -15%

OA            230            207        -10%
OB            230            194        -15%

Should I worry?  It is on the ragged edge but everything sounds WONDERFUL!!!!

Thanks a bunch!
John

You got NO PROBLEM.

It's a good illustration of the complexity of making good tests. The points you identify are the driver plate voltage. Because the driver is running under fixed bias, this point will vary more like 20% or even 30% - 2 or 3 times the variation expected at more stable points. This is because of variations in individual tubes. It's just too confusing to try to say this in the manual.

If you look at the schematic, you'll see that my theoretical values for these points were 200 volts. The actual first production unit measurements were 230 volts, so that's what is in the manual - we use real measurements rather than theory. Your measurements are much closer to the theoretical value, which is based on the published tube curves, so I think you have just validated my theoretical calculations - thanks!

:^)

 

[howardnair]

thanks for the explanation-in the schematic it does  read 200v--even more interesting to me is if i put  in a nos  mullard cv4024 terminals 2/14 and OA /OB are 186 v/166v -i put the EH back in -if i left in the mullard  will someone  explain what exactly is happening -one] to the tube and two] with the circuit--it sounds perfectly fine with  either tube- i took out the sovteks and am running the sophia electric  meshplate 2A3's -[which you can buy on ebay from sophia for 99 each ]  - i sat and did some listening this morning [i am out of work at present]-classical music from a fm-3 -every thing was there -really very nice-but what else does one expect from paul and doc--