Oh okay, I thought you were asking what the value of R702 was above.
Rail voltages are +49V and -49V (+/-48.93 at this moment)
Voltage between R703 and R705 is +10.66V
TR703
B: +31.03V
C: +22.45V
E: +31.63V
TR705
B: +22.45V
C: +33.37V
E: +21.85V
TR707
B: +4.79V
C: +18.48V
E: +4.23V
TR709
B: +18.48V
C: 0V
E: +19.12V
TR711
B: +17.71V
C: +33.32V
E: +17.70V
TR713
B: +17.72V
C: 0V
E: +18.36V
TR715
B: -0.813V
C: 0V
E: 0V to -0.06V
Hopefully that’s what you wanted.
Dan
Dan, lots of efforts but worth.You measured all the bjt's in the tone amp, but only TR701, TR703 and TR705 are relevant. I'll check the others later too.
So the dgs values of TR701 are missing.
But now something important appears:
TR705... C: +33.37V
This IS the real supply voltage of this (pre tone) amplifier, designed to boost the signal 10 times (R721--R709) to compensate for the loss (-20dB) in the Baxandall adjustment network. It is +33.4Vdc, tapped from the right side of R062 in the power supply regulator circuit.
Now the midpoint of R703-R705 of 10.7Vdc makes sense ( (47/147)*33.4 = 10.68Vdc.
This is the formula to predict the voltage at the emitter of TR705, assumed Id is near zero and Vgs as calculated from earlier post being 1.8Vdc and 1.6Vdc (L/R channels).
VeTR705 = { R707*R717*(([R709+R711]+R721)/[R709+R711])* ((R705/(R703+R705))*Vrail + Vgs) + [R713+R715]*R721*VbeTR705 + R717*R721*VbeTR703 } / { R707*R717 + [R713+R715]*R721 }
approximated to
VeTR705 = 1.9937*(10.679 + Vgs) + 0.6045
yielding to
Vgs = 1.8Vdc -> VeTR705 = 25.53Vdc
Vgs = 1.6Vdc -> VeTR705 = 25.09Vdc
Measured: TR705 E: +21.85V
The difference is due to the aformentioned "assumed Id is near zero", which is not so: TR703 B: +31.03V
Id ~ ( (Vrail - VbTR703) / R707) + IbTR703 is approx 240µA +IbTR703, say 245µA.
Extended calculations are available through motivated pm only, but one is welcome to correct me if I'm wrong.
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ps:
Dan,
If you're curious as to why I puzzled the formula, I happened to be invited to a very select poetry happening last night, to perform some of my own breed, Vogonian style. But I hate to listen to other Vogonians, because it is utterly boring.
mp
Dan,
If you're curious as to why I puzzled the formula, I happened to be invited to a very select poetry happening last night, to perform some of my own breed, Vogonian style. But I hate to listen to other Vogonians, because it is utterly boring.
mp
Lol, well I really appreciate all of the assistance you provided, even those pretty much all of what was in your previous post went right over my head lol. I know the poetry my be boring to you, but I know of hobbies/habits much less healthy.
Dan
Dan,
If taken into account the current from the fet (approx 245µA), the output voltage drops to a lower calculated value of 21.9V, which is very close to your measured value of 21.85Vdc.
Now we now into what limits this output must stay, try your alternative jfet and measure the output again.
If taken into account the current from the fet (approx 245µA), the output voltage drops to a lower calculated value of 21.9V, which is very close to your measured value of 21.85Vdc.
Now we now into what limits this output must stay, try your alternative jfet and measure the output again.