I need a suitable sub for the obsolete J231 (marked with blue stars in schematic). I don't know transistor circuits very well and as best I can tell, the J202 was the closest I could find in a TO-92 package. I've included a schematic for the circuit they are used in, and the data sheets for the J231 and J202 are attached in separate posts. Will the J202 work without major problems?
Assistance needed in selecting JFET subsitution
- Thread starter lassoharp
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yup 202 looks closer,,, the pinch off voltage is what varies the most in JFETs
JR
JR
I don't think it is very critical. (If it was, Fender could not build these.)
Idss, Vgs(off), and their *ratio*, should be in the ballpark. But even the J231 specs a very wide ballpark.
In that ballpark, Gm will be 2000-3000uMho or 400 Ohms.
This will influence gain, but R8 R18 R25 are larger than 400r and will dominate gain, gain varies little with FET change.
Q2 is biased rail to ground but with source resistor to -15V. For "any" JFET this WILL bias-up exactly as designed, drain at 60% of V+. Which is not particularly critical (this is a low-level stage).
Q1 Q2 are biased with Gate at -10V (through R4 and R3+R5 R19 R21 gate resistors), source resistors returned to -15V. Since Id is probably a half mA (33.2K with ~~15V across it), which is much less than Idss (2-6mA), we can assume Vgs is 2V or 3V. Making 5V+2.5V= 7.5V. 7.5V across R23 17.8K is 0.42mA, which makes 14V drop in R22 33.2K, which is half the total 30V supply, which seems a good op-point. Looking at R7, it says "1.78K", twice, but this leads to an absurd 3mA or 4mA, which in 33.2K drain resistor is 100+V drop, so that can't be right. R7 is surely another 17.8K part.
Check: Q1 has gain near R6/R8 or 20. Q1 should take 0.5V peak input. 20*0.5 is 10V peak. Biased as analyzed (drain near +1V and source near -8V) Q1 can do 8V 9V peak. 8V=10V for practical purpose. Q3 drives power amp which needs 2V input, which it can do easily with MasVol on "10", even turned down to "6". Lower MasVol settings clearly imply a desire to schmear the sound in Q3, and run the power amp at less-than max level. Q2 can never overload until Q3 is in 10X overdrive.
I have assumed +/-15V supply from D1 D2. If these mystery-parts turn out to be 12V or 18V, everything changes in nearly the same proportion and operation isn't much different. They can't be over 18V or the '4558 dies. IMHO they can't be down at 5V or the input headroom is too small for rock-n-roll.
They sure are hitting that reverb spring hard. First-glance Q13 Q14 eat full output-stage voltage, tank impedance is similar to speaker impedance, tank gets similar power to speaker. Actually the '4558 limits drive to like 8Vrms, added resistance puts tank near 6Vrms, which is still over 3 Watts.
I hope you are not building that output section; it has NO short-protection. (The $7 transistors will blow to protect the $0.20 fuse.)
Idss, Vgs(off), and their *ratio*, should be in the ballpark. But even the J231 specs a very wide ballpark.
In that ballpark, Gm will be 2000-3000uMho or 400 Ohms.
This will influence gain, but R8 R18 R25 are larger than 400r and will dominate gain, gain varies little with FET change.
Q2 is biased rail to ground but with source resistor to -15V. For "any" JFET this WILL bias-up exactly as designed, drain at 60% of V+. Which is not particularly critical (this is a low-level stage).
Q1 Q2 are biased with Gate at -10V (through R4 and R3+R5 R19 R21 gate resistors), source resistors returned to -15V. Since Id is probably a half mA (33.2K with ~~15V across it), which is much less than Idss (2-6mA), we can assume Vgs is 2V or 3V. Making 5V+2.5V= 7.5V. 7.5V across R23 17.8K is 0.42mA, which makes 14V drop in R22 33.2K, which is half the total 30V supply, which seems a good op-point. Looking at R7, it says "1.78K", twice, but this leads to an absurd 3mA or 4mA, which in 33.2K drain resistor is 100+V drop, so that can't be right. R7 is surely another 17.8K part.
Check: Q1 has gain near R6/R8 or 20. Q1 should take 0.5V peak input. 20*0.5 is 10V peak. Biased as analyzed (drain near +1V and source near -8V) Q1 can do 8V 9V peak. 8V=10V for practical purpose. Q3 drives power amp which needs 2V input, which it can do easily with MasVol on "10", even turned down to "6". Lower MasVol settings clearly imply a desire to schmear the sound in Q3, and run the power amp at less-than max level. Q2 can never overload until Q3 is in 10X overdrive.
I have assumed +/-15V supply from D1 D2. If these mystery-parts turn out to be 12V or 18V, everything changes in nearly the same proportion and operation isn't much different. They can't be over 18V or the '4558 dies. IMHO they can't be down at 5V or the input headroom is too small for rock-n-roll.
They sure are hitting that reverb spring hard. First-glance Q13 Q14 eat full output-stage voltage, tank impedance is similar to speaker impedance, tank gets similar power to speaker. Actually the '4558 limits drive to like 8Vrms, added resistance puts tank near 6Vrms, which is still over 3 Watts.
I hope you are not building that output section; it has NO short-protection. (The $7 transistors will blow to protect the $0.20 fuse.)
Thanks John and thanks PRR for the excellent breakdown!
That's interesting. They followed this amp with the "II" series which are the ones most people seem to remember. I wonder if the corrected that in those. Seems ironic too given they added a speaker fuse. I can't recall ever seeing that on any other Fender amp. Worried of DC?
That's interesting. They followed this amp with the "II" series which are the ones most people seem to remember. I wonder if the corrected that in those. Seems ironic too given they added a speaker fuse. I can't recall ever seeing that on any other Fender amp. Worried of DC?
The tube amps generally could not hurt themselves, no output fuse.
Later SS amps had more and better output transistor protection; Fender used a lot of power chips which had essentially everything-protection designed-in.
It is these intermediate technology transistor power amps which are prone to destruction. You are right though: WHEN these amps fry an output device, they typically fling themselves to one of the 45V rails. 40+V in the speaker is more coil heat than even over-over-driven guitar signal, and the voice coil toasts. So the $0.20 fuse may be to protect the speaker; bitter experience is that it won't protect the transistors.
Later SS amps had more and better output transistor protection; Fender used a lot of power chips which had essentially everything-protection designed-in.
It is these intermediate technology transistor power amps which are prone to destruction. You are right though: WHEN these amps fry an output device, they typically fling themselves to one of the 45V rails. 40+V in the speaker is more coil heat than even over-over-driven guitar signal, and the voice coil toasts. So the $0.20 fuse may be to protect the speaker; bitter experience is that it won't protect the transistors.
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