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Old 5th November 2010, 12:20 PM   #5191
Salas is offline Salas  Greece
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So everything is oriented as I wrote? If yes and still the same voltages, make your B+ 25V now using the trimmer in the reg. What is the voltage on Q2's collector and Q3's drain after that?

P.S. Is your R5 well connected to Q3's gate and ground? Any possibility of having a 27k instead of a 2k7 R9 or another one wrong like R1 for instance? The voltages are weird around Q2 for Vbe drop even.
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Old 5th November 2010, 10:18 PM   #5192
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Made B+ 25.1V and took 2nd reading with a different DMM because the first one was taking a long time to settle down (even with a new battery - time to discard it because it has already happened before). The new voltages are marked in the attached diagram. I haven't studied them but there are a few changes from the first readings. Thanks for your patience.
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Old 5th November 2010, 11:59 PM   #5193
Salas is offline Salas  Greece
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That one looks logical. Stronger but logical. Shows your Q1 fets like being 6mA jobs, 5.8mA with slight degeneration of the 3R3. The 2nd stage and output look rather normal now it biased up as a whole. Basically it will play. You should be able to hear music. You need to bring Q3 voltages together at 12.5V. Use 100R trimmers instead of R6s. Experiment with some mismatched 4R3-6R8 range for R2s to bring Q2 at 12V in both channels. So to keep the gain at bay a bit and match up channel outputs. Those first 2 stages fets are upper limit GR it seems. Surely not 4mA IDSS and not well matched between channels, but you will bring everything together by doing the moves I just mentioned. Keep us posted.
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Old 6th November 2010, 12:46 PM   #5194
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OK Salas, the Lady sings... the blues! Only one channel is working, but I haven't done the changes you suggested. I'm sure we'll get around it. Well done Salas!! It's the channel whose voltages are shown second in the various voltage readings in my last attachment. Incidentally, as Chief Moderator you'd better remove the attachment in post Simplistic NJFET RIAA #5188 because it can mislead readers and cause trouble. Thanks for now and regards. Will keep you posted. Joe A
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Old 6th November 2010, 02:44 PM   #5195
Salas is offline Salas  Greece
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Hoping that you haven't got a killed Jfet in the other channel. I removed that attachment as you requested. You are near success it smells. Not bad for troubleshooting till now. Keep us posted.
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Old 6th November 2010, 06:11 PM   #5196
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Luckily I didn't kill anything... that channel was dead due to a bad coupling output capacitor. I inserted the 100R trimmer in place of R6 (33R) and now Q3sees 12.5V as you suggested. Biasing Q2 (BC550) has been trickier. I inserted 4R7, 5R6, etc up to 13R in place of R3 (3R3) and all I managed to get at the junction R1-Q2 was a maximum of 8.33V. (Pcb now looks somewhat like a breadboard!) I stopped there, thinking that maybe R1 (2K4) needs to be tweaked/changed. All other components and positions remained intact.

Anyway, the amp is playing music right now and initial impressions are very good. It does compare very well with the Steve Bench amp. The sound is clear, clean, detailed, strong and wide, with no trace of harshness. It's still burning in and maybe I'm biased because I'm happy for getting it working. Layout is in a shoe box cover so in due course I must put in a proper metal case.

I'm not sure it's worth bothering about the voltage at Q2. I'm sure you know better. So far, very good, so Well Done, Salas and many, many thanks for your help!!
Regards.
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Old 6th November 2010, 06:21 PM   #5197
Salas is offline Salas  Greece
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That's nice. Mysteriously you have shown 9.7&10.7V at Q2's collector in your second attachment. You are measuring from C Q2 to ground again, right? Make R1 2k2 then and arrange R2s accordingly so to have something 10-12V at Q2's collector. Channels need be matched at those points so to have same gain for best stereo. Let us know more soon along with pics etc. Also try your input load trimmer to find best tone with your cart. Congrats!
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Old 11th November 2010, 09:30 AM   #5198
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Spent the past few days tweaking the amp to get the required voltages. Finally I got it sounding to my taste and prepared to put it in a case (a humble one at that!) This is the version I'm listening to and it sounds great. No hum or whistles. Output caps are not identical but I like the sound of the (large) Russian PEPT 3.3uF better. Later on I'll change the other (SCR) cap. Meanwhile, I attach a photo of innards of my Salas nj-fet riaa. I encourage other vinyl lovers to build it. Cheap to build (compared to valves), lot of help on this thread, fun (and a learning experience in my case), and above all GREAT SOUND!!! THANKS A LOT AND WELL DONE, SALAS!!!
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Old 11th November 2010, 10:07 AM   #5199
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I like the hula-hula heatsink.

- keantoken
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Old 11th November 2010, 10:18 AM   #5200
Salas is offline Salas  Greece
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Wicked DIY from a valves projects constructor but with heaterless pentodes and NOS USSR caps. Nice one Sonata. What is the best input trimmer load for the vintage design Japanese MM of yours?
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