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Clone preamp section of JC-120
Clone preamp section of JC-120
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Old 1st September 2018, 02:14 PM   #1
john65b is offline john65b  United States
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Default Clone preamp section of JC-120

Anyone have thoughts on this? Don't want the distortion or amp section, just the clean guitar preamp with volume / treble / bass / mid tones controls...

Send output to an amp of my choice (ss, tube, class d, etc)...

A waste of effort or worthwhile?
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Old 1st September 2018, 02:19 PM   #2
john65b is offline john65b  United States
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Looks to be four 2SK-117 and a single 2SC-1815? Connection to 5 / 7 is a bit confusing...

Also noticed that the JC-55 / 77 etc are all opamp based and sound, supposedly, exactly like the famous JC-120...
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Old 2nd September 2018, 02:25 AM   #3
PRR is offline PRR  United States
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5/7 cross-feed two input to two power amps. If you want the one channel, ignore this.

Yes, the JFET RJC is one of the sweeter amps ever built, and I do not think the opamp ones are just-as-good.
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Old 2nd September 2018, 12:26 PM   #4
JMFahey is offline JMFahey  Argentina
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Build the plain top channel, sections involving Q1 to Q4, up to the left side of R29, place the output jack there.

Personally I would add a unity gain Buffer , a single 100% NFB Op Amp is perfect for that, so output is lower impedance and can drive longish cables and most any Power Amp input.

You will need to build a clean 27V (nominal) supply to feed the preamp, anything from 24V to 30V will do, and if you add the Op Amp buffer (a single TL071 is enough) you can also feed it same single supply as the rest of the preamp, no need to get +/-15V just for it.

If needed, ask me for a simple single supply Op Amp buffer.

You will end with a very nice, loud and clean "Jazzy" sounding amp, great also for Pop, Country, Funky, etc.

In fact, sort of an SS "Twin Reverb" sound type of amp.
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Old 2nd September 2018, 12:51 PM   #5
john65b is offline john65b  United States
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Looks to be a pretty basic design... I should have something bread-boarded in a jiffy. Just gotta get up with Digikey and make an order...Mybe couple this pre onto an ICEPower 200ASC as it has a 47VDC output pin....add a couple zeners and I should be able to get the 27V and 43V for the pre...

I just purchased a JC-55 combo and a JC-77 chassis (both not working) to see what they sound like.

I am liking Solid State guitar amps more than tube - I picked up a broken Fender Princeton 65 (the solid state Princeton, not tubed) for peanuts, and was shocked on how good and clean it sounds, and how well it took pedals. I have since sold off a couple tube amps (two Peavey Valvekings, a 100 watt Peavey Windsor, a Fender Blues Deluxe Reissue, and Hot Rod Deluxe (all purchased off Craigslist, broken, that I fixed) to "Fund" my new foray into Solid State Guitar amps, hence the two JC's. I was going to pick up a JC-120 (with "issues") local but it is HUGE and HEAVY)

Thanks PRR - you probably dont remember, but you help me sort through a 6L6GC / 6SL7 / 5U4 amp that was literally given to me back in 2005. I joined this forum to try to fix this amp (turned out to be bad input 6SL7) and learn about electronics. I asked some incredibly foolish (and dangerous) questions.

Hundreds of fixed amps later, I am here...

You RAWK PRR!

My wife, on the other hand, wishes I never joined this forum. Bah!
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Old 2nd September 2018, 12:59 PM   #6
john65b is offline john65b  United States
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JM - leave the R29 and R33 out? I was going to terminate after R33 with input to Amp...

I can also add your buffer with another "copy" of Q2 also, correct?
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Old 3rd September 2018, 02:12 AM   #7
PRR is offline PRR  United States
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OK, with all the 10K power amps around, I agree with a buffer.
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File Type: gif JC-120-JFET-cut.gif (17.0 KB, 107 views)
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Old 14th September 2018, 09:49 PM   #8
john65b is offline john65b  United States
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Thanks PRR!

Can I sub a 2SK170 for the 2SK117 without any other changes?
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Old 14th September 2018, 10:31 PM   #9
JMFahey is offline JMFahey  Argentina
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Agree with PRRīs buffer, too much in a hurry to open the Service manual now but if R29 and R33 are the channel mixing resistors yes, they are not needed now.

As of FET replacements, given that their characteristics vary a lot from unit to unit, best is to buy a few extra and try them, leave the ones which work closer to expected and save poorer ones for non critical stuff ... such as unity gain buffers.
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Old 14th September 2018, 11:23 PM   #10
john65b is offline john65b  United States
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I vaguely remember that the 2SK170 should not be used over 18 vdc rail...hmmm.....
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