Just got my first JC120 (UT) and I play Telecaster (vintage fralins).....the treble is over bearing. I can almost adjust the treble pot to my liking but I hit a certain point and it falls off too much. The amp is playing and functioning properly but geez its bright. What can I mod in CH2 to tame it some....C4, C16, C8 ?.....or is there somewhere else I should look. speaking of C8, what is 0p exactly.
congrtulations
C4 and C8 negative feedback so increasing them would give less high frequency since it gets cancelled out.
about C16 you could play around with tonestack calculator to get a sort of idea where to go..
in the schematic it looks like the treble pot is linear maybe a logarithmic pot would give you a better feel.
yeah, I looked at it on the TSC.....but I like the sound of the amp....minus the extended highs. Thats why i'm trying to avoid the tone stack. I'm going to start with the others and setup a clip in/out rig. Wasn't sure which direction to move, up/down, but since they are NFB, now I do. Thanks for the info !!
Don't take offense if you've already tried this, but have you fully explored the interaction between the mid EQ knob and the hi EQ knob? I have been getting more of an appreciation for this recently as I've switched guitars and drifted into cleaner and cleaner tones. I'm finding that lower hi EQ settings combined with raised mid EQ settings often give me enough "bright" to have detail, without being too shrill. It helps when I run the bass EQ knob a little lower than I otherwise would, too...don't really need it, it eats headroom and with it reduced a bit, you can dial out some of the shrill without turning the tone into detail-less mud.
Or you could try what I read about in a Jeff Beck interview once...set it up LOUD and BRIGHT, so that it will pretty much take you off at the neck if you open up the guitar's volume and tone knob...but then play with those two controls rolled back somewhat. Some guitars sound better than others doing this, but it's fun to have tonal and dynamic control right at your fingertips like that, and while JB was doing it with a strat and a Marshall, it works on loud clean amps too. If the amp set to "borderline unuseably bright" with the guitar tone pot full out, there is probably going to be a reduced setting on the guitar that sounds good and not too dull.
I have been playing guitar enthusiastically for more than 30 years and I can still find things to discover in a simple 3-band passive EQ...maybe you can too, without having to mod anything.
'Course maybe modding it is the attraction in the first place...
Your amp sounds great with a Gibson L5 ;)
Your amp is not bright.
If anything, it's not "warm" ... no news since the power amp is classic unimproved SS.
But your Tele is.
I'd use slightly thicker strings, which to boot will give you more power and sustain.
FWIW your amp is nothing but an "SS Twin" , specially same EQ (only treble cap is somewhat lowered in value, you can test original "Twin" values if you wish ... not surprising since a Twin used to be the king of the hill in 212 combos when JC120 were designed back in the day.
And as men tioned, with a "Jazz" guitar it sounds fine.
Well, problem solved ! Removed C6 from the bright switch circuit. I don't know how much of that signal could have made it through the 1M resistor (R6)..but anway, don't need it. Changed treble tone stack cap C16 from ceramic to silver mica. Replaced C4 with silver mica and raised it to 47p. Bingo ! still nice and sparkly without the ear split. Thanks Everyone !!
Do me a flavor and play an ES335/L5/ES175 through your mod and see what it sounds like. Were the ceramic caps X5R dielectric? Are you saying the mica dielectric improved the sound?
Quote from: gbono on March 02, 2016, 11:58:18 PM
Do me a flavor and play an ES335/L5/ES175 through your mod and see what it sounds like. Were the ceramic caps X5R dielectric? Are you saying the mica dielectric improved the sound?
I would gladly play one through it if I had one. I'm would assume they were XR5's....I don't see them using XR7's......if size is an indication of temperature range....these were really small...about 3mm across. YES ! the silver mica's greatly improved the sound....I could tell instantly. However, I did replace the bass and mid tone stack caps with CDE polyester 150's but I did this BEFORE the ceramic caps and it made little change. It went from a ratty top end to much smoother....just as you would think a mica would do. JC120 is one of the easiest ss amp I've ever worked on.....the lay out is excellent. Every aspect of the amp has it's own individual pcb....and simple to pull out. CH1, CH2, effects, Ins/outs/effect loop and power section. Nothing double or triple stacked....that's why the amp is so large.....unstacked pcb layout. I'm in the process of removing all of the ceramics, in favor of micas....I'm also replacing all of the .068uf coupling caps with a higher grade and replacing all of the small electrolytic that are in the signal path with Nichicon FS series caps (audio grade). This may have an adverse effect of too sterile....at which point I'll backup. I did this same thing to my Fender Ultra Chorus but eliminated several of the small electrolytic caps for non polar polyester caps and......It.sounds.amazing!! The biggest hindrance of that amp is the 1000pf volume pot bypass cap.
ok update for anyone curious, this amp responds VERY well to upgraded components....which I suppose any would. So I've taken the CH2 preamp pcb out and replaced all 5 of the .068uf coupling caps, and the lone 47uf electrolytic cap. This really brought the amp together and was a great improvement. I'm not going to bother with the distortion section of the signal....I'll never use it. Next, replace the components of the effects pcb (reverb and Chorus). If anyone is interested here are the caps used. Vishay MKT469 PN: BFC246921683 for the .068uf and Nichicon FS for the 47uf.
It's always nice to know the type of caps you used for replacement.
But even more than that, what everyone is always curious about, and no one ever reports:
what was the capacitance measurement (and ESR) of the old parts, and of the new parts you replaced them with? ;)
Quote from: g1 on March 04, 2016, 12:26:50 PM
It's always nice to know the type of caps you used for replacement.
But even more than that, what everyone is always curious about, and no one ever reports:
what was the capacitance measurement (and ESR) of the old parts, and of the new parts you replaced them with? ;)
I measured the new parts, dead on.....all of the old parts and they were dead on, no drift at all....BUT....a big BUT, the originals were the cheapest kind out there. You know, the small little chicklet size, that you can actually see the foil wrap through the sub standard coating they were dipped in. So, ESR ? I wouldn't even know where to begin with those cheapies but the replacements, I'm almost certain are better. They were this type....not the actual ones but like this.
QuoteI measured the new parts, dead on.....all of the old parts and they were dead on, no drift at all....BUT....a big BUT, the originals were the cheapest kind out there.
If new parts were dead on value and old parts too were dead on, to boot had not drifted after all these years, I fail to see *what* could have caused
any sound change at all.
As of price, how do you know?
Just curious.
Quote from: J M Fahey on March 04, 2016, 08:04:50 PM
QuoteI measured the new parts, dead on.....all of the old parts and they were dead on, no drift at all....BUT....a big BUT, the originals were the cheapest kind out there.
If new parts were dead on value and old parts too were dead on, to boot had not drifted after all these years, I fail to see *what* could have caused any sound change at all.
As of price, how do you know?
Just curious.
Well, you also weren't sitting in front of it before and after....so your comment, in my particular case is based on "your" theory. Now if you are saying that parts are parts, then maybe my ears are different than yours. I saw you on another post...do you come around just to contradict people or what
Quote from: cejay825 on March 04, 2016, 08:18:09 PMdo you come around just to contradict people or what
No, do you?
Could even be fresh solder connection on the signal path helped out a bit too. You just never know sometimes when it comes to solder connections. Honestly all of these tiny caps in the signal path are basically all cheap parts but if they measure right then they work.... Well sometimes lol. As I recall checking ESR on very small caps 0.1uf and smaller always prove impossible on my blue ESR meter. I will have to check that out again though no doubt.
Actually my ESR meter does read even those small signal coupling caps :duh. I think the thing with those caps is that I am not testing them as much for ESR. Mostly I am tracing signal and the signal does not come out the other side so I just pull it out and toss it aside. :trouble I find the ESR meter really comes in handy when I am testing for bad electros. So if you measure a 0.1uf cap and it say 10-80 on the ESR scale it is probably good still. However, if i measure way higher than that then it is definitely a bad one. I will have to start testing those bad coupling caps soon for sure. :dbtu:
X5R for a small value ceramic capacitor - I don't think so - try C0G. X5R has a really nasty voltage coefficient and is microphonic due to the piezo effect. Do you have the BOM for this amp design?
Maybe it is easier to listen with one's eyes.
Got the amp sounding great BUT this chorus thing is driving me crazy. With the chorus off it sounds just like it should but when the chorus is engaged, the right speaker/channel (looking at the front of the amp) has a volume drop....I would say in the order of around 25-30%. I'm assuming this is the dry signal channel because you can plainly hear the chorus effect in the left channel. The chorus works and sounds great but theres the slight volume drop. Is this normal ? This would be a great hindrance when playing live.....
I wouldn't call that normal.
One thing that occurred to me earlier. if the amp sounded overly bright, as in the lower range sounded less than optimal by comparison. And now one side sounds weak. Try reversing the speaker wires on that weak side, just as a test. Any difference?
The amp in clean mode - ie no chorus - should send the same signal to both channels. In chorus mode, it turns off clean to one side and runs the chorused signal there, keeping the other side dry. I can see some element in the routing circuit could be resistive or otherwise attenuating. Good place to use a signal tracer.
I'll try a speaker lead swap test. By the schematic, isn't the chorus being injected into the ("Power Amp R") right channel ?
I don't have the schematic open. One side has chorus only (no dry) while the other side has dry only (and always). CHorus working against dry makes a much better chorus effect than the wet/dry mix from something like a chorus pedal.