1982 JC-120 clips very early--what's wrong?

[naturalsnotinit]

First post 

So I have a JC-120 (it's a weird transition model...small logo, MN3004, chorus trimmer pot) and I love it... Except for the fact that it seems to go from super clean to mildly disgusting pretty early on the volume pot. On low input it's usually around 4, and on high it seems to before 2. I took it into a tech who told me it was all totally perfect and that JC's are always like this... What gives? When I use my Peavey T-15 it seems to do it earlier because the pickups are hotter... But overall, this makes me quite sad because it doesn't seem like it should be doing this and I sincerely doubt that all of them are like this. Thank you in advance for any help!

[J M Fahey]

If it puts out 60W/ch (and I guess the Tech checked that) , then that's the way it is.

Maybe your guitar is too hot for it.

SS gives you very good cleans ... as long as you don't let it clip at all, or are excellent to amplify good distortion if you previously created it but in general don't gracefully clip on their own.

Some are designed with this in mind and pre-equalize signal so if the amp clips, it does not fart or sound buzzy, but not all.

I guess the JC120 designers thought everybody would use them clean (that's why they fitted a useless distortion circuit to it) .

A similar amp which works much better is the Marshall Valvestate 8240 , with killer distortion, and 2 Celestion G12T75 and to boot is cheaper.

[naturalsnotinit]

I'm not sure I'm following you here. In theory, are you or aren't you supposed to be able to take the volume pot all the way up (conceivably, and within reason I guess) without having clipping? I just feel like I should have more headroom, unless it's the result of a linear taper pot or something like that?

Also, I got it for $380 which I suppose isn't too bad considering it was the last revision before they combined chorus and vibrato oscillators into a single waveform and stopped using JFETs for the gain stages... idk... I can't really go back on the purchase.

Also, what are some good ways to prevent clipping? I just feel like I should have way more headroom... 

[g1]

  Clipping of the power amp will occur when the amp achieves it's full power output.  Where the volume control is set does not completely control it, it also depends greatly upon the level of the input signal.
  A very weak pickup will allow a higher amp volume setting, but a "hot" pickup will drive the amp to clipping at a lower amp volume setting.
  Many amps will clip at around 4 on the volume control.  Turning down the volume control on the guitar itself will allow higher amp volume settings.

[Roly]

Hi naturalsnotinit, welcome.

SS gives you very good cleans ... as long as you don't let it clip at all, or are excellent to amplify good distortion if you previously created it but in general don't gracefully clip on their own.

THIS should be cast in stone somewhere.   :dbtu:

In theory, are you or aren't you supposed to be able to take the volume pot all the way up (conceivably, and within reason I guess) without having clipping?

No, you aren't.  To make g1's point in a different way; traditionally the typical power amp (output stage) produces full output power for around one volt of signal in (this is at the master volume between the power amp and the preamp).  Whatever the actual sensitivity of the power amp for full output, this is a fixed maximum voltage, and the volume control is used to scale the input voltage to this level - it determines the overall preamp gain between guitar signal and the power amp input.

What the position of the volume control is actually telling you is how high your input voltage is compared to the one volt required by the power amp, a high setting for full power (clip) means a low input signal voltage, and a low setting means a high input signal voltage - it's about normalising the input signal voltage for the power amp.

In rack amps it is common to find the volume control is not marked 0-10 but in dB starting at perhaps -60dB and proceeding down through negative values until you get to 0dB at full blast.  This highlights the role of the volume control as a signal attenuator - excess gain is provided and you wind some back to match the incoming signal to full output.

what are some good ways to prevent clipping?

A compressor/limiter pedal.

If you look at a lot of guitar amp circuits you will notice that some sort of clipping stage, e.g. diodes in anti-parallel, is often found in s.s preamps, yet is almost unknown in valve amps.  The idea is to introduce signal clipping in a controlled manner in the preamp to avoid having the output stage clip in an uncontrolled manner ('coz it sounds better).  In a valve amp the output transformer make all the difference to what happens in the output stage during clip, the upshot of which is that a clipping valve amp output stage can sound "rich", while a clipping s.s. output stage sounds "harsh".

I actually run a homebrew Twin-50 s.s. of my own design for keyboards, and a valve Playmaster 117 for guitar.  On those occasions where I do run guitar through the Twin-50 I have to be very careful not to push it too hard because there is a "wall" at clipping that sounds pretty terrible.

[J M Fahey]

there is a "wall" at clipping that sounds pretty terrible.

That's the point on most SS amps, unless they were carefully designed to minimize that.

FWIW I consider the Roland JC series the most overrated amp in the planet.

Mind you, not bad, just overrated.

They are always mentioned as an example of clean amp, while there are tons just as good ... and many better.

I guess it's the only SS amp Tube/Metalheads know, after all "it was used by Metallica" ... and a few others.

Same era SS Yamahas were at least as good, and many modern amps too, from humble Peavey Bandit or Crate or Fender amps on.

I already mentioned a Marshall Valvestate which is very good and handlespickups better ... because it was designed so by people whobuilt their reputation on distortion.

Certainly one big asset of JC amps is the killer stereo chorus , no doubt, but as far as clean sound ... not bad ... not amazing either.

You want CLEAN and LOUD and NO distortion whatsoever?

Follow the Steel Guitar guys.

They need exactly that, so some specialized amps were designed and built for them.

They started using Twin Reverbs, but they were not loud and clean enough  , so they opted for the ones loaded with JBL speakers , loudest and cleanest of them all ... still not enough.

So in quest of power (>100W on tubes becomes a nightmare) they went SS .
They found the same problem as you , so the next logical step was to add very tight compressors/limiters so signal NEVER distorts.

One excellent example is the Peavey Nashville 400:

which comes with all the goodies:
1) >200W RMS ... SS of course.
The same poweramp used in Peavey Bass amps and Powered Mixers
2) a Black Widow speaker, Peavey version of JBL.
Monster magnet, huge 4" edge wound aluminum ribbon voice coil, aluminum dome attached directly to it, curvilinear cone, shattering power and efficiency.
3) clean preamp with modern active equalization instead of another refried copy of old worn Fender type EQ
4) an incredibly tight compressor which does not allow >1% distortion, if that much.
Not the classic level detecting one, but their IOC type, Input Output Comparator which does exactly that: while output tracks input faithfully it does nothing; as soon as a slight difference appears, the compressor pads it down, to the exact point where they match.

How it works?
1) the power amp input pair gets the input signal on the base of Q3 and a sample of the output on the base of Q4.
2) Op Amp U8b is getting input signal through R77 and output signal through R79 and compares both.
While they are the same, it does nothing.
When they are different (output clips, even a little), it triggers and sends a strong error signal to U8a which amplifies it and sends it to pin 5 of U7 (through a couple resistors and diodes) which is a compressor IC (I think it's a CA3080 hidden behind a Peavey part number) which tightly controls amp gain, allowing NO distortion.

[gbono]

there is a "wall" at clipping that sounds pretty terrible.

FWIW I consider the Roland JC series the most overrated amp in the planet.

Don't forget the often neglected Peavey stereo chorus  :tu: