Hacking a Jet City Jca2112rc

[J M Fahey]

Sorry but I guess you are mixing DC voltages and AC voltages thinking they are the same thing.
R82 biases Q5, has no measurable effect on AC voltages there.

(i did a test of another 4,7m resistor to see how it drops voltage from my 320v b+ and got 56v)

How did you measure that?
Let me guess: you soldered one end of the resistor to +320V and touched the free end with the multimeter probe, to check how much voltage 4M7 drop, and you measured 56V on some scale. Is that so?
Another question: what signal source are you using to test? Applied where?
What AC voltage do you measure at J2 hot pin?
What happens if you plug your guitar straight into J5?
And if you plug in the preamp input and use no pedals in the loop?
Thanks.

[shreditup]

Let me guess: you soldered one end of the resistor to +320V and touched the free end with the multimeter probe, to check how much voltage 4M7 drop, and you measured 56V on some scale. Is that so?

uh, yeah.

Another question: what signal source are you using to test? Applied where?

well if you're talking about voltage then from the ground to the measured object, like the v2a plate.

What AC voltage do you measure at J2 hot pin?

1,7v(from it to ground)

What happens if you plug your guitar straight into J5?

clean sound.(and btw its now as loud as i expected)

And if you plug in the preamp input and use no pedals in the loop?

then no sound occurs. Thats because i didnt solder the hots of the fx loop jacks(b1) because the signal with delay is going to loop back in to the input and delay again, and because of that no matter what the feedback knob on the delay pedal is set to the delays are endless nonetheless.

I guess i'm really confused in this stuff, but want to get to the bottom of it.

Thanks.

well actually Thank you! You're helping out a lot!

[shreditup]

And btw i just measured the voltage between v2a plate and ground and got 320v. Strange.

[J M Fahey]

Well, don't know the best way to say it, but basically I see that you need to know more basic theory, *because* what you are trying to accomplish is quite ambitious.
There's *tons* of people around who get by without it, and seem to be quite successful, typically having built a lot of pedals and/or many tube classics, such as 5E3, "18 watt", etc.
Fact is , they are building them "by the book", following an established and proven schematic , an already debugged layout, etc.
Fine.
But when you want to travel away from the established path, on when trying to service something and want to know what the *out* of script voltage or part resistance mean, unfortunately you lack the means to do it.
When Musicians ask me about what to read , i often suggest starting a with very unglamorous an in theory boring (in fact it's not) Physics book.
Can't suggest a specific name or author, it varies in every country, but it's the "Electricity" section, which they usually teach you in ¿high school? when you are around 17/18 y.o.
They explain *the concept* of Volt, Ampere, conductor, insulator, resistor, capacitor, inductor, transformer, magnet, potentiometer, frequency, etc., plus basic Ohm's law and others.
Besides the "parts" explanation, you'll learn how to measure and calculate things and why a multimeter gets to be a *multi* meter.
Granted, they'll actually explain the galvanometer (the old analogic needle meter) and how by a clever combination of resistors, switches and batteries you can measure voltage, current and resistance, but once you understand it, you'll *really* understand the digital ones, which for all practical means emulate them.
As you see, *everything* is useful there.
You'll have a lot of "*d4mn !! *now* I understand it from *inside*" moments.
Electronics, which is basically one application of that knowledge, will be seen in a new light and actually become far less complex than it was before.
Just my 2 cents.

[riz]

Heed the advice in the previous post. I recently got a copy of Electronics Designers' Handbook. No, I haven't read the whole thing, but it has really helped me a lot! Some really, really fascinating reading. Plus tons of stuff I'll never digest in a million years... GREAT chapter on tubes. Any book of this type, that really explains how stuff works, will be good. I found I was getting too far ahead, doing repairs and such, without really knowing how the stuff works. I'm feeling ready to start tackling some very basic circuitry from a design point of view, but just fixing stuff using a schematic is pretty straightforward most of the time.

[jmaf]

For tube knowledge, nothing beats old books. The ones nobody wants anymore, because they won't teach you digital electronics. I found this old hardcover book in a relative's library. I asked if I could have a read during a week and then return it. I got it as a present!

[shreditup]

yeah, you're right, i need to learn some theory. I'll find a book and start reading it, but in the meantime here's what i've done today-
I've made the whole circuit cleaner, and used an rg147 coax wire for connections. Then i rebuilt the power supply ala legacy, and somehow the effects loop works now(and the amp sounds really cool now)! Though this one problem came out: when i play light, all comes out alright(random rhyme) but if i strike hard then the sound gets really trebley and shrill just for that moment. Kind of like the amp has a threshold of voltage, and if the signal surpases this threshold, it amplifies the higher freq's(at least this is how i understand it). I am guessing its a cap problem somewhere in the effects loop, but all seems fine.

[shreditup]

Or the transistor needs replacement.

[jmaf]

Does it not sound like "normal" overdrive? Just earlier today I had this guy over who was complaining his amp would never be clean, it just happened his Seymour Duncan humbuckers guitar put out 3 to 4 times more signal than his Strat....so unless we reduced the gain on his amp, he'd never get any clean out of his Les Paul.

[shreditup]

well it is like a normal overdrive, but its really abrasive and is a lot louder than the normal signal.

Also, i just measured my dc and ac voltage out of the rectifier and the results are 338v dc and 748v ac. Thats kind of abnormal, right? Why is the ac so high?

[J M Fahey]

Also, i just measured my dc and ac voltage out of the rectifier and the results are 338v dc and 748v ac. Thats kind of abnormal, right? Why is the ac so high?

It is not.
You have a cheap multimeter, only suitable to read voltage from a transformer secondary, or a wall socket , nothing fancier than that.
They are really "only DC" multimeters, which for "AC" just add a diode in series and multiply the reading by 2.5 or 2.8 to "compensate" for the fact that you are reading 1/2 of the actual wave, plus it being presumed a senoid.
If you connect your probe to , say, a plate, it will indicate an AC voltage which is not there.
It will do to measure transformer secondaries.
Very confusing.
Real AC voltage meters, are AC coupled through a capacitor, and couldn't care less about DC voltage present.

[jmaf]

well it is like a normal overdrive, but its really abrasive and is a lot louder than the normal signal.

Also, i just measured my dc and ac voltage out of the rectifier and the results are 338v dc and 748v ac. Thats kind of abnormal, right? Why is the ac so high?

I have no idea why you'd get such readings. Looks like a measurement error of some kind to me. Are you sure it reads 748 VAC and not mV ac(just ripple)?

[diagrammatiks]

I don't understand how you're even doing the loop.

The Legacy is using two sides of a 12ax7 as the driver and recovery stage for the reverb. The jca does that with 2 12ax7s.

If you converted the jca too legacy specs...

You'd have to make sure that the loop take off points are correct and that you are taking and putting the signal back into the same place.

Which channel did you convert into? The lead channel?

The Jca20 has v1, v2 with v2 hardetched as a cathode follower. I'm assuming you cut the traces on v2?

The loop then goes through Q5 which acts as the driver and is recovered through v3a.

What are you using as the recovery stage for the loop?

[shreditup]

Well its like this, i use v1 and v2 for the 4 gain stages of the lead channel(all of the traces on v3 were cut), then the signal goes through the tone stack and master volume, then to q5 as the driver then to half of v7 as the recovery(obviously all the traces and components from the jca reverb have been removed), then to phase inverter v4. I didnt do the reverb circuit,cause i dont really need it.

And yeah, it must be my cheap multimeter, i also couldnt believe the fact that it measured 469v ac on v2a plate...

[shreditup]

uhh, well i solved it. Instead of using the left triode of a 12ax7(pins 1-5) i used the right one(pins 6-9). Strange..