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Thread: Hacking a Jet City Jca2112rc

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    Hacking a Jet City Jca2112rc

    Hey,
    So i have a Jet City Jca2112rc and recently started to mod it. I did some mods but then i fcked up somewhere and saw smoke coming from the PT, so i turn it off. Well i took it to a tech and he fixed it, luckily neither of the transformers/tubes were blown. So when i took it back i noticed 2 strange things.
    The master volume became overly responsive, to get at the same level as 2,5 on the MV pre-mods i have to turn it only to like 0,04-0,05 now, and it increases just like that afterwards(i've only tried it to ~3 on the "new" MV, didnt go higher, cause i guess i might overstress the tubes or the transformer or something).
    And the presence control is at maximum(9/9) right now, and it doesnt sound shrill at all here(9/9 on presence pre mods was hell). In fact if i turn it counter-clockwise it gets shrill, then after like 1/15 turn it unleashes this high pitched squeal at maximum master volume(much like a feedback, but it happens even if the guitar isnt plugged in).It stops if i turn it to maximum again.
    And all the mods i dont were in the preamp and i havent touched the master volume and the presence control, just got more gain out of it and changed the voicing.
    Any idea what the problem can be?

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    Maybe the negative feedback loop on the power amp has opened up (due to resistor burning out). Got a schematic?

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    What exactly did you mod in the amp? Almost sounds like the problem when the OT leads are reversed causing the NFB to become positive.

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    Click image for larger version. 

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    JCA20H service schematic.pdf
    Click image for larger version. 

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    Well my amp is a jet city jca20h with a reverb, but the jca schematic isnt as close as the astroverb. And JCA20H_mod_1,1 is the basis for my mods.

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    well i dont know if the tech reversed the transformer leads, but i didnt do it certainly.
    Here are some schematics, and JCA20H_mod_1,1 is the basis for my mods.
    Click image for larger version. 

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ID:	12678JCA20H service schematic.pdfClick image for larger version. 

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    "it unleashes this high pitched squeal at maximum master volume" Sounds like an oscillation! Best to go back to the original design and get your feel on firm soil.

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    Supporting Member Jazz P Bass's Avatar
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    It sure sounds like the output transformer primary leads where reversed.
    Hint #1: Presense control is reversed.
    Hint #2: It squeals.
    The best test for this condition is to run the amp into a dummy load at a set output amplitude.
    Then the feedback resistor is disconnected.
    If the amplitude rises, then you have a negative feedback condition, which is correct.
    If the amplitude goes lower, then that is positve feedback, which is incorrect.

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    Thanks guys, but i somehow fixed it. If i remember correctly then one of the speaker jacks connected with the ground via chassis(cause i cant take out the pcb without unsoldering the transformer leads, so it just hangs there). But i've gone far from that, now made a kind of jcm800 clone out of the amp, sounds good!

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    Not enough b+

    Hey, so i have been modding my amp(jet city jca2112rc) for like 5 months(and devoting a lot of time into it). I have modded the preamp for gain, then made it a jcm800 clone and modded a lot, then a bogner xtc red channel clone, and recently a carvin legacy clone. The problem i encountered was that per carvin schematic it had 385 b+ on the preamp tubes. The amp is el84 based so the max i can get is 330v, and with 330v preamp tube b+ and the schematic values i get 100,124v plate voltage on v1a and v1b when they should be 165-180v. As a result(at least i think so) i have insane amount of gain and the amp is oscillating on higher gain settings. So i though about lowering the anode resistor from 220k to 100k on both stages and got ~177-184 plate voltage and the amp got a bit brittler sounding and sounded more midsy and pushy(in a good way), BUT the oscillations were huge. I cant even play the lower gain settings now.
    I really have no idea how to cure this. Any advice would be helpful i guess.

    Edit:And my guitar volume pot started sounding really scratchy.

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    Last edited by shreditup; 04-12-2011 at 05:29 PM.

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    you won't be able to get 385 b+ unless you switch out the transformer.

    The most under load is going to be about 360 on the plates which is what I run mine at.

    In order to raise the b+ on the preamp tubes you'll have to change the dropping resistors.

    The JCA stock uses something like 10k for each stage. I get about 345/350 on the first preamp tube using 840 ohm power resistors.

    I've got 4 preamp tubes in mine though so you'll probably want to use a bit more, around 1k or 1.5k.

    Use 2 watt or more resistors for those.

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    Well to get those 330v i took out the dropping resistors completely(and put a wire in their place). And do you have a jca20 or a jca2112rc because i even measured the place right after the diode bridge(before any kind of dropping resistors, if there are any left lol) and got 332, not sure how you got 345-350 there tbh.

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    You need dropping resistors for filtering or else your amp will oscillate.

    I've got a jca20 head.

    Measured right after the rectifier with no load at 380vdc. Rectified with 4 soft recovery diodes.

    Your transformer should have a sticker on it saying 270-0-270 as secondary output. Thats 380 no load after rectification

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    fuse- 100v-120v fuse 120v
    output: 266v 150ma
    27vac 100ma
    6.3v 2000ma

    thats the pt. Anything of interest?
    And what about those dropping resistors? I still have those oscillations at med-high gain, so i'll consider any variants. Where and what resistance do i need to put them?

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    The pt should do 370-380 unloaded.

    The dropping resistors should be r31 - 10k, r49, r45, and r3

    you can reduce those to whatever you need.

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    Old Timer J M Fahey's Avatar
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    so i have been modding my amp(jet city jca2112rc) for like 5 months(and devoting a lot of time into it). I have modded the preamp for gain, then made it a jcm800 clone and modded a lot, then a bogner xtc red channel clone, and recently a carvin legacy clone.
    i have insane amount of gain and the amp is oscillating on higher gain settings.
    BUT the oscillations were huge. I cant even play the lower gain settings now.
    I really have no idea how to cure this. Any advice would be helpful i guess.
    I guess your amp suffers a *severe* case of mod-itis.
    "low B+" is the least of your problems.
    The illness is so advanced I can't even offer a solution, besides scrapping it and buying a "real" Bogner or Carvin or whichever amp catches your fancy.
    Following faithfully a schematic and getting the "right" parts is only half the journey; the other half lies in layout, grounding, etc. , which might not be physically possible inside the Jet City chassis.
    Sorry.

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    Quote Originally Posted by diagrammatiks View Post
    The pt should do 370-380 unloaded.

    The dropping resistors should be r31 - 10k, r49, r45, and r3

    you can reduce those to whatever you need.
    well if it should then why is it not?
    and if i can reduce them then why cant i take them out completely? whats the theory behind this?


    Quote Originally Posted by J M Fahey View Post
    I guess your amp suffers a *severe* case of mod-itis.
    "low B+" is the least of your problems.
    The illness is so advanced I can't even offer a solution, besides scrapping it and buying a "real" Bogner or Carvin or whichever amp catches your fancy.
    Following faithfully a schematic and getting the "right" parts is only half the journey; the other half lies in layout, grounding, etc. , which might not be physically possible inside the Jet City chassis.
    Sorry.
    Well its i'm on a tight budget, and shipping here is problematic. And secondly its my newfound hobby to mod my amp.

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    The Valve Wizard

    there go you.

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    Noodle of Reality Steve Conner's Avatar
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    Quote Originally Posted by shreditup View Post
    well if it should then why is it not?
    and if i can reduce them then why cant i take them out completely? whats the theory behind this?
    Well, let's say you got too fat and decide to go on a diet. You can eat less food, and feel healthier. By that logic, if you ate no food at all you would feel great. Whereas in reality you would die.

    And so it is in engineering, trends in the behaviour of a circuit under component changes can only be taken so far before they fail, and the circuit starts to do something unexpected. (Dies, maybe.)

    Well its i'm on a tight budget, and shipping here is problematic. And secondly its my newfound hobby to mod my amp.
    So, undo all the mods one by one until the oscillation stops. If you remember exactly which mod caused the oscillations to start, undo that one first.

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    "Enzo, I see that you replied parasitic oscillations. Is that a hypothesis? Or is that your amazing metal band I should check out?"

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    Old Timer J M Fahey's Avatar
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    Fine with me, it's your time and your hobby.
    Anyway, as a general rule (not only here but in many other problems) I suggest you change 1 variable at a time (say gain/cathode cap/divider chain/wiring layout/whatever) so you know what it did, decide whether it's worth keeping or not (or try to tweak it), and easily backtrack if it causes any problem.
    If you change 3 or 4 things in a single step (or wholly rebuild a new stage) you'll never know *which* mod did exactly *what*.
    Otherwise it can get frustrating real fast, driving you away from a truly fascinating hobby.
    Go step by step and you'll get better results, going slow is not a bad way to travel if you are enjoying it.
    Good luck.

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    +1 John.

    The general rule in the aircraft industry was to go one step at a time, test, and evaluate. If necessary, revert to the original configuration with known good parts and then start from a clean sheet of paper.

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    yeah, thats what i'll do next time. I actually did it this time, but unfortunately split the whole process into 3 big blocks, instead of 1 variable at a time. I did the whole preamp, then rebuilt the tonestack(this one uses the bauxandall) then the power section.
    Well now i removed most of the unused stuff and did made everything cleaner, and i lost some of that constant uuuuh sound, although by a small amount, but the high freq oscillations didnt stop. And having 400ohms as dropping resistors didnt work(it made the amp sound worse actually).

    Btw i actually rebuilt the preamp yesterday, the oscillation didnt stop even there. I suspect that the v1 socket or anything related to the v1a and v1b stage is the problem.

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    Valvulados.com jmaf's Avatar
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    My few cents:

    1) You need the dropping resistors, or the unfiltered pulsed DC will resonate and beat with the guitar signal, it's horrendous.

    2) To increase voltage while still filtering, use a DC choke instead of the 10k resistor. Its ripple attenuation is exponentially higher when compared with resistor pi filters. And it should only drop a few DC volts.

    3) Your lead dress is ultimately important, if you posted good quality photos you'd get much more useful feedback.

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    1) what resistance? and do i need them after each stage?
    2) what kind of choke do you have in mind(how much henries)?
    3) well i have but they're really crappy quality unfortunately. I'll see if i can get better photos on weekends.

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    I don't see how you're going to figure this out unless and until you revert to the last known good configuration. You have a schematic, yes? And/or you took notes and carefully documented what you were doing with a camera as you went?

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    Valvulados.com jmaf's Avatar
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    Quote Originally Posted by shreditup View Post
    1) what resistance? and do i need them after each stage?
    2) what kind of choke do you have in mind(how much henries)?
    3) well i have but they're really crappy quality unfortunately. I'll see if i can get better photos on weekends.
    1) 150 to 220 ohms, give or take. see merlin's article on it: The Valve Wizard
    2) 4 to 6 henries should be enough. anything over 80 to 100 mA wiring

    Edit: Sorry for answer #1, I was reading another thread along with this one and replied here. Blame it on the beer. So I'm leaving the reply there in case anyone wants to read Merlin's text about rectifiers.

    So, for #1:
    1) Probably a few k per filter, 10k, 4k7, 2k2 and 1k would be a plausible filter chain. Your power tubes are fed right after the first capacitors and before the filters, out of raw +B.

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    Last edited by jmaf; 04-16-2011 at 05:20 AM.

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    Effects loop question

    So im in the middle of a Jet City Jca2112rc->Carvin Legacy conversion, and now i have this problem that i couldnt quite figure out.
    http://www.carvinworld.com/crg/crg/s...ics/VL100F.pdf
    I have already done the preamp, tone stack, phase inverter, and modded the power amp and now i have been trying to do the effects loop. Also i have decided to skip the reverb section, and this is where the problem starts.
    So the input and output for the reverb are b4 and b5 respectively. So if the reverb isnt used then we can disregard b5 completely, but what to do with b4? I have disregarded that also, and built the loop, but when i turn on my delay pedal in the loop the volume drops a lot and the sound gets really degraded and there's no delays there. So i measured ground to the input of the loop where via schematic it should be 240mv and i get around 10-20v so i logically if i dumped more signal to b4, the signal voltage would be less(right?) and i would hence get less voltage in the out and in of the loop. So the question is: How much signal to dump to b4(that'll be the ground now i guess)? Or if my theory is incorrect, How do i lower the voltage out of the loop? Resistors i guess? Or increasing the anode resistor so that the collector voltage will be lower hence the emitter voltage will be lower as well?

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    Last edited by shreditup; 04-25-2011 at 01:40 PM.

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    Old Timer J M Fahey's Avatar
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    Dear shreditup, you sound real confused ... and your explanation is not clear.
    If you don'y want/need the reverb, just omit everything between B4 and B5, period.
    I don't get your:
    I have disregarded that also, and built the loop
    What does it mean?
    By the same token, I don't know where you are hooking your reverb pedal.
    The J2 Send J3 Return Jacks run at a pedal-friendly 240mV signal level.

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    Quote Originally Posted by J M Fahey View Post
    Dear shreditup, you sound real confused ... and your explanation is not clear.
    If you don'y want/need the reverb, just omit everything between B4 and B5, period.
    I don't get your:
    What does it mean?
    By the same token, I don't know where you are hooking your reverb pedal.
    The J2 Send J3 Return Jacks run at a pedal-friendly 240mV signal level.
    by omiting everything between b4 and b5 do you mean that i also skip 180k/33k voltage divider?
    and the problem is that the send and return jacks run at 20v signal level, which is why my Delay pedal isnt working in the loop. And i want to know why is it so big and more importantly how to lower it to 240mV or so?

    I thought that the signal level is so big because the b4 takes out a lot of the signal voltage, then omits it to ground when the reverb is disengaged. I might be wrong though, but then i have no idea why is it 20v.

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    Old Timer J M Fahey's Avatar
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    y omiting everything between b4 and b5 do you mean that i also skip 180k/33k voltage divider?
    No, that's the direct/"dry" signal path, what you don't use is the delayed signal path.
    In a nutshell: sound from both channels gets to B4.
    Part of it goes straight to B5 through R20. Fine.
    Other part goes to the reverb circuit: not used.
    Signal on plate of V2a *may* reach up to 60 to 90V RMS , which padded by the tone control loss of 20dB (10x) can't have more than around 6V RMS.
    You have also signal loss through the interaction of Volume and Presence pots, and around 5x attenuation thanks to R20/R21.
    Q5 provides no gain so there is no way you can have 20V RMS there.
    You can have 1V (tops) with V2b reaching clipping, so the 240mV specified on the schematic looks perfectly reasonable.
    Follow the signal path I described and measure voltages along it, they must be reasonably close to what I suggest.

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    well the voltage on v2a plate is 170v. Yeah, it gets attenuated a lot, but then there's this r82 from the collector to base of the transistor, that probably drops the voltage but still leaves some, which is why there's 20v(i did a test of another 4,7m resistor to see how it drops voltage from my 320v b+ and got 56v). But then again without the resistor the sound is bad. I'm guessing the signal voltage is way to low without the resistor.

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    Last edited by shreditup; 04-26-2011 at 04:02 PM.

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    Old Timer J M Fahey's Avatar
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    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.

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    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!

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

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    Old Timer J M Fahey's Avatar
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    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.

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    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.

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