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Thread: Hacking a Jet City Jca2112rc

  1. #71
    Valvulados.com jmaf's Avatar
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    I searched and Yahoo says the JCA2112 circuit is a Soldano Astroverb.

    soldano_astroverb_16.pdf

    Does it match your circuit somehow? Indicates a -16 VDC bias voltage. You probably need twice that for 6v6's.

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  2. #72
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    yeah it looks pretty much the same. And yes, that secondary winding only gives 27volts, which is not enough and which is why i want to make marshall style bias from the hv tap.

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  3. #73
    Old Timer J M Fahey's Avatar
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    The sign on the ot says 1.8k 0 1.8k, so i guess that's 3.6k plate to plate.
    No. That means it's 7K2 plate to plate.
    The ratio is quadratic so 2x the turns means 4x the impedance.
    As of the other problem, *if* that EL84 power stage gets too much signal , that can simply be attenuated with a resistive divider, you are not *forced* to fully rebuild the output stage.
    Of course, it's your time and money.

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  4. #74
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    Well the main reason is that el84's sound too raunchy and aggresive for the legacy. Also i like to experiment, and these 6v6's are 1buck at a local market(they are soviet, so wont sound as good as jj's and the likes, but still good for getting a general idea about their tone and i wont feel really bad if i brake them, though i have yet to do that). But still, what are the ways of attenuating it?

    http://www.carvinworld.com/crg/crg/s...ics/VL100F.pdf So it's the od preamp->pi->pretty standart fixed bias dual el84 power section with 4,7k grid stoppers and 150ohm screen resistors(and 288/285v on plates/screens). Clean channel, reverb circuit, footswitch relay and negative feedback omitted. I think that lowering b+ or plate resistors on the pi is the best way to attenuate the signal for el84's, but i dont know if that's the best idea and more importantly i don't know how much to attenuate, so help here is needed.


    By the way i disconnected the fx loop because, if it's connected according to the schematic, it lowers the volume a lot but sounds good(i can easily talk over it cranked), but if connected marshall style doesn't sound right. I guess it's because a pair of el84's put out a lot less power than a quad of el34's, which the legacy was designed with. How do i do make it work with the el84's i have now? By lowering r91?

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  5. #75
    Valvulados.com jmaf's Avatar
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    Quote Originally Posted by shreditup View Post
    they are soviet, so wont sound as good as jj's and the likes
    I'd buy all the 1 dollar tubes from your local market and test them all. If someone like the folks at Groove Tubes are reading this, I bet you they're buying air tickets to Latvia right now....

    Sometimes we devalue what we've got in our tiny village, when what you've got may be gold. One dollar tubes? Get them all.

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  6. #76
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    Well i haven't checked but i think i heard the seller say 1 buck for small tubes(el84's, 12ax7 and the likes). Bigger tubes are more pricey(a 6l6 costs 3-4 bucks, haven't checked the others), but still that's cheap. Though if i had the money i would def buy most of them then sell em for more.

    The main concern right now is that i need a way to attenuate the signal hitting the power tubes because it's just really unpleasant and unplayable if you hit the strings hard.

    Basically the sound gets much louder, thin and shrill while the strings that i hit hard are ringing, then when they decay or if i'm playing soft it sounds good, so i guess it's a way too high signal amplitude for the el84(even if i plug my guitar in the fx loop in, i still get the shrill and really overdriven sound when i hit the strings hard, especially considering its only 1 gain stage, pi, and 2 el84's from that point). So how do i attenuate the signal hitting the power tubes?

    I've tried lowering the b+ for the pi, didn't work, and tried a larmar ppimv, which helps but dulls the sound. Any other alternatives?

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  7. #77
    Old Timer J M Fahey's Avatar
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    Respectfully I suggest you scope that amp output, and different points along the signal path, to see what you *really* have there.
    a) you may have the EL84 grids rectifying signal and blocking themselves with high signal peaks
    b) the amp may be marginally stable, and oscillate on parts of the cycle
    c) sometimes you may have grounding issues, "negative" feedback becomes "not so negative as you think", it causes some visible (ugly sounding) kinks in the waveform.
    d) maybe it's not an *EL84* issue but a *PI* issue .... or even something with the last triode preamp before it.
    Since today your only way to judge sound is hearing it through the speaker, you are hearing all 3 stages I just mentioned mixed and at the same time.
    It's hard to decide which part of the problem comes from where.
    e) you name it. A high gain amp is a complex beast.
    Good luck.

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  8. #78
    Valvulados.com jmaf's Avatar
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    Quote Originally Posted by shreditup View Post
    Well i haven't checked but i think i heard the seller say 1 buck for small tubes(el84's, 12ax7 and the likes). Bigger tubes are more pricey(a 6l6 costs 3-4 bucks, haven't checked the others), but still that's cheap. Though if i had the money i would def buy most of them then sell em for more.
    One dollar el84's? Sell your car and buy them all.

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  9. #79
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    Well i dont have a scope, and i guess it's not easy to diy one.
    a) Tbh i dont really understand what you mean
    b) Strange considering it only happens when i hit strings hard... Or maybe not. Confused
    c) Feedback disconnected. But it may very well be a grounding issue, dont know where though.
    d) well it is most likely with the last triode because the strange amplification nearly completely disappears if i bypass the fx loop. Though the amp is not loud enough without it.
    Then again a larmar ppimv solves that to a (large) degree, so i guess it's most likely a) or the way too high signal swing, which needs less tone sucking attenuation.

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  10. #80
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    What if i use a oscilloscope software and use my (embedded low quality) sound card? What additional components do i need and how do i do it? And should i do it at all, since i figure it wont be very accurate and all.

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  11. #81
    Valvulados.com jmaf's Avatar
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    Quote Originally Posted by shreditup View Post
    Well i dont have a scope, and i guess it's not easy to diy one.
    a) Tbh i dont really understand what you mean
    b) Strange considering it only happens when i hit strings hard... Or maybe not. Confused
    It's not strange at all, it's all part of the same issue. When your signal swing drives the output tube grid positive, the phase inverter impedance is very high, so the signal can't drive any current. When the output tube grid goes positive, it conducts towards the cathode, grounding your signal. Positive grids are like diodes down to ground.

    With a scope you would be able to see the waveform and see if the positive swings are getting clipped.

    What you describe sounds like excess gain, driving the output tube grid positive. Getting the distortion when hitting the strings hard is an indicator.

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  12. #82
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    Wont measuring the dc voltage at the output tube grids with a multimeter when i'm playing help?
    And yes, this excess gain is pretty close to what i had in mind. What gain are you speaking of and how do i reduce it to el84 friendly levels?

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  13. #83
    Old Timer J M Fahey's Avatar
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    Yes, it's basically that, coupled to the fact that the now forward biased diode (grid=anode; cathode ... well ... cathode) conducts very well in one way, not at all the opposite , so the coupling capacitor gets charged to a very high *negative* voltage, the peak of the highest signal hitting that grid.
    When the signal level lowers somewhat (meaning everything else except the peak) that tube has a *huge* negative bias which turns it off, only passing choppy farting signal peaks.
    Horrible.
    That is called "grid blocking distortion". It's clearly seen on a scope.
    A couple months ago my reliable Kikusui Scope died, obviously on a Saturday afternoon when I needed it most.
    In desperation I downloaded some Software Scopes, a couple actually worked very well.
    The only *important* precaution is that since it's the direct board input, it has low impedance (think 10K) and needs around 200mV, period, or you blow it.
    I kludged a rotary 5 position attenuator which *always* had some resistance in series with the actual board input, and added a couple antiparallel 1N4002 diodes across it, so you never go over safe 700mV peak.
    You may use (hot crocodile).022x630V/220K/47K/10K/2K2/470r/100r (to ground crocodile)
    I put a .022x630V capacitor in series with the attenuator, to separate it from plate DC (which anyway you can't measure or display)
    The other problem is that it's AC coupled, so you can't see DC values, but to see clipping points, waveforms, oscillations (you can't see the actual HF waveform but its envelope) and assorted stuff, it saved the day.
    Better than nothing and you can't beat the cost.
    I found this one very good: Winscope
    You have a similar one here: Oscilloscope
    Good luck.
    PS: remember to protect your soundcard input, high voltage can *easily* burn it.

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  14. #84
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    Ok
    I have never worked with an oscilloscope before, but i guess the plan is to use signal generator from the software oscilloscope via speaker out into the input of the amp, then instead of a speaker connect a cable to the output jack with the signal side in series with this .022 hv cap then a 220k(or the others that you listed) to from signal to ground, then into a mic/line in on the pc and observe. This is probably wrong, but it's a guess. Obviously i wont try it until you approve(the fact that the amp output has high current that'll go into the mic in on the pc kinda frightens me).

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  15. #85
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    I forgot to mention that the effects of the strange amplification are apparent only at lower levels. As the volume gets beyond a certain point, about loud conversation levels, the sound loses these loud farty sounds and gets pretty normal, apart from lots of presence, which may be normal. The loud farty sound is more or less constant in volume and and is heard only if the normal sound is not loud enough to overshadow the farty sound. Also i get sound even if i turn the mv down(more volume with the loop, almost none if the loop is bypassed). This i guess sounds like a signal from the ground somewhere in the preamp get's amplified and even more if the loop is engaged, if that is even possible. But then 2 things don't add up:
    1) When i plug my guitar in the fx loop in the sound is normal until i crank my guitar volume pot, which is when i hear these strange whistle noises along with the normal sound(which not that normal because it's very gainy).
    2) Why does the ppimv lower the effect a lot?

    I'm thinking it's a phase inverter issue. I'll check all the components and wiring related to it tomorrow.

    Meanwhile on the oscilloscope
    http://tone-lizard.com/images/oscilloscope_setup.gif
    I found this here pic. So i should use a 16ohm(is 15ohm ok?) 100w(or less, i wont need to crank the thing at first) at the amps output jack, then i don't understand what comes next in between it and the line in. Can you draw a schematic or explain more in detail?

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  16. #86
    Old Timer J M Fahey's Avatar
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    Yes, something like that.
    Some slight corrections:
    1) suggested attenuator, an image is worth 1000 words.
    Click image for larger version. 

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    Not Lab precise, simply to lower the intended signal into something the board can handle safely.
    The diodes are there in case you touch something you shouldn't.
    2) You still will *always* leave your speakers connected, your amp needs them and they are the "real world " load.
    An important side effect is that you'll very soon educate yourself to match what you see and what you hear.
    It's one of those OH! moments.
    3) Start by using your guitar as usual.
    Play the "ugly" chords and see why they sound ugly.
    Then, as a diagnostic tool, inject some sinewaves at various levels, which you have previously found using the guitar.
    I personally don't use the PC as a signal source and at the same time, as Scope.
    To simplify things I download a 1KHz MP3, play it on my MP3 player and set it to loop (repeat endlessly).
    Most can produce 200mV signals, which are ample for this.
    If you want your hands free, download some clean guitar solo (guitar only, no backing instruments) and use it ... or record one yourself.
    IMPORTANT: although I included a 500V setting, it's a very dangerous voltage level, it's there only if the signal at a triode plate is somewhat larger than the screen.
    DO NOT USE IT ON POWER TUBE PLATES OR SCREENS.
    You have practically the same waveform, at safer levels, across the speaker terminals.
    BEWARE THAT A CROCODILE CLIP IS TOO SMALL AND YOUR FINGERS WILL GET TOO CLOSE TO WHAT YOU ARE MEASURING.
    Get one of those probes that look like a multimeter test probe, but have a small spring loaded hook on their tip, so you can clip them to component legs or socket pins.
    Good luck.

    PS: or do what I did when I was a young broke student in the early 70's: I went to the "Universidad de Buenos Aires , Facultad de Ingenierķa" (substitute by any approppriate University or Technical School close to you) where I was studying, asked permission *very* politely to the Lab Manager, and he let me use all those wonderful 500 Kg , trolley mounted Tektronik or HP Scopes they had, plus signal generators and assorted measuring instruments; at idle hours of course.

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  17. #87
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    I'm still a bit confused by this.
    1) These voltage numbers are more or less imaginary, right? Afaik, the output transformer outputs ~15-20vac with el84's, it's not possible to get 500v there.
    2) I don't understand the need of all these attenuation combinations in my application. I should attenuate the given 15-20v signal to 500mv(more and my audio card goes dead, right?) And i dont have a any kind of multipole switch(should get one though).
    3) I also don't have 1n4002 diodes. I do have some 1n4007 left from my jcm800 clone build, and some 1n914's. The 1n914 suits, right?

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  18. #88
    Old Timer J M Fahey's Avatar
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    The attenuator I suggested is the full version, what I built for my own use, lets me see signals from 1V to 500V *anywhere* on a tube amp.
    Don't worry, if you *only* want to see what's getting into your speaker, measuring across its terminals, you can use:
    Click image for larger version. 

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    Yes, you can use *any* silicon diode: 1N400x , 1N4148, 1N914, etc.

    Now that I notice it, you can capture your screen (search for some small program or add-on to do it) and show us your waveforms.
    Good luck

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  19. #89
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    Should i also use the .022 cap in series before the attenuator?

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  20. #90
    Old Timer J M Fahey's Avatar
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    Not in this case, since you are clipping your test probes across the speaker terminals, where there's no DC.
    Even less so in a tube amp.
    If you want, you can add a .1 or .22uF x100 or 160V in series, in case you want to test an SS amp someday and it happens to have DC on the output.
    In any case, PCScopes do not show DC voltages, only Audio/AC.
    Just apply a nice middle of the road frequency such as 1000 or 440 or 400 Hz and test .
    *It is not a real scope*, but it saved my bacon many times and the cost is zero.
    There are some full function USB digital scopes, but I'd save a little more and get a surplus "obsolete" real one .... even if tubed

    EDIT:
    I searched and Yahoo says the JCA2112 circuit is a Soldano Astroverb.

    soldano_astroverb_16.pdf
    Is that an official schematic?
    Or some reverse-engineering by a Musician?
    Because it has some obvious mistakes.
    Please re-check it.
    Good luck

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    Last edited by J M Fahey; 07-21-2011 at 08:11 PM. Reason: opened schematic PDF

  21. #91
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    http://img833.imageshack.us/img833/3858/yeahq.png
    Upper left idle, right playing soft, bottom left striking hard. That's with the volume control on zero.
    I doubt any of that was of use, just to see relative levels of the horrid distortion compared to the normal signal, so it's time to test out a 1khz/400hz signal.

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  22. #92
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    random.wma
    Forgot to mention, i used the whole circuit into mic in instead of line in because my line in appears to be not working. The audio file is made using sound recorder.
    Still 0 master volume. I dont think you would the soft playing, but you def would hear the part when i'm striking hard.

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  23. #93
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    Ok, i'm kinda sleepy so tomorrow(or in about 12 hours) i'll post what the oscilloscope showed while playing the 1khz, 400hz test signals.

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  24. #94
    Old Timer J M Fahey's Avatar
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    Fine, very good and useful.
    To begin with, congratulations on setting this up and making this work.
    Now on what you posted:
    by seeing your waveforms, even before hearing your audio I already knew what to expect.
    What I told you about matching "ear to eye".
    Your first screen shows the typical low level background hiss or noise.
    It may be actual noise from your amp or digital noise generated by he sound board digitizer.
    By it's appeareance it's called "grass" because it looks like some earth with some grass leaves growing out of it.
    Turn your amp off; whatever dissapears comes from it; whatever stays is digital noise.
    Screen2: a low level guitar sound.
    Too small to see details but it doesn't look bad.
    Screen3: you have the low level sound, plus bursts of sound, probably farty, chopped and ugly (also sounds like it has a bad quality noise gate) 10x the normal signal. Horrible.
    It looks like a horribly biased stage which is cutting normal sound until some high level overrides it, or an way overbiased output stage or what happens when you put your volume on "0" and some sound still bleeds over, either by poor grounding, lead dressing or poor supply filtering.
    Anyway, congratulations, we are moving beyond the "horrible/farty/etc." labels which help but mean different things to different people.
    As of the soundboard settings: get into control panel or whatever device manager you have and enable or select the line input.
    If you don't succeed, you still need to check off the "+20dB" setting some Mic inputs have and the "AGC" setting .
    Good luck.
    PS= for clarity relabel your soundtracks as "shreditup01.wma" and so on and your images as "screen01/02/03/etc".png or whatever.

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  25. #95
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    Ok, sorry for not replying, i decided to take a different route. Since that oscillating crap varies with moving certain(hard to see which ones exactly) wires and the fact that it's a real mess in there i'm making a pcb for the legacy. The plan is to initially use 5 boards: PSU and bias, OD preamp, tone stack and master volumes, effects loop, PI and PA(and then add a clean preamp, in case i decide that i need it).
    So made the OD preamp layout. I used EAGLE 5.11. Since that is the first time i have ever done something like this, there are likely mistakes there, so if anyone here could help on this, it would be great! And i dont quite know how to print it in 2 layers, i guess after i print one side, i should flip it and print again?
    Pics and Eagle files
    Notes:
    1) The position of the gain pot, tubes, and input jack are fixed(due to using my jca chassis)
    2) Left top pin is the ground, Center(a bit left of center) top pin is the b+, Right top and a bit lower are the heaters, Bottom left 4 pins are the input jack, output is the other leg of c6.

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  26. #96
    Old Timer J M Fahey's Avatar
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    Dear shreditup.
    I guess you will be happier and reach your ends sooner by building your new preamp board using some insulating base (fiberglass if available) and crimped eyelets.
    Don't forget to add extra eyelets on free spaces, for future mods.
    And wiring pots, jacks, switches, with flying wires.
    This way you retain flexibility; you can move, add or delete parts easily, change grounding points, supply filtering, whatever.
    PCBs are justified when :
    a) the design is relatively simple, and the first try is already good enough or
    b) you want to make 100 or 1000 copies and want all to be exactly the same, when built by average skilled workmen.
    You should know that commercial amp PCBs go through a good number of revisions , from prototype to final product, with a lot of them thrown in the garbage.
    For an individual builder it's the *long* path.
    And if you finally tweak your design, by cutting 10 tracks and adding 23 wire bridges ... you are back to square one.
    You might as well have used eyelets from the beginning.
    Good luck.

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  27. #97
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    Hey,
    I decided to try pcb's(curiosity), and made the layouts, but i have this question on my mind right now.

    I am in the process of finishing the conversion, but have been thinking lately:
    The pa is gonna be 6v6 based, and since i want full b+ on preamp and pi plates as in the legacy(415v and 390v respectively) and these 6v6 that i got(6p6s) are the russian equivalent of 6v6gt, so they have a maximum of 350v on plates and screens. But while the 6v6gt can be easily(or not as painful) overdriven to 415v on p/s, the 6p6s is known as not that sturdy, and more than 370v/350v(plate/screen) is not recommended. These 2 are the best solutions i have come up to so far(i will be referring to part names on this schematic):

    1) Have a transformer to produce 420-450v(don't know how much voltage it will drop with a load) rectified and use b+ dropping resistors on output tube plates and screens with otherwise unmodified legacy psu schematic. This was my original idea, but i have been thinking that that'll introduce too much sag(since it's fixed bias, power tubes draw more current as they get driven harder, especially on the screens, as i see in the datasheet, hence i get more voltage drop from the resistors, and get less voltage on chords/ riffing and stuff and less bass response for sure)
    2) Use 2 separate transformers for pa and preamp. Filtering up to r56 for the PA. And for the purpose of not changing the tone of the amp too much, use the same filtering for the preamp as in the original legacy schematic, just without connecting power tubes.

    So which is the better option? Extra transformer cost, weight, placement is of course a factor, but it's not the deciding one(tone is), because i just happen to have a transformer laying around for the job, and don't care if it'll take space and the amp will weigh more.

    Also i wont be using a center-tapped transformer, so i cant use the original bias schematic. Can i just use a -50v from a transformer tap at r54 and continue from there? Or do i have to search for a different schematic for the bias?

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  28. #98
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    are you still building in the jca chassis?

    you're not gonna fit two transformers in that.

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  29. #99
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    Yes i can. Even if the second one is the same size as the original, there's still plenty of space. But the preamp will only draw like 10-15ma, so it's power consumption is 3-4,5w, so a 10w transformer is theoretically up for the job. I am obviously not gonna find a 10w transformer that puts out 300vac on the secondaries, so i thought about finding a random 10w mains transformer, and rewinding the secondary for the desired voltage. Also, i got an idea from another forum of rewinding the secondaries on a random 70w transformer to have 4 secondaries: 300vac preamp(20ma), 290vac power amp(120ma), 50vac bias(150ma), and 6,3vac heaters(2300ma). Is that possible to do at home?

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  30. #100
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    Hey,
    So the pcb's for the legacy build are done, the psu seems ok, but i get this random problem with the pa board, that even if, for example, the heater wires are disconnected from the board, i still get like 50vdc on pin9 and 10v on pins 4 and 5 of the 12ax7 pi socket, and ~20-80v on heater pins of the 6v6 sockets. All is relative to ground. Also if i connect the heater wires, the transformer starts humming, overheating like crazy so that the glue(or whatever they use for sealing) starts melting and popping to outside.
    This might be a clearance problem, but it would be illogical to make such sockets that pins would pickup voltage from other pins via air.
    The sockets themselves are fine, checked with a multimeter, the pins do not connect.
    Most likely its some chemical left after etching or some other process i've done. I used fecl3 for etching, acetone for removing ink, might have used white spirit to clean the board(don't remember if i did this step), glycerol on whole board, so that the tin sticks better, and used tin on traces. And that's it, then i started soldering. Maybe i should clean it, what should i use here?

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  31. #101
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    Just checked, some spots indeed do connect, don't know if it's via glycerol or copper or whatever. For example pins 5 and 6 of a 12ax7 socket connect, and have a ~1,9-2megaohm resistance between them, when obviously they shouldn't connect at all. I tried cleaning the board with white spirit, that made it worse actually, pins 5 and 6 now have ~470k resistance between them. What should i clean it with?

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  32. #102
    Old Timer J M Fahey's Avatar
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    Dear shreditup.
    I wrote an answer yesterday but didn't post it because, although sincere, you might find it impolite.

    You are trying to do alone and with limited means something that even for a "big" Company with time, $$$$ and a full design team would not be an easy task.
    Hey !!, not even mighty Carvin themselves fit such a high gain preamp into a tiny JC2112 size chassis !!!

    I would design and build an excellent sounding and time tested JCM800 type preamp (or something of equivalent complexity and gain, not more) , and if necessary add external gain to drive it to the Moon and back.
    Which gain could come from silent, hum free Op Amps, *or* an external tube preamp if you wish so.

    Said preamps should be built on problem-free and better sounding eyelet fiberglass boards.
    Presto !!, no etching or cleaning problems !! Best insulation you can have !!
    Good luck.

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  33. #103
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    Well i wont stop now, that's for sure. Nothing else other than the legacy will suit too. One thing i will consider if the pcb route fails, is the fiberglass eyelet idea. But what i need now is a way to clean off glycerine from the board. I have tried it with soap and hot water, didn't work, so I'll try using isopropyl alcohol. The main problem here is that i used glycerine as a flux about a week ago so it might be hard to clean off, but other than that i don't see much problems here.

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  34. #104
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    interesting thread , shame none of the damn pdf files open

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  35. #105
    Bent Member Chuck H's Avatar
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    It's a four and a half year old thread. A lot can happen to files in that time.

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    "Take two placebos, works twice as well." Enzo

    "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

    "Being born on third base and thinking that you must have hit a triple is pure delusion!" Steve A

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