Hacking a Jet City Jca2112rc

[shreditup]

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.

[shreditup]

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.

[shreditup]

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.

[J M Fahey]

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.

[shreditup]

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.

[J M Fahey]

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.

[shreditup]

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?

[diagrammatiks]

are you still building in the jca chassis?

you're not gonna fit two transformers in that.

[shreditup]

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?

[shreditup]

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?

[shreditup]

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?

[J M Fahey]

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.

[shreditup]

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.

[Jack1962]

interesting thread , shame none of the damn pdf files open

[Chuck H]

It's a four and a half year old thread. A lot can happen to files in that time.