Hacking a Jet City Jca2112rc

[shreditup]

ok, so the only problem remaining is the pot on the guitar is scratchy. It must be dc coming from the amp, but i have no idea where from or how to fix it if i find it... Any ideas?

PS: you all have been very heplful, thanks!

[jmaf]

Some folks decouple the amp input, especially high distortion amps like the Tiny Terror. A large cap should offer less impedance, it's the only solution I know, unless of course the pot IS scratchy.

[shreditup]

no, the pot isnt scratchy, i tried a bunch. What cap should i use with minimum tone loss?

[jmaf]

no, the pot isnt scratchy, i tried a bunch. What cap should i use with minimum tone loss?

The bigger you can accommodate the better.... I'd put in 200nF at least. Good news is you can use physically smaller low voltage caps.

[shreditup]

Just tried a jamicon 47uf 500v, really thinned up the sound. Also tried a 500uf 6,3v random cap, lost some highs and didnt get rid of the scratch. Is there another (less tone-sucking)way?

[J M Fahey]

Just to check that some DC is there: put all controls on 0 , plug a guitar cable on the input and measure DC voltage on its free end.
Try the 200mV, 2V and 20V scales .
If you have no DC, then it's just a *very* dirty or even cracked pot ... or your amp is oscillating wildly or is very unstable and entering/leaving the oscillating state is often heard as similar to scratching, the actual high frequency oscillation can't usually be heard.

From deep inside my heart: I would
1) I would restore that poor amplifier to its original conditions
2) If not satisfied with gain, bite, body, or some other sound characteristic, (obviously you are not), I'd try some *relatively* minor mods, such as different value capacitors here and there, some changed resistors, which should appreciably change the sound, added or pulled clipping diodes, etc. but I would *not* try a full "rip all and rebuild" operation, which usually opens a can of worms.
High gain amps are tricky, seemingly minor details as lead dress can cause all kinds of trouble, and they are not shown on the schematic.
Even if you get the original layout image, it will not physically match the chassis you have.
I think that adding some external gain between guitar and amp will add a lot of fire and an EQ in the loop will add balls and bite; both "external" add-ons will be more controllable.
Good luck.

[shreditup]

Well you know, after 4 months of (intense)amp modding, that just aint going to happen. Plus the tone i'm now getting is seriously insanely cool, and that scratchy pot isnt going to take that away.. Although i will be building a new amp in summer, that is going to be jcm800-based, for that 80's rock sound.
Oh, and how can i make my volume control on 0 make the amp fully silent, cause right now its not? And why does that happen? I though that when the MV is on zero the signal is fully sent to ground.

[shreditup]

NFB volume loss

Hey, so i'm finishing my jet city jca2112rc-> carvin legacy conversion and i have this one problem with the nfb loop i cant figure out.
http://www.carvinworld.com/crg/crg/s...ics/VL100F.pdf
So the schematic shows that the loop connects to the same grid(2nd pin in my case) of the phase inverter, but while it does sound good, it lowers the volume dramatically, and i can crank up the mv in my bedroom without my ears bleeding(in contrast thats about 3/9 on the master volume without nfb). If i remove the nfb, or connect the nfb the traditional, marshall way to the 7th pin of the phase inverter tube the amp gets its volume back, but the tone is not that good. Any ideas on why the huge volume loss?

[TransLucid]

The schematic says there's 12 dB of feedback, so that's about how much extra gain you should get if you disconnect the loop. You can decrease the amount of feedback by increasing R91. You should also adjust (decrease) the parallel capacitor if you want the same tonal characteristic.

Since the feedback currently goes to the input to the PI, the output is out of phase with the PI input, so you can't get negative feedback connecting it the normal way (otherwise it would be positive feedback). I'm not sure why your amp didn't oscillate when you hooked it up that way - could be because the amount of feedback in that configuration was very small.

[Chuck H]

NFB will reduce voltage gain but not power. The amp should have less volume at lower settings. That's part of what NFB does. But the amp should be just as loud at the point of clipping with or without NFB. If not then something is wrong. You should check the value of R91 (360k) and C70 (10pf). C69 is informally indicated as a value "10". I don't know if they mean 10uf?, 10pf?, It can't be 10uf because that would have to be a polarized cap and no polarity is indicated. And 10pf wouldn't make for a very effective NFB loop. So good luck with that I suppose. Incorrect values can dramatically alter the loop gain. It's also important that the NFB circuit is on the 8ohm tap of the OT secondary. This can also dramatically effect the loop gain.

I don't know what you mean about NFB connecting to pin 7 in a typical Marshall. Most Marshall amps have the NFB connected to the cathode circuit on pin 8. I would think that connecting your NFB circuit to pin 7 would create a positive feedback loop for an actual increase in voltage gain.

[shreditup]

Finishing touches

Hey, so i have pretty much finished my jca2112rc->legacy conversion, but there are things that i still haven't done, like adding a clean channel, and things that are stock, like the bias and filament circuits and b+ voltage. I now have my 100v/120v/220v/240v multi-tapped power transformer set to 220 or 240 right now(see below) and i was wondering

1)Whats the best(safest) way increase the voltage right out of the rectifier from 320-330v to about 420? I have tried this and i got ~650-700v b+, but i also the transformer hummed while the power was on and i blew a fuse after about 10 secs. I suppose there is either a way to increase it by wiring it somehow OR i think there are multiple unused taps in the transformer itself, like here, with which i can make it more accurate, but i'm not sure these taps are there and dont really want to unscrew the transformer unless i know they're there. Is it common for these kinds of transformers to have stuff like this?
Or if that doesn't work, assuming i use my config where i got 650v b+, can i decrease the voltage before the input of the transformer to get somewhere near the mentioned 420v b+? If so, then how?

2)What will replacing the bias and filament circuits of this
with those here do? Will i gain sonically(less hum or something)? And if it is worth doing, then how do i adjust the legacy bias circuit for el84s? By lowering r53 or by using r52 of some value? And what should i do with my violet transformer output coil(where the current is dropped right now) if i drop the current into b+, like on the legacy?

Just a few things though:
No, i'm not restoring the amp to its original specs and not going to the tech to get it done, because i have been modding it from january, am almost done, and planning to build a new amp, so i need all the info and experience i can get.
And yes, if i manage to increase the b+, i will use a dropping resistor for el84s screens, dont know the voltage that i'm aiming at yet though.

Any help appreciated!

[Alex/Tubewonder]

Taps on transformers primary winding are for making it easy to adapt to your local naminal voltage in the wall socket. They are not for raisng or lowering secondary voltage as it will easy lead to overloading of the primary side, which you already experienced when the fuse blew and protected you from a more serious chain of events. Australia has nominally 240V with a tolerance of -10%/+5% so you should use the 240V tap on your power transformer. A 230V tap would be within reason but I wouldn't use a 220V or lower tap.
You could use a voltage doubler on the secondary side but in this case I don't think it's a good idea.
You could get a 2nd smaller transformer that has a 24V or 48V secondary rated at ca 500mA and connect it in series with the HV secondary winding on your existing transformer. But only if you really know what you're doing, which I honestly doubt.
My best advice: get a replacement transformer, either a custom made or I would think something from Hammond catalog will surely fit your needs.

[shreditup]

Well firstly i live in Latvia, its 220v here, which is why i thought by rewiring from 240 to 220 i would increase the b+ by about 30v, still not enough but better.

You could get a 2nd smaller transformer that has a 24V or 48V secondary rated at ca 500mA and connect it in series with the HV secondary winding on your existing transformer. But only if you really know what you're doing, which I honestly doubt.

Yeah, i didnt quite get your idea. Can you explain it more in detail?
I also got an advice at another forum by having another transformer parallel to the main PT for the preamp and PI, since i would still lower the voltage for the power amp. Preamp doesn't stress the transformer a lot and doesn't require a lot of current, so i guess this is a good option. I would get ~550v dc at the b+, use a dropping resistor to about 415 for the phase inverter, then use the rest of the schematic. But still i want to know what your idea was because these 24-48v secondary are available even here in Latvia, so please do tell.


PS: where did the idea of me being Australian come from? From my use of b+ instead of the european Anode and the fact that i have 220v mains i guess?

[Alex/Tubewonder]

Well, you linked to a transformer photo on Australian manufacturers site.
Before we proceed:
Why do you need well over 400V for the output stage? I wouldn't recommend running EL84 at more than about 330V. Unless you have a bulk box of them and are willing to replace them daily. Only if you had NOS 7189A or 6973 tubes you would be safe running them at 400V plate supply and get ca 25W output.
What's wrong with the original bias circuit? Unless you have the above mentioned 7189A or 6973 that need -25V bias. Then you only use a voltage doubler on existing bias winding.
EL84 on 400V or more - not a good idea.

[J M Fahey]

because i have been modding it from january

Well, that's an excellent reason to keep butchering it, isn't it?
There's an old proverb about not throwing good money afted bad one, but maybe it loses something in translation.
I had already suggested building a full Carvin Legacy from scratch, if no other way to have one.
Obviously in a proper sized chassis, with proper transformers, etc.
It would take time and money, of course, but the end result would be more predictable and its construction less stressful, and it would be a project of which you could be quite proud about, and with good reason.
I think you had an excellent little amp, which would have gracefully accepted a couple reasonable mods, either to clean it up or to make it somewhat more aggressive (you still had some 10 or 15dB extra gain available, by tweaking the built-in signal padding/attenuation), but you are trying to cram into that little chassis more things than physically fit into it.
Anyway, good luck, but don't try to defy Physics laws.