Hacking a Jet City Jca2112rc

[shreditup]

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?

[pdf64]

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

[JoeM]

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.

[shreditup]

JCA20H service schematic.pdf

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.

[shreditup]

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.
JCA20H service schematic.pdf

[Firechief]

"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.

[Jazz P Bass]

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.

[shreditup]

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!

[shreditup]

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.

[diagrammatiks]

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.

[shreditup]

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.

[diagrammatiks]

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

[shreditup]

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?

[diagrammatiks]

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.

[J M Fahey]

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.