Hacking a Jet City Jca2112rc

[shreditup]

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?

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.

[diagrammatiks]

The Valve Wizard

there go you.

[Steve Conner]

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.

[J M Fahey]

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.

[Prairie Dawg]

+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.

[shreditup]

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.

[jmaf]

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.

[shreditup]

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.

[Prairie Dawg]

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?

[jmaf]

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.

[shreditup]

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?

[J M Fahey]

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.

[shreditup]

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.

[J M Fahey]

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.

[shreditup]

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.