Hacking a Jet City Jca2112rc

[shreditup]

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.

No, you misunderstood. I am aiming for 385v for the preamp b+, 415vdc for the PI(as mentioned on the schematic), and use my somewhere around 330v b+ on the power section(like i have right now), which i am going to obtain via having a tap on b+ like its done on the schematic of the legacy(H3a.B screen), but use a large dropping resistor. And what i meant by bias and filament circuits was that they obviously differ on these 2 schematics, but is it a 2 different paths to the same result? I think not, which is why the question of what would i gain if i copied the legacy filament and adjusted bias circuits?
Now what was your idea about those 24/48 transformers?

[shreditup]

Well, that's an excellent reason to keep butchering it, isn't it?

Well not really, no, lol.
But seriously if i had the money i would definitely build one from scratch, but i dont, which is why this little amp has been my modding platform.

[J M Fahey]

I am aiming for 385v for the preamp b+, 415vdc for the PI

I think you are mistaken here.
You do *not* need those specific voltages, since to begin with, you are not building a Carvin Legacy but, at best, something "inspired" by it.
Voltages, parts values, etc., are not "magic" values but whatever the designer calculated was necessary there to achieve some specific end.
You can't just graft parts of different amps, stitch them together and think it's done.
*They* must drive some EL34's, and they have, say, 430V available, so getting 415V is easy and natural.
*You* are driving EL84's , which need much lower grid signal, you also have around 330V B+, and the, say, 300 or 310V you'll easily get for your PI must certainly be enough.
Must be, or all other EL84 amps wouldn't work, would they?
Work within what your power transformer can deliver.
Good luck.

[shreditup]

Yes, but all these voltages, parts values, etc. make an amp sound like it does, and i like the legacy sound and want to make it as close as possible(also the fact that i'm getting way too much gain and things sound mushy at all gain settings) within the limits of my amp(the chassis,the el84 power section, the output transformer, most likely the power transformer also. I am getting a new pcb done for the legacy, mostly because i upgraded to star grounding here and all these wires are pretty annoying, but still the amp is dead quiet now), and i would like to know how to get the mentioned voltages and answers my bias and filament circuit questions.
And as i said i am going to drop the voltage for the power section. It will be pretty unusual and not something you see everyday, if that's what you're telling me, but then again- is that an obstacle?

I dont really understand your position of you trying to stop me converting the amp and turning it back to original, so here's my story. You see, I got the amp, my first tube amp, then after a while i understood that's not the sound that i'm after(i didn't even like it, it was thin and shrill and didn't have enough gain, surprisingly you don't see this on many youtube demos). It was too late to return it, and i was thinking about selling it, but then again there would not be anything that i liked much in that price segment. So i then started modifying it. Got a better sound, but didnt really have a concept of tone, so the tone i was getting was pretty off(dont know the best way to describe this part). So i modded it into a jcm800(actually i was thinking about copying it into a legacy at that point, but it was far too different and far too many things to change), liked the sound, then i decided a bogner xtc was better. The bogner was almost perfect, but had kind of scooped mids and was thin at that department. Well then i guess i was ready for the legacy and here i am now close to the finish, so i just need to do these more or less final steps to converting it to an amp i like, but i dont know how, so i'm asking for your help.

[shreditup]

Also, what about using an step up autotransformer before the main pt(or on the HV secondaries, or for the preamp)?

Edit: Nvm, they're too expensive..

[J M Fahey]

(actually i was thinking about copying it into a legacy at that point, but it was far too different and far too many things to change)

Glad you noticed it.
My suggestions are good natured and wishing you the best, same as any other suggestion you'll receive here, but of course you are the ultimate judge of your own destiny.
Yet I'm quite sure nobody will suggest you anything dangerous, such as overvolting your power transformer which is what you are asking in your last question.
And no, it's not a *cost* problem.

[shreditup]

So there's absolutely no safe way to increase the b+ without buying a power transformer with higher voltage secondaries?

[shreditup]

Legacy

Hey,
So i've converted my jet city jca2112rc to a carvin legacy except for the power amp, which is still el84. It's been pretty good, but the sound is really harsh and i get power tube distortion way to easily, and it sounds as if it's being driven too hard. I didn't really notice that until recently, and i guess that the legacy preamp outputs a larger signal swing than the el84 can handle. After trying different attenuation techniques, i've decided(and mainly because i dont like how el84 sound in this amp) to convert the output section for 6v6's(i even saw a post on some forum that said that one guy successfully ran a quarter of 6v6's in a legacy combo so i guess no attenuation is needed). I guess that my output transformer with a 3,6k primary will suit if a drive my 16ohm speaker in a 8ohm secondary, but what about the bias circuit? How much negative voltage do i need and are there any 6v6 fixed bias circuits that i can copy from, preferably without using a pt tap?. And what plate/screen voltage should i aim at?

[Chuck H]

Yes, a pair of 6V6 run on an 8ohm secondary to a 16ohm load will be fine if the primary Z is 3.6k. Why would Jet City be using a 3.6k primary for a pair of el84's??? are we sure about this number?

6V6's could need more than double the bias voltage that EL84's did. I don't know the jca2112rc circuit, but if it is fixed bias now you'll need to change the voltage divider ratio in the bias supply OR if the amp is cathode biased you could just keep it that way. Cathode bias will sacrifice some dynamics and headroom. Some players like that. Without using voltage from some PT secondary the only way to bias the amp is by adding a transformer for the bias voltage or cathode biasing. Or are you asking if there is a bias circuit that doesn't require a dedicated bias secondary?

[shreditup]

Yeah. The sign on the ot says 1.8k 0 1.8k, so i guess that's 3.6k plate to plate.
Well i was thinking of doing the bias like they do on marshall's and legacy's, among others, using the hv tap.
http://mhuss.com/Plexi6V6/Plexi6V6c.gif Does this work without a (grounded) center tap on the hv secondaries?

[jmaf]

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.

[shreditup]

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.

[J M Fahey]

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.

[shreditup]

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?

[jmaf]

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.