JCA OWNERS!!! (RE)JOIN THE JET CITY LOUNGE!!!

[käsebrot]

With 1,8k/10u you will get frequency gain reduced to 18,7 Hz.

Why would anybody want this in a guitar amp? Those ultra low frequencies even could not be delievered by a guitar speaker.

For comparison: 1,8k/1u is at 183,7Hz, 2,7k/.68 (Marshall Style) is at 232 Hz.

[shep]

i simply used the ampbooks cathode cap calculator. also studied other amp
designs like mesa and marshall. i experimented with the bypass cap and it seemed to sound good to my ears. i apologize if you disagree with what i did.

[käsebrot]

If this works for you, why not. Nothing to apologize.
I just think, that there will be better ways to get rid of the fizz.

[shep]

I love each and every one of you mother fuckers. nohomo

Thanks guys. And sorry, I should have mentioned from the get go it was a JCA22H. I'll get it sorted. I am semi-new to all of this and will eventually get my shit together. Right now I read "try changing R26 from xx to xxx for a larger penis" and I know where to look and what to do. Sometimes the stuff you guys toss around like childs play is very difficult for me to comprehend, but I am learning. Oh, and for what it is worth, changing R26 won't make your pecker any bigger. I tried several values and still nothing

i would guess that some one on here would say that r26 does make it bigger

[Tortuga]

Does anyone have the full schematic for the 5012 with the phase inverter and power tubes?

I had a full set of schematics at one point - searching for the 5012

It's not same board as 50H, eh?

[Meelocheck]

With 1,8k/10u you will get frequency gain reduced to 18,7 Hz.

Why would anybody want this in a guitar amp? Those ultra low frequencies even could not be delievered by a guitar speaker.

For comparison: 1,8k/1u is at 183,7Hz, 2,7k/.68 (Marshall Style) is at 232 Hz.

So you are referring to the plate resisters he mentioned earlier? And yea, I don't think the human ear can hear anything below 20hz...They can feel it though! If I at least know I can vary the value of those resistors and get some different tones it might be worth checking into it and maybe adding a switch or two to add that different flavor. Who knows. That 's why I love this stuff - you can do what you want and there really isn't a right or wrong, just a finished product that sounds good to you in the end.

Anyway, I am going to mess with this thing eventually when I have some time but I do appreciate the input. Some times it is more fun to play the thing rather than constantly poking around inside of it

[shep]

http://www.ampbooks.com/home/amplifier- ... capacitor/ according to this calculator higher frequencies than what he is mentioning are amplified including those of the low e (82Htz)

[käsebrot]

1,8k is the cathode resistor on V1 (bypassed with 1u cap), not the plate. The plate resistor is 220k.

[shep]

I love each and every one of you mother fuckers. nohomo

Thanks guys. And sorry, I should have mentioned from the get go it was a JCA22H. I'll get it sorted. I am semi-new to all of this and will eventually get my shit together. Right now I read "try changing R26 from xx to xxx for a larger penis" and I know where to look and what to do. Sometimes the stuff you guys toss around like childs play is very difficult for me to comprehend, but I am learning. Oh, and for what it is worth, changing R26 won't make your pecker any bigger. I tried several values and still nothing

we were referring to, c1, the bypass cathode cap for v1a(the first preamp stage). besides changing r6 from 220k to 100k, i also changed c1 from 1uF to 10uF. i found it pleasing but that is only an opinion.

it is also difficult for me to understand. and im still in the learning stages just like you. i doubt i'll ever get out of it.

[Meelocheck]

What about R53? That made a slight difference in one of my other amps. Same application anyway. Any point to messing with that? Also, what about diodes. Changing the IN4007's to 1's? I believe the 1's give you more gain and are grainier? Best to stay with what I have but I was just curious about that one.

I appreciate all the tips and this thread is tits for noobs like me. Thanks again guys. Rock On.

[shep]

according to jca22 schematic r53 would
change the voltage to the entire preamp. it may do
something pleasing as the jca22 uses a transformer with less voltage off the bat than its relative the slo. slo100 uses 350v or 360v. while the jca22 uses around 280v. but i wouldnt try it without first calculating what the voltage change will be if you change it. that makes thing more complicated. so for now i wouldnt suggest doing it.

changing the diodes may change voltage. and may change tone. as far as distortion/gain is concerned i dont believe it will do anything. i dont know enough about that to say. i personally wouldnt try it. but maybe someone else will give some good insight on the subject.

you could always look at data sheets of both types and see what the differences are. also look in similar amp designs to see what kind they use. mesa recto, mark series, marshal, slo, etc.

if you wanna calculate the voltage change for r53 to make it closer to slo feel free to ask. i may just be so inclined to help you. not sure how close it is now though. i'd have to check.

[midnightlaundry]

So I changed out the 2-220k plate resistors. What a huge difference. It totally opened up the amp. I tried changing the matching cathode resistors to 1.5k, but I thought it took away some of the harmonics. So I changed R27 back to 1.8K and it's more balanced.

I put a JAN 5751 in V1, and I like it even more. It's even more open sounding and feeling, but there's still plenty of gain left.

[shep]

So I changed out the 2-220k plate resistors. What a huge difference. It totally opened up the amp. I tried changing the matching cathode resistors to 1.5k, but I thought it took away some of the harmonics. So I changed R27 back to 1.8K and it's more balanced.

I put a JAN 5751 in V1, and I like it even more. It's even more open sounding and feeling, but there's still plenty of gain left.

what kind of speaker are you using?
i suggest changing c1 here is what it does. it really does boost bass and mid range bass. it was as drastic a tonal change as changing the resistor to 100k. changes low e from 30.64db to 35.27db. and ups the mid range bass which i think this design originally lacked.

[shep]

.

[midnightlaundry]

It sounds great thru a 4x12 of V30's. I had the resonance off for that. Ill run it half way with the Eminence combo speaker and it sounds pretty good there. Really I dont think this amp needs more bass. I have the 50 watt btw.

[shep]

lol my bad i thought you were a different user. glad you like it though. i was impressed with it myself. im glad you pretty much felt the same way about what it does to the tone. so, to know im not totally tone crazy.

[shep]

it'd be cool if they threw this in the mods section of the forum.

i also made a real clean channel out of
the amp. but switching for it is fairly complicated.

[Meelocheck]

So I changed out the 2-220k plate resistors. What a huge difference. It totally opened up the amp. I tried changing the matching cathode resistors to 1.5k, but I thought it took away some of the harmonics. So I changed R27 back to 1.8K and it's more balanced.

I put a JAN 5751 in V1, and I like it even more. It's even more open sounding and feeling, but there's still plenty of gain left.

R43 and R44?

Shep, sounds like leaving R53 alone for now is a safe bet, lol.

[shep]

So I changed out the 2-220k plate resistors. What a huge difference. It totally opened up the amp. I tried changing the matching cathode resistors to 1.5k, but I thought it took away some of the harmonics. So I changed R27 back to 1.8K and it's more balanced.

I put a JAN 5751 in V1, and I like it even more. It's even more open sounding and feeling, but there's still plenty of gain left.

R43 and R44?

Shep, sounds like leaving R53 alone for now is a safe bet, lol.

he is using the jca50 so numbering is different. also he changed another plate that was 220k to 100k but it doesnt apply to us because we use the jca22. other 220k is already 100k on our amp.

[midnightlaundry]

LOL I changed that cathode resistor back to 1.5K. Now that I had a little time away from the amp, I went back with fresh ears, and decided that the amp is plenty bright to handle the slight bit of warmth that a mere 300 ohms difference adds. 100k/1.5k is a classic gain stage. Nothing wrong here.

I'm still digging the 5751 in V1. It has a nice wide breakup and feel.

[käsebrot]

If you change the 1,8k cathode resistor to 1,5k, do this at V1 (R49), not at the 3rd 1,8k (which you've done at R27).

[kash]

I have a Hovercraft Falcon 100, which is just a JCA100H with a lot of mods. Not sure if anyone has already brought this up, but I'm having some problems with heat because of the cabinet being designed the way it is: with no vent on top and a very small vent in the back. I've actually had the logic circuit for the channel switching start to go a little haywire at the end of a rehearsal, and the sound get slightly thinner (hopefully I didn't ruin the power tubes, because I just bought them). I'm considering adding some cooling fans that blow on the transformers and then the exhaust would kind of wrap around and come back out the middle through the power tubes, just enough to get the heat out, but not significantly cool the power tubes. I'm thinking of using an external DC power supply, rather than trying to tap off of the amp anywhere to avoid noise problems. I will most likely use 12V fans and run them at 9V to knock back the CFM a bit. Has anyone else done this? Thoughts?

[shep]

just be careful about the fan interfering with the audio circuit. should be fine. if there are any ic's in the circuit you can add heat sinks. sounds like you have the right idea.

[DocRawk]

Right I'm going to add this here in case this is useful to anyone using a Boss GT, or anything similar in 4Cm with the JCA 50/100. This is my setup, and the reasoning for my levels - hopefully someone may find it useful

Here I am assuming a Boss GT-xx where you've set the output to +4db rather than -10db

The signal path basically goes like this (with impedance in Brackets)

1...........................Guitar (-10db) -> GT-xx Input (-10db)
2........FX Send on GT-xx (-10db) -> Amp Input (-10db)
3............FX Send on Amp (+4db) -> FX Return on GT-xx (-10db)
4..................GT-xx Output (+4db) -> FX Return on Amp (+4db)

Cables 2 and 3 define the preamp - with the loop ON - the JCA preamp is active, and when OFF then a Boss GT preamp must take its place since with the loop off you are only using cable 1 and 4. The levels should be roughly the same as without a GT in place as you're going direct into the FX return.. there is where the fun goes on when you match levels!


In this case, we note the only mismatch is cable 3 of the 4cm - where the Amp FX send is sending a hot signal to the FX return on the GT and needs to be attenuated. I noticed in the "GT8 Brilliance" document that someone used S/R levels for the loop of 100/20 - where a return level of 20 attenuates 12db, which was nearly right for me. In testing between crunch channel and a BOSS GT preamp in test (in this case the Soldano) I found an optimal level of 100/10 which means I can run the BOSS GT preamps at about 60-80% level, and it gives enough headroom for some of the clean preamps as well. I also set up the GT output to around 2pm to get the cleans clean with no clipping that I think I can tell. I am about to test it real loud this weekend in my local rehearsal place but I managed to level match at home running at about 95db (good and loud!) using a decibel meter on the amp - so if I redo the above with relevant levels I get.

1.....................................................Guitar (-10db) -> GT-xx Input (-10db)
2.........FX Send (Send Lvl 100) on GT-xx (-10db) -> Amp Input (-10db)
3.....................................FX Send on Amp (+4db) -> FX Return (Return Lvl 10) on GT-xx (-10db)
4..................GT-xx Output (output ~ 70%) (+4db) -> FX Return on Amp (+4db)

I chose +4db for the output level as if you set it to -10db you end up with mismatches on cable 3 and cable 4, and as cable 4 is the final output that would mean you would get a reduced level..


Hopefully this may be useful to others...

[thesockmonster]

Ok, before I dig into the boards and start swapping, what's the roll favorites as far as tubes? I'm guessing new tubes benefit these tremendously?