Guitar Gear Forum

Didn't record all I wanted from GABfest, but here's what I got:



(Videos are taking a while to process uploading for some reason)

[video]https://youtu.be/3ZYEMdqzV80[/video]

[video]https://youtu.be/xqa699WSsXc[/video]

Hello Guys,

Just joined the forum, glad this is here.

I had scored a JCA5012C from Guitar Center here in FL, however Im having issues with the amp that seem to be normal for what at least some of the issues are.

The amp is noisy without anything plugged in, this will hopefully be solved soon as im doing the heater mod now actually.

The other issue I have seen in the amp are volume drops especially when the volume for the OD channel is raised from 2-4.... if I leave the master on at 5 or so the volume drops happen once in a while if I chug out heavy rhythm riffs. Looking into this further, I replaced all tubes and have a new set of known working JJ el34l's from a Mesa 50 watter I have. I have set the bias to about 41ish (read 437 plate volts across pin 3 / ground, I like to run my tubes slightly hot ; 25watts / 437 = 57.208 100% dissapation. X's by .7 and .75 I get a bias windo of 40 to 42.. so not quite 75%). I first started noticing the issue when I was biasing the amp. I notice that while playing the amp with my eurotubes bias probes plugged in (I have 2 DMM's with 2 probes installed), the bias readings jump from idle (41) to 50 and above on the right tubes.. the left tube does the same thing but the oddity is that I read 0 or a really low value on that tube sometimes (0-12-40-50 then back to 0). Is this normal to have the bias readings go to 0 sometimes? I have the dmm set to the 200 scale so I dont think that is the issue. I think it may explain the volume drops that happen at random as well.

Any ideas on what to check? Im thinking it may be one of the tranny's, but im unsure of where and what to test.

Thanks for any help!

Brian R.

You're running EL34s? I'm not totally familiar with the conversion process, but did you do anything with the SGRs?

I'm a little curious as to why you didn't take the amp back - shouldn't be having issues right out the gate.

I did the heater mod to mine, but ultimately wish I hadn't. I don't think I gained anything from doing so, and pretty much made it unsellable

Hello,

Nope, im an idiot lol.

Im running JJ 6l6gc's.... oops!


There is a noticable hum coming from the amp thats why im doing the mod. High gain amps will have hiss / hum a bit, but this one I cant mic in a PA as its just that loud.

Otherwise, Im used to modding my amps by tweaking to taste if need be. The only mods I see this thing needs are the heater mod (if it works) and to find out why I have volume drop at random. Its not a huge volume drop, but makes me wonder if there isnt an issue coming up down the road.


I got the amp for $306 man, for the tone I would rather work with it

Thanks!

Brian R.

i did the heater mod and it was helpful. the volume drop sounds like a tube issue, but im not sure, as you said you replaced the tubes. because the jca50 has a ridiculous amount of gain, you will not be able to ultimately get rid of the hum, only tame it. i have the jca2212c and it is considerably less noisy. i use a noise gate on all of the high gain channels on all of my amps. out of all my amps, the jca50 is the noisiest. it has more hum than even my home brew mark2c+.

anyways, doing the heater mod should tame the hum to a usable level, especially with a noise gate. it works perfectly fine for me. i also put an artificial center tap on the heater supply. i made sound comparisons of before and after both of those mods. there was a difference in applying both of them.

the other option is to run a dc heater supply which should get rid of most of it, but its a more complicated solution.

having your amp hum really loud when nothing is plugged into its input jack is another issue all together. the jack should ground itself out when there is nothing plugged into it. i don't think there should be any noise in that scenario.

first ive heard of biasing tubes while someone is playing through them. is that a common thing to do?

Hello,

So I did the mod and it seems to help with the humming issue for sure. Still a bit of hiss but this is expected and manageable.

Problem is, the amp still has the issue of losing volume of which I have traced to the bias pot on the pcb. If I apply some pressure to the pot, the volume is restored and the issue goes away. So im not sure if the pot needs a cleaning but I would prefer to just have it replaced.

When I was testing for the volume loss I had 2 DMM's setup to read the plate voltage going to both tubes... on pin 3 of the tube on the right (looking from the rear of the amp) I see that the plate voltage goes from 437 to 340 to 220 and back to 437 at random times. THe odd thing is when applying pressure to the bias pot the issue still happens, yet the volume issue dissapears. Not sure if I have a bad trace on the pcb or if the power lead is cut somewhere... the tubes are new and I have swapped them with 2 other sets and im still having the same issue.

Any thoughts on the issue? The amp sounds great for a 1x12 so im on board with just having it repaired at a local amp shop if need be. I could replace the bias pot but I think something else is at play here.

Thanks

Brian R.

If when pushing the biasing pot it fixes the issue every time you push it, I would say that's your problem. You can set the pot where you'd like, read the ohms of the pot at that setting, and replace the pot with a resistor of that value. Or just replace the pot. I'd try replacing it with a resistor first, just to see if it's the pot. How ever you'd like to do it, though.

Check the solder joints for the pot to the board. It would be a good idea to heat the pads and add a bit of solder to fix any bad connections that may not be so apparent.

I forgot to mention, check the ground wire connection to the board. Also, check other ground connections, such as the chassis ground. When I first received my amp, the hum was very bad. I realized there was a bad joint on the ground wire to the board. It was hard to tell because there was a huge ball of solder on the pad.

As far as the dmm. Your probably just getting bad connections with your dmm. That would explain why it jumps around. I think biasing should be done while the tube is at rest. So there aren't any fluctuations in the readings.

Man, a DC heater solution is so easy a cave man could do it.I did as follows on my 50h. There's a two wire jumper between the heater trace for the power amp and the heater trace for the preamp. Unsolder the jumper from the power amp side. Drill hole in back of amp (somewhere sensible, ye ying yang!) and install a wall wart psu jack, the standard usual size. You'll need to solder a cpl of wire leads to the jumper wires cause they ain't long enough to reach the jack. Solder positive to tip and negative to ring of the jack. Also Run A 2nd Wire Lead FROM THE neg. Wire to ground. I used the ground leg of a close by resistor. Order a 4amp 6vdc wall wart psu from amazon or wherever. When ye get it, plug it in, grin when the preamp tubes glow, turn amp on and be happy as a tornado in a trailer park cause yer amp is 60Hz hum free.

wait - so you power your heaters with an external wall wart??
I would FOREVER be forgetting to turn that thing off.........

Hello,

Yea I would want the external heater supply to be built into the amp and fully tested for ground loops that cause hum.... there is another post on the board about this using a power supply inside the chassis but I guess it generates too much heat. His solution was to mount it on top of the chassis near a transformer and that to me is acceptable as long as there isnt noise induced.


For me the heater rework helped, but its still noisy... but not anything I dont get from various Mesa, Evh and Friedman modded Plexi amps I have (Its a high gain thing). I have gotten used ot just switching to the clean channel to kill the noise anyway. Thanks

I suppose you could get a bit more creative and use a relay (triggered by the amp mains) to turn off/on the wall wart. Regardless, I almost like it better than cutting the traces (as I did), which permanently alters the amp and ultimately makes it less reliable, due to having wires simply soldered to copper traces that can work themselves loose over time.

Hello,

I did the mod also and prefer it over the traces due to the lowered noise floor.

Also by doing the mod, its easier to do the heater dc supply mod since everything is now available on the pcb without having to take it out again, just my opinion of course.

I would prefer to find a dc source inside the amp, but if an external power supply is required and there is enough isolation from emi to not induce more noise I guess its the way to go.

I recently got a jca100hdm and the amp doesnt seem to have the problems that my 5012c has... however its interesting as the tone of the 5012c seems better to me as its a pushed type of tone where my 100 is ok but is loud! To get the sag / pushed tone I would need ot crank it to at least 4, but that would be melting the paint off the walls. Even in 50 watt mode its loudddddd.

well if another PSU that also has to be plugged in the wall works for you guys,
i guess it's whatever catches your fancy.

i currently have that dc mod set up the same on my mark2c+ homebrew. thought of taking the walwart apart or just trying to mount it on the chassis and wire it to the 120v wires coming into the amp. just put heat shrink over the wall plug section. or just get a 12v transformer and build a little dc supply circuit inside the amp. to lazy to get around to it, so i have two plugs for it. it is kind of a pain in the butt that way, though.

Brianwrym wrote:Hello,

So I did the mod and it seems to help with the humming issue for sure. Still a bit of hiss but this is expected and manageable.

Problem is, the amp still has the issue of losing volume of which I have traced to the bias pot on the pcb. If I apply some pressure to the pot, the volume is restored and the issue goes away. So im not sure if the pot needs a cleaning but I would prefer to just have it replaced.

When I was testing for the volume loss I had 2 DMM's setup to read the plate voltage going to both tubes... on pin 3 of the tube on the right (looking from the rear of the amp) I see that the plate voltage goes from 437 to 340 to 220 and back to 437 at random times. THe odd thing is when applying pressure to the bias pot the issue still happens, yet the volume issue dissapears. Not sure if I have a bad trace on the pcb or if the power lead is cut somewhere... the tubes are new and I have swapped them with 2 other sets and im still having the same issue.

Any thoughts on the issue? The amp sounds great for a 1x12 so im on board with just having it repaired at a local amp shop if need be. I could replace the bias pot but I think something else is at play here.

Thanks

Brian R.

I had the same problem on mine with the bias pot. I ended up installing an external bias pot and killed 2 birds with 1 stone. I wanted to be able to easily adjust bias without taking it apart anyway.
Also, floating the heater wires helped mine too, but still a noisy amp. I may consider the DC power myself

G-SPACE wrote:I suppose you could get a bit more creative and use a relay (triggered by the amp mains) to turn off/on the wall wart. Regardless, I almost like it better than cutting the traces (as I did), which permanently alters the amp and ultimately makes it less reliable, due to having wires simply soldered to copper traces that can work themselves loose over time.

I would probably just connect directly to the power switch on the amp, so when the amp is on, so is the wall wart. But I'm sure there's a much more sophisticated way to do it LOL. I'm kinda redneck that way.

Hi all

I have never liked very much the lead channel of my JCA100 (too much gain, too dark) and after having tried several mods over the years, I found out something really nice by pure mistake.

My lead gain pot got horribly scratchy and needs to be replaced. But It takes time to order the new pot and since I wanted to keep using the amp for gigs, I have simply bypassed the whole lead gain pot circuitry by connecting the V1b grid directly to V2a grid (after removing the 470k grid resistor).

Now, there is only pot to set both channels gain but... soundwise... holy shit !!!
the lead sounds now ultra crisp, crunchy and nasty as hell
(very tight too, also because i have changed the coupling caps V1-V2 to 0.002uF)
is very easy to do, try it for fun

Has anyone here done Stump's JCM 800 2204 mod? If so, how was it? I'm debating doing it to my JCA50H.

I've actually done something pretty close to it. I didn't change all the values to what stump posted, I let a few parts he suggests jumpering, and I think it gets really close to the JCM 800 sound. I did it to get that Jawbreaker sound, and it really gets close. I should add I bought a kit from Epic Amplification. There are a few differences from stump's parts, but it saved me time sourcing out all my own parts.

R13 + r14 are part of the Phase inverter and most people have said to leave them stock. I would recommend the same because of the differences between EL84 and EL34's.

So while I was biasing my Jet City JCA50H yesterday, I decided to try a small tweak while I had the amp opened up. I removed the bypass cap C19 on the cathode follower, as discussed in this thread, and man, it really cleared the amp up. The dynamic range increased and the gain on both channels was decongested a bit and slightly cleaned up. Such a simple mod, but damn I love how my amp is sounding now.

Next week I'm going to try lowering the values of the plate resistors on V1 and V3, another mod suggested in this thread. Hopefully the components will be in by rehearsal next week so I can put the amp through it's paces after tweaking it. I'll report back my results.

I forgot to tell you hommies I joined the ranks. This amp is bad ass. Reminds me of my Avenger and the Hot rod 50+ I had in many ways. I totally stole this thing as well. Like seriously cheap.

^ My bro

So I've been absent from the forums lately. I've been working like a mad man and haven't had much free time to do much. BUT, I have been doing some damn fine upgrades to my Amelia and Friedman 20H, and this weekend I'll hopefully have some clips and pics to show off.

With the Amelia, I have been working with Bryce/Epic Amplification testing out and tweaking a High Gain mod that he developed. It's fucking badass. Some peeps here heard my stock Amelia at GABfest, and this mod takes to a new level. More gain, more clarity, more balls! He granted me naming rights, and I decided to name the mod "Firehorse" - a nod to the nickname Amelia Earhart gave to the plane she flew over the Atlantic.

Along with the high gain mod, I did a full cap upgrade with Sprague/Orange Drop 225s and Mallory 150s. Also did a few small circuit tweaks to my personal taste (and it's a never ending process lol), but the meat and potatoes of Amelia and the Firehorse mod are still there. I gigged with it a couple weekends ago and was absolutely floored.

Next on the project list is making the Overdrive function footswitchable like it is on my Earhart. Speaking of, the Earhart is now officially retired now that I have my Amelia breathing fire

Friedman 20H now has choke, cap upgrade, Depth control, NFB control, and OT upgrade. Working on cloning the basic BE voicing into my stock 20H whenever I have time for it

Hey,

I’m about to buy me a jca20h. I was after the vintage version but this came pretty cheap and thought I could mod it in to something I like.

I saw this video on yt and was completely blown away;
https://youtu.be/ZoEKjZmCBZM

Now I have a few questions:
I want to start with the most common mods like tube swapping, maybe (in part) jcm values, caps etc... but what are the differences between the h and hv? Maybe someone would like to share a schematic via pm? And I was wondering about the depth switch... there’s no clear documentation (at least, I couldn’t find any) on how to do this...

-Lars

Hello dear Jet City fellows, i am new to the club.
Well at least i want to join your club and buy a used Jet City 20 Watt Head and mod it with the Friedman BE Mod.
So some question before i take the shot:
The 20h and the 22h are two different beasts right? So i should get the 20h to do the Friedman Mod?
The 20HV is a different animal too, or can i mod the 20HV to the original Friedman Mod (because it is cheaper used)?

Last but not least: Are there any schematics out there that show the Friedman Mod in detail?
I cannot find any, would appreciate your help.

Thank you and Regards

20H is easier to do the Friedman mod than the 22H. There is a tutorial of sorts that replicates it but not sure if it's 100% accurate. I'm actually in the beginning stages of doing it to mine (I also have a Friedman modded one as well). Might be a week or two until I can report on how well it turned out.