Guitar Gear Forum

Hey guys, I've been watching the posts for a while and I need your help to solve a problem with my JCA50H.
It's non-modded, biased at 46-37 mA, with stock preamp tubes and EHX 6L6 power tubes. The biasing process did relieved the amp from that high-end fizz everyone complains about, but I haven't been able to get rid of a annoying hum in the amp. The hum is present even with everything in my room turned off and/or disconnected from their respective outlets. The hum is there when I turn on my Polytune (drops volume of the hum, but still there). I have two guitars, one with mini humbuckers and the other one with active pickups (humbuckers too) and no dice. I've checked for ground loops and it seems there are none. The amp and my pedalboard are connected to a single outlet in my room to which is connected to one of those multiple outlet thingies. I've verified it's not the 60 Hz hum (the note of the humming is B1), so that's ruled out. Also, the hum is present on both channels and even when my guitar is the only thing in front of it (meaning I'm just using my guitar without my pedalboard). Here's a clip you can hear (keep in mind I recorded this with my cell, so the other buzzing :

https://soundcloud.com/jack-montfort/humming-noise-in-my-jca50h

I understand since the JCA50H is based on Soldano technology is bound to have some level of noise (being a high gain amp and all) but this hum is really annoying and I want it gone BAD. Please help! I don't know where else to go!

B1 is 61.7 Hz so that is pretty close to 60 Hz hum.
Power line noise can enter the amp several ways: through proximity to the power transformer, Through the guitar pickups or through the tube heaters. If the hum goes away when you have nothing plugged into the amp then it is being picked up by the guitar or a poorly shielded cable.
120hz hum/buzz is from the internal power supply after the 60hz line frequency is rectified.
Unbalanced bias in the power tubes can cause hum in the output. Sometimes a new set of matched tubes will reduce the hum for awhile but it will return as the tubes drift with age. The only way to set and keep the bias balanced is to have individual bias pots per tube.
Try some preamp tube swaps to see if there is a noisy tube in one of the preamp positions

Thanks ironman28, but the hum does not goes away when there's nothing plugged in it. I went back to the unbranded stock tubes, no difference. Everything is stock again. The only thing I changed in the amp is the bias, which is 36 mA. How can I treat power line noise?

I should clarify that "power line noise" is 60Hz hum .

Does the hum go away when both masters are all the way down?
If yes then the hum is coming from the preamp-remove each preamp tube one at a time starting with V1(power off before you remove a tube)V1 is nearest the input jack.
If the hum goes away after removing a preamp tube that is the stage where the hum is entering.

If the hum does not go away with the masters all the way down then the hum is entering the power amp someway.
Try reducing the bias in small increments to see if the hum can be reduced.
How did you bias the amp? with a bias probe? transformer shunt?
Make sure the amp is not near any other amps or power transformers that could induce hum.

If all the above does not localize the hum or reduce it you may need to open it up for further trouble shooting.

I biased the amp with a probe. Now, troubleshooting report:
As per instructed, removing one of the preamp tubes while leaving the rest untouched, starting by V1, gave the following results:
V1: Hum noise diminished greatly. I had to turn up the master to 8 or 9 to hear it. Still the same note. Clean channel was incredibly clean and I had to turn up the corresponding master to hear the hum.
V2: Same result as V1, but hum noise almost disappeared. Both masters were at 4 and I couldn’t hear a thing. Again, when turning it up to 8 or 9, it would appear, but otherwise, sweet silence.
V3: Something strange happened when this tube was removed. On either channel, from 4-8 on the master the hum would appear, (increasing volume as I went up) but when I reached 9 and beyond it went dead quiet. The hum was very low, manageable and what I can describe as “normal”. Also, when turned off, the usual “pop” or fading sound of whatever you’re playing is gone.
V4: Same result as V3.
V5: NO. HUM. WHAT. SO. EVER. Even with both masters and volumes at their max, there was no humming sound coming out of the amp. And when turned off, also dead silent.

So, what is the diagnosis here, ironman28? Thanks!

Try some different tubes in the V1 position to see if it reduces the noise as that stage seems to have the greatest effect.
It may be you are hearing the normal residual hum as it builds up through each gain stage.

There might be other issues inside the chassis that a tech can fix for you.
High gain amps use other techniques to reduce hum like elevated or DC filament supplies usually for at least the V1 position.
A good tech can do these mods to your JC.

I live in Panama, Central America and there's no one I know that does that kind of work. Can I assume a full retubing should be a solution?

If the power tubes are relatively new, then try some new preamp tubes. The stock Chinese tubes are usually decent, but they are not screened as strictly as something you could get from a vendor.

How loud was the amp when you recorded that clip? I have actually heard of many accounts of noise like this. Does yours have the metal toggle power switches or the Marshall-style blue rocker switch?

anyone have a jca5212? i got one in a trade and it sounds really weak ive put new tubes in it and it made very little difference my dual rectifier clean channel has more gain and low end does it sound right ?

@ironman28 Mine has the plastic blue toggle. Why?

Because for some reason, most of the accounts I have heard about excess hum have been related to the versions with the blue rocker switches. There were some minor design changes when they made the switch to the different power switch, so I'm curious to know what has changed besides a few OD channel circuit differences.

Well, I couldn't be able to tell you but I really want to fix this. I love how JCA amps sound like and while I know high gain amps will never be without some level of hum, it should be low and not like this; like when you're playing riffs, if you pay attention, you can hear the hum in the background of the music you're playing.

I just recently unloaded my third Jet City amp. I had one of the 20 watt heads when they first came out, Then a JCA 2112 R C, and most recently a JCA 2212C.
The 2112 was a dud to me but there was nothing wrong with the tone and "niceness" of the other two. The only reason I got rid of the 2212 C is because it didn't fit the layout of my "small combo shelf". Tone was tops.

mjtripper wrote:I didn't see that anybody had posted an updated schematic for the picovalve so here is one I put together a while ago. Some of the values might still be wrong but at least the signal path and component names printed on the pcb are corrected instead of using the univalve ones. This is from tracing out the one I have that I've done mods to.

It's, um... a bit small. Can you post a link to the full size version?

So, guys any other tips I can try with my humming issue? Besides retubing, what else I can do? What about shielding all the pre amp tubes with shields? Will that help?

GRIMESPACE wrote:Looper:
TC Ditto Looper

Pedals that do NOT work in the loop:
EHX Small Stone and Small Clone

Add Electro Harmonix Cathedral to the list of the pedal that don't work in the loop.
Cathedral Reverb works well in the Crunch Channel, but When I switch to Overdrive channel, I gives a large volume drop.
And same problem with Ditto Looper, remove it from the list of pedal that work.

The manual of Cathedral states that the input impendence on Cathedral is 2 M Ohms, I thought the input impendence to run on Line level was 1 M Ohm.

Does anyone know what practically the difference is between the effects loop in the 22H and the 20HV? I know that the 22H is tube driven and the 20HV has a MojoTone loop, but what will the practical differences be, if any?

I got a chance to test drive a 22H and 20HV at a local shop yesterday. I liked the 22H quite a bit. Having a delay in the loop instead of in front of the amp is way better. Unfortunately there was some problem with the 20HV where it would keep cutting in and out, like the cord was bad and someone was constantly wiggling it.

I was hoping to trade my 20H (with JJs) towards the 22H, but I'm not sure that they're going to give me anything for it.

Check it - Mike Soldano modding a JCA50H to run EL34/KT88 with minimal changes (ie: no transformer changes)! Looks very doable. Might consider giving something like this a try, although I still want to work with Jay to try out some better iron at some point...

[video]http://youtu.be/IVmrvqX1vV4[/video]

...and, just gotta say - lotsa love for Douglas White!

No Transformer changes? Seems odd since he has been saying not to run anything except 6L6s in these amps. I wonder what made him change his mind all of the sudden? 6550s/KT88s would just require changing a few resistors, and EL34s would require resistors and grounding pin 1 on the power tubes. If they are using stock iron to run EL34s or even KT88s, then you should be able to use KT77s with just a bias resistor change if you want a simple way to get some EL34 type tone.

I just purchased a JCA 20H, I have never had an amp that was this low wattage. I am running it into a 4x12 mesa cab with V 30s. I think it sounds a bit "mushy" on the low end...I don't know much about tube amplifiers and am wondering if this is a sign of tubes being worn out, or just the way these low wattage amps sound with the 4x12? Any help would be appreciated as I am a rookie for sure...

Should be a good combination - everything I've read is positive with JCAs running V30s

What settings are you using?

Is the amp new (new stock tubes), or did you buy it used? Pics are always good.

Generally, swapping out the stock tubes and getting the bias set properly is the first step. My JCA50H really changed for the better after doing so. There's also plenty you can do to mod the amp, if you're so inclined.

Welcome to the club!

rainsk8er wrote:I just purchased a JCA 20H, I have never had an amp that was this low wattage. I am running it into a 4x12 mesa cab with V 30s. I think it sounds a bit "mushy" on the low end...I don't know much about tube amplifiers and am wondering if this is a sign of tubes being worn out, or just the way these low wattage amps sound with the 4x12? Any help would be appreciated as I am a rookie for sure...

That's just how low wattage amps are. You do get enough volume out of them but not a lot of headroom and since low frequencies need more wattage to be properly reproduced than highs, the low end gets a little mushy. It's not a bad thing and some playing styles actually benefit from it.

rainsk8er wrote:I just purchased a JCA 20H, I have never had an amp that was this low wattage. I am running it into a 4x12 mesa cab with V 30s. I think it sounds a bit "mushy" on the low end...I don't know much about tube amplifiers and am wondering if this is a sign of tubes being worn out, or just the way these low wattage amps sound with the 4x12? Any help would be appreciated as I am a rookie for sure...

Billy is pretty spot on, but maybe there are some things that we can help you with to get closer to your goal. What kind of guitar, pickups, tubes, FX and what kind of Mesa cab do you have, the Recto OS or traditional cab? Music are you playing? How loud?

Jay wrote:No Transformer changes? Seems odd since he has been saying not to run anything except 6L6s in these amps. I wonder what made him change his mind all of the sudden? 6550s/KT88s would just require changing a few resistors, and EL34s would require resistors and grounding pin 1 on the power tubes. If they are using stock iron to run EL34s or even KT88s, then you should be able to use KT77s with just a bias resistor change if you want a simple way to get some EL34 type tone.

Well he probably knows the actual ratings of the heater windings on the chinese iron. They might have also upgraded the specs on new production to allow EL34 heater currents. You can test the heater winding by loading it down to simulate the EL34 current to see if the voltage drops more than 10%. The real problem is the rise in core temprature under those conditions.
I added a small 2A filament xformer for $17 to my 50h to run the preamp tubes and use the stock winding for the PA tubes only.
With some 1k sg resistors I can run almost any power tube I want. Also DC elevated the heaters for the preamp tubes.
I'm not confident in the reliability of the stock pwr xformer under stress for long term. It would be nice if JC actually published the design specs.

ironman28 wrote:

Jay wrote:No Transformer changes? Seems odd since he has been saying not to run anything except 6L6s in these amps. I wonder what made him change his mind all of the sudden? 6550s/KT88s would just require changing a few resistors, and EL34s would require resistors and grounding pin 1 on the power tubes. If they are using stock iron to run EL34s or even KT88s, then you should be able to use KT77s with just a bias resistor change if you want a simple way to get some EL34 type tone.

Well he probably knows the actual ratings of the heater windings on the chinese iron. They might have also upgraded the specs on new production to allow EL34 heater currents. You can test the heater winding by loading it down to simulate the EL34 current to see if the voltage drops more than 10%. The real problem is the rise in core temprature under those conditions.
I added a small 2A filament xformer for $17 to my 50h to run the preamp tubes and use the stock winding for the PA tubes only.
With some 1k sg resistors I can run almost any power tube I want. Also DC elevated the heaters for the preamp tubes.
I'm not confident in the reliability of the stock pwr xformer under stress for long term. It would be nice if JC actually published the design specs.

I like the idea of the filament tranny. $17 isn't bad. How small is it? Shouldn't be too difficult to wire in. I had also contacted Heyboer to make some replacement style Transformers for both PT and OT for the 50H and 100H and had them designed around the SLO spec, but with a higher filament rating. I gave them the dimension of the core and the footprint of the endbells, and they said it could be done, even to make the Hi-Def OT like the SLO has. Prices were VERY reasonable, but I just have not had the time and resources to getting around to doing it.