JCA5012C Master Volume Mod

[mrfret]

So, I decided that the only thing I really didn't like about the JCA5012C was the lack of a master volume. I know there's lots of info out there about adding a MV several different ways, but I couldn't find anything specific about THIS amp. So I asked a friend and did a LOT of research online (I'm completely new to this level of modification) and lo and behold, it works!

So what I plan to do here is just show what I did in case anyone else needs it. Feel free all you gurus out there to pick apart what and how I did it. I'm sure there's bound to be better ways than what I came up with. (Did I mention I'm new at this? LOL)

Parts:
1 dual gang 500k LOG pot
2 68k resistors
2 .022uF Caps

I decided I had the most room beside the power switch


R53 with .022 cap and 68k resistor


R55 with .022 cap and 68k resistor


C10 and C11 unsoldered from board and wired to MV


Here's a modified version of Bruce Collins' schematic showing what I changed


I put it all together and luckily it worked the first time!
Now tell me what I did wrong. I'm still learning!

mrfret

[mesa3077boogie]

Nice work, did it add any new noise?

Does the fly heater wires really help with noise all that much for your?

[mrfret]

I flew the heater wires a while back and it did help a lot with the noise.

[mrfret]

Well I played out with it Friday night and it works ok until it gets loud. I started getting this microphonic type noise. I may not have had it if it weren't a combo, but the vibrations made the noise. I can even tap on the top of the amp and make the noise. Almost like tapping a spring reverb, but not really.
Any suggestions?

Might need to check the tubes. Reminds me of tapping a bad tube.

Someone please look at what I did and make sure there are no glaring mistakes. I am pretty new to this.

[clipless bumper]

Did you try tapping the tubes?
Like you said, it sounds like a microphonic tube.

Another possibility could be a solder joint that heated up, and either shorter or opened, but that doesn't sound likely for the symptoms.

[mrfret]

Did you try tapping the tubes?
Like you said, it sounds like a microphonic tube.

Another possibility could be a solder joint that heated up, and either shorter or opened, but that doesn't sound likely for the symptoms.

I haven't had time to troubleshoot yet, but that will be the first thing I check. Been playing all weekend. Good thing I have multiple amp options! LOL
I didn't think of a solder joint, but I'll be checking that too.
Thanks!

[mrfret]

OK, so I finally had a chance to mess with it last night. I left all of the controls where they were from playing out last. It was loud! LOL. Anyway, got to tapping on tubes and found the V1 was noisy. I swapped it with the tube from V2 and made sure it was seated good, and V1 is still slightly noisy but not as bad. I probably should let it get good and hot and test some more.
I also tapped around inside with a chopstick to see if anything else made noise, and nothing.
I have a few older 12ax7's I'm gonna swap in also to see what happens. I have a feeling I just need tubes.
I always planned to change out the cheap chinese tubes anyway, but it wasn't broke so I figured I'd wait.

Did anybody see anything wrong with how I did the MV mod?

[mrfret]

After further testing I think it's just the cheap tubes. After swapping the v1 with v2 I could not get it to repeat the microphonic noise no matter how loud or hot it was. So I ordered new tubes

[mrfret]

Had to remove the MV mod. Even with new tubes I had trouble with microphonics on v1. Also, the MV itself would have an ungodly squeal between 8 and 10 even if the other volumes were low. I hope someone takes a look at what I did and can tell me where I went wrong.

[Tortuga]

Missed this earlier - interesting!

I'm not sure I've seen a master set up like this before. I'm curious - what wattage dual pot did you use? I wonder if it needs a heavier-duty one than what would normally be used earlier in the signal chain (ie: pre-phase inverter). And what's with the unbalanced resistors leading to B+? Is that normal? Also, your amp is a 50 with 2 power tubes, right? Wouldn't that possibly cause some of the circuit values to be adjusted in that area (where your dual-master is)?

I guess the bigger question is why did you feel that you needed another master volume pot, when the amp already has masters for both channels?

[mrfret]

Missed this earlier - interesting!

I'm not sure I've seen a master set up like this before. I'm curious - what wattage dual pot did you use? I wonder if it needs a heavier-duty one than what would normally be used earlier in the signal chain (ie: pre-phase inverter). And what's with the unbalanced resistors leading to B+? Is that normal? Also, your amp is a 50 with 2 power tubes, right? Wouldn't that possibly cause some of the circuit values to be adjusted in that area (where your dual-master is)?

I guess the bigger question is why did you feel that you needed another master volume pot, when the amp already has masters for both channels?

As for the why of the resistors I don't know! LOL. I don't know enough yet to know why. I was just following the instructions (as I understood them at least) laid out by people smarter than me.
As for why in the first place, well, I just wanted the convenience of turning both channels up or down evenly with one knob. Maybe I'm just lazy, but my blackheart bh100h has 2 channels each with it's own master, then a master master volume also. It makes it easier than having to do the volume juggling act during a gig.
The mod worked, mostly. Just had the microphonics and the squeal I wasn't expecting.
Maybe if I used different values on caps or resistors?
Here's the pot:
https://www.amplifiedparts.com/products/R-VA2X500KA
Says 1/4 watt. I don't remember seeing that specified in anything while researching. Maybe I should have gone higher?