Amp popping noise when switching pickups

[Amish Mel]

I have a vox ac4c1-bl and I added the bitmo voxmo15 kit. Afterwards I tried it out and it sounded great, no problems. It worked as it should.

I then experimented with some different preamp tubes. During this time I noticed popping when I switched my guitar pickups. Also scratchy volume pots. I plugged into a different amp and there were no problems. I returned the new tubes back to the AC4 which were fine beforehand.

I'm new to modding amps, and I'm thinking I messed up somehow. I may have overheated a cap when soldering, and perhaps it failed after running the amp for a bit. Or that I inadvertently damaged something while disassembling/reassembling.

Other changes I made to amp-
I did the "bright caps mod" where you cut a leg of the c20 and c6 caps to improve the brittle sound. I had done this last week and did not have any problems.
I also replaced the stock speaker with a greenback. The speaker is not the issue however.

Keeping in mind that I have little experience, can someone give any advice on how to track down the problem? Thanks for any help!

[Jazz P Bass]

BitMo VoxMo15 Mod Kit for the Vox AC15

No schematic for the Vox ac4c1-bl amp?

No schematic for the BitMo?

Which caps did you cut out?

The AC4c-B had a small value capacitor across the input jack.
If you removed that one, try putting it back.

[catalin gramada]

Hi. From you description I suspect a grid leak resistor problem. Check all inputs of the preamp valves to the ground and do all efort to tie it in the same point where cathode was grounded. So v1 leak resistor to the ground point of v1 cathode.sometimes the input reference to ground is done through the volume pot. Tie the pot pin to the ground in the ground point of cathode for the drived stage.eg if you have a pot în front of v2 you have to ție it in the cathode of v2. Hope understand. Excuse my bad english. Also it worth to check for any dc leaks over coupling caps. Late edit. Signal grounds retour for large impedance path are sensibile. Keep the wiring as short and pose it sticked to chassis. You pots also could be rubbish keeping the wiper in intermitent operation but scratched noise is usually asociated with dc leakage over the signal

[Dave H]

I have a vox ac4c1-bl and I added the bitmo voxmo15 kit. Afterwards I tried it out and it sounded great, no problems. It worked as it should.

I then experimented with some different preamp tubes. During this time I noticed popping when I switched my guitar pickups. Also scratchy volume pots. I plugged into a different amp and there were no problems. I returned the new tubes back to the AC4 which were fine beforehand.

Did you notice any popping with the original tubes? If it only pops with the 'different' tubes one of them could be faulty. Try the known good tubes from the other amp to see if it pops with those.

[Amish Mel]

I replaced the stock tubes with new and tested tung-sols. Ive been using them all along, but last night I tried out some older 12at7s just to see how they sound. I didnt care for them over the others so I then replaced the tung-sols. Around this time I noticed the popping, BUT perhaps it was there before and I just didn't notice or I did something when switching out tubes....checking the v1 and v2 sockets I see no damage or foreign debris etc.
With a plastic probe I have tapped on my solder joints and on the tubes while the amp in powered on. All of my connections seem fine.

[Amish Mel]

Ive heard this sound means I am leaking DC back into the guitar. Not sure what that means, I am searching online...

[catalin gramada]

Ive heard this sound means I am leaking DC back into the guitar. Not sure what that means, I am searching online...

It is more over the pot. You amp pots should never conduct any dc current through it.

[Amish Mel]

sorry for the low quality

The bitmo instructions say Do not redistribute- I'm not sure of the legal ramifications....

I cut the C6 120pf ceramic and C20 100pf ceramic a week before...no popping then

I did not remove any other caps or anything else at all.

[Mick Bailey]

You can check for DC on the input (which would cause the guitar pots to be scratchy) by just unplugging your guitar and checking between the guitar lead sleeve and tip with the other end still plugged into the amp. There should be nothing there.

[Jazz P Bass]

[ATTACH=CONFIG]37160[/ATTACH]

sorry for the low quality

The bitmo instructions say Do not redistribute- I'm not sure of the legal ramifications....

I cut the C6 120pf ceramic and C20 100pf ceramic a week before...no popping then

I did not remove any other caps or anything else at all.

Where are you getting that information from? C6 & C20.

That schematic (unreadable as it is) appears to be the stock amp.

Scan & post the Bitmo instructions. (No lawyers here!)

[Amish Mel]

You can check for DC on the input (which would cause the guitar pots to be scratchy) by just unplugging your guitar and checking between the guitar lead sleeve and tip with the other end still plugged into the amp. There should be nothing there.

I did as you suggested. As I said I'm a novice at modding amps or anything electronic. I know enough to realize that you meant using a multi-tester, so I tested DC at different settings and the needle never moved. I wasn't sure which probe to use on the tip or sleeve, but did both ways and nothing.

[g1]

When you returned the tung-sols back into the amp, did you put them in the exact same positions as they were before?
If not, it may still be a tube issue, and the iffy tube may have been in a spot where it's defect was less problematic.

[Amish Mel]

When you returned the tung-sols back into the amp, did you put them in the exact same positions as they were before?
If not, it may still be a tube issue, and the iffy tube may have been in a spot where it's defect was less problematic.

You know what? You are right. I replaced the tung-sols with the stock chinese tubes and the pop is gone....I must have somehow damaged at least one of the tung-sols moving them around. I'm kicking myself for not investigating the tube angle earlier- I just assumed that the tung-sols are new so they wouldn't be the problem.

I suppose I will just figure out which one (if not both) of them are bad....I have some old assorted tubes to work with.....wish I had a tester.


At least I know it wasn't from shoddy work on the mod!

Thanks to everyone for their help, hopefully I won't need to report back unless to mention continued success.

[g1]

You can use the amp as your tester. The V1 spot will be the one that made the pop when switching pickups.
So leave the chinese tubes in except V1, and try the tung-sols in that spot. You should be able to find the defective one that makes the popping when switching pickups.

[Amish Mel]

You can use the amp as your tester. The V1 spot will be the one that made the pop when switching pickups.
So leave the chinese tubes in except V1, and try the tung-sols in that spot. You should be able to find the defective one that makes the popping when switching pickups.

Yes, I did just that. Discovered that BOTH cause the popping. This must likely mean that its not a defect, I just got these from Yen Audio, and they come pre-tested.

I must have fried them somehow in the amp?

I don't think I physically damaged them, especially both. Oh well, I have a Groove tubes (v1) and an old RCA (v2) in there right now. I have a NOS IEC Mullard on the way to try in v1 as well. I may get a Mullard reissue for v2.