JMP 50 Popping Sound

[Riffmonster]

Hi all,

I just completed the build of a 50 Watt JMP clone. I followed this layout from Modulus Amps https://www.mediafire.com/file/x0ctrzgp ... layout.pdf but sourced my own parts and used star grounding.

The start-up went smooth, no smoke or smell. The amp sounds fantastic too. However, something is wrong. There is a popping sound coming from the amp in intermittent but freuent intervals. The sound comes with or without the guitar plugged in. It kind of sounds like a cap discharging, a high pitch pop almost like a firecracker. Can someone help me where I should start looking first? I unfortunately do not have a oscilloscope. My first bet would be the filter caps?

I already tripple checked all connections and resoldered them, but this did not make a difference. My voltages are a big high but do not look odd (if I am not wrong):

Pin 1 Pin 3 Pin 4 Pin 6 Pin 7 Pin 8
V1 175v 1v 223v 2v
V2 181v 1.1v 319v 181v 183v
V3 242v 45v 231v 45v
V4 454v 451v 454v
V5 453v 451v 454v

Here are also some pictures. I can send more detailed ones if needed. The amp is a bit dirty now after all the troubleshooting.

Many thanks for the help, much appreciated.

IMG_0815.jpg

IMG_0816.jpg

IMG_0814.jpg

[Littlewyan]

Could be a loose connection or a bad valve.
- Try removing valves in order to narrow it down. Start with V1. Still there? V2 and so on.
- Once you've narrowed it down to a particular section of the amp, try swapping that valve out for another (or both valves if it's the output valves, I've had noisy output valves before).
- If a valve swap doesn't fix it, it's a circuit issue. Get yourself a wooden chopstick and prod about in the problem area, see if you can make the noise happen. Could be a bad solder joint.

Also if you can supply a recording of the sound that would be helpful.

[danman]

It may be a long shot but several folks over the years have reported issues with those black silver mica caps. Some have been faulty right from the get go and others have been damaged from excessive heat while soldering. I have gone to using quality ceramic caps these days because of this. I also would not rule out a bad tube if you have not already swapped them.

[gktamps]

+1 on Littlewyan's comments.

[Riffmonster]

Thanks a lot for these quick answers, that is helpful.
- Pulling V1 does not make a difference
- pulling V2 makes quite a difference,
- pulling V3 makes it almost disappear.
- pulling V4 makes no difference
- pulling V5 makes no difference
- pulling V4 and V5 makes it go away

I tried different tubes for all positions without success.
I was not able to reproduce the sound while chopsticking the amp. See below the video. Turning off the standby switch makes a similar sound sometimes. Could there be some arcing going on somewhere?

[pdf64]

Wow, that’s weird!
Do the HT node voltages at the reservoir cap (OT CT) or screen grid node (output valves pin6) ‘twitch’ with the pops?
How about if the standby switch is linked out?

I’m a bit wary of soldering to what are obviously push on spade tabs, I’d rather use stuff with lugs intended for soldered connections.

[Riffmonster]

Could I just omit the standby switch for this circuit (given that it is solid state)? EDIT: It is not the switch, problem persists without it too.

Second EDIT: Voltages do not twitch or change at pin 6 of the output tubes or the OT CT reservoir cap when it pops.

Not sure if understand your other comment (apologies, not a native english speaker here). The board is a eyelet board and the ground connections are lugs too. What exactly do you mean? Thanks.

[statorvane]

I think he is referring to the connections to the power and standby switches. I solder to these connections all the time, no issues, but the spades on my switches are brass.

[Riffmonster]

I don't think that is the issue since I have used these switches in many builds in the exact same way.

Really frustrating issue, can't figure it out at all.

[bepone]

The sound comes with or without the guitar plugged in. It kind of sounds like a cap discharging, a high pitch pop almost like a firecracker.

can you put in parallel with marked 100k any other resistor value, 150-220k 2w, and verify that there is no popping? or change that 100k for 68-82k anything for the test.

unt1.png

[Littlewyan]

So even after pulling the Phase Inverter valve the issue still occurs? What changes? It gets quieter? Or happens less frequently?

On the left I can see a lot of what appears to be little bits of solder on the cap can, I'd clean that up. And what's that on the right hand side? An unused PT winding? It sounds to me like something shorting out or arcing.

[Riffmonster]

Thanks both for your tips.

1. The problem persists and even got worse when I put a 150k 2 watt resistor in parallel to the 100k resistor depicted.
2. Yes, I cleaned that up on the filter cap right after the photo. The wires are unused PT wires, you could use the PT also for 120 and 220v. I triple heatshrinked these leads.
3. I double checked and actually the problem is gone when pulling the phase inverter V3.

[bepone]

so there is a problem in V2 circuit. i was suspecting too high cathode voltage in cathode follower and some leaking there..but looks some other thing (oscilations out from hearing freq.range?).

[bepone]

or this treble mica cap connected to this 100k resistor, can be easily replaced with ceramic 500v for the test

[bepone]

oscillations if they are can be easily seen in oscilloscope on output(speaker terminals)