Crackling sound at high volume

I was playing through the 2203 clone I just finished up and a crackle started occurring when I played a low A on the guitar...especially muted. This was at very high volume, 6+ with a medium amount of gain.

I swapped out the power and pre tubes... No change
I tried a different speaker in the same cab to rule out a speaker issue

I then took the chassis out of the enclosure and flipped it over to see if I could see any arcing....nothing.

I then switched from my 8ohm cab with V30's to my marshall 16 ohm cab and the sound was gone.

Right as I was about to say, ok, this is a prob specific to 8 ohm, POP, the 1 Ohm bias resistor on one of the power sockets exploded.

I replaced the resistor but still have the crackling at 8 Ohms. At this point, I need some further direction as to what to check for.

Thank you for the help.

jason
 

Galu

Member
2018-04-17 6:50 pm
Do you have a schematic for the clone?

I attach the original Marshall 2203 schematic for comparison - may help elicit a response.
 

Attachments

  • Marshall 2203 Schematic.gif
    Marshall 2203 Schematic.gif
    146.4 KB · Views: 118
The only thing that is jumping out at me, especially when trying to compare voltages to other findings, is that there is an extra 10k resistor in series after the choke. This different than the schematic of the actually 2203, which only has one 10k resistor.

I am still pretty new at reading and working with schematics, so somebody please correct me if i am wrong.

At this point, I cant find anything obviously wrong. The voltages seem to be reasonably inline, considering the extra 10k.

I am really at a loss here, so any help would be greatly appreciate.

Here is a pic of the final wiring. If that helps?

DS4_4107.jpg
 

PRR

Member
Paid Member
2003-06-12 7:04 pm
Maine USA
www.diyaudio.com
...clone I just finished up and a crackle...

"Crackle" says to me an intermittent connection.

1 Ohm resistor BLOW-up says to me a MAJOR EL34 fault.

While it does not fit all observations, I would look VERY VERY hard at the grid bias connections. Momentary loss of bias is "crackle". Prolonged loss of bias is MELD-DOWN, of tubes unless the resistor quits first.

Yes I know all your connections are always perfect. Check anyway.
 
"Crackle" says to me an intermittent connection.

1 Ohm resistor BLOW-up says to me a MAJOR EL34 fault.

While it does not fit all observations, I would look VERY VERY hard at the grid bias connections. Momentary loss of bias is "crackle". Prolonged loss of bias is MELD-DOWN, of tubes unless the resistor quits first.

Yes I know all your connections are always perfect. Check anyway.

I will be quadruple checking. I will post an update shortly.

The two things that are curious to me is why it would only crackle at higher volumes and only at 8 ohms?

Either way, checking.

Thank you for pointing me in a direction.

jason
 
I spend two hours today checking connections, resistance and voltages and I cant find anything that seems alerting.

The only thing I noticed is that the 1 ohm resistors really read 1.8ohm. Which is a little weird because they are 1.2ohm 1% vishays.

Anyone have any other ideas?

Thank you,

jason
 

PRR

Member
Paid Member
2003-06-12 7:04 pm
Maine USA
www.diyaudio.com
> ...There is no resistance between the common and any of the different outputs on the OT secondary. I am not sure if that is normal or not?

"No" as in ZERO or "no" as in "INFINITY"?

ONE lead should be hard grounded. The other leads may read less than 1 Ohm to common (they need to be much less than speaker impedance).

> the 1 ohm resistors really read 1.8ohm.

Most bench DMMs do not read sub-Ohms accurately. Press the probes firmly to each other. This "should" be ZERO. It never is. There is probably 0.5 to 1.0 Ohms of stray resistance in meter and leads. And with any tarnish on probe or part will add a variable extra resistance depending on contact pressure and location.
 
> ...There is no resistance between the common and any of the different outputs on the OT secondary. I am not sure if that is normal or not?

"No" as in ZERO or "no" as in "INFINITY"?

ONE lead should be hard grounded. The other leads may read less than 1 Ohm to common (they need to be much less than speaker impedance).

> the 1 ohm resistors really read 1.8ohm.

Most bench DMMs do not read sub-Ohms accurately. Press the probes firmly to each other. This "should" be ZERO. It never is. There is probably 0.5 to 1.0 Ohms of stray resistance in meter and leads. And with any tarnish on probe or part will add a variable extra resistance depending on contact pressure and location.

"No" meaning close to zero resistance. Now this may be because of the reasons you mentioned above, but resistance between common and each tap is reading .4 ohms.

jason
 
Another thing to add...

I was measuring my plate voltage while reproducing the crackle...

Here is how that went.

Plate voltage was about 470v and then i hit a note. DMM popped....see ya. Fried! Also, the 1A fuse blew on the amp.

Now, it was one that self adjusted its range, so that may have been the issue, but i have no idea.
 
I have never used a scope before, but this is what i did...

I ran a 100hz sine through the amp and measured the grids at each stage.

I can see the crackle in the sine wave all the way back at to the point just past the coupling cap past V1B. This is even before the gain pot, I believe?

The crackle only occurs when the volume of the amp gets loud enough.

Any thoughts on where to look next?

Thanks,

jason