loud popping volume.

[rymac]

I think my problem started after I took the amp off standby. Realized the volume was turned up with a guitar cord plugged in but no guitar.
Oh no!! That sent a loud pop and hum through the amp. I quickly turned it back off. Now when I turn the volume knob I hear a loud pop and crackle even with nothing plugged in.
What did I break??

[mikeboone]

What kind of amp is it?

Do you have test gear? A DVM? You can start with simple things.

Unplug the speaker cable from the output and measure the DC resistance across your speaker(s).

Plug your output into another amp cabinet to see if the problem follows the amp.

Plug your speaker(s) into the output of another amp to see if the problem follows the speaker.

Be mindful of speaker loading and the output power from the amp you ise to test with to ensure we dont break something else.

I always start with the easy stuff and then ask these guys when I get in too deep. They are good group of guys on here who will give you lots of information and things to try.

[rymac]

The amp is a 5F6A circuit. It's a 2X12 cabinet. I measured 3.6 ohms on the speakers. I don't have any thing else to plug the amp into at the moment. I'm suppose to record this weekend .

[mikeboone]

It could possibly be an unrelated issue. Try removing your preamp tubes one at a time starting from V1 to see where the issue stops. Turn it off during tube removal and replacement.

It may well be a funky preamp tube or one needs to be reseated.

Its worth a look.

[pdf64]

Does the amp sound ok? It may be just the shorting connection on the input socket not shorting, which is an inconvenience but not a breakdown. Peter.

[rymac]

I will check that out. I notice with nothing connected to the input when I turn the volume all the way up it starts to feedback. It also pops when the guitar is plugged in with guitar volume down?
The amp does sound OK.

[rymac]

I replaced both preamp tubes. No change. Nothing plugged in
Turn volume knob loud popping.
The amp worked with no problem through an entire gig the day before.
Problem started after I turned on the amp with guitar cord plugged in and volume turned up but no guitar. IDIOT!! Maybe it's not related I don't know.

I'm going to open up the amp next and check ground connections starting with input jack.

[rymac]

Checked connections to ground on input shorting jacks. Tested below 1ohm.
Around .3 ohms. Not sure what to do next?? Couldn't see any cracked solder joints

I also tested the resistor that is soldered to the bias pot. From the resistor lead to ground I measure 3.6 ohms. I'll clean up my connection between the bias pot and chassis. I don't think this is my problem.

[rymac]

I cleaned and retensioned the preamp tube sockets. The loud popping seams to be gone.

I have a shielded wire coming off the input jacks from the 2 68K resistors.
I have the shield connected to the plate of that tube.
I got this from a Gerald Weber book. Any thoughts?

Rymac

[rymac]

I thought I had it fixed but guess not.
I tried cleaning the volume pot. Maybe I didn't clean it well enough.
Maybe I need to replace the volume pot. I don't know.

With nothing plugged in I turn the volume knob and I get a loud popping sound.
I can hear it in my sleep. Please help.

Rymac

[Enzo]

Furst off, plugging a cord into an amp without a guitar at the other end won;t hurt the amp. At the very worst it makes a loud noise come out the speakers. Every time I play the guitar a loud noise comes out the speaker - it won't hurt it. And even if it somehow could hurt it, it would hurt the power section, not the preamp anyway.

You can turn down the volume and not hear it. Turn up the volume and there is is. That means the noise is in the first stage. Is this a real 5F6A or something someone built? The Fender 5F6A has two channels - bright and normal - and each channel has a separate volume control. is that what you have and is only one channel noisy or both?

Pick a channel to be noisy. Pull the first tube. Does the noise go away or remain?

Aside from the popping, is the volume control scratchy sounding? That is to ask, is there any DC voltage across that control?

With the tube installed, I don;t know which one in your amp, but pins 2 and 7 are the grids. Take a clip wire and ground to chassis each of those grid pins. Does that kill the popping?

In this amp the two sides of the tube share a common cathode resistor, they operates off the same B+, and have the same plate load resistances. The plate voltages should be the same. Plates are pins 1 and 6 - are the DC voltages there the same as each other? Do they stay more or less steady or does one rock and roll? ROughly 150 volts.

The 100k plate load resistor could be noisy. That is the resistor connected to pin 1 or 6. Try replacing it.


Are you saying there is a loud popping WHILE you are turning the volume? Or it pops AFTER the volume is turned up, and turning it down turns the popping down?

Of the pot itself is popping while turning, check again for DC voltage on it, and the only place that can come from is a leaky .02uf couplng cap

[rymac]

The amp is something I built. I just have one channel. I'm only using one side of the V1 tube. The unused pins of the V1 are connected to ground.
I hear a little bit if of static and loud popping "while" turning the volume knob.
I think I did hear a random pop or two while playing.
I will do everything you have suggested and report back.

Thank you Enzo
Rymac

[rymac]

Here are the results of my testing.

Tubes installed.
Grounded pin7 of V1 did not change popping.
grounded pin2 of V2 (which is connected to volume knob) killed popping.

V1 plate measured 196vDC with 5751 and was steady.
V2 plate measured 322vDc with 12AX7 and was steady.

The coupling cap that goes to the volume knob I measured between 1mv Dc and 20mv DC NOT STEADY. I'm guessing this is the bad cap.

I tested the two .02 caps in the tone circuit. I measured 186vDC on the side connected to the 56K resistor. On the cap connected to the mid knob I measured 00.0 with meter set to mvDC. The cap connected to the treble knob measured between .1 and .9 mvDC. Is this cap bad?

How can I test the coupling cap from Treble knob to the driver circuit?

When I set the bias I use the transformer shunt method. I measure one side of the output transformer. Meter is set to milliAmps.
Even after the amps is warmed up the current is not steady it goes from -36.1mA to -34..8mA. Doesn't seem right.
I'm wondering about the quality of these orange drop caps in the tube driver circuit also.

Where can I get high quality caps? Is there something better than the orange drop. I can't remember where I got these.

Thank You
Rymac

[rymac]

Well I replaced the .02 coupling cap that connects to the plate resistor and plate of V1. The other end connects to the volume knob.

The volume knob is still popping "while" turning the knob. On the volume side of the cap I still measure voltage going up and down between 1mvDC and around 20mvDc. Is this cap bad also. I've had the cap for a couple of years but never used orange drop rated at 400v. Maybe I'll replace the 100K plate resistor?? Please help.

Rymac

[rymac]

Just thought I should end this thread. After much testing and replacing parts the problem was the volume Pot. It's a Mouser 1meg. Unfortunately another Mouser was all I had to replace it with. What's a better brand of pot for Fender amps??

Rymac