Could it be this simple? (Fender Dual Pro)

[PRNDL]

I'm fixing a Fender Dual Pro for a local musician and music teacher.

The amp pops and crackles when you turn it on. Removing the first preamp tube stopped the noise, but changing tubes didn't stop it.

I went through it and found one of the B+ voltages was very low, so I recommended a cap job. The tubes were fairly new and all tested good on my Sencore.

Today I replaced the electrolytic caps only to find the noise was still there. I measured resistors in the preamp section.

Then I noticed the two 10K resistors on the input jacks had their joined tails very close to the ground terminal strip. I moved the tails so they were parallel to the jacks, out of the way of the input coax connector and away from the ground bar. Viola - no more noise!

Could it be as simple as that?

This is a spectacular amp and I'm going to have lots of fun testing it to make certain it's really fixed.

The other question I have is the 60W light bulb limiter stayed on bright (not very bright, just normal) even with standby off and slightly brighter with it on.

Is this common in newer amps?
(Usually the bulb goes bright for a second while the caps charge then dims).

Thanks!!

BTW ... this is one of the best designed newer amps I've seen Fender make. It's a circuit board layout similar to an old board, but with two switches.

[PRNDL]

One of the tubes was microphonic (V4, which is gain for the volume pots).

I'm going to recommend a fresh set of preamp tubes. These were Fender Groove Tubes with the Sovtek stamp on them.

Has anyone got a preference for similarly priced tubes?

JJ/Tesla, Sovtek and EH are all around $9 for a 12AX7.
Fender/GT is $3 more.

The replacement I had on hand is a Sovtek.

Thanks!!

[Tom Phillips]

...Then I noticed the two 10K resistors on the input jacks had their joined tails very close to the ground terminal strip. I moved the tails so they were parallel to the jacks, out of the way of the input coax connector and away from the ground bar. Viola - no more noise!

Could it be as simple as that?...

Entirely possible. I have seen similar things many times. Did the B+ come back up after the cap job?

Regards,
Tom

[PRNDL]

It may be the effects return jack

The popping noise is, unfortunately, intermittent.

It happens with the new tube, although it does seem to be something to do with V4, which the effects return goes to.

The effects send is a two-tab and return is a three-tab jack. My meter reads variable resistance between two tabs on the return jack, even after cleaning it. Tapping it with a chopstick causes the same popping noise.

I've heard that effects jacks are notorious for this sort of issue.

Is the best solution to use a short coax between the effects send and return?
(That seems to work, but I'll have to test it for several hours to be sure.)

Or, will a shiny new return jack be better?

Thanks!!

[metalangst]

One of the jacks may have a bad switch. Either the tension is sprung, or the contacts are dirty. Depending on the type of jack, you can check the tension of the contact with a probe. If it's good, use a burnishing strip to clean the contacts.

[PRNDL]

I have a brand new switch that has the same issue: varying resistance (on the 2 ohm scale).

The switch in the amp had a lot more variance that dropped down quite a bit after cleaning. It's about the same as the new switch (variance < about half an ohm).

I can hear some minor crackling (nothing like the huge ones before the cleaning).
It's dead silent with the coax jumper cable between the send and receive jacks.

At this point I'm wondering if standard Switchcraft jacks really aren't meant for this.

[daz]

I've been in a sh|tload of newer fender tube amps and i've seen a lot of hard to find problems like you described, often finally finding it was a jack or such. I don't know what it is, but they seem to be using some flaky hardware, jacks especially. One thing to look for on that jack is to check it and see if the threaded part that is basically pressed on holding the entire jack assembly is tight. If you can twist it at all replace it. I found that to be an issue with a few of them that somehow caused noise problems like crackling and such. I'd replace it with something other the switchcraft type.

i swear, i've never seen so many intermittent noise issues like that with any brand/amp as i have with those newer fender tube lines. A lot of it ends up being connection issues like the jack, bad tube sockets, even a pilot lamp believe it or not cause an issue that i can't recall at the moment. Don't even ask !

[PRNDL]

It's much better after cleaning and letting it dry overnight with the coax jumper in.

Daz, I've heard of these issues, which is why I decided to post.

Also, my feeling is that these jacks simply aren't designed to pass signal through the switch. Otherwise they'd have better contact material.

My current question is whether the 12AX7 tube was microphonic, since I tapped it pretty darn hard. I changed it for a new Sovtek since this is a pro musician's amp who wouldn't like a failure to occur at an inopportune moment.

[tubeswell]

If the pre-amp tube was microphonic, the amp would howl like a banshee when on and nothing plugged in. Grasping the suspect tube with a dry towel at this point should quell the howling somewhat. Or take the suspect tube out and hold the base of it firmly between thumb and fore-finger and flick the glass with your other hand's fingernail while holding it up to your ear. If the tube makes a loose jangling sound like a jingle bell (instead of a solitary 'dink') when you tap it, its haddit.

But an intermittent popping sounds like a bad connection somewhere, either bad socket like daz said, or loose/badly soldered wire or jumper lead, or loose socket pins. Does it perform the same trick with the amp turned on when you thumb the cab hardish?

[PRNDL]

I think you're correct ... it's fine with regular testing.
I moved the socket by hitting it that hard.

Thanks for the great replies. As many of you know, I'm not exactly a beginner, but I've noticed it does take a lot of experience.

I'm going the extra mile with this amp.
It's in great shape and has that lush tone.