Amp Tech Thread / Ask a tech Q

[Pissy]

Internal fuse would be my guess.  No smoke right?   If not that then something else with power delivery.  Cracked solder joint or something. 

Does it have a breaker like thing to reset?  Pics?

[mortlock]

Not mine but this is the one i have. Internet pics.
No smoke, no pop, no nothing.

[mortlock]

No obvious breaker to reset.

[RAGER]

I would give everything the wiggle test. Wiggle cord at outlet when plugged in, wiggle power switch.  If that doesn't show you anything then it's time to get the screw gun out and partially disassemble to do a visual. NOT PLUGGED IN. There is almost definitely a fuse near where the power comes in by the power transformer/power cable entry.

If you're not comfortable with that or even going further with the chopstick test with power on DANGEROUS then you gotta seek a tech.

[Muffin Man]

if you're concerned about your prostrate just wait till you start digging around inside an amplifier pretending to do your own tech work - things could really puff up quick. Nice piece of iron haging off the back of it though.

oh my
https://music-electronics-forum.com/forum/amplification/guitar-amps/maintenance-troubleshooting-repair/35747-ampeg-b100r-power-transistors-saturating

[mortlock]

how would i convert that combo so it just functions as a speaker cab? how do you install a 1/4 in jack to the existing speaker?

i want to pull out the head and power the cab with a different head.

[RAGER]

It's very possible to salvage pretty much everything you need to do that from parts from the amp section. Mono input jack, screws, backing plate material and the speaker wires are probably  long enough too. You just have to possess the know how and done soldering skills and the tools of course.

There's plenty of stuff online regarding how to wire a mono speaker jack.   I'd come over and help but you got Covid probably. And you live 3,000 miles away.

[mortlock]

if you remove the tone pot from a fender p bass. how does the pickup naturally sound. is it treble side, bass side or in the middle? what determines that?

[RAGER]

if you remove the tone pot from a fender p bass. how does the pickup naturally sound. is it treble side, bass side or in the middle? what determines that?

Probably the value of the capacitor.

[Pissy]

So a resistor in parallel with a capacitor creates something called an RC circuit.  An RC circuit has a unique property related to AC signals that allows passage of the signal through the capacitor from a specific frequency and beyond. 


The values of both the capacitor and the resistor determine where in the frequency spectrum that point occurs. It's called resonance.

In a guitar, the signal from the pickup is full spectrum- basically- until it gets to the tone RC circuit.  Since the resistor in that parallel RC circuit is a potentiometer, the frequency point changes based on where you twist the knob.  With a tone pot, those upper frequencies are peeled away from the signal and allowed to pass through the RC "gate" to ground effectively removing them from the signal going to the volume pot, and ultimately the amp.


If you pull the tone pot and that cap out, it's much like turning the tone knob all the way up.  But not 100% exactly because the presence of the components colors the sound a little, for better or worse.

Most all tone circuits on amps or guitars or even stereos for that matter use this same principle.  Coloring the sound by being selective about what is removed, vs adding boost to any specific frequency range.

[mortlock]

question. on a peavey combo amp the ground prong on the plug snapped off flush. the amp still turns on when plugged in but im wondering what will happen if its used without the ground prong. im assuming its not safe??

[Pissy]

It's not technically safe, because if a high voltage component within the amp, say the main 110 from the wall, touches the chassis of the amp, it will energize the guitar and you.   If the ground is there, all (or most all) of that current will get run straight to ground, tripping the breaker, blowing the fuse, getting really hot... something other than electrocuting the user.  

Having said that so many houses i used to live in had two prong outlets, and anything that needed the ground lug got an adapter that is supposed to hook into the screw in the middle of the outlet, which I don't think many people do.   It's never happened to me, but its feasible.  Replace the cord or the plug on the end. They're cheap (replacement plugs) from HD or Lowes and easy to do with a screwdriver and an exacto knife to strip the wires. 

[mortlock]

Help. My peavey micro bass head is making a weird noise. It seems fully functional but im getting a weird clicking chirping noise out of it through the cab. Its sounds digital and the clicks and chirps repeat in a pattern. It gets louder as i turn the gain up. What is wrong?

[Pissy]

Bad chip somewhere I'd guess. Difficult to troubleshoot without shotgunning (the act of blindly replacing in hopes of finding the offending component) parts.  But at least you know it's before the volume stage.  

[mortlock]

hey pissy. so i narrowed this down. its def not the amp because the same thing happened to me with a different amp. i took all the pedals out of my signal chain and plugged strait in to the amp.

its my bass. with my tone rolled to full bass its fine, once i roll the tone to more treble its starts to happen, and if i max out the treb it gets more intense.

whats the problem? a ground wire?

[mortlock]

the bass is a fender jazz bass with a single bartoloni jazz pup in the bridge position. the neck pick up has been removed. mods gone wild.

[Muffin Man]

Mod sounds okay. Some of those are real Bartolini and some are, not so Bartolini, when they went budget. If you like it, then settled.

I'd gut it as a matter of annual maintence. Level and true it up, keep yer fingers happy. If Pissy will take it in, just have the whole axe on the bench for a tune-up. That's always smart. I'll send $25 bucks at it  /for free-jazz.

[Pissy]

Active or passive electronics?  Assuming they're passive, My guess is the tone potentiometer.  

When you go full bass, you're basically taking the resistor out of the circuit bleeding all of the high frequencies to ground.  When you go full treble, you're using all of that resistor, bleeding nothing to ground.  When your issue is at its worst, is when you're using all of that resistor, when you get the problem partially, only some of it, when you don't have the problem at all, none of it.   Change the pot.   I think Fender jazz pots are 250k.  They're smaller physically than would go in say a Telecaster, if I recall correctly. 

[mortlock]

Thanks man. Im on my way over with the bass for you to fix. Save me some brisket.

[RAGER]

Do you wear a watch? Or is it your phone in in your pocket or near your bass/amp?