Unspecific Peavey 5150 question...

[declan]

Hi all,

I'm registered here cause I liked the pickup makers forum (I was entertaining thoughts of doing such a thing over the summer but my ambition exceeded my budget), and I thought perhaps you amp guys might be able to help me out with something.

I've got an older Peavey 5150 that's got more than a few years on the road under its belt. I bought it about 2 years ago and it had just been retubed and bias-modded. Right around that time, the band I was in broke up and I haven't used the amp a whole lot until recently when I started another group heavy enough to require such amplification.

Anyway, the amp is probably due for new tubes, but I've got a pair of problems I'm wondering if I should address before I blow a hundred bucks on a new set of tubes--

1.) I fired the amp up at practice the other day and my bass player immediately noticed and mentioned that it sounds very little like a 5150. It doesn't have that trademark "huge" sound of the 5150, though it does have all the gritty distortion. There is no irritating hum or buzz, but the tone is exceptionally thin and unsatisfying. Is this simply the tubes being old? Could it be a poorly adjusted bias? How can I adjust this? Is there an easy way to locate the pot? What should i NOT touch if I open the amp up?

2.) During periods of extended use at ANY volume, the amp will suddenly lose roughly 75% of its volume, with no obvious external stimulous. The tubes are glowing as they should, and if I leave it alone, it is sometimes better after a couple of hours. Or days. Is this a tube issue or something more insidious?

If this is something simple, I'll probably do it myself. If not, fortunately, there's a really good amp guy in town, but he's expensive and I'm poor.

Disclaimer-
I'm a guitar guy. You've gotta explain slowly
Thanks!!!

[Enzo]

First, try a different cab. Even a cab you don't care for should be enough to tell if the amp is running right.

When the amp drops out like that, ball up your fist and whack it hard on the top. if it reacts at all, there is a loose connection somewhere.

When it drops off, take a spare cord and plug it into the effects send jack, and the other end right back into the efects return jack. If this restored he sound, the return jack needs service.

I sincerely doubt it is a bias issue. I have no idea how the mod was made. I myself would recognize the bias adjustment if I went inside, but I do this for a living. I have no way to tell you what to look for. You'd have to have a means of measuring the tube current to adjust it, either a meter and the skill to use it or a commercial bias meter.

The voltages the amp runs on can KILL YOU. Don't touch anything inside until you know what you are looking at. Always work inside with the amp unplugged and left to sit for 10-15 minutes

[declan]

It's the 60w combo version-- I'll try running the speaker out into a 4x12 and see what happens.

The amp is unresponsive to violence. Believe me, I've tried!

The effects loop is working fine- I've been running a 31 band EQ in my rig to get a slightly thicker tone out of the amp.

The bias mod I'm referring to is a well known mod for the 5150- somewhere, a resistor has been replaced with a pot to change the amp from fixed to adjustable bias. This is supposed to change the sound quite a lot, but I don't know how to adjust it so that I can play with it. It's supposed to remedy the "harsh" highs and "flabby" bottom end-- I have no bottom end at all and the sound is really thin. Sounds like it got adjusted in the wrong direction at some point...

[Enzo]

I still doubt it is the bias. Bias is just not that critical. Improvements to tone through bias adjustment are in nuance, not gross sound changes.

I am familiar with modifications to the amp, but every tech makes his layout his own way, using his own selection of parts and his own way of mounting things. SO I have no way of telling you what to look for. When i do it, I drill a hole and mount a round control with a screwdriver slot accessible from outside the chassis. SOme other tech might use a small "trimmer" style control glued to the circuit board. And that is just two examples.

My effects loop possibility is now moot if you are using the loop, the problem I was aiming at occurs when the loop is NOT in use. However, how did you determine the external EQ and the cables to and from it are not involved in this problem? Remove the EQ from the system to verify it is not the cause. One of my rules of troubleshooting is this:

Never think up reasons NOT to check something.

If it is not the speaker, your description is starting to sound to me like there is a break in the signal path, and what you are hearing is actually just crosstalk in the amp. That tends to sound weak and thin.

[Amp Kat]

Make sure the Post control is above 3. It will get real thin if it's not. Does it act like this in the lead and rhythem channels ?

[declan]

The problem happens at all volume levels and gain settings. The amp sounds horrendous without the EQ unit- even weaker and buzzier/tinnier. It still gets loud enough to rattle the windows when I crank it up, it's just that the sound has absolutely no bass, even with the bass on the amp's EQ cranked, as well as the bass on the external EQ... the Resonance knob makes no appreciable difference.

I've used different cables in different positions and I'm 99% sure it's not the cables. I'll double check it tonight with my friend's cables to make sure that it's not simply that ALL of my cables are equally shot (This is not out of the question, but still unlikely.)

Also, the rhythm channel won't get as clean as other 5150's I've heard. I know it's not a Roland or anything, but it's still hard to get a true "clean".

Is this thin, buzzy sound attributable to old tubes or shot speakers? These are things I can probably fix. If it's DEFINITELY not just tubes and/or speakers that are causing the problem, I'll go get it checked out.

I know it's hard to approach this methodically as a technician without having the amp in front of you, and I appreciate the help so far. Is there anything else you can see that might be causing this issue?

[Enzo]

OK< so it sounded the same through that 4x12?

Are you sure the two speakers in the combo are IN PHASE?

SImple test. Unplug the speakers from the amp chassis. Touch the tip of the plug to one terminal of a 9v battery, and lower the shaft of the plug to touch the other battery terminal briefly. the speakers should pop or thump with each touch. Look closely at the speaker cones, when touching the battery, both cones will move a little and try to stay in that new position until you let go. Both speakers MUST move the same direction.

[Gtr_tech]

OK< so it sounded the same through that 4x12?

Are you sure the two speakers in the combo are IN PHASE?

SImple test. Unplug the speakers from the amp chassis. Touch the tip of the plug to one terminal of a 9v battery, and lower the shaft of the plug to touch the other battery terminal briefly. the speakers should pop or thump with each touch. Look closely at the speaker cones, when touching the battery, both cones will move a little and try to stay in that new position until you let go. Both speakers MUST move the same direction.

What he said. Do that test first. If one goes in the opposite direction then just reverse the wires on *one* of the speakers....leave the other alone.

I'm a little hesitant to advise further. The combos have thier own little problem areas that are beyond your ability to service. The "clamp" switching circuit is problematic. The FX jacks are on a pcb, but its pretty small so I wouldn't think there'd be cracked solder problems, but you don't know 'til you check.

BTW, the "clean" side of a 5150 is really not intended to be Fender clean. Just more of a crunch/rhythm tone. The only way to get them close to clean is to max the output master and use the preamp vol as the main vol. Even then its not that spectacular.

[declan]

Didn't get to check out the 4x12 yet but I'll do that speaker test tomorrow night if I get a chance. If the speakers are going the opposite direction, does that mean the signal is out of phase?

On the clean channel- it's not even really a "clean" channel strictly speaking... i guess the intention was just for it to be less lethal. I figure with hi-fi EMG's and a light touch it should be relatively clean though.



Still no answer on the tubes! Could this be responsible for any of the problems?

[Enzo]

If the speakers move opposite directions from each other, it means the SPEAKERS are out of phase. The signal is just the signal, it can't be out of phase with itself. Speakers out of phase means one has its wires backwards.

Tubes would not be what comes to my mind for the symptom as described, but anything is possible, I guess.

[Mars Amp Repair]

Just to chime in...try lightly tapping the tubes with the handle a small screwdriver. Obviously don't tap them hard enough to break them, but you do need to tap them hard enough to elicit a reaction, if there is going to be one. If the issue only occurs when the amp has been running for a while, then try this cold & hot. Tubes can be very intermittent as well as temp sensitive, especially the tubes they make today.

Also, try pushing slightly on the tubes & SLIGHTLY rocking them in a circular motion. to determine if there is either a bad connection on the tube socket or possibly a loose socket pin. Don't get carried away with the rotating as YOU can be the one to loosen the pins & create more problems. The idea is to try to get a reaction if there is a problem there.

Listen for crackling or any obvious variance in volume or operation. Other than substitution, that should tell you whether or not the tubes are causing you're issues.

I agree with Enzo, don't try to think up reasons for NOT trying things. We all have spent hours looking for the problem elsewhere because we were sure it couldn't be right where it ended up being in the first place! glen

[gribnick]

How much use did the tubes get in the day(s) you were actively using the amp? Tubes get used up and go bad with use but not usually by sitting in a closet. I have some tubes over 40 yrs old that rip just fine!

Volume drops and thin sound have many possible causes but one that WILL occur by sitting in a closet is the main filter caps can go bad and can cause all of those symptoms you refer to. When an amp sits for awhile, the electrolytic caps (and I forget the exact temrinology here) de-generate so the electrolite in them goes back to its original state. They have to reform again when the amp eventually gets powered up again and they don't go through that cycle as gracefully when they are older since caps definitely have shelf life. I've had a 10 yr old amp with damn near exploded caps and one leaking crap all over the place (crate blue voodoo). Older ampegs also have had that issue a lot (I've recapped all 3 of them I've ever had). Many production amps have inexpensive caps in them..

Filter caps aren't usually difficult to change out but they are the main thing in the amp that will send you to the afterlife if you don't do it right since they hold several hundred volts for even many days at a time in many amps. While a visual inspection won't insure you the caps are good, taking a look at them (big sucker caps in the power supply usually near where the power plug is) may show some to be bulged on their tops and/or leaking which definitely means they are evil and must be replaced..

You are in Rochester (as in NY)? If yes, I'm guessing your resident amp guy is Jerry at FJAMODS? He is one of the best there is. His repairs are fairly reasonable as he can usually crack something like that out pretty quick with not much bench time. And being local, you'd save all the shipping charges I had. Expenses with him are usually in the mod areas where you can run up a good sized bill pretty quick. Good stuff all but it does cost. Send him a note and talk to him about your repairs .. you may be surprised what you can afford.. 5150s are definitely his specialty. If you are in MN or any other Rochester for that matter, same thing applies but you can add about $80-$100 for both way shipping (total) depending on where you live.

[sphreak]

i have the same problem

...with my 5150 head. after having the amp on for awhile, the bottom end just disappears & it sounds scratchy and thin. Did you ever get an answer as to what would be causing this? this happens on both channels, any volume....turn it off awhile and later its back to normal. ????help!

[gribnick]

Dropping/changing volume can have a lot of causes, including the caps but is also not infrequently a bad connection that is affected by heat -- as the amp warms up and things expand, the connection(s) get worse and lose contact. Besides closely checking solder joints (preferably with an illuminated magnifying glass), the best check for this sort of issue is "the chopstick test" .. invert the chassis and keep it inverted while it is running (and I will pause here to again say to do this RESPECTING ALL SAFETY RULES FOR HIGH VOLTAGE -- mistakes here are permanent). Using a chopstick (bare wood, no paint, we don't want anything conductive!!!), poke around in side the amp .. disturb the wires .. poke the molex connectors (I've found bad ones before!!) poke everything .. if you find the sound changing when you poke something you may have found your problem.

If that doesn't find something for you, I don't know that I can explain in a forum further steps to take. Perhaps others could .. I've also not seen the inside of a 5150 so cant really advise further specifics..

[ColinBecker]

I have the same problem!!!

MY 5150 has the same issue!!!! EXACTLY. I have replaced all the pre and power tubes. Checked the Cab all the cables several times. I need help with this aswell. I have been searching for an answer all week. HELP?