Clipping in the lead channel, need answers:(

[sdre]

Hello all. Im working on a Peavey 5150 combo and am having issues with the lead channel. It clips and cuts out when post lead volume is up at three or higher. any ideas what may be causing this?

Thanks in advance

Jeremy

[Enzo]

clips AND cuts out? Can you be more specific - as I read it it sounds like the sound goes away and at the same time clips.

SO scope the signal on the top end of that control. When this happens, is the signal there stable, or does it cut out too?

Is the pot itself OK?

Go through the circuit and see where the interruption occurs - isolate the problem.

[sdre]

Well If I strum hard it acts as if its getting too much power and then chokes. If I let a palm mute ring out it sputters. sounds almost digital. I have a feeling its a relay switch but Im not quite sure.

Im not sure what you mean when you say scope the signal. Are you referring to an oscilloscope? If so, I do not have access to one.

As for the pot itself, i cleaned it with contact cleaner and the problem still remains.

The problem is only apparent when the lead post is at three or higher. anything below that it sounds amazing. no noise.

Thanks.

[Enzo]

Yes, oscilloscope.

Well then look for DC voltages in that circuit that are not right. A grid sitting at a wrong level, a cap charging up in the signal path.

Use your ohm meter and check the pot for proper action - remove it if you have to for this.

Isolate the problem. Send the signal from the effects send to another amp for a listen. Does it act the same coming out the other system? Likewise, take a preamp out from some other amp and feed it into the effects return on the 5150C. Does THAT work OK or does it still crap out on you? This will narrow it down top a preamp or a power amp problem.

It may seem like a lead channel only problem, but it could be the signal that hits the power amp varies enough to matter there.

[txstrat]

I once had had an amp acting like this too. It was the 470k resistor in front of the grid (to ground) of the specific triode. It was open (I had that on a reverb circuit cathode resistor too -why me).
I changed the resistor and the amp worked fine.
Are the voltages on the plate and cathode within specs? I found that they were much higher with no grid resistor. Although that would be the same like Enzos hint - grid at wrong level.

Hope this helps

Matt

[gbono]

Are you overdriving the input with the guitar(??) that you are using to diagnose/cause the problem? Have you tried changing the level on the guitar?

[sdre]

I once had had an amp acting like this too. It was the 470k resistor in front of the grid (to ground) of the specific triode. It was open (I had that on a reverb circuit cathode resistor too -why me).
I changed the resistor and the amp worked fine.
Are the voltages on the plate and cathode within specs? I found that they were much higher with no grid resistor. Although that would be the same like Enzos hint - grid at wrong level.

Hope this helps

Matt

The plate voltage is reading around 498 and 508 And when you mention cathode readings do you mean in MA? if so its between 37 and 47 MA.

I will check that resistor but not to sound dumb but whats "grid to ground" Im still learning about amps and their terminology.

[txstrat]

I had two triode systems fed after the same B+ resistor. One plate measured 155v that was ok, the other measured 256v. The cathode had a DC voltage of 1.5v in the system that was working. The other measured 3.6v. That made the voltages about double for the not ok triode. After I fixed the grid resistor both triodes measured about the same. With grid to ground I mean the resistor that is applied to the grid with the other end going to ground.

Hope this helps

Matt

[tubeswell]

whats "grid to ground" Im still learning about amps and their terminology.

Grid leak resistor

See this page for a quick guide on triode terminology

http://www.diystompboxes.com/pedals/tubedummy.html

[HRocks]

hey guys. I am having the same problem right now, but with an original 5150 head. If the post gain goes past 3, if I give it a lot of "power" (a big hit, big palm mute) it dies and sounds like a flubby electronic sound.

I took it to a place to have it looked at, and they just called me back saying they couldn't find anything wrong with it, but a full service should do the trick. They said my preamp and power amp tubes are bad.

The power amp tubes I just replaced about 3 weeks ago, and the preamp tubes came with the head. I bought it used about 3 months ago, and it was fine until a few weeks ago. Previous owner claims the preamp tubes were only 2 months old.

So needless to say, I am pretty confused and I need to call the guy back. What should I say? Could this be the problem? He said the amp had some wear on it, and cleaning all of the contacts should fix it. I cant really drop $300 into this right now and have it not be fixed.