5150II Noises (and lack thereof)...

[senorpapacabasa]

I know, you're thinking "Oh great, another 5150 problem"....

I need help diagnosing a problem with my 5150 II. For a while, i've noticed a hum in my lead channel, but recently, it has gotten alot louder. A "hum" really isn't an accurate description. Its more of a rumble, like a jet is flying by overhead. About the same time as this "hum" got really loud, I noticed a drop in volume of my lead channel. Low volumes are the same, but when I want to try to play with a full band I have to crank it up to 5 or 7, I used to be able to play on 4 with a full band.

The rhythm channel is working fine, no noises, no volume problems. Its only on the lead channel. Does this rule out power tubes as a possible problem ?(I hope so, I just replaced the power tubes a few months ago)


I tried easter egging with a brand new preamp tube. When I replaced the v2 preamp tube, I the rumbling was still there, but I noticed a slight improvement. But the volume still wouldn't crank way up. I was told to check the screen grid resistors. After looking at a schematic, I assume these are the resistors just outside of the tube screens... they are labeled R203, 205, 207, 209 with a value of 100W5 (100 ohm, 5w?). Any other suggestions are much appreciated!

[Valvehead]

I know, you're thinking "Oh great, another 5150 problem"....

I need help diagnosing a problem with my 5150 II. For a while, i've noticed a hum in my lead channel, but recently, it has gotten alot louder. A "hum" really isn't an accurate description. Its more of a rumble, like a jet is flying by overhead. About the same time as this "hum" got really loud, I noticed a drop in volume of my lead channel. Low volumes are the same, but when I want to try to play with a full band I have to crank it up to 5 or 7, I used to be able to play on 4 with a full band.

The rhythm channel is working fine, no noises, no volume problems. Its only on the lead channel. Does this rule out power tubes as a possible problem ?(I hope so, I just replaced the power tubes a few months ago)


I tried easter egging with a brand new preamp tube. When I replaced the v2 preamp tube, I the rumbling was still there, but I noticed a slight improvement. But the volume still wouldn't crank way up. I was told to check the screen grid resistors. After looking at a schematic, I assume these are the resistors just outside of the tube screens... they are labeled R203, 205, 207, 209 with a value of 100W5 (100 ohm, 5w?). Any other suggestions are much appreciated!

Is the noise like a real low frequency? Try pulling out V4 and see if it goes away.

those screen resistors are on the bottom of the power tube board. you can see them if you look real close--they are about an inch long, white rectangular. you can test them from the top.
To replace them, you have to drill out the rivets and remove the power tube board. Why Peavey mounted it this way is beyond me....

[senorpapacabasa]

I wouldn't say its super low, but it is a low frequency. I will try that out, thanks.

Well that sounds like fun, in that case I hope that isn't as issue!

[Amp Kat]

I would be willing to, bet it's the power tubes.

[Enzo]

If the rhythm channel works fine no noises, etc, then the power amp is not involved. The screen resistors included.

I would be looking for a noisy or way off value resistor in the lead channel, since you already swapped tubes around.

[Amp Kat]

If the rhythm channel works fine no noises, etc, then the power amp is not involved. The screen resistors included.

I would be looking for a noisy or way off value resistor in the lead channel, since you already swapped tubes around.

He swapped the preamp tubes around but not the power tubes. He replaced those several months ago. I misread the part about the clean being unaffected. I could have sworn when I first read it he said that both channels did it. So now we need to find out if the send out works ok and if another amps preamp into the return works ok.

[senorpapacabasa]

So patch the send from my effects loop into another amp's return loop?

I will try another amps preamp out into my return also. Thanks guys!

[senorpapacabasa]

Besides tubes, what are some other likely suspects to cause a lack of volume? Like I said earlier, the volume level on the rhythm channel is full, but the lead channel does not get as loud as it used to.

[senorpapacabasa]

Is it common for preamp tubes to go but, but not be microphonic? I tapped them all several times and I'm not getting any strange noises or pops out of the speakers, can I assume that they are not microphonic??

Last night, I opened up the chassis to take a look around, not only because I am having problems, but also because I haven't checked it out or had a tech check it out since I bought it (used) last year. Fuses were all good, no leaky caps, solder joints looked good (the ones that I could see), what else should I check out??


The hum isn't really whats bothering me, its more of the lack of tone and volume (still on in the lead channel though). If no one has any ideas or suggestions, I guess its off to eurotubes to order some preamp tubes... I'll cross my fingers and hope that fixes it!