Quick backstory... I've been kicking around the idea of building a clone kit of either a Plexi or JCM 800. I figured I'd start with a few pedal clones to get my soldering down and gathered all the necessary equipment. Last night I ran across a deal on craigslist I couldn't pass up. Today I bought a Peavey 5150 combo for 100 bucks. The guy selling told me it had a crackle to it, so I figured it has a bad component or two, or maybe even just need the tubes reseated. The amp is covered in thick dust and frankly, I need to clean it before bringing it in the house. I tried it out in the garage when I got home and was pleased to find no crackle sound whatsoever. The rhythm channel works just fine. The lead channel however seems like it ignores the post volume control. It's as if the post volume is dimed no matter the position of the knob. When I get it cleaned up, I'll probably record a couple of clips.





I'm currently searching through any google  pages I can find on possible causes. If anyone has had a similar experience, let me know how it was resolved. Overall I'm feeling good about my chances of either fixing it, or finding someone who can. The speakers are the stock Sheffield 1200s. The power tubes have been replaced with Groove Tube GT-6L6-GE and I'm assuming the preamp tubes are stock since they are unmarked. I'm fairly new to tube amps so I'm researching which tubes I'd like to replace them with. I'll probably update with pics as I tear it down and update on progress. I'm pretty excited since I've always wanted a 5150 since they came out when I was in high school.





Also, is the reverb control supposed to work with both channels, or just the lead, since I only notice a difference on the jacked up lead channel? I'll update as I come up with more questions.



 

I used to have a 5150 combo, but never had any problems like that.
I'm guessing you may have a dirty pot, a fried pot, or some other short that is causing the offending pot to be bypassed somehow.
There's other issues that are common with several Peaveys, including your's.
- Check the Grid Resistors. Peavey had a bad habit of using 5W 100 ohm resistors, which are inadequate for 6L6s. 5W 470 ohm are what you need for those 6L6s.
- Peavey also had a habit of skimping on the Rectifier Diodes, they used 1n4003s, which like the Grid Resistors are inadequate. 1n4007s are more like it.
- Peavey also designed your amp with a fixed bias. IIRC it's set at about 14ma per tube, which is very cold, regardless how many volts are running across the plates. This is more of a preference thing though. Here's Weber's Bias Calculator page, it comes in handy when figuring out where your boas should be in relation to plate voltage. http://www.webervst.com/tubes1/calcbias.htm
I recently swapped out the Grid Resistors and did a couple mods, including a bias mod, in my long dead VTM120. It got it working for awhile but quit on me again. I still need to do the Rectifier Diodes (probably why it quit on me again), and need to try some different resistor values in the bias circuit.
If you do a Google search for "taming the peavey nasties" there are a couple threads on other boards that go into greater detail.
As far as tubes go, I like JJ 6L6s. In the preamp, I like to start with a Tung-Sol 12AX7 in V1, a JJ 12AX7 in the Phase Inverter position, and then I experiment with the other tubes. The Tung-Sol is a very lively and articulate tube, so I love it for the input stage, and the JJ is a great PI tube because it gives the amp a little extra grunt & growl that others don't. Another one that's worth mentioning is the Mullard Reissue 12AX7. I have a couple that I tried in my amos, and they sound good, but were too dark for my tastes. But one of them is now in my Radial Plexitube pedal and turned to be just what it needed, because it took alot of the fizz out of the pedal's tone.
Oh, and unless you like a thinner tone, don't bother with Electro Harmonix 12AX7s. I have 2 or 3 laying around that I tried that fond their way back into their boxes.



 

Cool, that gives me a starting point. I'm going to clean it up and bring it in the house tomorrow to start pulling it apart. Hoping the offending component(s) will be obvious. All the tubes look fine, glow evenly, no burn spots. If nothing else, I have half a 5150 for 100 bucks.

Killer deal,  good luck getting it 100%.

Cool.

 

Having any luck with that 5150?

To be honest, I haven't had a day off since I got it to really look at it. I may drop it off with a local amp repairman who says he's worked on a few before. If I have time Friday I'm going spray some contact cleaner in the pots and swap the tubes with some known good tubes to see how it acts. If some light cleaning doesn't do the trick, the overtime I got this week will more than pay for any repairs needed.I did pull the amp out of the cab and aside from the cobwebs and dust, it really didn't look bad on the inside. No burn marks or visual signs of anything burning up or swollen caps or anything like that. Tomorrow I'll have a little time at work to research how to test a pot. I'm guessing I'll be looking for a certain value with a multimeter, but I'm going in as a total noob. I will for sure assume power is hot and make sure the amp is discharged. One of my hurdles to overcome is electricity. I prefer something I can flip a breaker and know isn't hot anymore.

I don't have any insight beyond commenting that $100 for a 5150 2x12 is a straight-up amazing deal. Even if all electronics were bricked, the loaded cab alone would be worth the price. My combo was $300ish on craigslist and filthy, and I thought that was a bargain. On the rare times I'm actually playing guitar, it spends half of that as an extension cab for one of my Mesas.

Peavey still makes the same thing in the 6505, so it should be possible to replace everything up to and including the printed circuit boards. Unlike a ratty old 1980s Mesa 50+, which would probably push past $500 for the much needed million mile tune-up and repairs with obsolete parts. I haven't wielded a soldering iron near high voltage for many years, so my amp repair skills are currently "Visa or MC, credit or debit?"

If time permits this week, I'll monkey around with mine and see how the reverb behaves. When you get it working, priority #1 is to get a good noise suppressor/gate in there. Mine must be about the noisiest amp I've ever owned. I don't say that lightly - I'm not coming from a Jazz Chorus standpoint. My past and current valve amps have included fairly gainy things like a JCM900 SL-X head, 100W stereo tube rack setup that was sustaining feedback heaven, Mesa 50 Caliber+, and Mesa Mk5. An ISP Decimator G String tames the 5150 nicely.

I'll have to admit I chickened out on messing with the amp myself and brought it in for repair. A previous owner had clipped the ground prong off the power cable so I had that replaced with the computer style power cable. The guy said he traced the channel problem to a bad cap and once replaced it worked perfectly. Also gave the insides a good cleaning. I pick up the amp on Saturday and am anxious to hear the lead channel. Part of me regrets not trying the repair myself since it was a single bad component, but another part of me feels like I'd still be trying to narrow it down and generally being lost. Maybe once I learn a little more about the guts and what/how/and why they work I may try with another fixer upper. I'll also keep that ISP suppressor in mind.

Nothing I can interject except

I had a Peavey Classic 212 that thing blew me out of my house. I sold it after keeping it a few years, I got it for 80 dollars sold it for 250. I knida wish i still had it it was awesome.