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Vintage SS Peavey Amp Troubleshooting


MegaFuzzBass

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I've got a '70s Peavey solid state Series 400 head and something funky is going on with the output stage. It sounds unnaturally fuzzy through speakers... the preamp out sounds just fine. I've been able to isolate the problem to the power module panel on the back... swapping it between my other series 400 confirmed that. It seems the louder I play, the clearer it sounds, and when I play softer, it sounds fuzzier. Almost like one of those boutique fuzz pedals with a voltage starve or bias knob... Which leads me to suspect an issue with transistor biasing? No parts on the board seemed fried or broken just by looking, but I noticed some trimpots that might need adjusting? Also possible an output transistor might be shot i guess. I had a very reluctant amp tech say it might be an issue with an output transistor, and that it wasn't worth fixing. Some other folks on a diff forum said it might be the capacitors. What should I do? And please, don't say "just buy another one, they're only $200"

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  • 2 weeks later...
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Well, people are generally reluctant to help people with DIY amp repairs, but having said that, you're on the right track. Your problem DOES sound like 'crossover distortion' and yes, it's caused by mis-biased output devices (transistors in this case). I bought a 200 watt Crate bass head with this problem, and yes I was able to fix it with the turn of a trimpot...however it's critical that you have a multi-meter when making this adjustment, or you could damage your amp in a major way.

 

Right here is where we'd normally cut-and-pasted the ADMONISHMENT about non-professionals working inside a powered-up amplifier. Although not quite as dangerous as a tube amp, SS amps still have line voltage and often high DC voltages of 60 or 80 volts that can BITE YOU, burn you or even kill you. Be safe and read up on safe procedures if you're not already familiar with them.....

 

Get a schematic diagram for your exact model of amp. There's usually handy test point voltages called out on the schematic.

 

You'll notice looking at the schematic for the power amp that it's symetrical - the circuit is "mirror imaged". One side amplifies the "+" signal, the other side amplifies the "-" side of the signal. This keeps the power transistors "off" for nearly half the cycle, which keeps them cooler and helps them last longer. However, the trick to biasing is we want the transistors to be "a little bit ON" and not completely off. Crossover distortion is what you hearing when one side's transistor suddenly switches on, it makes the signal have a little jump that you hear as crossover distortion.

 

...to be continued.

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Those are nice heads when running right but they are old school designs which shouldn't be run when they develop issues because they don't have any safeguards. I had the same bass amp and mine blew its power transistors and voltage regulator sending 120V AC to the speakers and blowing my prized Altec Speakers. The power caps used in the power section are super low budget and given the age of that head should have been replaced by now, otherwise you're running on borrowed time.

 

The fuzz sound you're having is nearly identical to an issue I had on a black faced Peavey head from the same series and has the same design with the preamp on the front panel and the power amp on the back panel. The problem was common to all channels which saved time narrowing it down. The distortion was being caused by a bad coupling cap which was passing DC between stages. I still have that head but I don't run it because I know the power caps are leaking and I know I'd simply blow it up plugging it in.

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  • 1 year later...
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Hello , Im Hopng someone has some advise... 

Im using a peavey mega bass head and  peavey 15/2x10 cab. For years i thouht i was not patching it properly to the cab when in bi amp mode as  it always sounded distorted. iIusually just run the full range.

 

As of recent i have been trying to play a rickenbacker through the rickosound. and play throughtwo amps... Im not a tech.. and was told recently that the mega bass head had two amps, So.. i have tried over the last week or so to tri an biamp the peavey cab, and still all i hear is a the two 10s when i turn the low volum up , it does not work. i have tried moving cables to every postion and still no luck. i have been testing it on a Fender. as well as two other bases and no difference.

 

Has anyone had this problem.

 

thanks

 

Paul

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Is this the head? 

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If so this is a mono head.  There is no biamping of any kind.  mh2.JPG.9e139c4e29b1e9ecdf5495cf1bc9e0ff.JPGAs you can see on the back it says speaker outs with a single box saying 4 ohm minimum.  There is nothing there to indicate it has a stereo or dual power amp output. 

The DI output is NOT a speaker output.  Its a line level output for recording of connecting it to a mixer or the input of another power amp.  If you're trying to connect that to the speaker youd be shorting the head out.  I'm surprised you didn't blow the amp up completely.  

Whenever you're in doubt about things like this you should always download the manual and read it before you start messing. You may actually save yourself from blowing the amp up. These kinds of miniature  class D bass amps pack allot of power for their size but they are incredibly complex.  Your average amp techs are often clueless on how they work no less know how to repair them.  Once they blow its cheaper to buy a new one then it is to repair one.  I have one of those Ampeg Portaflex class D heads.  I try not to run it past half power. When run hard they overheat and shut down till they cool off. 

 

If you have one of the older Mark Bass heads like the Mark  8 it does have a crossover EQ and line outputs which let you drive a second amp.  The ones I saw did not have two built in power amps so its not much different then the amp above besides having extra line outputs which are connected to an EQ/crossover circuit.  With those heads, your set the built in amp to produce bass tones then whatever power amp and speaker you connect to the outputs you can set for mids and highs.  The connectors on the front are line level outputs NOT speaker outputs.  If you connect them to a speaker its like putting an 8 ohm resistor over the preamp output..  You virtually short out the preamp when you do that and feeding a speaker signal back into the preamp creates a feedback loop.  Very,  Very bad.  You need an additional power amp and speaker cab to use the crossover. 

If you had two of these amps, one with say 15" speaker cab and another with 10's and a horn, you could then use the high output to feed the power amp input on the second amp which bypasses its preamp.  The one head is then the master and the second is the slave.  This way one set of controls runs both heads and you can adjust the tone of the second amp with the crossover frequency and balance the level with the balance knob. 

mark8.JPG.95dc85d327fc959812d4332939bd683e.JPG

 

This one also has a stereo chorus output which can feed another amp.  looks like the extra output is high frequency so you could either drive a guitar amp or PA mixer direct if you wanted.  you just have to be careful with the levels not to overdrive the second amp.   

If you know specifically which mark head it is I can get you more specific details.  There should be manuals on any of these even though they are old, someone likely posted them.  if not most of it is the same with most amps and pretty easy to figure out

Peavey does make some pro gear that has dual power amps for bi-amping, but I don't thing the old blackface do.  Tahy may have made a separate power amp as an accessory but I've never seen a dual power amp in the old ones.  My buddy has an equipment endorsement with peavey so I get to see allot of their top end gear.   He uses one of their bass heads that has two power amps built in.  He has his cab wired for it where one power amp drives a15" woofer and the other drives 2X10's and a horn.  When both outputs are used the crossover separates the frequencies,  and drives the lows to the 15 and highs to the others using separate power amps.  When only the one speaker jack connected, both power amps drive the same cab full frequency and the crossover is disconnected. 

Like I said, that's a very expensive pro model that's about 1200 watts and very expensive.  Those old blackface are maybe 250W mono heads. 

 

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