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Old Peavey Classic Hissing

7/14/2005 7:07 PM
Jack Collins
Old Peavey Classic Hissing
I've got an old Peavey Classic on my bench.  
212 Solid State Preamp all the way to the power tubes. 2 channels brite and normal. Reverb, Trem.  
When the MV is turned up all the way it hisses  
pretty loudly. Even if the channel vols are all  
the way down.  
None of the other controls effect the hiss. I tried to work backwards from the MV and one of the  
first things I got to was a SPF558 which seems  
to be an n channel fet. I tried swapping it  
with a MPF102. The swap worked but it still hisses.  
I don't have a schematic for this. I know I can  
get one from Peavey.  
I tried to trace the hiss backwards with my scope  
but quickly became confused as to what connected  
to what.  
Anything in particular I should be looking for?
7/14/2005 7:25 PM

Is it the Classic, the Classic VT or the CLassic VTX? Each is different. The original CLassic has transistor preamp, the others use op amps.  
SPF558 can sub a J231 or a 2N5951. Watch out on FET subs since the pinout differs all over the map. Most any relatively low noise 40v N channel FET.  
You probably do have a noisy semi somewhere. But PV is back from summer vacation, so call them for the schemo. 601-483-5365 and go to parts or email
7/14/2005 8:03 PM
Jack Collins

It's the classic.  
I'll call PV.  
No op-amps. All transisitors.
7/15/2005 12:26 AM

Well, the circuit is simple enough then. The tone stack output feeds directly into the gate of the FET. The output from the FET then through a cap to the master vol pot. If the EQs have no effect, that pretty much limits where it can be, anda if the master turns down the hisss, that limits it further.  
The trem output also hits the gate of the FET through a cap. If the trem output FET is noisy it could inject there, but turning the trem depth to zero should eliminate that.  
The output of the reverb circuit mixes into the wiper of hte master vol pot. But again, if the reverb were the noise source, turning the reverb control to zero should kill it.  
Try a different N-FET. SOurce and drain don't care, but make sure to identify the gate lead.  
When you call PV, ask for the master semiconductor cross reference listing too. Always helpful with their numbers.
7/18/2005 5:01 PM
Jack Collins
Got the schematic/It's not the N-FET
Peavey emailed the schematic. I can see what you're saying. I pulled the N-FET out. It still  
hisses when I turn up the MV.  
Turning the trem and reverb all the way down does not decrease hiss if MV is dimed.  
What else should I look for?  
Could the hiss be coming from the ?4278 transistor  
whose base is fe by the MV through a cap?
7/18/2005 6:39 PM

If pulling the JFET does nothing, then yes, the next thing I would consider would be that 64278 driver. The MV goes right to its base via that cap. (Maybe the cap is noisy? Is it one of those white body Mallorys?) Turning down the MV grounds the base.
7/19/2005 10:16 AM
Jack Collins

Thanks Enzo.  
I asked Peavey for a copy of the semiconductor cross ref and the nice lady I talked to didn't know what I was talking about. Do you know what  
would sub for the 64278?

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