Peavey MKVI Bass

[stokes]

Looking for a schem for a Peavey MKVI bass amp.Can anybody point me to one?Or is there another model that might be close enough?

[Enzo]

Just contact customer service at Peavey.

[stokes]

Thanks Enzo,I had a feeling that would be the way I would have to go.I checked all the schem sources I have and cant find anything.As I've said before I usually try to avoid solid state amps,but this is for a very good friend of mine,so I guess its time I get into the 20th century.Sounds to me like it is a blown output transistor,but if not I am sure you will be hearing from me as you seem to have the most experience with Peavey.I dont have the amp yet,but he describes very low volume and severe distortion when he really hits the strings hard.I'll post back in the appropriate section if I have problems.

[Enzo]

There is no great body of online Peavey schematics because there is no need for it, they'll send anything they have.

I'd look for power supply and preamp problems. Also, look at the path between preamp and power amp. Wherever there is a jack to the real world, PV likes to protect the signal path with clamping diodes to the +/-15v rails. A shorted diode puts 15v DC on the signal path the signal has to fight. For that matter a slammed over op amp makes a tremendous hurdle for a signal as well.

[stokes]

Thanks for the pointers Enzo.While I have your attention,does this amp,if you know,have any kind of input protection?He has been using a simple tube preamp I built for him,not for overdrive,but just to give a little warmth to the solid states "coldness".It doesnt put out a very big signal,but it does have a level control that when dimed will put out about 1-2 vac.I warned him not to turn it up past a mark I put on it,but I am wondering if he did.I assume that if he blew the input,he would get no sound at all,but like I say,I dont know a hell of a lot about solid state.I have heard that some solid state units have no input protection and could be damaged by too big a signal in the input.He has been using the preamp about 3 years with no problems,but i have to wonder if it could be the cause.I dont actually have the amp here yet and am just thinking ahead.

[Enzo]

DOubtful. It is like most other PV SS amps. 1-2v is not going to kill it. The input jack pair is the standard hi/lo pair. There is a .1uf DC blocker, and at the input of the first op amp is the pair of clamping diodes I referred to. They are reverse biased to the 16v rails. SO if the input signal EXCEEDS 16v towards either polarity, the diodes become forward biased and shunt the over voltage into the low impedance of the power supply.

SOlid state amps are usually only threatened if the signal exceeds their power supply voltage. Otherwise they just overdrive like anything else.

Op amps are not happy when their inputs are at greater voltages than their supply rails. From the ICs point of view it is the equivalent of a reverse power application. SO PV adds these diodes - which are totally invisible to the signal - wherever the real world can get access to the circuit. ANy signal input jack, the footswitch jack, and also line outs. If a transient got on an output line it could reach back and affect the driving IC potentially. Look around the board for pairs of little 1N4148s.

On some older models like the ever present XR600, those diodes and ICs were wired right to the jacks, so you could stick a cord in the jack and measure for DC at the plug as a test for a shorted diode without opening the amp. They are wiser here, the diodes are all inboard of DC blocking caps, so all diode testing must be done inside. But you already have it open anyway.

Failed output devices usually cause DC trouble, rather than low output.

Isolate the problem as you would with tubes. You have a power amp in jack, you have preamp out jacks. If it is the preamp, the amp is easily traced.

[stokes]

Thanks again for the pointers,Enzo.I am waiting to hear back from customer service on the schem and for my friend to get the head to me.I'll post back when I get into it.