It sounds very similar to severe underbias in the output stage. Being a valve driven MOSFET output stage I'd guess there is very little negative feedback to elliminate crossover distortion. Most amps bias generator networks are wired so if the bias pot goes open circuit (usually the case from contamination of the tracks) the output stage will loose all bias. MOSFETS needing several volts of Vgs will suffer from severe crossover distortion as there will be a considerable amount of signal around the 0v crossing area on the output that will not be amplified. There are alot of distortion mechanisms that come into play in such an instance but let's not worry about them hey

You should be able to check across the source resistors with no signal applied and no speaker connected for a couple of MV of voltage. If you measure 0v then this is the case. If you can identify the gate connection then measure the voltage difference between the gates of two FETs from opposing power rails, if you measure 0v again it is definetly the bias generator that needs attention/ re-adjustment.

Thanks. I feel kind of weird saying this on a guitar forum but I never learned guitar  , drums was my thing. I do however know alot of peole who can play the skin off their fingertips who have been nagging at me to stop building PA gear and move over to musical instrument amplifiers. I love the sound of guitars and if any of my designs can help anyone else pluck, strum, thrash or wail away into the early hours then I am allways happy to help.

I must admit it is annoying having to wait until someone else plugs in to see what my creations sound like.

You should certainly get a very clean sound from a class A amp, of course the drawback being the limited power output. It really depends on the volume levels you wish to play clean at, if using SS then there's no reason why a well designed class B amp operated below clipping won't sound clean.

If you want a good easy to build Class A SS design then I have plenty kicking about, just add pre-amp of choice and away you go.

Hi there, got the itch to build a rack mount guitar head for use with a seperate pre-amp and this is the end result. Came across this forum and thought it worth while posting the end result and some schematics. About to start on the pre-amp but have spent so much time on the amp wanted to post some pics just cause I was pleased with the result so far.

Inside the case is a 300 Watt into 4 ohm (400 Watt at ~10% THD) triple EF output stage BJT (Bipolar junction transistor) amplifier with short circuit / overload protection. Fast latching DC protection that activates at less than +- 2v offset, active balanced line reciever signal input, mains soft start and fan controller.

The amp has two 325VA transformers each with a CT 70v secondary and a single 15v secondary. The main secondarys each goes to a BR then BR to caps allowing parallel operation without directly connecting the less than perfectly matched secondary voltages. One transformer 15v secondary goes to a full wave bridge rectifier then via an LM7815 15v reg then feeds the fan controller and soft start. The other 15v secondary on the other transformer goes to a voltage doubler dual 1/2 wave rectifier then via LM7815 LM7915 creates a dual 15v rail for the balanced line reciever and DC speaker protection.

Through carefull conection of the multiple 0v points the amp even with no attenuation network (volume controll) does not hum or buzz and can only be heared to hisssss pleasantly when an ear is pushed gently onto the speaker. Input is via XLR and output is via binding post or 4 pole Speakon. Case was a really badly built Skytec dodgy PA amp, don't have the fan controller schematic as I designed on the back of an envolope around components I had in the scavanged parts bin so as I can't remember what the values of those where can't re-draw it on the computer.

If anyone wishes to build the amp I am happy to give some pointers, am really looking forward to getting the hybrid transistor-valve pre amp built. Looking at profusions digital delay board (guitar optimised version) amongst other things to stick in there.






Main amplifier schematic


Essential DC protection


Active balanced line reciever


Soft start allows 1 amp of mains current for about 1 second then allows full current draw, prevents nuisance fuse blowing/circuit breaker tripping and protects caps and BR's from massive current inrush.

That's all for now folks, Look forward to your opinions, Cheers Liam.