Lateral/Vertical mosfet conversion

[DrGr33n]

Hi. I was hoping someone could anyone tell me if this circuit is suitable to convert from Vertical to Lateral mosfets and what changes would need to be made.

I bought it secondhand and was told it wasn't running stable. I had intentions of using the case and transformer for another project. Turned out neither case or transformer were up to scratch, so I thought I'd attempt to get it running.

Seems the previous owner tried to replace the original Vertical output transistors with non pin compatible Laterals and made a mess re-routing the output transistors with wire.

So far I have recapped the amp, changed resistors to .5w 1%, new multi-turn pots, changed the 0.47ohm wirewounds to 0.22ohm low inductance type, new zener diodes and tidied up the birds nest.

It now appears to run and sound okay. But I have a feeling it may need additional changes to optimize for 2sk135, 2sj50.

Any advice would be great.

Cheers

[tommy1000]

Have a look at Ostrippers post 6 in this thread instead.
Audio (Lateral) MOSFET's for Class AB - Overview, P-Spice URL's

[nigelwright7557]

Unless your getting oscillation or poor bandwidth I wouldnt change anything.

[DrGr33n]

Thanks for the advice so far.

I wasn't exactly sure how to set/check the bias so have just set it by having 20mv across the 0.22ohm resistors, 2sk135 are 20mv but the 2sj50 are 18mv. Is that because of higher input capacitance? Is it okay/safe to leave at that? Would changing the gate resistor values fix it?

Hope somebody can make sense of that. I'm still rather new to this.

Cheers

[Mooly]

Interesting...

Firstly, lateral FET's do not need thermal compensation like verticals do. So the bias generator which is everything around TR6 isn't strictly necessary and certainly mustn't be in thermal contact with the outputs (which it would be with the verticals). Laterals have a negative temperature coefficient, verticals have positive.

100ma per pair is about right for laterals, so thats 22 mv across each 0.22 ohm. No load on the amp when you measure this by the way.
Reasons for the variation will be because the output (which should be at zero volts) is feeding current into/out of those 2k2 resistors and the other ends of those wil be at "non equal" voltages. That current adds to the FET bias current but is non symetrical. Also tolerance of 0.22 ohms.
The differing gate capacitance of the FET's plays no part at DC. Only at extreme high frequencies is it of relevance. Gate resistors have no effect at DC, even if they were as high as 10meg ohm.

[DrGr33n]

Thanks for the insight Mooly.

It got me thinking, since this circuit isn't designed for laterals. I might buy some suitable verticals to put in this and use the 2sj/2sk for another project.

I seem to recall seeing a design of yours with laterals and tl071 around here. Behaves well and sounds very good from what I remember.

Cheers

[Mooly]

Hi,
You recall correctly,
My MOSFET amplifier designed for music.

It does indeed behave well and sounds very good... if you read all the thread some have commented that it sounds like a very good valve amp.