Hybrid Guitar Power Amp 45W

[mensur]

Hi guys, I was designing guitar amp for my self, and I need some advice from more experienced guys.
So here is the schematic, please tell me if something wrong or need to be changed?
Here are the questions:
1.Can somebody give me formula for calculating bias current of
MOSFET-s
2.Where to measure it,
3.Could somebody calculate me bias current on the MOSFET's?

Thanks

Hybrid 45W.pdf

[Steve Conner]

Hi,

1. There is no fixed answer, it depends on temperature, variations between MOSFET batches etc. If you're actually asking "What should I set the bias current to" then the answer is usually about 100mA or so. It doesn't really matter, as MOSFETs have lousy harmonic distortion at all bias levels, unlike BJTs which have a sweet spot of minimum distortion.

2. Measure the voltage across R9 or R10, and use Ohm's Law to calculate the current.

3. No, see 1. You have to adjust it to the proper value once the amp is built, and check that it stays stable as the amp heats up.

Also, I'm not sure an ECC83 cathode follower will give enough current to drive the MOSFETs fully at high frequencies, and I think using a MOSFET for the thermal sensor may be a bad idea. A BJT should work fine, especially if you're using lateral power MOSFETs. If you're using VMOS, then see Rod Elliot's article on the subject:

http://sound.westhost.com/articles/hexfet.htm

[octal]

MOSFETs do not really need thermal compensation per se as unlike BJTs, they have a negative temp coefficient. I agree with Steve that a BJT would be a better idea for the bias setting device.

[Robert M. Martinelli]

So here is the schematic, please tell me if something wrong or need to be changed?

Hi,
maybe it's only a typo, the schematic is missing a node between R9-R10 ( 2SK/2SJ source resistors ) and the output line IMHO.

I'd try an ECC82/12AU7/5814 as a CF.

HTH

Best regards

Bob

[mensur]

Here's the new scheme:
Hybrid 100W
But I'm worried that R15/R16(220K/10K),R14/R13/Offset are correct values for the VBE multiplier?As I understood R15/R16 and VBE multiplier form voltage divider that will drive the mosfets to the on state and set bias?
(230K / 10K{VBE}=1.25V for driving 4 MOSFET's that need about 8V/per side for the on state,I think it should be 27K/10K{VBE}=about 8V/per side).
Or am I wrong?And what should be the value of bootstrap current source cap, after it's scaled?
I know ECC83 is not good drive for HiFi, but for guitar amp some irregularities in drive can be very musical,also guitar amps doesn't do anything above 6KHz.

Thanks.