Help on Nemesis amp

[RichardJones]

On the schematic they have it looks like a bias point of 2.45volts. The current going to the OPT is 0.8 amps.

Looking at old datasheets for the 2SK135 this cannot be possible.

What is the actual bias and quiscent current for the amp???

Thanks.

p.s. I want to use the BUZ900P series instead.... what should I change?

 

[RichardJones]

Datasheet

The datasheet is at:

http://www.bonavolta.ch/hobby/images/audio/2sk135j50.gif

Thanks!!!

 

[djk]

If you just look at the spec sheet the bias looks high.

But the RDS goes up from 1R0 to 1R7 over temperaure.

This is enough to require a higher gate voltage to keep the current in the ball park.

Running 30W through a 100W part is smokin' hot, I'll let you calculate the junction temperature.

 

[RichardJones]

Its ok?

So the 2.4v bias will work then??? Looking at the spec sheet this looks way over 0.8A.

Is there something I am missing?

Oh and I have run 125W MOSFETS at 40W diss each with NO problem.

Anyone else have any suggestions?

 

[RichardJones]

No more suggestions???

Come on.... there has to be someone else out there with suggestions about this circuit.

Planning on using 68,000uf of capacitance (or maybe 136Kuf) and battery supply.

Have some coupling caps... use Riken Ohm wherever I can.

EP OPT.

etc....

Can anyone explain the bias issue yet???

Thanks!

 

[djk]

You could make it with 'gasp', adjustable bias.

This is a 'who cares' thing.

Obsolete, expensive transistor. Hard-to-find, very expensive output transformer.

 

[RichardJones]

hmm....

putting your smart *** comments aside....

I probably could do adjustable bias. The output tranny is not expensive... not as much as a good tube one.

The MOSFET can be easily replaced with a BUZ900 with just a small adjustment in bias.

Thanks anyway.

 

[RichJones]

question

what is the purpose behind the resistor and cap across the primary of the opt? and the purpose of the resistor and cap from drain to gate?

Thanks!

 

[Tube_Dude]

what is the purpose behind the resistor and cap across the primary of the opt?

Is a zobel network...for limiting the high frequencyes and making the primary of the tranformer look less inductif to the mos- fet.

and the purpose of the resistor and cap from drain to gate?

Is to introduce some AC feedback and limit the distortion...that even so will be high...

 

[RichJones]

values?

how do I determine the values??

 

[AudioGeek]

question on 1M feedback resistor.

In the design there is a RC combo using a 1M resistor and a cap to the gate.

How does this local feedback affect biasing??

Also, If I were to take out the cap input and just have a 2:1 input transformer.. how would I bias the mosfet and would I adjust the 1M resistor??

Haven't decided on what mosfet to use yet... something with low gate capacitance.

Thanks!!!