High performance VVR for cathode & fixed bias BIG Amps - Enjoy

[kagliostro]

This is suitable also for big Amps than 50W

the in series 100k resistors are 1/4W

the 47k trimmer is to be set with the voltage set at his max

Enjoy

K

[HotBluePlates]

It's easier to have a 2nd MOSFET to share the dissipation.

Sorry, I don't have a schematic for it; Kevin OConnor showed it in several of his books where he talked about power scaling.

[kagliostro]

But put two Mosfet in parallel hasn't the same drawbacks of putting two diodes in parallel ?

The principal characteristic of the circuit I posted is to have automatic bias adjust for each output level

K

[HotBluePlates]

They wouldn't be in parallel; the MOSFETs would be in a cascode arrangement.

Either way, the idea is when the amplifier power goes up, add a second MOSFET device to share the power dissipation, so each device dissipates less heat.

[PRR]

Still, the special feature of the plan kagliostro has shown is the semi-self-compensating "fixed" negative bias.

IIRC KoK's plan uses a 2-gang pot to control both B+ and C-?

This one uses an op-amp (Q1 Q2 Q3) to invert and scale the voltage on G2. P1 does all in-use changes. (P2 is set-up, not used on stage.)

A parallel or series pair can easily be added to this plan, just like any other. For parallel operation, duplicate R6 also, to urge current sharing.

[HotBluePlates]

Still, the special feature of the plan kagliostro has shown is the semi-self-compensating "fixed" negative bias.

IIRC KoK's plan uses a 2-gang pot to control both B+ and C-?

No.. going from memory, KOC derives a separate bias supply then has the output of the power scaled B+ coupled through some transistors to a portion of the bias supply. The end result is that as B+ gets less-positive, the C- gets less-negative.

I don't remember the specifics, as it has been maybe 6-7 years since I looked at that circuit.

But KOC might have drawn out simpler plans in some of his books, when he presented the nuts n bolts of powerscaling, but was still generally trying to protect his final circuit topology.

[kagliostro]

I've tried to figure how would be a circuit with a doubled mosfet as to increase dissipation

but I'm not sure about it, particularly about the presence of D2 and R6

K

 

[PRR]

.

[kagliostro]

Thanks PRR

I follow you only partially

this way the voltage drop on the firs Mosfet is fixed or variable ?

I sense it will be variable, but I'm not sure about

Is correct to maintain R6 or with in series mosfet it isn't really necessary ?

K

[HotBluePlates]

The same current has to go through both mosfets. But properly arranged, each drops only half of the voltage difference between rectified a.c. and Vout. So each gets half the total power dissipated as a result of load current * voltage drop.