F5 Turbo Builders Thread

[nikwatt]

F5 v_1

[nikwatt]

scheme v1 from guide

[cubicincher]

Hello Nicola,


you use a 600 VA toroid with secondaries of 2x25V~.


After the bridgerectifier and capbank you will 'land' anywhere around +-32,5 V?
(25 V ~ x 1,3 = 32,5V - rule of thumb/ could be a bit higher?).


The voltage rating at 35V of your caps (15000uF / 35V) seems a bit low for me.
Not a lot of headroom! In other words - close to none. I would have used 50V
types minmum to have a safety margin.


Greets
Dirk

[nikwatt]

Ok....I have other caps
Kermet 15000 40v

[verticalmammal]

When I'm using the cascode voltage formula to determine desired resistance values for R25-28, what cascode voltage am I actually shooting for in my F5Tv2 build? Maybe shooting for just under 24V? Using the lowest suggested value of 4.75 across the board I get 16V of cascode voltage. Is that where I want to be or should I look at possibly dropping the resistance of R25,26? Or perhaps I'm misunderstanding something?

Thanks again!

[Bfpca]

Most people seem to aim at between 11 and 16 volts. I don’t think it is super critical. You are lowering the voltage on the input Jfets to a safe level and also reducing the power consumption, so the jfets run cooler.

[6L6]

Just make the cascode resistors all the same so your cascode voltage is approx 1/2 the rails.

[verticalmammal]

Originally Posted by Bfpca

Most people seem to aim at between 11 and 16 volts. I don’t think it is super critical. You are lowering the voltage on the input Jfets to a safe level and also reducing the power consumption, so the jfets run cooler.

Originally Posted by 6L6

Just make the cascode resistors all the same so your cascode voltage is approx 1/2 the rails.

4.75 across the board for 16V on the nose it is then.

Are y'all sick of hearing me say "thanks again" yet? I hope not, I still have a few more parts to source before I start building

Seriously, I hugely appreciate your help.

[AudioSan]

16V is a nice place to lande. I don't agree with some others that the voltage is not important. Well, the voltage alone is not. But combined with the current they draw, it makes up the power dissipation. And thats the important factor. Keep it way below the limit.

[Shteve]

So I've finally got to biasing my F5T V2. Made a couple of mistakes, but managed to spot them before the smoke showed me where they were. Just waiting for the second mono block to get up to temperature and thought I should share some pictures.
I' m starting my bias at 0.25V (like Nelson did on his V1 build) which give a max heatsink temp of 42C (on the first mono block) and output offset seems stable under 2mV and 80W power consumption at the mains input. Rails ar at 36V. Cascaded with10k resistors to get 17V across the JFETs.
Like so many others, I send a massive thank you to Nelson for sharing this design and all the other contributers that have explained it clearly enough for even me to follow.