LND150 replacement options

[Ten Over]

Do you really want to propose a loop with 130V on a pot that needs to be touched by the player?
You will kill someone proposing that! Please remove that schematic!

Alpha shows a minimum dielectric strength of 500V.

[bepone]

1M send should go before 22nF ofcourse, ground pin to the ground like master volume, and then from the viper 22nF to IRF to existing schematic.

also there is 100k gate stoper on that IRF which is making more bad then good. it is easy to understand why, need to dig little bit about , enjoy

[Ten Over]

Power dissipation: 3mA on a source follower supplied at a standard 450V B+ makes 750 mW dissipated in a TO92. If you supply it with 300V it will be 500 mW.

So then with a 450V B+, you would have 250V across the MOSFET to get 750mW. At 300V B+ you would have 166.67V across the MOSFET to get 500mW. Neither of these scenarios are even remotely similar to the example you quoted. The example you quoted has 20V on the Source, so a 300V B+ puts 280V across the MOSFET. The 2mA idle puts the dissipation at 560mW. 560mW is what a 740mW device derates to at 55.4 degrees celsius.

[Ten Over]

1M send should go before 22nF ofcourse, ground pin to the ground like master volume, and then from the viper 22nF to IRF to existing schematic.

also there is 100k gate stoper on that IRF which is making more bad then good. it is easy to understand why, need to dig little bit about , enjoy

If you put the 1M Send pot there, then the load presented to the tone stack will vary from 157.33k to 1M. Not desirable.

100k gate stopper is appropriate. What bad do you see?

[Ten Over]

SOA: with 3mA you can't exceed 90V Vds, that means you will be safer around 70V Vds.

What safe operating area is that? It is certainly not anywhere near the SAO's of the four devices I suggested.

3mA is the peak AC voltage. The idle current is 2mA. 2mA is what you would use for power dissipation.

[Ten Over]

Don't make your bitterness hide your brain.

I didn't know that personal insults were allowed on this forum.

[bepone]

If you put the 1M Send pot there, then the load presented to the tone stack will vary from 157.33k to 1M. Not desirable.

100k gate stopper is appropriate. What bad do you see?

tehnically you dont even need send, only CF bottom part just to reconstruct.
if you want send then you can put it there (in CF bottom part divider, then send can be 10k pot)

100k gate stopper can be a big problem, already this mistake is rolling 15 years from forum to forum, from VVR schematic to every other...do some tests and you will find out.

[Ten Over]

tehnically you dont even need send, only CF bottom part just to reconstruct.
if you want send then you can put it there (in CF bottom part divider, then send can be 10k pot)

Like this?

[Ten Over]

100k gate stopper can be a big problem, already this mistake is rolling 15 years from forum to forum, from VVR schematic to every other...do some tests and you will find out.

I have thoroughly tested it. It works correctly. Maybe you had better just tell me what the big problem is.

[bepone]

Like this?

yes, maybe little bit fine tuning, i would go like this, serial loop classic

2204 SS Loop 14 PNG (1).png

p.s. before i was commenting schematic with IRF fet

[Ten Over]

p.s. before i was commenting schematic with IRF fet

Well, then cross out "LND150" on the source follower and write in "IRF820".

[bepone]

p.s. before i was commenting schematic with IRF fet

Well, then cross out "LND150" on the source follower and write in "IRF820".

actually i was reffering to gate stopper 100k it is not the same for LND/IRF. for LND you can leave 100k , for IRF to be reduced..also huge difference is material composition, but i'm sure that you know which one you need to take.

[Ten Over]

actually i was reffering to gate stopper 100k it is not the same for LND/IRF. for LND you can leave 100k , for IRF to be reduced..

Let's throw some numbers at the IRF820.
The total input capacitance on the data sheet is 360pF.
The reverse transfer capacitance is 37pF, so the Gate-to-Source capacitance is 360pF - 37pF = 323pF.
The Gate-to-Source capacitance for a source follower is reduced by the factor (1 - A) where A is the gain at the Source.
The gain at the Source is around 0.99, but to be conservative we will use 0.98 for the reduction factor.
323pF x (1 - A) = 323pF x .02 = 6.5pF = effective Gate-to-Source capacitance.
The effective input capacitance is the sum of the reverse transfer capacitance and the effective Gate-to-Source capacitance.
Cin = 37pF + 6.5pF = 43.5pF

fc = corner frequency = 1/(2 x pi x R x C) = 1/(6.283 x 100k x 43.5pF) = 36.6kHz

In testing, the frequency response was dead flat through 30kHz with a 100k Gate stopper. The response was down 0.15dB at 20kHz and 0.3dB at 30kHz with a 220k Gate stopper. Obviously, the corner frequency with 100k is somewhat higher than 36.6kHz. This is because the data sheet is quoting at 25Vds and we are running at around 130Vds and because we used a conservative gain. The input capacitance falls with increasing Vds.

Attached is a mathematical derivation of the effective input capacitance for a vacuum tube cathode follower. The same math holds true for a MOSFET source follower.

[bepone]

It is not about input capacitance..think more, you dont need any math, only logic.
Maybe some experience with HF circuits

[pompeiisneaks]

So you call that prehistoric when you in 2023 propose a very dangerous schematic with an IRF820 that is more than 40 years old?

It is not the prehistoric nature of dinosaurs that I am referring to. It is their extinction. STU9HN65M2 will soon be extinct. I would not have recommended a device that is being discontinued, but it is fine with me if you want to do that.

Also 40 years of age on an IC doesn't mean much, the 555 timer was invented in 1971 and still used to this day on circuits all day long.

I'm 52, that 555 timer is 1 year younger than me

Vacuum tubes are a LOT older than that and we still love them.

~Phil