MOSFET as first tube (V1)?

[ChopSauce]

How about using an IRF820 as V1, with:

- a 100k resistor between drain and power supply

- a (1k5, xxuF) pair between the source and the ground

as the Mosfet folies document suggests that the MOSFET can be substituted to a triode ; and if so:

- what about an input/gate resistor?

I've already assembled a vintage Vox AC4 inspired amp, as per Merlin "The Valve Wizard" Blencowe's instructions (the small signal pentode) that is with a triode feeding the EF86 on input.

This lefts an unused triode (V1b) and since the other half is V1a, there's absolutely nothing you can do with V1b but collecting noise or a parallel V1a//V1b, which you don't really need, for the EF86 is already easily overwhelmed by a single triode.

Hence the idea of using a MOSFET, which would be a lightweight solution with anything but drawbacks, at first sight, right

[Ben N]

I would thing the MOSFET would work fine, but one possible use for the other triode is as a cathode follower to drive a tube stack, which an EF86 does not like to do.

[Manfred]

as the Mosfet folies document suggests that the MOSFET can be substituted to a triode

Not really because the tube has a different overload behavior, especially with a positive input signal where the overload is caused by the grid current limiting.
In this case the overdrive of the first stage by e.g. a booster is not like a tube.
But a first stage with the high voltage Fet LND150 as a booster or preamplifier can be found in various applications.
Here the IFR820 is not suitable for a common source circuit because firstly its forward transconductance is much too large and secondly the range of the signal AC voltage is limited by the reverse diode between drain and source.
However, the IFR820 is good as a source follower and can be found in many applications.

[ChopSauce]

Yes, I've already used an IRF 820 to drive a tone stack after the EF86, with success. I've just copied a circuit designed by someone else, to be honest.

I thought that the IRF820 could make for a "normal" gain stage, but I'm not aware of the difference between the IRF820 and the LND150, sorry. One more thing to learn.

But a first stage with the high voltage Fet LND150 as a booster or preamplifier can be found in various applications.

Yet the subject seems to be too complex for me at this time, I'd like to see an example of circuit like that. Overdrive is not what's needed. Just a little boost to feed the EF86.

Thank you for your consideration, anyway!

[Manfred]

Have a look at these projects:

[Vitallka]

Hi there. Here are 2 schematic solutions for the first stage on the MOSFET
https://imgur.com/JdnL8Rj
https://imgur.com/S95tLv4

https://imgur.com/HyprekI
https://imgur.com/MlJs9xQ

[Manfred]

I am amazed that the common source circuit should work with the IRF820.
Ok, the P-spice simulation seems to work, but have you tried the circuit in practice?

[Vitallka]

The first scheme - working and the cascade was used as a return in the fx-loop .
Similar to the second circuit I used in the amplifier, but with a different jfet transistor at the signal input.
https://guitar-gear.ru/forum/index.php? ... h_id=37697
https://app.box.com/s/ujk84fd9c1rpczkp61g4ahbkutbpf6bq

[Manfred]

The first scheme - working and the cascade was used as a return in the fx-loop

I find that interesting what gain and what maximum undistorted output amplitude did you achieve with the single IRF820 in the stage you showed.
That the cascade circuit of power MOSFET and JFET works is well known,
but with the source circuit with the single power MOSFET as a signal amplifier I had my doubts so far, I have to try it again myself.
Maybe there are other members of the forum who have tried this circuit and can say more about it.

[ChopSauce]

Awesome! Thanks to both of you. My plan was to use a FET to boost the signal before the EF86 and hence make a vintage Vox AC4 circuit work with more sensible - as per Merlin Blencowe's article : http://www.valvewizard.co.uk/pentode.html - settings for the EF86.

Unfortunately my friend appeared to have no real interest in that project, afterwards, and I do not intend to build that for myself. So, I leave the subject to your discussion and keep note of that as a mean to convert to a more "vintage" tone one of these reissue AC4TV amps, which most likely have just enough power for two tubes.

Anyway, now I'm guessing at how I could drive a reverb tank from a solid state amp (Z_in of order 100 Ohms) within an all tube circuit - that is using still only a 200-250V DC power supply: using FETs?

Maybe that could be a subject for another post...

[george giblet]

How about using an IRF820 as V1, with:

The input and feedback capacitances could cause some loss of high. A cascode connection would help but you still might have to drop drain resistor value. The output impedance of a tube stage is less than the plate resistor so technically there is a right value or the MOSFET drain resistor to match that. The capacitance loading details would need to be looked at but it might just make it.