MOSFET source follower question

[Zozobra]

I'm putting ideas together for a preamp (attached) for a 30W Class A PP EL34 amp and I'm thinking that this is a good a time as any to try out a MOSFET source follower to drive the tone stack. From R.G.'s MOSFET Follies article I see that a 220R gate stopper is needed (R19). Should I just go ahead and use 100k for the cathode/source resistor? I'm going to use an IRF820 as they're easy and cheap to source.

[jazbo8]

Yes, 100k will work, don't forget the gate-to-source protection Zener (I used 12V).

[soundmasterg]

I used a MOSFET source follower in a trem circuit to isolate the oscillator from the power tubes they were modulating and it worked well. It didn't sound as good as a tube follower, but I didn't have room to add the tube and I did have room to add the MOSFET. I plan to use it again. I also used the IRF820.

Greg

[NateS]

A number of HV power mosfets are available with built in gate zeners now. My selection process goes something like this:

DigiKey -> (Absolute spec requirements) -> sort by preferred attributes, In Stock.
Set desired Qty to weed out the price that's only available to Foxconn and Samsung
Sort by price.

Then I pick from the top of the list. Making sure to check the qty on hand. If DigiKey only has 10 of them, I take that to be a sign that I'm likely deviating unnecessarily from the happy path. (I have designed circuits in the past only to discover that I built it around some oddball part that was obseleted 20 years ago.) If part A has 100 on hand and part B is only a few cents more but there's 300k on hand - I pick the high volume part.

[Gregg]

I'm putting ideas together for a preamp (attached) for a 30W Class A PP EL34 amp and I'm thinking that this is a good a time as any to try out a MOSFET source follower to drive the tone stack.

It's actually not as simnple as that. You should have in mind that in order for the MOSFET follower to "copy" the clipping behaviour of the tube CF it needs some changes which are described in detail here:

Some nuances of using mosfets as followers in tube guitar amplifiers

Also here:

http://www.amtelectronics.com/downlo...e_follower.pdf

[R.G.]

On the other hand, it *is* as simple as that if you're not driving the cathode follower into clipping and aren't looking for that particular sound.

[Gregg]

Looking at the schematic it will be clipping for sure at least at higher gain settings.

Maybe this is a ready to go solution:

https://www.facebook.com/amtelectron...type=1&theater

[R.G.]

Schematics don't have signal levels. Circuits do.

[tubeswell]

I used a MOSFET source follower in a trem circuit to isolate the oscillator from the power tubes they were modulating and it worked well. It didn't sound as good as a tube follower, but I didn't have room to add the tube and I did have room to add the MOSFET. I plan to use it again. I also used the IRF820.

Greg

Me too. Done this in several amps now. No more worrying about high h-k voltages.

[Zozobra]

Thanks for the stimulating input, guys

It's actually not as simnple as that. You should have in mind that in order for the MOSFET follower to "copy" the clipping behaviour of the tube CF it needs some changes which are described in detail here:

Some nuances of using mosfets as followers in tube guitar amplifiers

This looks interesting and I may well give it a shot. Have you tried this out btw?

[Gregg]

No, but I read comments from people who have done A/B listening tests between MOSFET and a tube and according to them it was hard to distinguish between the two.

[Alan0354]

I was looking on the scope just the other day on the waveform of the cathode follower, I saw more of the slant on the bottom than the top. The top was flat if I remember correctly. The bottom is slanted downward because of the current is sunk by the 100K resistor to ground. Driving a tone stack with the resistor of 50K rather than 100K of the Fender exceeds the current sinking capability of the 100K cathode resistor when you try to swing down to 100V.

The top of the waveform at the grid of the cathode follow is limited when the Vgk goes to 0V or even slightly positive when driving too much cathode current. I don't see any of the waveform presented in the Victor Kempf pictures.

Seriously, until it is clipping on the extreme, you really think it makes any noticeable difference? Are they making a mountain out of a moll hill?

Far as I concern, the main advantage of using any follower is to present a low output impedance. So if you drive a typical Fender/Marshall type of 3 pot tone stack, it lower the interaction between the treble path and the bass/mid path. In Fender type of stage, the treble and bass interact with each other because the impedance driving the tone stack is 100K//Rp (plate resistance ). I was thinking about using Darlington BJT as follower to get even lower output impedance.

Using MOSFET or BJT has the advantage of not having to deal with grid current of the cathode follower and you don't have to deal with the swing limitation as I described above.

Hell with all the diodes, just put the diodes that needed to protect the MOSFET!!!!

The one big thing about using any SS device is you cannot claim it's all tube amp. Don't say this is silly, every one of us ultimately do want to sell our amps regardless we really pursue it hard or not. People pay a lot more for a "all tube amp"!!!! Or else, there are advantages to mix in some opamp strategically and still keep the tone affecting circuit all tubes.

[Gregg]

I can't comment from experience whether Kempf is right but what I know from experience is sometimes slight changes in a waveform can be clearly heard. So I guess the only way to make a practical decision is to listen between all 3 configurations and make a decision for yourself.

[Zozobra]

The one big thing about using any SS device is you cannot claim it's all tube amp. Don't say this is silly, every one of us ultimately do want to sell our amps regardless we really pursue it hard or not. People pay a lot more for a "all tube amp"!!!! Or else, there are advantages to mix in some opamp strategically and still keep the tone affecting circuit all tubes.

The all tube amp thing is a bit silly IMO and it's not something that concerns me in the slightest. If adding the much feared silicooties improves the tone and/or functionality of the amp then they're going in! If I don't add power scaling then I'm going to tinker with Zeners in conjunction with the cathode resistors as mentioned by Chuck H in another thread.

If it sounds good it is good.

[Alan0354]

The all tube amp thing is a bit silly IMO and it's not something that concerns me in the slightest. If adding the much feared silicooties improves the tone and/or functionality of the amp then they're going in! If I don't add power scaling then I'm going to tinker with Zeners in conjunction with the cathode resistors as mentioned by Chuck H in another thread.

If it sounds good it is good.

I agree, ask the people that pay.