fender harmonic vibrato with MOSFETs

[cbass]

So I was looking at this http://www.geofex.com/Article_Folders/s ... remolo.htm
I don't see why I couldn't use MOSFETs for the oscillator and followers and a tube for the "mixer" part.
One thing I don't understand is what the far left MOSFET is doing? Regulating the voltage to the rest of th circuit?
[img858]http://www.thevintagesound.com/ffg/sche ... _schem.gif[/img]

[martin manning]

One thing I don't understand is what the far left MOSFET is doing? Regulating the voltage to the rest of th circuit?

Exactly. The string of Zeners sets the output voltage and keeps it stable.

[R.G.]

Yeah, as Martin notes, the first MOSFET is just a power regulator.

You could rework the third MOSFET to have the right in/out phase LFO signals on it and tinker with the biasing to get it at the right DC levels for this, or add a fourth MOSFET to do the LFO phase splitting to make the DC levels setup easier.

It might take some tinkering, but it's doable. And it saves a tube!

[cbass]

Cool thanks guys. I was just thinking if I could save some triodes I'd have room for reverb

[R.G.]

Yell if you need help adapting it.

[matt h]

Keep us posted on the development of this-- very interesting!

[eniam rognab]

subscribed

[cbass]

Yell if you need help adapting it.

Thanks RG I'll probably need it.

[JazzGuitarGimp]

I've been thinking about this for a few years now but haven't had the chance to put it to the test. I wonder if LND150's would be up for the task: I know they will handle the voltage, and I don't think there is of a much current requirement for the two-triod oscillator or the PI. If you try this, please post your progress. I am very interested.

Cheers,
Lou

[R.G.]

The LND150 is a nice device, but you really have to watch the power dissipated in it. The TO-92 package can only tolerate maybe 300mW. The LND says 700mW, but it's at a case temp of 25C, which isn't typically possible.

I like to use TO-220s for high voltage stuff. I just found some depletion mode TO-220s from IXYS that are a kV and 800ma rated. This ought to be good for the 2ma or so and 250mW that the PI does in that amp. The ones I saw are in stock at Digi-key for $1.67 each.

The LND150 and its ilk have the seductive nature of doing the self-bias trick like triodes do, but unless there is a huge savings to be had, using an ordinary enhancement mode device and a second biasing resistor works as well and is easier to design.


That particular amp design runs the LFO from about 360V. If someone could measure the size of the LFO output waveform peak to peak on the cathode of the second triode section, this would be pretty easy.

Post thoughts: It's interesting to see the high voltage MOSFET being accepted into tube amps after so long a time. When I posted the first MOSFET heresies back in 2000, I took a lot of public and private guff about how silicon makes your tone stink. I was gratified to see how many people have discovered the trick since then while I was searching depletion mode MOSFETs.

[cbass]

Doing some reading about MOSFET phase splitters sounds easy enough . so since I will already have my regulated voltage would it be as easy as using 56k resistor on the drain and the source like on the plate and cathode of a triode?

[R.G.]

Yes, mostly.

Notice that in the tube circuit, there is 70V across the bottom 1.5K + 56K resistances. That's 70/57.5K = 1.2ma flowing there.

The simple way to do this is to manage to place the gate terminal at any voltage that makes the source be at 70V +/-. For an N-channel enhancement device, you can use a resistor string that puts the gate at about 73V. For a depletion mode device, you tie the gate through a 1M to a resistor that drops the threshold voltage at that current, so there are a few more steps.

The output of the LFO from the amp schematic flows through an 8.2M resistor, a cap and into a 10M pot, from whence it feeds the PI through a cap. That can stay the same, and have the wiper-cap feed the PI.

[cbass]

Thanks RG I'm starting to see things clearer . I'm sure I'll have many more questions .
Not sure how soon I'll get this project going I have most everything I need cept for a choke and the actual MOSFETs and the tapped pots and a 10meg pot

[moj067]

It may close or far from what your trying to do. But this may help.
http://www.madbeanpedals.com/forum/inde ... ic=14282.0
Sorry if it's far off.

[cbass]

Heres a pic of the victim. Believe it or not it actually worked and sounded good. This was one of the first amps I worked on before I really had any idea what I was doing before I even really had internet access . I had converted it to a Princeton reverb type deal with tweed tone control and I left the very cool and unique sounding univox trem. Believe it or not it was quite and sounded good until the PT took a dump.