MosFET Source Follower for instrument pickups

[toshiba_nz]

Hello all.

I'm thinking of using this device 2SK1828 as a source follower inside my bass guitar (currently passive pickups) , along with a 9V battery, to drive to the input tube of my bass amp (a 150H choke loaded 01A tube) , without any HF roll off when the bass volume pots are right down.

Is 1Kohm source resistor ok (10ma?) ?

And, can I bias the gate with a small battery? or a resistor divider network?

 

[Mooly]

I would have though resistive biasing would be fine and if you know the input impedance and capacitive loading of the power amp you could get a better idea of what current to run the MOSFET at. 10ma sounds very high tbh and will kill a PP3 type battery in... well not all that long. 1ma should be enough for most applications I would have thought.

Why not use a small signal JFET rather than a MOSFET? Whichever you use the biasing resistors will need to be high, in the meg ohm region to avoid loading the pickup.

 

[toshiba_nz]

Why not use a small signal JFET rather than a MOSFET? Whichever you use the biasing resistors will need to be high, in the meg ohm region to avoid loading the pickup.

Thanks Mooly, I like the sound of mosfet (in SF mode)

 

[Mooly]

Fair enough 🙂 can't argue with that.

So something like this. Tweak the gate bias to get maximum symmetrical output swing.:

 

[cabirio]

Whichever you use the biasing resistors will need to be high, in the meg ohm region to avoid loading the pickup.

Actually the pickup wants (expects?) to be loaded. If you don't want to fundamentally change the sound that you get with your particular passive volume / tone controls + cable capacitance, you should emulate that load at the input of whatever active circuit you connect the pickup to.

OTOH, if this is just about preventing HF roll-off when turning the volume down, why not just use a treble bleed circuit? a 270k resistor and 470p cap in parallel across the input and output lugs of the volume pot works pretty well.

 

[Mooly]

Actually the pickup wants (expects?) to be loaded. If you don't want to fundamentally change the sound that you get with your particular passive volume / tone controls + cable capacitance, you should emulate that load at the input of whatever active circuit you connect the pickup to.

I wasn't 100% sure on that and was thinking something like a capacitive pickup...

Thanks 👍

 

[toshiba_nz]

Thanks again! Can I avoid 2 coupling caps?

Also can I somehow use +/- 9V supply, so can I have the output at -9VDC for direct coupling to the grid of the 01A tube (that needs -9V bias) ?

 

[Mooly]

The input cap is needed if the pickup is resistive... meaning it appears as a resistor across the input.

For biasing remember the actual voltage will vary with battery voltage. To get -9 volts is perfectly achievable (he says without trying it 🙂) and would at first thoughts need the circuit flipping over to 'positive earth' and using a P channel MOSFET.

 

[toshiba_nz]

Mooly,

I think the problem is that I'm using a 9V battery to bias the grid of my 01A tube like this

and as soon as I turn down the volume pots on my bass, the treble goes right down.

Maybe I should use filament bias on the 01A tube and see if that's any better, before I resort to a SF in my bass guitar

 

[Mooly]

So...

And caps aren't all bad you know 🙂

Just seen your post:

I think the problem is that I'm using a 9V battery to bias the grid of my 01A tube like this

As drawn that should work. Any reduction in the 1meg value caused by a pot should not alter the grid bias but if the signal (pot) feeds in at the other side (the grid) then it will.

 

[toshiba_nz]

Thanks for the above schematic, exactly what I was looking for!🙏😃

So the pot on the bass is adjusting the bias of the 01A tube?

 

[Mooly]

If the pot acts on the grid then yes, it will do, if it acts on the other side of the battery then it should not.

The grid draws essentially no DC current and so shunting the the 1meg even down to ground potential via a pot has zero effect on biasing. If the pot acts on the other side of the battery it reduces the grid bias by an amount determined by the pot setting.

For example if the pot was set to 100k then the grid bias would fall from -3 to something nearer - 0.3volts

Feeding time 🙂

 

[toshiba_nz]

Cool, Got it ☺

 

[gijser]

Thanks Mooly, I like the sound of mosfet (in SF mode)

You could use LND150. It's a mosfet, but not as you may know it: depletion mode, so acts like a Jfet. You could then build this:

https://www.jocidapark.com.au/circuits/circuits/buffercableschematic.gif

 

[toshiba_nz]

Thanks!

Because my 01A tube is capacitor coupled to the next tube (801A tube) , the HF roll off problems may be the 01A's ability to drive stuff, so I'm wondering if the SF should go on the grid of the 01A, or the anode.

Thanks again👍🙏

 

[toshiba_nz]

So...

And caps aren't all bad you know 🙂

View attachment 1352708

View attachment 1352709

Just seen your post:


As drawn that should work. Any reduction in the 1meg value caused by a pot should not alter the grid bias but if the signal (pot) feeds in at the other side (the grid) then it will.

Hi Molly,

Now I have the input tube with filament bias, so it's biased and the grid sits at 0v.

How do I arrange a mosfet source follower (for the input tube, preferably enchancement mode) to directly couple to the grid of the input tube ( grid at 0v / mosfet source at 0v) ?

And, how do I work out the components and voltages (if I can do this)

Many thanks in advance🙏