MOSFET buffer

[Fresh_Start]

I have one "extra" tube triode in a single-ended amp design and want to add an effects loop.  The insertion point would be between the "master" volume and the PI-like triode stage (i.e. power tube driver).

Kevin O'Connor has a really complicated solid state buffer for an FX loop.  His tube-driven buffer is less complicated, but he says that a MOSFET has too much input capacitance to substitute for the tube triode in the simpler circuit.

Merlin's site shows an AC Coupled Cathode Follower  Is it possible to substitute a MOSFET for the tube triode in that circuit?

Can someone point me to a "known good" MOSFET (or other solid state) buffer which does not require a separate power rail?

Ideally, I'd like to use a solid state buffer and O'Connor's overall circuit architecture with the tube triode as the recovery stage.  The series/parallel switch in his design has a lot of appeal.

If it matters, I would not be using any "line level" effects, just pedals.

Thanks in advance for any ideas.

Chip

[FYL]

Gate/source capacitance isn't really a problem if the mosfet is used as a source follower.

Check http://www.geofex.com/Article_Folders/mosfet_folly/mosfetfolly.htm for some ideas, and don't forget to add gate stoppers (RG hasn't drawn them).

[PRR]

> a single-ended amp design

You can steal a mA at 20V from the output cathode, use a JFET.

> Gate/source capacitance isn't really a problem if the mosfet

What's a good small MOSFET for 400V rail?

[Fresh_Start]

Mr. Keen suggested this little bugger for a DC cathode follower:
http://www.mouser.com/ProductDetail/Diodes-Inc/ZVN0545A/?qs=sGAEpiMZZMveMCOqFR6qCMipDFy%252b9W8VLXG6ikMZM%2fA%3d

I am truly ignorant when it comes to solid state electronics, despite reading through the section on effects loops in TUT 1 I don't know how many times.  O'Connor's solid state buffer is no doubt great - it just has a high parts count and I simply don't understand all of the circuit.

Could I use a ZVN0545A in place of the tube triode in the AC coupled cathode follower circuit Merlin shows?  (I'll be well below 450 volts on the B+ at that point in the power rail.)

Thanks again,

Chip

[FYL]

What's a good small MOSFET for 400V rail?

Zetex has some nice models such as the ZVN0540A (400V) and ZVN0545A (450V), 700 mW diss, 70p CISS. Available from the usual suspects.

http://www.diodes.com/products/catalog/detail.php?item-id=1433
http://www.diodes.com/products/catalog/detail.php?item-id=1434

[FYL]

Could I use a ZVN0545A in place of the tube triode in the AC coupled cathode follower circuit Merlin shows?  (I'll be well below 450 volts on the B+ at that point in the power rail.)

A common mosfet is an enhancement device which doesn't conduct with zero volt gate bias, you either need to use it in a DC CF, as RG shows, or add glue.

OTOH, you could use a depletion Mosfet, which conducts by default and is controlled by a negative bias, just like a tube. Supertex has some interesting models, alas sometimes quite difficult to source. Check for instance their DN2540: http://www.supertex.com/pdf/datasheets/DN2540.pdf

[FYL]

You can steal a mA at 20V from the output cathode, use a JFET.

Sure, simple and effective.

[Fresh_Start]

You can steal a mA at 20V from the output cathode, use a JFET.

Sure, simple and effective.

Can you show me a buffer circuit using a j-fet?

Is this a reasonable part choice?
http://www.mouser.com/ProductDetail/NXP/BF510215/?qs=me8TqzrmIYU2f0hDdp7vzw%3d%3d

I'm going to edumacate myself, starting with http://ist-socrates.berkeley.edu/~phylabs/bsc/PDFFiles/bsc5.pdf

I'll need a regulated low voltage supply since this amp has a fixed/cathode bias switch.  There are plenty of references for that - more than enough in fact

Thanks for your help guys!

Chip

[FYL]

Can you show me a buffer circuit using a j-fet?

From http://www.muzique.com/lab/buffers.htm

Is this a reasonable part choice?

Do you like surface mounted components? SOT23 packages are some of the smallest, with 1.4 mm spacing between the legs...
 

Get some J201's or similar generic jfets in TO-92 package, cheaper and much easier to use.

http://se.mouser.com/ProductDetail/Fairchild-Semiconductor/J201/?qs=sGAEpiMZZMs8pq%2frIcncIDuloSKJeQKQyjKPsc1lhn4%3d

[PRR]

> ZVN0545A

As FYL says, "Gate/source capacitance isn't really a problem if the {small} mosfet is used as a source follower." The ZVN0545A's 70pFd "input" is not frightening, and in a Follower that is bootstrapped by roughly Gm/Rl or 500/50K or 1/100, giving 1pFd. Crss adds another 4pFd in Follower configuration. This 5pFd total is about what your wiring stray capacitance will be in any case, much less than the 100pFd of a 12AX7 gain stage or 3 feet of cable.

The MOSFET won't self-bias same as a depletion-mode (vacuum) device, and has lower dissipation ability; do this:

[PRR]

> you could use a depletion Mosfet..., alas sometimes quite difficult to source.

Huh. Mouser has LND150, 4,000 in stock, under a buck.

Capacitances are truly negigible.

The nominal static zero-bias is ~~1.5mA, just right. However the spread is 1mA to 3mA, a bit broad for thoughtless production. A 1K "self-bias cathode resistor" would bring this near 0.5mA for most samples. Then 100K load would work, and run cool even if B+ were at 500V.
> regulated low voltage supply since this amp has a fixed/cathode bias switch

Fixed-bias does not have the advantages in SE that it may have in P-P.

A simple fix-bias is to replace the cathode resistor with a string of 5V Zeners, adjusted to suitable current (monitored across 1 ohm at the bottom of the string).

The buffer does not need a regulated supply. It may want filtering, but buffer current and voltage is low, 2K and 100uFd 35V ought to be ample.

[Fresh_Start]

Many thanks!

Now I have to actually build the amp - without the effects loop or buffer first - and then come back to this.  The important answer here is that it can be done relatively simply.  Those articles FYL linked to from the AMZ Lab Notebook are really helpful.

Assuming that the fixed bias/cathode bias switch turns out to be useless, I can always turn it into a pentode/triode switch.

Chip

[jubal81]

> ZVN0545A

As FYL says, "Gate/source capacitance isn't really a problem if the {small} mosfet is used as a source follower." The ZVN0545A's 70pFd "input" is not frightening, and in a Follower that is bootstrapped by roughly Gm/Rl or 500/50K or 1/100, giving 1pFd. Crss adds another 4pFd in Follower configuration. This 5pFd total is about what your wiring stray capacitance will be in any case, much less than the 100pFd of a 12AX7 gain stage or 3 feet of cable.

The MOSFET won't self-bias same as a depletion-mode (vacuum) device, and has lower dissipation ability; do this:

I'd like to give this a shot. What voltage rating would be safe for the .2uF output cap?