mosfet D-lator ?

[10thTx]

I was thinking about building a D-lator that would sit in the bottom of a combo cab similar to how a reverb pan might. The idea would be to velcro it to the cab bottom hooked up to a delay pedal also velcroed to the cab bottom.

I was thinking that a tube standing up might be problematic, so I was pondering how to build it so a tube is not an issue?

I've been reading about the Klein-ulator needing 36v (instead of 9v) to sound "better" at gigging levels. That got me to thinking about a higher voltage mosfet being considered.

I was thinking about the idea of using a IRF820 mosfet as a triode and having higher voltages than the Klein-ulator uses? I've successfully used the IRF820 mosfet as a cathode follower but not as a gain stage triode.

Any thoughts on this as an idea?

It may just be another way to make a D-lator where the mosfets could be put into the board layout?

EDIT: revised and reposted schematic later in the thread

With respect, 10thtx

[markr14850]

I don't believe that an enhancement mode MOSFET like the 820 will self-bias the way a triode will. Maybe look into a depletion mode MOSFET like an LND150 - folks seem to like using those in effects loops.

Edit: R.G. has a nice write-up of many of the issues here: http://geofex.com/Article_Folders/mosboost/mosboost.htm

[10thTx]

OK, what about this design instead?

With respect, 10thtx

EDIT: reposted schematic with layout later in thread

[markr14850]

I think that you might have bias issues with the source follower as well. Enhancement mode devices like the 820 are easy to use when DC coupled, but need more support when AC coupled. You probably would need a divider from B+ to bias the gate positive relative to the source. (Or run a negative rail.)

[10thTx]

I know zip about mosfets other then they worked fine in the CF designs I've tried.

I did run across this post on another forum:

As a source follower, the IRF820 will be fine, so using it in the send is going to work. In the return, you have this setup to amplify and send signal off of the drain. The large input capacitance has the possibility of killing your high end. You may want to consider other devices for this. ZVN0545 and the LND150 are two devices that would do this one well.

This is the 2nd thing I've read that would suggest the IRF820 should work as the source follower? But like I said, I really don't know.

With respect, 10thtx

[markr14850]

Yeah, DC coupled source followers can replace cathode followers quite well (except for the cases where the cathode follower was causing grid current based distortion, as Merlin has well documented). The only difference there is that in the CF case, the cathode is a few volts DC higher than the grid - whereas in the enhancement mode SF case, the source will be a few volts DC below the gate. When DC coupling, the grid or gate is at such a high voltage, this difference can be ignored. We always stick a coupling cap after it, so the exact value of the DC idle voltage doesn't matter.

When AC coupling, this over/under voltage difference changes everything. Because of the input capacitor, the gate is essentially at ground. The source would then want to idle at (approx) -4v. Obviously, it can't - so it will sit at zero, and the only "signal" that will pass through are any peaks above 4v.

So, to use an enhancement mode mosfet, you either need to add a positive bias voltage to the gate (after the input cap, before the gate stopper) of some reasonable amount (B+/2 maybe?), or provide a negative rail for the source instead of ground. The former is usually easier, and more common. The latter might be overkill for this project, but is a good learning experience for building power tube drivers, ala SSS.

[tictac]

Here's a FET circuit that can do what you want...

You can put another FET after this gain stage if you want a source follower to drive the Loop preamp out...

TT

[10thTx]

Given my severely limited knowledge of mosfets ..........

Is it being suggested that the first mosfet also be a LND150?

Unfortunately, I'm not able to understand the majority of what was explained .........


With respect, 10thtx

[tictac]

Is it being suggested that the first mosfet also be a LND150?

Yes, the mosfet you are trying to use cannot be used like a tube triode; the LND150 can... the LND150 circuit I posted is like a mosfet replacement for a 12AX7 triode...

TT

[10thTx]

Yes, the mosfet you are trying to use cannot be used like a tube triode; the LND150 can... the LND150 circuit I posted is like a mosfet replacement for a 12AX7 triode...

Thank you! With respect, 10thtx

[surfsup]

Tictac, for the send, a gain stage is not necessary, usually the signal needs to be attenuated down to about 1volt pp. so the irf820 should work, yes? The lnd150 for reamplifying the return (gain stage) for this i understand.

[tictac]

Tictac, for the send, a gain stage is not necessary, usually the signal needs to be attenuated down to about 1volt pp. so the irf820 should work, yes? The lnd150 for reamplifying the return (gain stage) for this i understand.

No it won't work as shown in the schematic. It was already mentioned that an enhancement mode mosfet cannot be used as a drop-in replacement for a tube triode. The depletion mode devices like the LND150 can.

You could use the IRF820 with a 2N5485 FET to make a cascode gain-stage but now you're talking about a completely different circuit and a bunch more parts.

Do a search for "Mosfet Follies" and you'll find some good info and different guitar amp circuits you can build with mosfets...

I've read good reports about the LND150 circuit I posted... You should be able to remove bypass cap across the source resistor and adjust the resistor value to make a source follower for the first stage of the loop...

TT[/img]

[10thTx]

It was already mentioned that an enhancement mode mosfet cannot be used as a drop-in replacement for a tube triode.

As I am understanding this, your saying the IRF820 will not work as a "typical triode gain stage"?

And that you are not saying the IRF820 will not work as a cathode follower?

Is this correct?


With respect, 10thtx

[tictac]

The IRF820 WILL NOT WORK... in the circuit you have drawn; not as a cathode follower and not as a gain stage... It won't do ANYTHING in that circuit.

Do a Google on "Mosfet Follies" if you want to see how enhancement mode mosfets CAN be used as gain stages, cathode followers and the like...

Or you can use the circuit I have shown you already... It will do EVERYTHING that you want...

TT

[10thTx]

IRF820 WILL NOT WORK... in the circuit you have drawn; not as a cathode follower and not as a gain stage

OK, thanks for the response and clarity! I do appreciate knowing that the LND150 would be a appropriate choice for that circuit. I am grateful for your help.

I am understanding then that you are talking about that specific circuit that I have drawn & you are not saying an IRF820 will not work as a cathode follower in other circuits.

What has been confusing to me was that I was given a FX schematic that I understood to be a working model that had an IRF820 in that first position and that I have built several amps successfully with the IRF820 in a cathode position.

I have read Mosfet Follies several times before, but despite my best efforts I only was able to understand portions of it.

With respect, 10thtx