Mosfet CF FX

[tubenit]

I have found no tonal difference between a mosfet CF and a triode CF.  The mosfet CF works fine for me.

As I understand it,  the FX design I use has a CF in the 1st gain section.  Could a mosfet be substituted there and not mess with the tone.  Think that would work?  And sound OK?

With respect, Tubenit

[sluckey]

I think it would be fine. Follow your own ears because you can't trust mine.   

[kagliostro]

Till yesterday I would say yes, today I say there are many probabilities it sounds good

I've read something that pointed my attention on the fact that used as a SourceFollower (CF) in DC copled way may be different from an AC coupled circuit

http://www.geofex.com/Article_Folders/mosfet_folly/mosfetfolly.htm

However, if we use a MOSFET that can take the voltage, we can literally replace the triode section in the follower with the MOSFET, connecting drain for the plate and source for the cathode. The gate is connected to the preceding plate through a 220 ohm gate-stopper resistor. No other changes are needed! A 12AX7 triode will bias with the cathode about 1V more positive than the preceding plate; a big MOSFET like an IRF820 will bias with the source about 2V more negative than the preceding plate - the 3V difference is utterly inconsequential.

"But wait!" I hear you yell. "The MOSFET has a big gate-source capacitance. Won't that suck all the high frequencies out of the signal?".

No, it won't. The big gate-source capacitor is there, OK, but when the device is hooked up as a follower, the apparent capacitance seen by a circuit driving the MOSFET gate is reduced to a very small level by the fact that the source is following the gate almost perfectly, so the apparent capacitance is reduced by the local feedback to an inconsequential level. Even a high impedance 12AX7 plate is not affected in the audio range by this capacitance. Even the big power, big capacitance MOSFETs do this one OK. I have done this, and what's more, done the necessary measurements to find the effect on the response of the composite stage - and there is no detectable change in frequency response below 30kHz. None.

--

As I'm sure you have easily an amp on the bench, give a try an let us know your opinion

Franco

[tubenit]

OK, continuing to explore options in my thinking .................

IF a clean or OD stage has a CF in it's last gain stage, then I am wondering IF the CF (1st stage) of the FX is really needed or redundant?  Any significant advantage in having it there. 

I have noted Hoffman has used an FX with Marshall type amps that has only a "recovery" type gain stage.  This is noted in ARCHIVES under FX loops.

With respect, Tubenit

[kagliostro]

To me you have a big loss of signal at TS

so the following CF isn't redundant

of course that is if you send the signal somewhere that isn't the following gain stage

K

[jojokeo]

I tend to agree w Kag but it may also be of greater benefit to have to act as a buffer from any loading affects from long cables/effects put between the send/return. Best way to confirm is installing it maybe temporarily and bypass it then see what you get?

[tubenit]

installing it maybe temporarily and bypass it then see what you get?

Yeah, that's where I am probably headed. However, it may be a couple of wks out before I can give this a try as I have other projects going.

With respect, Tubenit

[tubenit]

To me you have a big loss of signal at TS  so the following CF isn't redundant

Unfortunately, I don't know much about the cathode followers. 

My impression is that the CF/send of the FX had little to no gain factor to it?  And I was thinking that "IF" that is the case, then maybe it is redundant?

I guess I can find out when I have time to experiment.

With respect, Tubenit

[kagliostro]

The necessity of a CF on the send is due to the fact that a CF has a low output impedance and can supply a relatively high current

this things do of the CF the perfect item for the Send circuits

the presence of a CF in a send circuit isn't strictly obligatory
(in some circuit you can find an anode follower)

but is highly recommended

Ciao

Franco

[HotBluePlates]

As I understand it,  the FX design I use has a CF in the 1st gain section.  Could a mosfet be substituted there and not mess with the tone.  Think that would work? 

Sure.

But why do you still have the 1MΩ (grid reference) resistor and the 1.8kΩ (cathode bias) resistor? Now that you're not using a tube, it seems neither of these are needed. Or at least they weren't needed in your other source follower stage.

My impression is that the CF/send of the FX had little to no gain factor to it?  And I was thinking that "IF" that is the case, then maybe it is redundant?

Why did you need the cathode follower in the circuit as it is now? Did it do anything over not having it? [Hint: you could probably connect that 0.05uF going into the cathode follower grid to the top of the 100kΩ Send pot to find out]

None of the cathode followers you've used have any gain to them (in fact, a slight loss), but they've all done something to your circuit, right?

So gain wasn't the feature. Instead, cathode followers can buffer one circuit from another. Tone stack causes loss with your high impedance plate output? Stick a cathode follower between them to give a high-impedance load to the gain stage, and a relatively low source impedance to the tone stack.

In your case, you're coming out of the tone stack, going to a 1MΩ level pot, then you also need a 100kΩ Send level pot leading to your effects. A follower between the pot buffers the channel level from the send level (to minimize interactions), and helps restore a lower driving impedance that would be wrecked by the 1MΩ pot.

Remember that the cabling between the amp and effects will have capacitance, and the effects could have a somewhat low input impedance. Both of those will knock down treble and/or overall signal strength, and the effect could vary with a changing setting of a pot (especially if you have the 1MΩ channel level directly feeding the effects).

Regardless, it shouldn't be too hard to test for yourself (by simply omitting the existing tube cathode follower). How bad any impact might be varies with the characteristics of the specific effects used, as well as the length and quality of cabling between amp and effects.