Pentode with local feedback - is it the way for the Tone Control that follows

[kagliostro]

To see a Tone Control that follows a pentode is something that happens really not frequently

surely it is due to the fact that a Tone Control is a too heavy load for a pentode

But I've seen this Fylon BK30 schematic and ... there is a Tone Control that follows the pentode







the unusual thing is that there is a 1M8 resistor that (to me) acts as a Local Feedback taking the signal from the output coupling cap and feeding the G1 of the tube

Is this a trick to permit to the pentode to have a Tone Control as load ?

Thanks

Franco

[HotBluePlates]

To see a Tone Control that follows a pentode is something that happens really not frequently

surely it is due to the fact that a Tone Control is a too heavy load for a pentode
...
the unusual thing is that there is a 1M8 resistor that (to me) acts as a Local Feedback taking the signal from the output coupling cap and feeding the G1 of the tube ...

That's not a"pentode" it's an "operational amplifier" (opamp)!

Now that there is negative feedback and a series resistor to the input, the opamp's gain is Rf / Rin = 1800 / 68 = ~26.5.



Others see "EF86" but we see "single-section 12AY7 with too many parts."

Why don't they use an actual 12AY7?  Probably because they only needed the 1 section, not 2 sections.

[kagliostro]

Why are they using an actual 12AY7?  Probably because they only needed the 1 section, not 2 sections.

No mojo on the use of a pentode ??

Thanks HBP

Franco

[PRR]

> the unusual thing is that there is a 1M8 resistor that (to me) acts as a Local Feedback taking the signal from the output coupling cap and feeding the G1 of the tube ... Is this a trick to permit to the pentode to have a Tone Control as load ?

Yes. The pentode gain is over 100. With the resistors it is less than 1M8/68k or 26. At least 4:1 of gain given-over to NFB. The Pentode's ~~200k output now looks like ~~50k. Hey, that is a lot like the ~~40k of a 12AX7 with 100k plate resistor. The 12AX7 tone stack will work well enough.

[kagliostro]

Many Thanks PRR

I was thinking something like that

yes, there is poor gain but load can be higher

perhaps it would be worth checking out if it wort's the effort or, as HBP say, better to use a 12a*7 tube to obtain the same result

Thanks Again

Franco

[pdf64]

Additionally, the inverting opamp configuration throws away the normal high input impedance of a common cathode stage, and it becomes roughly equal to the 68k series input resistor.
So the guitar input gets loaded down a bit, and so won’t sound as trebly.

Of course if a buffered pedal is used, eg a tuner, the guitar then sees a high input impedance, and responds accordingly. But that can be confusing for a user.

Regarding pentode mojo, it may be seen that NFB applied an active device acts to linearise its operation, and by that, perhaps reduces whatever mojo it might otherwise impart

[kagliostro]

Also this info are useful to me

Thanks pdf64

Franco