V1 grid leak Resistor upper limit?

[G._Hoffman]

So, I'm thinking about making an acoustic amp, and one of the big issues is that the output impedance of passive piezo pickups is very high.  Because of this, I would like to make the input impedance of the amp very high, and I'm just wondering how high you can go with the grid leak resistor before it causes problems.  Like, the B-15N can be as high as 5.6MΩ - but could one go, like, a lot higher?  Because 20-40MΩ would have some real advantages with some pickups.  The other alternative would, of course, be a FET input, but I'd like to avoid that if I can.

[Willabe]

KOC in his books said you could go as hi as 10M for the grid R to deal with passive piezo pickups Z.

[G._Hoffman]

KOC in his books said you could go as hi as 10M for the grid R to deal with passive piezo pickups Z.

Not quite as high as I might like, but it'll do.

[tubeswell]

Merlin has a bootstrapped input for Piezos in his 2nd edition preamps book. For more info on how to alter this, see the book.

[sluckey]

...

[pdf64]

I suggest to at least try grid leak biasing, eg https://el34world.com/charts/Schematics/files/Fender/Fender_deluxe_5c3.pdf
Just swap the 5M grid leaks for 10M, and don't forget the DC blocking caps at the input.
It's so simple!

[jjasilli]

Also checkout the Mesa Boogie V1 Bottle Rocket Schematic.  Ibeleive it's posted here. It has a 10M input resistor.  I love mine.  I put orange drops and Fender blue caps in the signal path.  Sounds to me like a Frnder BF/SF preamp.

[G._Hoffman]

Merlin has a bootstrapped input for Piezos in his 2nd edition preamps book. For more info on how to alter this, see the book.

Damn.  I have the 1st edition, but now I have to buy another book.

[tubeswell]

Merlin has a bootstrapped input for Piezos in his 2nd edition preamps book. For more info on how to alter this, see the book.

Damn.  I have the 1st edition, but now I have to buy another book.

Yes all his books have a lot of good stuff in them. I have the complete collection.

[j-b-c]

If you must avoid the obvious jfet for aesthetic reasons the nuvistor tube was its predecessor and used for hi-Z inputs on instrumentation and hi-end audio.  It was used in the original Neumann U47.

RCA specifically recommends it for piezo input stages.

Typical operation: Rg 100MΩ.

http://www.bitsavers.org/components/rca/nuvistor/2A1087_RCA_Nuvistor_Brochure.pdf

They are cheap enough ==> https://soviet-tubes.com/product/6s52n-v-subminiature-triode-nuvistor-equivalent-to-the-7895/

Their sockets cost quite a bit more.

[j-b-c]

Adding ...

The AKG-C12VR mic uses the common 6072 at 200MΩ according to its schematic in the service manual found here ==> http://www.sdiy.org/oid/mics/AKG-c12vrSM.pdf

[HotBluePlates]

So, I'm thinking about making an acoustic amp, and one of the big issues is that the output impedance of passive piezo pickups is very high.  ... Because 20-40MΩ would have some real advantages with some pickups.  The other alternative would, of course, be a FET input, but I'd like to avoid that if I can.

If you must avoid the obvious jfet for aesthetic reasons the nuvistor tube was its predecessor ... It was used in the original Neumann U47.

RCA specifically recommends it for piezo input stages.

The AKG-C12VR mic uses the common 6072 at 200MΩ according to its schematic ...

This is all crying out for a buffer, preferably placed inside the guitar (isn't this mostly what is done anyways?).

Why?  Because the high impedance will wreck your treble response as it interacts with cable-capacitance.  The old condenser mics mentioned all had their amplifier/buffer inside the mic-body, 1-2 inches from the capsule that needed the high-impedance charge-pump designs being used.

There are also good piezo preamps/buffers/EQs on the market, which begs the question again about building what might turn out a more-expensive & less-performative solution in a tube amp.

[G._Hoffman]

This is all crying out for a buffer, preferably placed inside the guitar (isn't this mostly what is done anyways?).

Why?  Because the high impedance will wreck your treble response as it interacts with cable-capacitance.  The old condenser mics mentioned all had their amplifier/buffer inside the mic-body, 1-2 inches from the capsule that needed the high-impedance charge-pump designs being used.

There are also good piezo preamps/buffers/EQs on the market, which begs the question again about building what might turn out a more-expensive & less-performative solution in a tube amp.

Certainly, the most popular pickups ARE mostly active, with preamps in the guitar - but not all.  And a couple of very beloved old systems, such as the LR Baggs LB6, are completely passive.  And one of the most popular systems on the market these days, the K&K Pure Mini, is passive, though they don't recommend a very high input impedance for it (I think it's because the three piezo elements in parallel drop the output impedance sufficiently to drive the signal a bit better, but I'm not sure). 

The idea, in this case, is to make something versatile.  I'm still in very early head space design - not even block diagrams on paper yet - but what I'm imagining is something with possibly three inputs - one very high impedance for the purely passive pickups, one in the 1M range for active and magnetic pickups, and a third with a proper tube mic preamp for mic level signals.  Because of that mic pre, I'm also contemplating a two chassis solution, with the power supply in the other end of the cabinet.
But as I say, still early days on the idea.  I've got at least 2-3 other amps I want to clear off the bench and the design process before I spend too much effort on this. 

[j-b-c]

Its not the resistance that is the problem. 

The problem is the capacitance of the transmission line between the microphone and its amplifier.

https://en.wikipedia.org/wiki/Electrical_reactance#Capacitive_reactance

The amplifier wants to be as near to the source as possible.