Alter Preamp Input Impedance

[jimbopicks]

Hello everyone, let me start out by saying that I don't have a ton of experience with amps, but I want to ask a question about altering the input impedance of a tube preamp. I have a homebrew Alembic F-2B clone that I picked up several years ago. These days I use it a lot with acoustic instruments, i.e. piezo equipped guitars, mandolins, etc. It sounds pretty good for that application but I tend to use a buffer in front of it, as the input impedance is a little low (it's 1 megOhm stock) for that application.

As far as I understand it, the input impedance is basically set by the 1 megOhm resistor on the input jack, correct? Could I safely swap out that resistor for a higher value (say 3 or 5, or even 10 megOhms) without having a detrimental effect on the rest of the circuit? That would allow me to just plug straight into the F-2B clone and get the sound I want without carrying my little buffer box.

Appreciate all of your help, thanks.

-Jim

[martin manning]

I would try a 2M2 or 3M3 and see if that works for you.

[M Fowler]

Add a switch to the input to go from 1m up to 10m either toggle 3 position or rotary type for more resistor options.

I have done this increase for steel guitar amps.

Mark

[Stevem]

If you do the impeadance switch thing the proper way to do it is to wire both resistors in series and then use a single pole single throw switch to short out the 10m resistor.

Also be sure to use a mini type toggle switch to minimize the potential issue of oscillating or picking up near by radio broadcast!

Also in above regard use minimum lead lenght on the resistors.

[Tony Bones]

It seems to me that grid current through a 10M resistor will bias the tube very cold... until you plug in a guitar with ordinary pickups. Then the grid current will go through the pickup and restore normal bias - probably not a big deal. I don't know what will happen when you plug in an acoustic with piezo pickup. Possibly no sound because the input tube is biased into cutoff? Probably depends a lot on the individual tube.

Most people solve this problem by using an active buffer on the acoustic pickup. That can be as simple as a FET source follower.

[Ten Over]

It seems to me that grid current through a 10M resistor will bias the tube very cold... until you plug in a guitar with ordinary pickups. Then the grid current will go through the pickup and restore normal bias - probably not a big deal. I don't know what will happen when you plug in an acoustic with piezo pickup. Possibly no sound because the input tube is biased into cutoff? Probably depends a lot on the individual tube.

Most people solve this problem by using an active buffer on the acoustic pickup. That can be as simple as a FET source follower.

I think the cathode must be grounded for grid leak bias to work. Once you put a 1.5K cathode resistor in, you will be hard pressed to detect any change in operating point even with a 10M grid leak.

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A high impedance source and a cable with a lot of capacitance will roll-off the high frequencies. I don't know much about piezo pickups, but I think there is a very good chance that you will lose your highs if you run a cable directly from the pickup to the Alembic clone.