piezo pickup

[buildingampproject]

What would be the difference between a tube amp designed to amplify the signal from a mic, compared to a piezo pickup? I understand this is in the preamp that could be either integrated or apart but, I am guessing the difference is in the circuit. If you know please comment, thanks.

[Geek]

Hi,

Input impedance. Piezo need high input impedance (tubes are a natural for them) to the treble isn't extinguished.

They are also capacitive, rather than inductive, so it's possible an L-C input network is needed to remove RF pickup.

Cheers!

[EWBrown]

As I recall, piezo pickups require a fairly high first grid / loading resistance, on the order of 10 megohms, in order to not lose their full frequency response.

Usually the first high-mu amplidication stage will have a 10 megohm grid resistor, and the cathode is directly connected to the signal ground, with no cathode biasing, the grid gets its bias voltage a combination of "grid leak" and "contact" potential. Think very "old school" for this design.

I haven't experiment with piezos and their perticular requirements and issues, in a LONG time....

/ed B

[buildingampproject]

So, if I understand for piezo is needed a high input impedance so, to get a signal from an acoustic guitar using a microphone is the opposite low input impedance, correct?
Then, using a microphone you don't want to lose frequency response either, how do you do in this case?
Think that my questions are in order to design one or two tube amplifiers for acoustic guitar using a piezo pickup or just a microphone, and I believe only one amp will be good for just one guitar. And my aim is low distortion "Point One" to amplify just the best the guitar can do by itself. I guess this is what "pure" means.
Thank you both!!

[dhuebert]

You can build a normal input circuit but make the grid leak resistor 10 megs. It works, I've done in on most of my guitar amps. You can't do it on anything with a capacitor coupled output tho since the cap will charge on the positive going signal but won't dicharge enough on the negative cycle and the tube will slowly go into saturation. Look at the front end of the Bass-O-Matic.

Don

[Gingertube]

The 10M (well >= 4.7M) is recommended for PASSIVE crystal pickup.
This is actually a bit too high for normal guitar pickups and you will hear evidence of peaking from the pickup.
I use a series 1M and 10M resistor as the grid leak with a switch across the 10M. Switch ON for "NORMAL" (1M input Z), switch OFF for "PASSIVE CRYSTAL" (1M +10M = 11M)
Cheers,
Ian.

[Geek]

Beautifully simple solution, Ian

Cheers!