Optimize tube preamp for piezo string bass

[quayhog]

I'm looking for suggestions for circuit modifications to use a piezo equipped string bass and a tube amp. I have a lot of PA heads I can play around and tweak to perfection.  Amplifiers from Dukane, a B-15 orphaned- no cab and blown PT, Grommes and Altec.


The string bass is equipped with http://kksound.com/products/doublebigtwin.php pickup and it produces a 1 meg impedance. I'd rather not use an outboard preamp and keep it contained on one chassis.  I have plenty of amps that I could dedicate for this single task.


Suggestions welcome on grid load values, tone stack types and any other tweaks. I'm offering up a chassis for a mod mule to play around with until I get it right.  I have a closed back vented 15" loaded with a D-140.  I have a spare reconed D-130 and a big CTS to play with too.


Thank you in advance for your help.


Jonathan

[Mook]

I'm about to do this myself and I'm about to use 6SL7 input, 6SL7 PI and KT88x 2.

My first stage of filtering is two (in series) 470uF caps at 400V.    Next filter is 2 (in series) 250uF at 300v each.    OT (Hammond 1650P) has ultra-linear taps.   

[quayhog]

Mook, are you going to use this for an acoustic bass with a piezo or an electric bass?

[Willabe]

If your using a piezo, try using a 10M resistor for the input grid leak, per KOC.

[tubeswell]

Merlin Blencowe has a very high impedance design for piezo pups on page 89 of his 2nd-edition pre-amps book which will work fine.


The input jack is AC coupled to a modest grid stopper which is tied to the grid pin. The grid is bootstrapped with a 2M grid load which is bypassed with 500pF. Rk = 1k5,  Ck = 10uF, and there is 10k tail between the bootstrap resistor and the ground. The plate load resistor is normal.

[DummyLoad]

use a j-fet amp or a bi-fet op-amp buffer.


--pete

[PRR]

The company's own preamps seem to be all 1Meg input. I dunno why you don't just use a 1Meg first grid resistor.