more distortion from alembic tube bass preamp

[kpestoso]

Hello, I just finished an alembic tube bass preamp as for http://www.geofex.com/FX_images/alembpre.gif. This is my second tube adventure after a Real McTube II.

The only mod I made were 100 microF cathode bypass capacitors instead of 25microF. It sounds great, no hum, no hiss, just good clean bass with some tube coloration. Anyway, I would like some more grit, more distorted sound, so I should push some of the stages into saturation for a warmer sound.

I think I know how the theory goes: bigger plate resistor, smaller cathode resistor, etc. but any experiences or suggestions?. Any real resistence values? Would it be better to work on the first stage? or the second?.

Thanks.

[puretube]

both stages;
try 220k plate-R;
try 820 ohm to 2k2 cathode-R values;

one by one.

move vol-pot to output-cap 2nd tube.
(use bass-volume for "drive", and preamp-vol as "master".

(double tonestack R/pot values & halve C-values.)

[kpestoso]

thank you, puretube

I´m sorry I won´t be able to work on the preamp in a couple of days, but I promise I will report back.

[kpestoso]

It took me more than a couple of days  to made some changes to the preamp.

I put 220k anode resistor and 800 ohm cathode resistor on both stages and it hardly improved the grit (gain). I decided to change the 100k resistor on the tone stack to 33k and then I got some more gain, but I think I would like a little bit more.

I guess the tone stack is taking too much gain off between stages.  So I´m planing on removing it and check the gain or moving the tone stack after the second triode.

Another option would be to use a second tube and one or two more stages...

Stay tuned...

[Eb7+9]

You might try using a Grid biased (grounded Cathode) circuit in the second stage with 10Meg in the gird circuit and 470k in the plate circuit - this minimizes the input headroom in that stage ... even then, you can't get much OD from cascading two triode stages ... and as you say - the tone stack defeats the gain of the first stage - so even less here ... probably  realistic to stick some grounded diodes or LED's at the output and moving the tone stack following the diodes so you can drive second stage and diodes harder ...

~jc

[kpestoso]

I´ve finally found some time to modify the preamp. Now I´ve moved the tone stack (and 1M volume pot ) after the second stage, with 1M pot as gain between stages, and now it works quite well  . You can get clean tones as well as overdriven ones playing with the gain and vol pots.

My next step is to change the plate/cath resistors values and try to find the most pleasant combinations regarding to clean/dirt ratios.

I'll try to put up a web pages with some clips as well as my eyelets layout, just in case someone is interested.

Thanks everybody for your comments.