some lousy tube pre-amp project of mine

[km-r]

Hi,

Im planning to build a high gain tube preamp. I had difficulties looking for a schematic of a good [reputation] high gain amp to copy the pre amp stage from. The next thing that crossed my mind is to convert the dr boogey schematic back to its mesa boogie tube preamp form.
So this is how im going to do it:



1.   Replace the fets with 12ax7's
2.   Remove the 220pf miller caps [no longer needed, I think?]
3.   Replace the 100k trimmers with fixed 100k 1-watt resistors
4.   Voltage ratings of coupling caps increased
5.   Since there are only four gain stages, I'll be using two 12ax7's and a mosfet to drive the tonestack.
6.   Please see power supply rails changes...
7.   Will be powered from a low current 220VDC PSU. See red "+" sign.

Oh, btw, that is mr gaussmarkov's schematic that I messed up with. I don't know if its illegal to do that...

So, so far, does it look correct?

Thanks!

[Somicide]

IIRC, the miller caps were a throwback to tube style.  Could be wrong, though.  I'm most interested in seeing how this progresses.  Now if only we had a bogner Ueberschall pedal...

[anchovie]

You might want to start with the original amp schematic, rather than working backwards from the Dr Boogie. Gain stages 1 and 4 had 220k resistors rather than 100k.

http://www.tubefreak.com/mesa.htm

[soulsonic]

Check out the Sloclone forum. http://sloclone.yesterdaysrevolt.com/index.php
They have several great projects up there. That stupid Mesa Recto is just a lame ripoff of the Soldano anyway, so you may as well start with the REAL DEAL.
Check out the preamp for the OD3 project:
http://sloclone.yesterdaysrevolt.com/OD3/OD3.pdf
Here's the Soldano X88R Preamp - obviously the inspiration for the circuit used in the OD3:
http://sloclone.yesterdaysrevolt.com/schematics/x88r.pdf
The "Panther" Preamp - a Soldano derivative by a hobbyist:
http://www.thinlizzy.de/preamp/panther.gif

This is probably the best place to start, there's much information to be found on those pages.

[km-r]

great help guys!
you see i want to study wiring up a high gain tube circuit...

ill have to browse those pages first.

[d95err]

220VDC seems a bit low for a high-gain preamp. From what I hear (haven't built one myself), high voltages is very important to get a good high-gain sound. Around 350VDC is common in amps like the Soldano etc.

If you want to use a lower voltage, you should probably adjust the circuit to account for the lower headroom (e.g. change voltage dividers to drop more voltage). Also, you may need to adjust the bias of the gain stages to get the right working conditions. High-gain circuits are very sensitive to how each stage is biased in order to get the right type of clipping.

[soulsonic]

My experience is that the biasing is really key. Given a certain biasing, the actual plate voltage isn't too crucial as long as it's within a certain range and the ratios of everything is right. That being said, I've found tube circuits to be far less sensitive to biasing weirdness than BJTs and certainly JFETs. It seems to be that as long as it's close, it'll work - unlike solid state where if it's off a little bit you might not get any sound at all. I've had tube stuff that was way off before, and it definitely sounded bad, but it was pretty easy to get it right once I figured out what I was doing. Like anything, it takes some trial and error, but once it all clicks in your head, it becomes some of the easiest circuits to do, IMO. Of course, you can agonize all day about voltages and little tweaks, but let's start with first things first.... first thing you want is sound; after you've accomplished that, then you can drive yourself crazy trying to get that "ultimate tone".