Soldano preamp FET emulation

[black mariah]

I'm working on an emulation of the lead channel in a Soldano X88. It's pretty straightfoward, a much simpler circuit than you'd think from a high gain monster. There are a couple of points I need a little help on though.



Am I correct in thinking that all power inputs should just go to +9v instead of separate ones (B and E) like they're marked on the original schematic, or would they be looking for different voltages?

Also, does it look like the biasing of the FETs would be fine as they are? I'm considering just putting some trimpots in so I can change the bias up like that but if I don't have to go to the trouble, all the better.

[PaulC]

There's a couple of things to look at when doing the tube to fet thing.  First off is in the biasing/source resistor values.  The tube values will not give you the same type of wave shapes when used with fets.  Q2 in the soldano has a MAJOR asym clip going on with very little gain.  I'd think you'd do better changing that out to asym diodes (something like a LED on one side and a 4148 on the other - it's that extreme).  You'd want the 4148 to clip the top part of the wave.

Resistors R2 & R10 set up a first order hi cut filter with the miller capacitance of the triodes while also effecting the biasing of the tubes.  You want to add about 150pf to ground at the gates of Q1 & Q3 to add this roll-off.  You wouldn't add this to Q2 if you use that because Miller capacitance is gain related, and the 39k unbypassed resistor used in the tube stage has very little gain - very little millercap.

Also the headroom of the triodes is much different than the fets.  You'd want to mess around with the voltage dividers between the stages to get the "timing" right when it comes to when one stage clips in relation to the next.

These circuits can have some good sounds, but if the goal is to try and nail the soldano preamp what you'd end up with would look pretty different when it comes to the values used.  If my goal was to nail it I'd build the full tube version first, and then use that to scope out so I could get the wave shapes and gain issues close with the fets.

Later, PaulC

[John Lyons]

Paul

Thanks for insight. In the Dr boogie/dual rectifier posts (which are almost exactly the same circuits folks) your advice was very valuable in getting the circuit under control. I don't think you got credit of this though...thanks!!

For the cold clipping stage you mention replacing the FET with an asymetrical diode pair. Is the clipping lestened by lowering the 39K ?
in the Dr boogie the value is 3.9k but I wonder if that was a substitution or a typo?  That stage would still be biased cold but a long way from 39K... I'll have to sub in a pot there to experiment.

Black Mariah

I've wondered about the power supply as well. In the real amp the different stages get slightly different voltages because they  are separated by resistor capacitor sections. In all the FET similations I've seen, each stage is feed from +9v directly then through a Trimmer pot instead of the 100k or so plate resistor.
The value of the source and drain resistor is what biases the FET. Like Paul C is saying. There is a gain and eq curve to each stage and the stages are set up in the real amp so that the gain is controlled per stage. Putting in the values shown in the amp schematic in with the FETs might sound great but it would not necesarily be along the same lines as the real amp.

If you can I would build the pedal on a bread board and tweak it for maximum tone.

John


John

[black mariah]

I'm not really looking to nail the tone. I'm basically just looking for something that will provide enough gain and has a three band EQ. If it can get in the general Soldano ballpark I'll be happy. No matter what it'll end up tweaked to hell and back. 

My initial thought was like you said. Just build the thing using tubes. Honestly I'd still like to do that... hell, I might do that. I was warned against it because there were so many tubes but if I just build this section without the input and output buffers it's only two tubes. No more parts than a small amp. Certainly fewer parts than some pedals I've done.

Thanks for the help. I need to read up on Miller capacitance since I have absolutely no freaking clue what the hell that even is. Getting in over your head is fun. 

[John Lyons]

Black Mariah
Miller capacitance is inherent at the input of all tube stages.
When you put a resistor in line with the input there is a capacitance that can be built on. The higher the value resistor the more high cut you get. This is used in some amps to kill oscillation and shape tone amp builds.

There is mountain of info here: http://www.aikenamps.com/ look under Tech info
Also check out the stuff at Runoffgrove.com under fetzer valve  (FETvalve)

John

[MetalGuy]

Another very interesting and innovative article on the subject /with diagrams, formulas, schematic and samples/ from our fellow russian DIY-ers. Check it out at:

http://www.sugardas.lt/~igoramps/article68/article.htm

Uhhh, yes it's in russian.

[JimRayden]

Wow... ingenious! I've been thinking of simulating that asymmetric preamp triode clipping and that guy seems to have done it.

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Jimbo