turn your diodeclipping solidstate amp into a distorting push/pull tube amp.

[Johan]

I did this mod to a solid state preamp I have, and thought I'd share.
I drew it as a stand alone overdrive, omiting the details on the powersupply/heaters. if you want to use it as I did, remove R1,R3 and short R2. and insert it inplace of the diodes to ground your solidstate amp almost certainly has
as reference, I did it on a marshall 9004. remove C2 and the LED's. insert the little amp in the holes where C2 used to be...I havnt tried it stand alone but it should work.

there are a few oddities I should mention.
first, the output transformer is a 1000:8 ohm one, but hooking up a 120 ohm resistor on the secondary instead of 8 Ohm's, it reflects 15K to the anodes. its a common little transformer you can get at a lot of places, like mouser or smallbear.

the lack of coupling caps and grid resistors are a way of overcomming "spacecharge" wich would draw the grids too far negative at this low voltage and turning the tube off. by feeding the grids straight from the opamp via the two 10k resistors, I got the grids to sit about 1/2 volt below the cathodes.(DONT try this on high voltage tubeamps.)

as a result, the opamp is constantly working to keep it's outputs at the reference DC(ground). This is sometimes claimed to force the opamp into class-A. I dont know if that's just salestalk or actually true. but it doesnt seem to harm the opamp any way, so..

the cathoderesistor is unbypassed to provide a bit of feedback without the need for extra components



as a sidenote: I first tried to make it selfsplit, like Doug's firefly, but couldnt get it to sound right. probably becouse of the spacecharge.

good luck..
J

[Steve Mavronis]

This is a cool concept. Do you have any before and after sound clips that do it justice?

[earthtonesaudio]

You can save one resistor and perhaps reduce noise by converting the op-amp section to an "instrumentation amplifier" front-end configuration.

[Zipslack]

Okay...I'm a little confused as to exactly how you are using this circuit.  You seem to indicate that you are connecting this where the clipping diodes WERE - does that mean that this is just a fancy clipping circuit (from pre-amp to ground)?  Or are you just connecting there for the input and then using the output of the tube circuit to feed into the power-amp section (which would involve a connection farther down the line)?

[Johan]

Okay...I'm a little confused as to exactly how you are using this circuit.  You seem to indicate that you are connecting this where the clipping diodes WERE - does that mean that this is just a fancy clipping circuit (from pre-amp to ground)?  Or are you just connecting there for the input and then using the output of the tube circuit to feed into the power-amp section (which would involve a connection farther down the line)?

yes, I removed the diodes in my preamp, connected this where the diodes used to be and let this circuit do the clipping instead. it starts clipping with an inputsignal ~1volt and the output is ~1volt, so all it does is replacing diodebased distortion with tubebased distortion...but who wants THAT?...   ...but I drew the schematic in a way that includes a gain controll, so it would work "stand alone" too.
J

[drewl]

This amp was a cheesy sounding 70's solid state Marshall.
I gutted it and turned it into a clasic Plex 1987 that you can footswitch in an extra gain stage making it a 2204 and it has two footswitchable post phase inverter master volumes.

[Paul Marossy]

Ha ha, cool way to recycle a cheesy sounding solid state amp!