modifying a schematic

[dosilegecko]

So I'm using this: http://www.schematicheaven.com/newam...at_minicat.pdf


I want to add a gain stage before this designs first stage.

I was thinking of 2 possibilities:

1) trying the "matchless" triodes in parallel input stage with 12AX7's, using 100k for the anode resistor and 1.5k for the cathode resistor with bypassing caps.

2) simple doing two basic 12AX7 gain stages with 100k as the anode and 1.5 as the cathode resistors, with bypassing caps.

The problem is I am unsure if I can use the existing voltages out of the power supply to drive these, I do not want to redesign the power supply.

[leadbelly]

I do not understand what #1 means.

The existing power supply should be be good for 1 more 12AX7. Make sure you have the heater current capacity.

#2 is good. However, since an EL84 doesn't take very much input swing to be driven to clipping, this design has enough gain already. If you still want more for whatever reason you haven't disclosed, it would be wiser to use just one half of the new 12AX7 as a grounded cathode gain stage, and use the other half as a cathode follower to drive the tone stack.

[Arnulf]

Originally Posted by dosilegecko

So I'm using this: http://www.schematicheaven.com/newam...at_minicat.pdf

Any particular reason for fascination with this crappy schematic ? There are literally tons of ECC83 + EL84 SE schematics avaliable online (use alternate tube names to increase number of search hits), most of which aren't nearly as silly as this one (I just can't get my eyes off that Master pot )

Here is one - skip the input attenuation selector and tone control section if you don't need those.

[dosilegecko]

I really liked how it sounded in the store, it was very pleasing to the ear!

Maybe I'll just keep the schematic the way it is now and not mess with it. I already have a very high gain amplifier after all.

[Enzo]

It's a guitar amp. He likes how it sounds. There may be "better" designs, but he likes this one. Matchless was a brand of guitar amp, they folded. The guy who started Matchless then started Bad Cat amps. A lot of people like those amps.

#1 refers to the input stage of certain model amps that featured the two halves of a 12AX7 in simple parallel - plate to plate, grid to grid, cathode to cathode. He is thinking of incorporating this idea into his own amp.

The amp may have enough gain as it is, but he wants to add a gain stage. That would allow overdrive and distortion opportunities he may not have now. Not much different really from using an OD pedal in front of the amp. WOuldn;t be my choice, but it isn;t my amp.

And the master volume control might make more sense if the missing ground connection to it were on the drawing.

[ChrisA]

Originally Posted by leadbelly

I do not understand what #1 means.

The existing power supply should be be good for 1 more 12AX7. Make sure you have the heater current capacity.

#2 is good. However, since an EL84 doesn't take very much input swing to be driven to clipping, this design has enough gain already. If you still want more for whatever reason you haven't disclosed, it would be wiser to use just one half of the new 12AX7 as a grounded cathode gain stage, and use the other half as a cathode follower to drive the tone stack.

Direct answer to your question: Almost certainly you would need to add a "C" output to the power supply. One more resistor and one more cap. Your new stage would be in-phase with one of the existing stages and couple through the power you'd get feedback. You need to isolate every second gain stage.

Don't listen to advice about building that hifi amp, someone did not notice you were building a guitar amp.