JCM800 standalone preamp

[fossilshark]

So to make a standalone preamp from a jcm800, would I just take the signal out right off the tone stack or would it be wise to add a buffer stage? I am open to using solid state components to make it more reliable.

Any advice is appreciated.

[fossilshark]

Also, I could just get away with a 350v (with center tap voltage) transformer?

[tubenit]

Is your stand alone preamp going into the power amp of another tube amp ............ OR ............. into a solid state PA system?

That's an important question to answer regarding the design.

I did a preamp for an acoustic guitar yrs ago that eventually worked out well.  There is a thread here about it:  Acoustic Guitar tube PreAmp & FX (el34world.com)

I've attached the schematic of what finally worked out well. Perhaps there will be an idea there that will be useful to you?
 


With respect, Tubenit

[ShoemanGB]

A 269EX would work.  I've been doing a similar project and scavenged my Firefly for the PT and chassis.  Tubenit's point is important.  There are ways to do both too.   

[fossilshark]

It is meant to go into a power amplifier but I would like to add a line out for direct to mixer stuff.

What would be the best circuit to slap on after the tone stack to do these things?

[tubenit]

Are you familiar with the Hughes & Kettner Red Box cabinet emulator?  You might benefit from reading about it.  One can use this to go into a mixer.

Hughes & Kettner Red Box 5 DI and Speaker Simulator | Sweetwater

Here is a recording I did of a mic'd tube amp speaker in a cabinet VS.  a H&K  Redbox into a computer.  See if you can tell which is which?   

https://soundclick.com/r/s8itsr

I've used this Red Box idea with a tube amp head that had a dummy load resistor instead of a speaker and then the Red Box went into a mixer sound board to a PA system for a "zero noise" stage.  It works well, IMO and is an inexpensive approach.

With respect, Tubenit

[ShoemanGB]

I have the Behringer DI(I know, I know, cheap crap) DI, the red one with the cab sim, and it does the job well IMHO, and the silver basic DI from them that lets me run a balanced line to a Torpedo Cab box and then into my mixer that gives way more options but it's way more $$.  The DI's have linked outputs from the input that you can connect to a power amp for live playing too, which I do if I don't have to do the headphone from the mixer "quiet in the house" thing. 

[fossilshark]

I am not looking for premade solutions. I think I just need a basic buffer stage coming off the tone stack for the INPUT TO POWER AMPLIFIER jack (intended to go into a peavey classic 50/50) and then slap a voltage divider on it for the lower db output (1/4" jack) for mixers. I am open to adding another tube stage if needed.

[fossilshark]

A line out intended for mixers would be able to be plugged into pedals to use as a type of effects loop? I'm sorry I am struggling so hard with this guys I've been out of the game for a while.

[PRR]

> I think I just need a basic buffer

[fossilshark]

Perfect PRR thank you I appreciate it alot. So that boy will put out an output suitable to go into a power amplifier (like a peavey classic 50/50) and then I can just blast a voltage divider onto it to have a line out for mixers? No impedance issues if theres the power amp plugged in as well as the mixer? And if I build a switcher box to go with it I should be able to plug the output into FX pedals and use it as a "send" for a fx loop situation?

[T Wilcox]

Wouldn't you want the line out to be after the power amp?
I have ran my preamps straight to my DAW but had to use a power amp in an amp sim to make it even start to sound decent.
Adding a line out to a power amp would require like a couple resistors and a pot, easy
As far as the preamp out goes I would just tap off the Master volume for a line level output.
Any FX between the pre and power amp will need to be line level. If you want to use instrument level pedals you would need to build in a SS FX loop or Dumbleator type device for them to work properly

[fossilshark]

yes I agree about the line out to mixers thing, and I will bring that up with my friend im building this for. Do you have any recommendations for a line level output circuit I can use in order to be able to run effect pedals between this preamp and a power amp? I am thinking this will have 2 outputs, one coming off exctly what PRR posted to go into a peavey classic 50/50 power amplifier and one to go into effects pedals that i can make a switchbox for to use as an effects loop.

Would the signal be too low coming off the effects pedal to go directly into the power amp? I probably have to add an FX in to a previous gain stage?

[fossilshark]

wayminute i think i got it, the FX loop would just be this circuit (minus a coupling cap on the input obviously) coming directly off the output of what PRR sent to make the FX SEND and then the second part of the loop would have a FX RETURN going into the final stage that is able to output to the power amp.

does this sound correct or am I tweaking?

[T Wilcox]

Yeah the problem with knocking the signal down to instrument level is getting it back up before hitting the PA.
For this I listed the 2 best solutions I am aware of which would be to put Zero loss/Metro/SS FX loop ( Be aware LND150's are hard to find at the moment) or a Dumbleator type FX loop.

Todd

[T Wilcox]

The tube FX loop you posted should work as well although I have not built that particular one.
Not sure you would need the tube buffer PRR suggested for any reason if you do the tube fx loop

[fossilshark]

That is a solid observation that just saved me from adding an extra tube  I appreciate the input alot i can start finalizing the schematic and get to drawing a layout.

Thank you everyone who helped me on this thread I appreciate you all.

[T Wilcox]

Good stuff! Let us know how if there are more questions and how the final design ends up!