ECH83 tube preamp

[rankot]

I am running it at 18V and it is fine. I will try 24V in next build. What about plate/anode resistors and potentiometers, would they cause any trouble in time?  Everything works fine for now.

[rankot]

This is most recent schematic, I hope it is clear:

Drive potentiometer is stereo B100k, but I don't have symbol for that, so I drew two standard pots.
Do I need C4 and C6? I just drew them, but I didn't put them on board.
R10 is there for test purposes - to see what happens when input impedance is lower.

[rankot]

Also, I found that gain pot is better if it is 500k. Pretty unusable from 500k to 1M.

[rankot]

I have settled down with 3M3 pull down resistor, and it makes huge difference to 47k! Really much more treble!

[aran.e666]

really cool timing, i just bought an ECH83 as in like 3 days ago and this post has been of huge help. i'm definitely going to make it and use it as an external preamp for my 12v valve amp

~Aran

[rankot]

This one works fine right now, but I have only one problem - if I connect it to power supply with other pedals (daisy chain, I know, I know... ) it produces some kind of hum sound. Connected to separate PS, works really fine. This is my final schematics, showing charge pump with 555. And I have added a trim pot so I can adjust the point where gain doesn't produce "choke" effect which I don't like.

[rankot]

I also experience a slight vibrato with this pedal. Does anyone have an idea why?

[jhsa]

This one works fine right now, but I have only one problem - if I connect it to power supply with other pedals (daisy chain, I know, I know... ) it produces some kind of hum sound. Connected to separate PS, works really fine.

You have a ground loop somewhere, that is what produces the hum sound when you use the same supply but not a different one.

João

[rankot]

It was actually a bad cable from bass to preamp - had a break somewhere in shielding Pre works fine right now, I am still experimenting with power supplies.

[kozmix]

Hi all,

Here's my go at tweaking the original schematic. I turned the original gain pot that sets the voltage on all those grids tied together into a trimmer (to be set at the point just before the volume breaks down when you turn down the resistance), and added a pot between the first and second stage to become the new gain pot. I found it was a bit easier to dial in the required amount of overdrive that way. I also changed the values of the first capacitor and resistor but I didn't find it made much difference. All in all it makes a nice overdrive pedal this way.

[amptramp]

If you modulate the voltage on the upper grids on the hexode section, you can make this into a tremolo.

[rankot]

I"m back to this one, because although it sounds nice, the GAIN pot is scratchy, so I wanted to change this completely and have a tone stack instead of gain.

However, during LTspice experimenting with the schematics, I've encountered a strange thing: when G1 is used as control grid, G3 connected to cathode, G2+G4 connected to B+ (actually used as a screen), everything looks normal if B+ is 18V, but if I change it to 27V, suddenly anode voltage goes negative, to -24V???



Is it a bad tube model, or the same will happen with the real tube. More important, if the same happens to real tube, will it be damaged?

[merlinb]

Is it bad tube model, or the same will happen with the real tube.

Tube models are unrealible at low voltages, so probably this.

More important, if the same happens to real tube, will it be damaged?

Lol, a tube would laugh in the face of your puny -24V!    But seriously, you won't get -24V in real life, the sim is lying.

[rankot]

Well, I supposed it was some kind of sim error. Actually, I will try it with a pedal and see what happens. 

[rankot]

OK, after half a day of experimenting, this is MY FINAL schematic for this baby (and I mean FINAL) 

http://docdro.id/Oj4Jc3D

I used Ra of 100k for triode and 82k for hexode; also kept gain pot from kozmix's schematic. 1N4148 diode used for cathode biasing, and added a DRIVE switch, so G3 can be connected to B+ or to ground (later is lower gain). G2+G4 are connected directly to B+, no screen resistor there, because every value bigger than 5k reduced final amplification.

[PRR]

> However, during LTspice experimenting with the schematics

There is no negative supply. HOW can it go wildly negative??

However I note your 2nd stage has no return for cathode current. A real tube would cut-off. A SPICE tube may have internal "current sources" which, unlike the things we call "CCS:, are actual SOURCES and will force node voltage to whatever to keep their assigned current flowing.

So just like all this "Don't work!" pleas: check your wiring!

[rankot]

There is no negative supply. HOW can it go wildly negative??

That's exactly what I though when saw the result, but it's better to ask someone with more knowledge.

However I note your 2nd stage has no return for cathode current.

ECH83 has a common cathode connection (labelled as "C" net on my schematic), so actually both triode and heptode share the same cathode connection to ground through 1N4148. It is not troubling LTspice, since results are as expected most of the time, but sometimes they just go crazy

Anyway, I rebuilt it using the last schematic posted (http://docdro.id/Oj4Jc3D) and it works really nice, most of the time as a warm, clean boost.

[jhsa]

Hey, is this preamp still sounding nice? I have a few ECH83. Built the distortion schematic we can find on the web, but wasn't that impressed with it.
I am looking for a nice, clean and warm tube sound. Would like to add a tone stack as well.. 

João

[rankot]

I'm afraid that tone stack will loose too much of a volume.

[jhsa]

So, is that the only problem? the tone stack? perhaps a buffer before??
I have a few of these tubes, as well as some 6N2P, and would like to build a nice starved plate, clean preamp. For overdrive, I have all my other pedals.

João