Tube preamp pedal

[Sheldon]

Hello,
I'd like to build my first DIY stompbox : a tube preamp. I'm quite a newbie even if I learned somes electronics basics at school and I assembled a few fet kits.
I don't know what will be my final circuits and gain stages, for educationals purposes I picked the schematics of a simple single channel amp that I know, the Orange Dark Terror. I found the schematics here :
http://guitar-gear.ru/forum/index.php?app=core&module=attach&section=attach&attach_rel_module=post&attach_id=22573

My 2 main issues for the moment are :

* Power Supply :
First solution is to use the 230Vac mains voltage, use the same circuit as the schematics above replacing the transformer with a 230Vac/12Vac transformer (heater tube voltage) and another 12Vac/230Vac (with the schematics rectifier circuit, for B+ voltage).
My concerns are that I would prefer to use a pedalboard 12Vdc voltage rather than 230Vac mains voltage, and using a 230Vac as the output of the rectifier will lead to work in the 15W mode with the schematics tranformer and to have 300V voltage for B+, maybe a 200V voltage (as for the 7W mode) would work better for a standalone preamp?

Second solution is to use a Nixie Clock type switching power supply (http://desmith.net/NMdS/Electronics/NixiePSU.html) to work with 12Vdc, don't know if it would work and if it's complex or not

* Preamp Output
I don't know exactly where to put my preamp output jack. Normally it would be the same as the effect send loop send. Is the V3b tube cathode follower necessary if I want to connect my preamp to a cab simulator (Torpedo, Palmer, A/DA...) ?
If so, to avoid using 3 tubes, is it possible to replace the tube cathode follower with Mosfet (LND150, IRF820, ZVN0545A?). I found some schematics on the internet, but I don't know how to adapt them to the V3b cathode follower of the Orange Dark Terror.

Hope you will be able to help me, thank you in advance

[tubegeek]

I think you are missing one important question: do you want to start with such a complex circuit, especially one that contains hazardous voltages?

With all due respect, you might want to get your feet wet with a simpler project first.

[Sheldon]

I already assembled a few FET mic preamps. I'm not an electronics expert, but I work in electrotechnics/electricity so I'm aware of what 300Vdc voltage is!
Maybe this project is too ambitious, maybe I will not finish it, but I really like learning things so I'd like to understand how it works!

[GibsonGM]

I hear you, Sheldon, but these are different than wiring in a new outlet or something.  It is VERY easy to put something together, maybe you unplug it, casually forget to discharge a cap and BOOM.    Or, you are playing the circuit very carefully, hear something that needs changing, and unplug the wrong thing.....reach into the board and BOOM you just got electrocuted.  The more you get 'used to' doing stuff, the more complacent you can become...you need to develop solid habits BEFORE reaching into high voltage, not *while* you do so.

I'd strongly suggest you check out some "Tube overdrive running on 12V" stuff (search forum), move up to the 40v, 50v range, build a preamp KIT, all while you learn.     I guarantee you there are about 5 to 10 pitfalls that can trap you, even if you DO know a few things about electrical safety.  We want you to develop the skills and get them ingrained BEFORE you start setting up 390V transformers on the workbench with loops of wire hanging off the side and stuff, LOL....

1 misplacement of a wire can kill you, and it is fairly easy to do it.  Not to create fear, but that's just the reality of it.  Go about this in the right order, and in 6 mos. to a year, you WILL be doing your own turret board stuff at high voltage.    There is a path in going about it, tho.   Please take some time with it!

FET mic preamps are cool, but they won't kill you or paralyze part of your body, generally speaking.  Or burn your house down.  "Awareness" is good...building skills is better!

[PRR]

How is a man going to *learn* something without getting bit??

Many of my best lessons left scars.

We hope Sheldon shows-up again still breathing, but we can't tie him to a comfy chair while his mind rots.

> avoid using 3 tubes

This thing is ALL loss networks, even more than tubes. Cut some of the fluff out. The "Shape" network wants a hi-Z load BUT then they cut-down 4:1 and 4:1 again. If we play the cards right we can have the same loss with a reasonably low (stage-work) output impedance, no buffer.



The supply voltage can be 100V to 500V and still "work". At the lower voltage a high-output guitar will not play "clean" unless you turn-down on the guitar. At the higher supply voltage it may not dirt-up without heroic strumming. 200V, 250V, 300V... anywhere in there will be fine.

The effective load on the power supply will be around 75K (for 4 triodes). So about 3mA or 4mA. That Nixie supply is good for much more (18, 40, 55mA). Switching supplies often have a "minimum load", related to their choke size and the overall ratios. Someone who understands these contraptions will have to validate my confusion, but I *think* the caveman fix for a too-low current is a higher choke value, perhaps 500 or 1,000uH. Logically Rsense would be larger (because we want to sense lower currents), and I'm tempted to say 0.5 ohms, but I may be very wrong.

There are gazillions of DC --> 2 bottle power supplies which may be more suited than a Nixie supply (Nixies need moderate voltage but considerable current to light-up many digits of eye-candy).

[GibsonGM]

How is a man going to *learn* something without getting bit??

Many of my best lessons left scars.

We hope Sheldon shows-up again still breathing, but we can't tie him to a comfy chair while his mind rots.

LOL, yeah, you're absolutely right.  One of my first projects was a Jacob's Ladder from a discarded 20kV oil burner transformer.  D&mn lucky I didn't use it to "power" some things I was thinking of, ha ha!

And I remember cleaning out the back of the family trash compactor with my finger...unplugged, sure....touched a BFC I didn't know was lurking in there, woke up across the room stupid for an hour.

So all that stuff said, disclaimer is out of the way.  Hope you find the right way to power your idea, and get it done safely and well, Sheldon

[Sheldon]

thanks a lot
Will it work with a 100K resistor instead of the 1M resistor and a 10K audio volume pot, so I have a 10K maximum impedance?

[Transmogrifox]

I don't see any reason you can't build the Orange Dark Terror exactly as the schematic shows this, and use the FX send output as your output.

In your other thread you talk about a FET for output buffer.  Just replace V3B with the FET you proposed and you're good.

The bias arrangement in the Orange Dark Terror schematic output buffer is exactly the arrangement I recommended in the other thread (R22, R23, R21).

Power dissipation with this circuit will be a little risky as RG warned, but the bias tells me the signal swing is small enough this probably won't be a problem.  Switching to a FET in a TO-220 package would eliminate all doubt with this substitution.

Adding the zener from gate to source would not be a bad idea as RG recommended, but otherwise this is a good candidate for direct FET substitution.

[J0K3RX]

If you're hangin out in the guitar-gear.ru forum then you will see variants of these often especially in the tube pedals...

[head_spaz]

This micro sized Nixie supply might help to simplify your project. Cheap too.



(Vertical Module)
SmartNixie HVPS-V
Includes 8mm Dia 330uF EEU-FC1C331 input capacitor
1.8V to 16V input, 150V to 200V adjustable output
4W @ 5V input = 23mA @ 180V
8W @ 12V input = 45mA @180V
0.65" x 0.80" x 0.45"
#1363 HVPS-V $13.95

(Horizontal Module)
SmartNixie HVPS-H
Includes 8mm Dia 330uF EEU-FC1C331 input capacitor
1.8V to 16V input, 150V to 200V adjustable output
4W @ 5V input = 23mA @ 180V
8W @ 12V input = 45mA @180V
0.95" x 0.95" x 0.35"
#1364 HVPS-H $13.95

[J0K3RX]

Might wanna have a look at this... Rej and John (GrindCustomsFX) over at madbean created this project and it looks spot on and really neatly laid out! Great if you want to just get to it and not have to design and all that sh!t... Probably also a great learning tool for that 1st time tube adventure  I know they put a lot of time, hard work and testing into this and the price is just silly cheap for this sort of thing!

https://dl.dropboxusercontent.com/u/28586266/GCFX%20Build%20Docs/STM800BuildDoc.pdf
http://www.grindcustomsfx.com/

[Sheldon]

I built the preamp, it works except one hiss problem (around 3kHz) when I turn up the gain pot above the middle :
http://www25.zippyshare.com/v/FmAHmgUl/file.html

If I disconnect the guitar jack and touch the jack, it sounds like this :
http://www65.zippyshare.com/v/gl8kghn9/file.html

It seems to be a ground problem, but I don't understand why. There is one common ground on my pcb, the DC power jack is isolated, the output jack is isolated and the input jack is not isolated, so the ground is connected to the aluminium enclosure in only one place.
Does anyone have an idea?

[GibsonGM]

Ouch.   I wouldn't call that hiss, I'd call it oscillation    I don't use switching supplies, so I'm not too much use to you there, but - can you provide a clip of audio (playing guitar) when it's NOT oscillating like that?  I mean - does it WORK otherwise?  My gut instinct makes me think not enough decoupling between your stages and power supply, and also in's and out's running close to each other, perhaps. 

Enough gain, and ANY circuit will feedback.  It's very hard to NOT meet the conditions required.   There are stray capacitances etc. at work all the time.   Your touching the jack suggests that is happening, but if you used a switching jack to take input to ground with no plug connected, shouldn't do that...did you? 

Some layouts are better than others, and experience helps dictate how you place wires and so on to avoid this.

Provide more data...what schematic are you using (the Orange Dark one?  Or Jok3r's suggestion?)..what do you measure (DC) for B+, and at your plates?  Cathodes?      Can you do a clip of you playing at lower gain, if it sounds good?

If you cross wires to the tubes, are you doing so at right angles....does using a dowel or non-conducting object to gently  move wires change the oscillation?   Are you using shielded input wire, connected to ground only at one end?

That will get us started

[Sheldon]

thank you for your answer. Sorry for the "hiss"/oscillation confusion, I'm quite a newbie in music equipment building, and english is not my native language!
So I'll try to answer your questions :
- On lower gain, it sounds pretty good. There is a very low volume oscillation, around 150Hz, maybe due to the nixie
- I don't use a switching jack to take input to ground with no plug connected
- I use a schematic quite similar to the Orange Dark Terror, with a 220Vdc nixie power supply (max1771) and a zvn0545a mosfet cathode follower
- B+ voltages are around 200Vdc
- I use no shielded wire

With further experiments, I think I have an idea.
I use an aluminium enclosure. Inside the enclosure, the pcb is on the lower part of the box, the 2 12ax7 tubes are on the top, input jack and DC jack are on the right side of the enclosure, output jack on the left side, and the pots are on the cover (cap/lid?) of the enclosure.
I decided to use this kind of connectors/wire for the pots : http://www.musikding.de/PCB-connector-4-pins
I think this is the error. When I move the wires near the tubes, the oscillation is huge, when I take the wires away the oscillation is lower, when I touch the wires with my hand the oscillation stops.
It seems I'll have to remove those "antenna" wire and replace them with shielded wire.

[GibsonGM]

Your english is very good, Sheldon   It just takes time to learn what words to use to describe symptoms!  No worries.

Can you post a photo of your build, so I can see how the wires run?  It is very common that different circuit 'blocks' couple to each other, and cause the symptoms you describe.  Where you have signal currents, you also have capacitive and inductive coupling...    The good news is that most of them can be solved by trying different things.

I believe your plans will help!   There are a few "rules" for high-gain builds such as yours.  Here are a few:

- Use a grounding jack for your input, so input is grounded with no plug inserted (ground the input with a jumper to test if this does something to the oscillation)

- try to use the shortest wires possible to go from circuit to tubes.  Try not to cross wires; if you must, cross at a right angle.  Be aware of what wires are more likely to radiate - your plates, runs to/from coupling caps...these are higher signal levels, which may couple to wires that carry smaller signals.  Keep power runs away from signal as much as you possibly can!

- be sure your heater wiring is coiled around itself, right up to the tube socket, and as far from other wires as possible.  Get it out of the way as much as you can.  Most important if your heaters are AC, less so if you are using DC.

- use shielded wire for the input line, with shield grounded at only one end.  Use anywhere you must run a high-gain signal over any distance (as in connecting to a pot).

- place your input grid stopper right on the valve socket itself.    Place a 10n cap from input to ground on the jack itself.

- use 'star grounding' when possible.   

- Each filter cap should be close to the stages it feeds, not placed in 1 location with wires running to the stages

These are some basic rules, and there are more!

Let's try moving the wires first!

[J0K3RX]

I agree with everything Mike said... I use the nixie type smps in my builds and I have to say that it is surprisingly silent vs. a transformer. I have some transformers that I mess around with but for these small to medium pedals I use the smps. I have tried different things with the heaters but I prefer to run traces that both travel along the edge of the board to the same side of the board, from there I used shielded cable to the power supply. I use DC heaters so this works well for me... If I were to use AC heaters then twisted and away from everything like you see in most amps would be the way to go.. One thing people often overlook is the gain pot.. If it is not on-board and or well placed it can become a receptor for interplanetary life forms If it is off-board and hanging by wires, Better shield that gain pot and install it in a good place! In a regular amp setup everything is usually well spaced and placed but in a small pedal you have to be extra clever with the way you route cables, wires, component placement, inputs, outputs, power etc.. 

[Sheldon]

thank you
I'll try to buy shielded wire and rewire as your recommandations

[GibsonGM]

Keep going, you have already done the difficult part.    Now you are only going through the process of cleaning up the layout, and you will learn all you need to know by doing this

I cannot say enough good things about shielded wire...ground one end only (I've said that before - because if you ground both ends, you have a big ground loop!)