Pedal not working - can’t find error

[frets]

Hi Guys,

I have been working on building pedals for the last 6 weeks.  I breadboarded the pedals and then translated them to pcb’s to be built.

I am presently working on one that is giving me considerable trouble.  It is a dual tube Valvecaster.

The problem is seemingly no power going into the pedal.  The LED doesn’t even light up - well, it did briefly when I inserted the guitar cable one time.  But only once.

I have triple checked my design for the board and everything is in keeping with the diagram at the conclusion of this post.

I have checked my wiring 4x and it is wired as you typically would for a pedal.

I know it is difficult to decipher the reason why a pedal does not function from a post on a forum.  

Probing shows no power going into the pedal.  I am confused by that one brief flash of the LED upon insertion of the guitar cable.

Would a jack input mis-wire interfere with the power flow?  I am using enclosed jacks - it is the first time I have ever used them.  I believe I have it wired correctly.  I have it wired in accordance with the following diagram on the switched lugs.  I do not know if this variable is associated with the lack of power flow.

From working on harnesses, my first thought was it is grounding out somewhere.  As you all know, I have limited experience building pedals and this project is my desire to expand my knowledge base.

Any help (as always) is greatly appreciated.  Again, I realize such a dilemma is difficult to diagnose from a mere post.

[newey]

Nov 25, 2022 14:13:46 GMT -5 frets said:

Would a jack input mis-wire interfere with the power flow?  I am using enclosed jacks - it is the first time I have ever used them.  I believe I have it wired correctly.  I have it wired in accordance with the following diagram on the switched lugs.  I do not know if this variable is associated with the lack of power flow.

I don't know either, but when I have used those enclosed jacks, you could tell which lugs were which by the sizes- the sleeve (ground) lug was the largest, both ring lugs a bit smaller, tip lugs smaller still. I don't know if thats true of the ones you have or not. And, did you test the switching by inserted/deinserting a plug, testing with your meter?

Also, your diagram seems to omit the grounds (on the assumption they are understood) from the 9V battery, as well as from the input and ouput jacks. Can we assume that you have used the battery negative to switch the power on/off at the input jack? Using the ring lugs?

[frets]

Yes Newey,
I have done that.

[newey]

OK, well then, I'm fresh out of ideas. Tagging JohnH may help.

A pedal with tubes seems a bit daunting to me as a first project.

[sumgai]

frets,

First things first: Congrats on biting off more than you can chew! It takes guts to attempt what you're doing, no one can take that away from you.

Next, I also have no clue what that jack might be doing, switching both the ring and the tip. So the "tip" for you is, "Measure every component before you set out to use it. And take notes!" Doing this first would've answered your question before you even sat down to ask for help.  (You can thank ChrisK for first postulating that little tidbit, here in the NutzHouse.)

And now we come to one of the few places left on the web where I can hold my own - tubes. My first thought was "oh boy,   here we go again." Long timers here will recall member dunkelfalke (Shadow Falcon, in German). His schtick was to put a tube pre-amp inside of a solid-body guitar. I could go look it up, but the project was never finished, or so I believe, because about that time, dunk slowed way down in his Nutz-ly participation.

I'll say here what I said there, in pretty much the same language - Why in the world would someone attempt to take a hi-voltage tube and feed it with 9 (or 12) volts, and then have the audacity to wonder why the bleepin' thing doesn't work? 

Let me cut to the chase here.  Without a deep understanding of exactly how a tube works, it's very easy to forget that the filament (pins 4 and 5) is also called the heater element of a tube.  (In this case, there are actually two filaments or heaters inside the glass envelope.) In simple terms, the current flows from the cathode to the anode (usually called the plate), but in order for electrons to actually flow, the cathode must be hot.  And I mean hot as in, too hot to touch, once warmed up.  The material plated onto the cathode requires that heat in order to emit electrons, as dictated by the demands of the circuit.  Short and sweet - no heat, no current flow. Period.

So here we have a pair of filaments, and the tube chart says that one of them runs between pins 4 and  9, and the other runs from 5 over to 9.  The first to characters of the tube's name are 1 and 2, which in tube-parlance means that the heaters need to see a total of 12 volts across the pair of filaments.  The designers of the tube were a bit crafty, and decided that since this was an upgrade from the older 6 volt heater tubes, they'd just use the same filament, and put in two of them.  It shouldn't take a rocket scientist to see that pins 4 and 5 will heat both filaments, correctly, with 12 volts, and that pin 9 can be left unconnected.  (This is not how most amp manufacturers do it, but it's legit.)

So what's wrong here?  Simple - 9v isn't enough ooomph to generate the required heat needed by the cathode, in order to get the job done - simple as that.  And here's the big design flaw - those filaments are generating heat, and we all know that doing so take power, as in, lots of power.  The standard 9v radio battery found in most pedals will drop over dead in less than an hour, given this job. Which then says that an external power supply is mandatory.  Which in turn says, 'what happened to the low cost, ease-of-use and portability, the whole idea of pedals in the first place?'

All of which explains why I won't bother to analyze and discuss the remaining circuit.  On the face of it, there's nothing really wrong here, beyond what I've already bitched about.  But there's a reason that the big-brains in the room don't do this kind of thing.... because solid state devices that operate at battery-friendly power demands already exist, and they do the job quite handily, TYVM.  To them, re-inventing the wheel in not very much fun at all.

Let's look at this from a manufacturer's standpoint.  Did anyone ever stop to think, why aren't amp makers doing this as a regular thing?  Shouldn't one think that they would benefit from getting rid of heavy (and expensive!) transformers, for starters...... if it were possible?  Hint: yes, one should think that.

Have I carped enough?

HTH





sumgai

[JohnH]

I was surprised to find this valve running at such low voltage. So I hunted the Web article down, and then followed a link to a thread about it. 184 pages so I'm sure the answer is there somewhere!


beavisaudio.com/projects/valvecaster/

www.diystompboxes.com/smfforum/index.php?topic=63479.0

I read that 12V may work better than 9V

[sumgai]

Nov 26, 2022 4:31:40 GMT -5 JohnH said:

I read that 12V may work better than 9Vs

Well certainly, getting the heaters up to 12vDC will get the tube up to its operating temperature, and that's a good start. But it's an exercise best left to the user as to how to overcome the battery drain.

The actual circuit itself will probably work, as it's designed to dirty-up the signal, not amplify it with any sense of cleanliness that would be the natural result of linear performance.

184 pages?!?!?!  Good grief, I've read award-winning novels written with fewer pages!

---

frets,

A word of advice:  If you're doing this for the learning experience, go for it!  But if you're doing it as a potential device to bring to market, be aware that others, with established brands, have already done so, and not all of them are still in business.  Things to watch out for: 

- Protect the tube. I mean, the pedal's chassis must provide for heat dissipation, and at the same time it must protect against fat-footed stumble-bums.  One "aw, crap!" moment, and it's back to the CAD computer for you!
- Be cheap!  I mean, damned cheap!  You're competing with a large pool of established companies, and you're already using a known schematic, so you have nothing to "up the ante" in terms of design.... all that's left to differentiate you is the retail price.

I'm sure others here could also think up some words of wisdom, I'll step back for now.

HTH



sumgai

[thetragichero]

hey i somehow missed this thread

besides what the other fellas have said, I'd certainly prefer to be running a tube off an ac power transformer. but then comes the practicality of all them volts on a pedalboard on the stage.... at about perfect level for beer to be spilled on it (either from the musician or the crowd)

now all that being said, you may wish to look into low power subminiature tubes (or nuvistors of you're feeling funky) for overdrive pedals