Do pedals that feature a 12AX7/ECC83 tube really create preamp tube...

[Taylor]

Indeed, tubelectron, I give your construction skills an

A+!

Those are some intense innards, nice job.

[Paul Marossy]

can't really agree with the "it has to be all tubes" sentiment

I built a shaka tube and it sounds very valve-ey to me (we call tubes valves in australia)

I built it in a night, it has a maximum of 40v anywhere internally so it is safe for DIYers.

I also agree on the "all tubes" thing. I built the Shaka Tube, and I think it sounds really good. It seems to me that the preamp tube is in that circuit only to give it that starved plate sound, or to make it tube-ish sounding. The majority of the distortion is coming from the opamp, not the tube.

Now if you want distortion coming from the pedal, a tube can do it, but if you only have 2 stages (like you would have with a 12AX7) you won't be able to achieve smooth distortion and decay. It works quite correctly with subtle overdriving but if you go further, it sounds like a fuzz with blunt decay. I speak for 100-250VDC powered units, never tried lo-voltage.

Using those preamp tubes at 12-24V makes them operate way way out of their linear operating range. That's what gives the "starved plate" tube circuits their characteristic sound. Some people like it, some don't. I personally think all tube distortions operating at low voltages sound terrible when you crank the distortion all the way up. Very fizzy and nasty sounding to my ears.

[petemoore]

  The cirucits I build begged for complimentary circuitry, I had some layin' around, pretty sure I tried most of the usualy [precompression, after distortoboosting etc. TC] even the 12au7 pleaded for more plate voltage than....IIRC that one was up to 36v for a while.
  I found it easier and fruitful to cajole Jfets, CMOS even bipolar to do LV tricks.
  9V bipolar boost into LV Jfet-voltage-starved-stage  [LM 317 adjusted]..this was making a cool punch-duck [strong attack made 'comression-folding' effect.
  I wanted to do further testing...see AMZ Lab Notebook

[tubelectron]

 Hi,

Indeed, tubelectron, I give your construction skills an


Quote from: tubelectron on Today at 01:39:50 PM
A+!


Those are some intense innards, nice job.

Thanks Taylor ! Yes, it is a quite compact unit : the box is 114x89x55mm ext.

Using those preamp tubes at 12-24V makes them operate way way out of their linear operating range. That's what gives the "starved plate" tube circuits their characteristic sound. Some people like it, some don't. I personally think all tube distortions operating at low voltages sound terrible when you crank the distortion all the way up. Very fizzy and nasty sounding to my ears.

It is what I am pretty akeen to think also, Paul, but unlike you, I didn't made trials on low voltage tube overdrives, as found on your website.

I found it easier and fruitful to cajole Jfets, CMOS even bipolar to do LV tricks.

I am accustomed to tube circuits for years, petemoore, but without any hesitation I state that the tube is not the "tone-providence" as often argued. I should even say that for Hi-Gain / Highly Distorted circuits, not vintage or moderate gain operation, the solid-state technology is unless exception way preferable than tubes. And yes, may be more easy and rewarding to tweak, too...

A+!

[petemoore]

  You'll hear the components if it 'uses components.
  Choose components chosen for preferred sound.
 
 

[brett]

Hi
I forgot to mention a key thing about using 12AU7s and 12U7s for distortion:  Don't expect them to distort much or very sweetly if fed a guitar-level signal (100 mV p-p).  Shove 1 or 2 volts into the grid.  The voltage turns weak/thin/fizzy into a much fatter and fabulous sound. 

I have has success with: pickup -> JFET buffer/booster (common source) -> 12AU7 -> JFET buffer (source follower) -> volume control.  In my limited experience, both JFET stages (or their BJT equivalents) are necessary for it to work. 

For tuning the bias and tone to personal taste, a variable resistor (pot) on the plate of the 12AU7 is handy.
cheers

[Paul Marossy]

Hi
I forgot to mention a key thing about using 12AU7s and 12U7s for distortion:  Don't expect them to distort much or very sweetly if fed a guitar-level signal (100 mV p-p).  Shove 1 or 2 volts into the grid.  The voltage turns weak/thin/fizzy into a much fatter and fabulous sound. 

I have has success with: pickup -> JFET buffer/booster (common source) -> 12AU7 -> JFET buffer (source follower) -> volume control.  In my limited experience, both JFET stages (or their BJT equivalents) are necessary for it to work. 

For tuning the bias and tone to personal taste, a variable resistor (pot) on the plate of the 12AU7 is handy.
cheers

Interesting findings. Sounds like that is a good formula for a nice sounding tube distortion.

[Pigyboy]

However, there are plenty of simple voltage multpliers around that are capable of turning your 12V DC into 100 to 200V DC at low current (a couple of mA, which is all you need).  If I were starting again, I'd look at that rather than a Real McTube type supply with 2 power transformers in back-to-back arrangement.

I know that exists compact integrated DC to DC 12/250 (or so) converters able to supply the plate voltage for a 12AX7 - I should search more info about that interesting alternative. Any of you had product references ?

There is this Nixie supply that I think is used in FrequencyCentrals Obsession amp.
http://desmith.net/NMdS/Electronics/NixiePSU.html

Do you have the schematic for your PS?

Nonetheless, a "reversed" transformer (P 12VAC to S 230VAC) radiates neatly less than a P 230VAC Xfo, so it needs a light shield only if it is close to the audio circuit, as shown below on my HCTO overdrive innards - which is noise and hum free at any setting, 250VDC plate supply and 12VDC heating, from an ACmains to 12VAC 0.5A wallwart adaptor.

[tubelectron]

Hi Pigyboy,

There is this Nixie supply that I think is used in FrequencyCentrals Obsession amp.
http://desmith.net/NMdS/Electronics/NixiePSU.html

Thanks for the information and link ! - Nonetheless, I thought that it would be a more compact device.

Do you have the schematic for your PS?

Yes. The wallwart is a stated as 1A, but a 0.5A model is convenient. You also have the detail of the TBP FSW operation with R.G. Keen's Millenium Bypass led indicator. Here it is :



A+!

[defaced]

What's the purpose of the 10 ohm, 3 watt resistors in the DC supply?  I can see the reason for the one on the positive rail, but not in the ground. 

[tubelectron]

Hi defaced,

What's the purpose of the 10 ohm, 3 watt resistors in the DC supply?  I can see the reason for the one on the positive rail, but not in the ground. 

Having an accurate look to my schematic, you will find that the DC supply to the heaters is balanced vs GND, which is taken at the centre of the serial heater operation. So that's why I used 2 identical resistors in symmetry. By this way, each heater has a common point to the GND. Well, it may be a bit "Art for Art", but it works reliably and perfectly : no smoky regulator, no hum at any setting.

A+!

[Paul Marossy]

Are you related to Ken Parker? That looks very much like his handiwork. 

[tubelectron]

Hi Paul,

Are you related to Ken Parker? That looks very much like his handiwork. 

Sorry... I don't know who is Mr Ken Parker ! Does he build electronics ?

I design all my schematic by hand. It's just a pleasure for me...

A+!

[Paul Marossy]

Hi Paul,

Are you related to Ken Parker? That looks very much like his handiwork.  

Sorry... I don't know who is Mr Ken Parker ! Does he build electronics ?

I design all my schematic by hand. It's just a pleasure for me...

A+!

Yeah. Well, sort of. He is the one who came up with the Parker "Fly" guitar. His wiring schematic and drawings in the owner's manual for those guitars look quite a lot like the style in your schematic there.

[Pigyboy]

Thanks for the info Bruno!

[merlinb]

I dont know about you guys but I made a pedal that runs a 12au7 , 12at7,12au7 ( in that order ) running on 9 volts and it distorts everything as much as I ever needed.  Just three valvecasters in series with a few little mods like input caps etc.

That's sweet! With those three valve lined up it reminds me of the Effectrode pedals! http://www.effectrode.com/website/effectspedals/effectspedals.htm

[tubelectron]

Hi merlinb,

I dont know about you guys but I made a pedal that runs a 12au7 , 12at7,12au7 ( in that order ) running on 9 volts and it distorts everything as much as I ever needed.  Just three valvecasters in series with a few little mods like input caps etc.

That's sweet! With those three valve lined up it reminds me of the Effectrode pedals! http://www.effectrode.com/website/effectspedals/effectspedals.htm

I think -but I may be wrong - that Effectrode's pedals works with high voltage on the plates, not 9V.

A+!

[merlinb]

I think -but I may be wrong - that Effectrode's pedals works with high voltage on the plates, not 9V.

Yes, they work off high voltage. (That wasn't the point I was making)

[zambo]

As mentioned before the starved plate tubes are a bit fizzy but i have noticed if you roll off some of the highs and lows in the circuit design it makes a decent sounding pedal. I tried one recently at 45 volts and it sounds pretty wicked. I will try and get a clip up. I was thinking of starting a thread for high gain all tube pedals. High or low voltage. I have been getting nixie smps for 13 bucks off ebay that run 200v and i was looking at the firefly preamp in a pedal....perhaps? Has anyone tried the cascode design in a hi voltage tube pedal?

[tubelectron]

Hi Zambo,

I quickly tried in a pedal the SRPP circuit (Shunted Regulated Push Pull - well known in tube Hi-Fi audio) on a 12AX7, which is a cascode variant.

It doesn't give much more gain than a 100K loaded 12AX7, but it's more linear and the Z output is lower. For a pedal I felt it had no real advantages, and some drawback : you need 2 triode section to have the gain of only one, the heater-cathode insulation fo the upper triode can induce ring noise or hum, so it's better to balance it between the lower and upper triode. Another unwanted complication !

But the real cascode circuit may be more interesting in gain performances.  (Note : I only use HV in tube design).

What is worth for audio is not always worth for pedals...

A+!