question on geofex tube wah

[panterafanatic]

Alright guys. i wanna do this. http://www.geofex.com/Article_Folders/tube-wah/tube-wah.htm hopefully at christmas, before if i'm lucky.

Couple questions. with the 12AX7, do i need two tubes? they scheme has the 2 halves that each have a common wire coming from it, but they aint electrically connected. i haven't done tubes before so my knowledge is rather lax. but i love working on wahs, so this is like top of the list.

Wouldn't the voltage affect the tube and FET? there's like a 100v difference.... would it not bias them completely different if i used 100 compared to 200v?

What wattage should the resistors be? 1/4w as standard on SS?

Do all the caps have to be rated for 200v or more or can they be lower? again, first time with tubes. i plan on doing more research before starting it.

On the powering scheme. why go down to 12v AC to bring it to 130v DC? also, how many transformers do i need? one or two? i was slightly confused by how it was drawn. the wires coming off the two parts of the secondary that go down to the right and say 12vAC, why are they there? where do i wire them? or is it just to say the first transformer by the 1k is says +130v, i'm assuming the arrow underneath is the -v?  why's there a box with the dotted line at the one side? just to say that goes inside the pedal whereas the wallwart goes to the outside?

[Ripthorn]

Okay, here goes:

You only need 1 tube, a single 12ax7 is a dual triode, meaning there are two triode sections in the one glass envelope.

The IRF820 is a high-voltage MOSFET, so it can handle the really high voltage in the circuit and many MOSFET configurations will self-bias at any voltage.  The tube's plates can take up to ~350V and be fine, but more volts = more volume typically.

For resistors, 1/2 W are good to be safe.  Those are what are used in typical tube preamp sections.

The caps should be rated typically for about 1.2 times the highest voltage in the circuit at the least.  If you go 200V, I would use at least 250V rated caps.  If you go 100V, I would use 200V rated caps.

Going down to 12V is to power the heaters.  Tubes require a very large heater current to charge up the tube (not really charging it up, but a good way to think of it).  The two lines that say 12Vac go to the heater pins of the tube.  That is why it is stepped down and then back up. Plus, you only rectify mains voltage directly if you want to die.  The box with the dotted line is actually in your enclosure and the dotted line indicates that the rest of the circuit is on the other side of that line, schematically speaking.  The far left part of the power section is a wallwart.

[panterafanatic]

thank you. i'll do 1/2 Watt then. should i just use 200v caps with the 130v output?

anyone know where i can find the walwart and transformers on mouser? i'm having a little issue finding it.

[R.G.]

I've had a few notes from people who built these, and they were very happy once they got the usual and customary building problems fixed. Just recently a guy built one from some submini tubes and tucked the whole thing, including power transformer inside a wah shell of some kind.

I figured most people would want a single tube build, so I cheated and made the final follower a MOSFET. The IRF820 is kind big and clumsy these days. It's a TO-220 package. Mouser stocks TO-92 MOSFETs by SuperTEX and Zetex that are 500V 0.5W devices that would replace the 820 easily enough.

12AX7s take 150ma of heater current at 12V, or 300ma if you use the split heater tap to run them from 6V. The plate current is typically down in the range of 1-10ma.

Let me be clearer: DO NOT TRY TO RUN THIS ON RECTIFIED MAINS WITHOUT A TRANSFORMER.  Even if you live, your bass player may borrow it and die, or your son may start tinkering with it in ten years...

[frequencycentral]

Low voltage version? Charge pump version? SMPS version?

[panterafanatic]

I've had a few notes from people who built these, and they were very happy once they got the usual and customary building problems fixed. Just recently a guy built one from some submini tubes and tucked the whole thing, including power transformer inside a wah shell of some kind.

I figured most people would want a single tube build, so I cheated and made the final follower a MOSFET. The IRF820 is kind big and clumsy these days. It's a TO-220 package. Mouser stocks TO-92 MOSFETs by SuperTEX and Zetex that are 500V 0.5W devices that would replace the 820 easily enough.

12AX7s take 150ma of heater current at 12V, or 300ma if you use the split heater tap to run them from 6V. The plate current is typically down in the range of 1-10ma.

Let me be clearer: DO NOT TRY TO RUN THIS ON RECTIFIED MAINS WITHOUT A TRANSFORMER.  Even if you live, your bass player may borrow it and die, or your son may start tinkering with it in ten years...

i'll use both transformers. i was just wondering as to why it needed two, now i understand.

so i bias the tube to go to 150ma with the 12v scheme?

Rick, what's a charge pump? SMPS? i'll google it anyway.

[Ripthorn]

I may have to build one of these now.  I have some small encapsulated transformers from Pulse that were cheaper than an SMPS and almost as cheap as a charge pump setup.  You can read about charge pumps at GEOFEX if you search for MAX1044.  Both articles are very good for understanding them and how to implement them.  Be aware that they get more inefficient the higher you try to pump the voltage.

[frequencycentral]

Rick, what's a charge pump? SMPS? i'll google it anyway.

Some useful links:

http://www.geofex.com/circuits/+9_to_33.htm

http://www.ledsales.com.au/kits/nixie_supply.pdf

http://www.diystompboxes.com/smfforum/index.php?topic=74088.0

http://www.diystompboxes.com/smfforum/index.php?topic=74729.0

[panterafanatic]

read first link, only a couple minutes left in class. with it, do i use a 12v regulator to lower it to 12v from 17v? also, will it be sufficient to heat the tubes?

[frequencycentral]

read first link, only a couple minutes left in class. with it, do i use a 12v regulator to lower it to 12v from 17v? also, will it be sufficient to heat the tubes?

Think of a 12AX7 as three seperate circuits in one, two triode sections and one heater:

The heater section needs 12 volts @ 150 ma to pin 5, and pin 4 grounded.

The two triode sections can be run at a voltage of your choosing (pretty much), and consume very little current compared to the heater section.

So the logical way to go about using tubes with charge pumps (or SMPS's) is to use a 12 volt DC regulated (and not a switcher) power supply, rated at say 500ma. This provides for the heaters power needs. Then you can look at the plate voltage. The first link you mention uses a MAX1044, there is another identical pin-out chip called a TC1044 which is more suitable as it can run at 12 volts (whereas the MAX's max is 10.5 volts).

To answer your question, the charge pump isn't used in the tube heater circuit, it just provides high voltage (at low milliamps) to the triode plates.

[panterafanatic]

ripthorn, totally missed your post a little earlier sorry. if i use a couple setups parallel will it add consistency?

am i able to provide the heater sections with additional current than needed but still use the tube to its full ability?

will i need lower resistors to bias the triodes if using a charge pump for the plate voltages? should i use say 100k trimmers until i get the proper current?

[panterafanatic]

so my modify link isn't showing up on my posts.. but which transistor should i use? they all have different ohmic values. (is ohmic even a word?)

http://mouser.com/Semiconductors/Discrete-Semiconductors/MOSFETs/_/N-6hpdi?P=1z0w951Z1z0y3zrZ1z0z60l

does mouser sell tubes? i dont see any. but i'm not great at finding stuff there. too many choices i reckon.(probably a good thing overall)

[aziltz]

Plus, you only rectify mains voltage directly if you want to die. 

Advice to live by.

[panterafanatic]

Plus, you only rectify mains voltage directly if you want to die. 

Advice to live by.

well i'd hate to die by ignoring it. 

can anyone help me with the mouser stuff though. i have no clue when it comes to those trannys

[panterafanatic]

hey, thought just occurred to me. how many tube pedals (say they all needed 130vDC) could run off of that transformer layout in the tube wah article?