Help with Marsha Valve Tube Schem

[ildar]

I'm going to take a shot at this pedal. http://aronnelson.com/gallery/vanhansen/marshavalve_tube_02
It's my first tube project, and I'm not 100% sure how to read the schem as far as the tube goes. It looks to me that only 6 of the pins are connected. Is this so? If so, which 6, and if not what am I missing?
Otherwise, it looks like a somewhat simple build, a good place to start with tubes.

[BrianJ]

You are correct,  It only shows 6 pin connections.  When people draw an opamp based circuit they omit the power supply connections for simplicity.  With tubes, heater connections are typically not drawn in the audio path schematic (their relatively simple connections are typically drawn into the power supply schematic).  The heater is a filament that heats inside of the tube- suffice it to say that it is necessary for a tube to function.  The heater is connected to 3 pins of the tubes base and can be operated in 2 "modes." 

12AX7 pinout  (also 12at7,au7,bh7... most 9 pin dual triodes)

Pin 1 Plate A (100k R)
2 Grid A (input)
3 Cathode A (2k7)
4 Heater (end 1)
5 Heater (end 2)
6 Plate B (other 100k)
7 Grid  B
8 Cathode B
9 Heater (center tap)

You can use either half of the 12AX7 for either part of the circuit (Do not however mix up parts of the two triode sections.  Connecting plate B to grid B be would makes this into quite a poor distortion pedal).  The heater can be wired like this:

A) 12.3v (AC or DC) across pin 4 to pin 5 with pin 9 unconnected.  This configuration will draw 150ma of current.
or
B) 6.3 volts applied to across 9 to pin 4/5 (they are tied together).  This will draw 300ma of current

You will be using choice A, obviously because the WHOLE circuit is running on 12V.  Most amps will use configuration B because POWER tubes typically only 6.3v heater supplies (higher currents as well) and having separate heater supplies would be silly.


Hope that helps, post again if you're still confused...

[ildar]

Yes, that helps quite a bit, thank you.
One other question, another simple one, with pins facing up, are they counted clockwise?

[BrianJ]

Yes, they are counted clockwise.  Most good tube sockets (use a socket) are numbered on the bottom.  I would also recommend drawing a small diagram of the bottom of the socket (with numbers) and a schematic indicating the internal parts of the tube to hang over your work space.  Trust me, it makes things much easier.  My drawing only moved once in the last year, and that was to make a pinout for you last night.


Good luck

[tennisdude]

Hi,

just to clarify, pin 7 is connected to the 470k resistor and .0022 capacitor in parallel, and pin 8 goes to the 10k resistor?

Also, what's a good way to get 12v? Would a 9v battery be sufficient, or would you need an external power supply?

Thanks
John

[ildar]

Also, what's a good way to get 12v? Would a 9v battery be sufficient, or would you need an external power supply?

Ooh ooh...I can answer this! 
Aside from using an external ps, you can try 8 AA batteries for 12v, although the ma draw may suck 'em dry quickly.

[tennisdude]

plus its a little bulky 

I may just go for the external ps route. thanks anyway 

[ildar]

It's definitely bulky, but some of the "major" forumites swear by using them, of course for different projects.

[BrianJ]

I wouldn't waste the money on batteries.  150ma would kill a 9v in three hours.  Get a 12vdc 500ma power supply and an apropriate power in jack. 

"just to clarify, pin 7 is connected to the 470k resistor and .0022 capacitor in parallel, and pin 8 goes to the 10k resistor?"

That is correct.  I would recommend googling for some other tube amp schematics, you wll definitely begin to see a pattern for the connections.  You will also see some of your options for modification.

[ildar]

Yes, external ps is something I'm going to more and more. Especially for some of the boxes I've built that are always on in my rig-clean boosts and such. I'd say for this build it's the best bet.

[ildar]

Double post

[Mihkel]

BTW, I did a vero layout for it this morning... Be aware that it isn't verified. If anyone spots any mistakes, please let me know!
http://aronnelson.com/gallery/Mihkels-layouts