Help with simple tube practice idea.

[MarkDonMel]

Hey all,

I am eventually going to build a Tube Driver or other similar pedal, and in the meantime I am reading up on theory and stuff to become more accustomed to the whole thing.  I have a decent book, it offers projects to go with the theory, but there aren't any for tubes.  Any ways, since I will be working with a 9-pin preamp tube I was hoping to start testing and fiddling with the tube.  Running current through it and stuff, to sort of illustrate the math I am doing...

One of the first things I wanted to do was heat the filament.  Can I do this with a 9v?  If I need less V can I just use resistors?  If I need more I guess I would need a step up trans?  I basically just wanted to light up the heating element without having the tube in a complete circuit.  If it needs more V or I do I need to use a walwart and a transformer to bring down the V ??

I guess I am hoping someone could give me a little direction as to how I would accomplish this.  It may be needless and being experienced you may think it silly, and indeed I may as well if I ever possess your knowledge, but unless this is dangerous...please humor me? lol

p.s. By the way, anyone know a link that has a data sheet for a cheap sovtek 12ax7wb?

Thanks much all.  Always appreciate the help.  Matt.

[aron]

The amount of voltage depends on the tube heater requirements usually found in the datasheet.

Typically 6.3V and 12V are used. The heaters usually draw a lot of current so you need a beefy power supply.

One way I have done it is to use 2 voltage regulators with a beefy 30V power supply.

One voltage regulator is a 6V one. 7806? and the other the V+ of my plate voltage i.e. 24V.

Another way is to check out the Shaka Tube schematic etc...

Also please read my simple tubes for dummies page.

Unfortunately I didn't cover the grid leak resistor to ground.... I need to add that.

http://diystompboxes.com/pedals/tubedummy.html

[smoguzbenjamin]

Hey aron that's pretty cool When I get my tubes from Ansil I will be embarking on a tube amp project. But I was wondering, whenever I see a tube amp schematic, I see high voltages, like 240, 220v. If the heaters only need 6v, and the rest of the amp needs 24, why the high voltages? :? Or am I confuzzled as usual?

[MarkDonMel]

excellent aron!

appreciate that, I thought I had read, or at least skimmed, all of the articles you had written...  I should go back and make sure because they are very informative.

I am going to read the tubes for dummies as soon as I get home from work.

lunch break is over lol, thanks again, matt.

[Peter Snowberg]

...I was wondering, whenever I see a tube amp schematic, I see high voltages, like 240, 220v. If the heaters only need 6v, and the rest of the amp needs 24, why the high voltages?

More voltage = more headroom and more amplification. With a distortion, you want to run out of headroom so the lower voltage helps that.

Take care,
-Peter

[The Tone God]

The standard preamp tube 12A*7s have a center tapped heater. You can look at it as two heaters with one common connection. Each heater needs 6.3v at 150mA. If you wire them in parallel using the center tap you need 6.3v at 300mA to heat the tubes. Using the heaters in series with no center tap you need 12.6v at 150mA. That will get the tube cooking.

The tube can run at a lower voltage but generally after you drop below 100v the characteristics that tubes are known for fade. IMHO if you use plate starved tubes you kind of miss the point of using a tube.

That being said you can still get a kick out of it and its something to learn. Have fun with it.

Andrew

[aron]

I would recommend trying low voltage wired tubes first. Then graduate to high voltage.

Hopefully for obvious reasons.

[gtrmac]

Simply put tubes need a high positive voltage on the plate to get the electrons to jump out into space.

[smoguzbenjamin]

Hmm okidoke. First a low-voltage amp. I don't know what Ansil popped in my package so I'll have to wait and see. :mrgreen: Cheers guys!