Shaka tube power section questions

[add4]

I'd like to build the shaka tube ( http://www.diystompboxes.com/smfforum/index.php?topic=18369.0 ) and i have questions about the power section:
I'm puzzled about the use of 12V AC to pins 4 and 5 of the tube. In the valvecaster type builds, 9v DC is used, sometimes with a charge pump to have 18v or more to the heater. But it's DC, not AC. The next part of the shaka tube section filters the current for the positive and negative part and provides DV to the rest of the build (if i understand correctly the role of the diodes in the power section).

So, why can't I use a MAX 1044 IC to get 18v DC to pins 4 and 5 of the tube, and then power the rest of the build with that?

I guess that my main question is : do i need AC power to the heater of a 12AX7, i've seen it done with DC.
thanks in advance for sharing knowledge

[Minion]

Firstly putting 18v to the Heater of a tube will greatly reduce it"s life expectancy and there is no advantage to useing a higher heater Voltage .......

Heaters can be run of both AC and DC , if useing DC you must use a very well filtered DC supply that is preferably regulated which adds to the complexity of the project .......

With AC heaters you just connect it and thats it , no filtering , no regulators no diodes just plain old AC ....... AC is just simpler and from my experience superior but everyone has their preferances .......

You must use twister pair when wiring AC heaters ......

[add4]

Thanks for your input.
I'm curious about your first statement. why would running a tube at 18v reduce it's life expectancy? is it too low? if it is, it's always better than the 12v of the original design, if it's too high, then it's surprising as the same tubes get a few hundred volts in tube amps.

Can you explain more?

[iccaros]

what you see if  is high voltage on the Plates and not the heaters. Tube heaters all have their own rating, but a 12ax7 can be ran in parallel 6.3v +/- 10% or 12.6V Serial +/- 10%.
Please read the tube datasheet to help  understand, that you are confusing the heaters with the Anode (plates) which can go to 350 + volts.
http://www.drtube.com/tubedata.htm

[Minion]

Pins 4 and 5 (and 9) are the heaters , they are run at 12.6v in series or 6.3v in paralell ....... They Must run at these Voltages for proper opperation, the Plate can run at up to 250v+ .......

[PRR]

> tubes get a few hundred volts in tube amps

A tube is two parts. A "heater" circuit and a "cathode grid plate" circuit.

The heater can be anything red-hot; we use electricity in a wire just like a toaster or a lamp. For multiple reasons, usually a lower voltage.

The cathode-grid-plate circuit is electrons in empty space. This is very different. Electrons do not like to fly though space. For best results we apply a high voltage. In other cases we try a low voltage and live with what we get. Low plate voltage can work OK for driving an amplifier input.

FWIW: the Shaka supply delivers 35V to the tube cathode-grid-plate circuit.

> puzzled about the use of 12V AC to pins 4 and 5 of the tube

12AX7 Heater is rated 6 or 12V (depending on connection). +/-10% is fully in spec, +/-20% will work but not as specified.

AC or DC does NOT matter: it's just heat.

Using 9V or 18V on the 12V connection is like your room-lamps run on 90V or 180V. 90V is very dim, poor result. 180V is VERY bright for a minute and then the lamp burns out.

Yes, some tube-pedals use 9V on the 12V connection. Again: we don't need the MOST out of a tube which is only driving the input of another amplifier.

> why can't I use a MAX 1044

The heater needs 12V at 300mA. Even the heftier MAX660 can only output 100mA.

[add4]

Great answers. now i understand much more about that, thanks!!!