School me on series heaters, please.

[mumford]

I built an amp last year that uses 2x 6SF5 tubes in the preamp. The 6SF5 is basically a single triode 12ax7 that can only run on a 6.3v heater (not 12).

The PT wasn't robust enough to run the heaters, so as a temporary measure, I've been heating the 6SF5s off of the 5v line. One of the tubes has developed a bit of hum after less than 10 hours of use--it's time to fix the heater issue.

A knowledgeable friend suggested I get a 12v 300mA radio shack transformer and use it in series for the heaters. Will I need some resistance in line to pull the voltage down to 6, or will just wiring them in series split the voltage to 6 each? I reread merlin on series heaters, but don't feel like I picked up on the answer there.

[Gibsonman63]

If I am not mistaken, a 12V power supply will only power two tubes. You do not need to add additinal resistance, because you will just drop more voltage. The resistance of each tube will be about the same, so you will drop about 6 volts on each one.

I would double-check the current requirement.

[Structo]

I don't think Radio Shack carries those transformers any longer as they were a favorite with the Dumble guys and when I looked into it they didn't carry them anymore.

It does depend on what kind of load is presented to the transformer to bring the voltage down to the desired voltage.

But you can buy smaller transformers that will do the trick.

Triad sells nice little filament transformers.

What you want to do is add up the heater current for all the tubes you will be heating.
The exceed that by about a 50% margin for safety.

The 6SF5 draws .3 or 300ma heater current so for two tubes it would be .6 or 600ma.

I would use this transformer.
http://www.mouser.com/ProductDetail/Tri ... fCSA%3d%3d

If after you install this transformer the voltage is above 6.6v then you can add a series resistor on the secondary or a zener diode to drop the voltage to the desired level.

[guitarmike2107]

You can run a 12AX7 off 12v or 6v..

what’s the current requirement for the 6SF5?

To run the two 62f5 in series

Connect the 12v heater supply to pin 7 on valve1,
Connect a link between pin 8 Valve 1 to pin 7 Valve 2
Connect the heater supply to pin 8 Valve2

That’s assuming the data sheet I saw was correct that the heaters are pin 7 and 8

easy

[Structo]

.3 amp for each tube

Heater is pin 7 & 8

[mumford]

Hmmm. I may just go ahead and switch those to a 12ax7. From what I'm told, a 12ax7 is literally 2 6SF5s, design wise. I've got the RS transformer, and it will run a 12ax7, but it won't have enough current for the 6SF5s.

BTW, they had the 12v transformer on the shelf at my neighborhood radio shack.

[Structo]

Well you lucked out on the transformer because they don't stock it anymore.

The 12ax7 draws the same heater current at .3 amps.

[mumford]

I'll just run it at 12v for 150mA, should be fine, I think.

Thanks for the help!

[Structo]

You can usually tell if you are taxing a transformer by the heat it gives off.
Warm is OK, but if you can't keep your hand on it for more than a second, you are probably exceeding the current draw it can provide.

[diagrammatiks]

5 vac with no centertap would give you around 6.3 if you rectified it with 2 diodes with .44 voltage drop across each diode.

[mumford]

5 vac with no centertap would give you around 6.3 if you rectified it with 2 diodes with .44 voltage drop across each diode.

I'm not sure about this. The merlin book says I will get a voltage loss if I rectify the heaters. (with a bridge rect plus a big cap)

[diagrammatiks]

That's all Merlin said? Nothing about how to calculate the actual drop at all huh.