6.9 VAC on heaters. Is this OK?

[nandrewjackson]

After reading up a bit on either using  the 6.3 V center tap, or making a "false" center tap with 2- 100 Ohms resistors, I decided to go with the resistors.

I guess if a tube goes phooey it's possible to ruin your PT if you're using the PT 6.3V center tap.  That's why I did it.

Well, anyway, I only had 140 Ohm resistors, so I used those and it has raised the V up to 6.9 VAC.
Acutally, I have some 100 Ohm R's, but they're tiny 1/8 Watt ones so I didn't want to use those. I thought they might get a bit hot.

This is only a preamp, with 4 -12ax7's.
Can I get away with this, or should I break the bank and get a couple proper valued specimens?

[Willabe]

It's fine.

12 _ _ 7 type tubes are rated/spec at +/- 10% of 6.3vac/dc. So 10% of 6.3 = .63 and 6.3 + .63 = 6.93 or 6.3 - .63 = 5.67. So any where from 6.93 to 5.67 if fine.

But if you are getting that measurement when your line (wall) voltage is low, then when it returns to normal or runs higher than normal you will exceed the heater voltage spec. 

Also I don't think using 140ohm R's instead of 100ohm R's as an artificial center tap caused the heater voltage to rise. Every PT is rated to be able to deliver a certain amount of voltage at a certain amount of current. If you under load the PT the voltage will rise.

Check any PT winding unloaded and you will see that it will be higher than if it were loaded. The more current the PT is able to deliver the higher the unloaded voltage will be from it's loaded rating.

Conversely if you overload a PT the voltage will drop. The more you over load it the more it will lower the voltage it delivers. (This is not a way of trying to control the delivered voltage. Because of over taxing/overheating the PT till it burns up.) It's all ohms law.


                 Brad          

[John]

A nice way to lower the voltage to your heaters is to use 2 diodes in reverse parallel, like below
-->-
-<--

Just on one side of your filament wiring. It will drop the voltage about .7 volts.

[tubeswell]

A nice way to lower the voltage to your heaters is to use 2 diodes in reverse parallel, like below
-->-
-<--

Just on one side of your filament wiring. It will drop the voltage about .7 volts.

Use 6A diodes on the heater winding

[Tone Junkie]

A nice way to lower the voltage to your heaters is to use 2 diodes in reverse parallel, like below
-->-
-<--

Just on one side of your filament wiring. It will drop the voltage about .7 volts.

Use 6A diodes on the heater winding

Thanks Guys good Trick just made my wall of knowledge.

[stratele52]

Artificial center tap made with resistor don't chance heater voltage. Is your meter is right ?  6.9 volts is not too much.

[nandrewjackson]

yeah, it's reading properly.

I didn't write down the voltage before I changed it from the center tap wire to the resistors, so maybe it was 6.9 before I changed it, too.  I dunno.

All of the other voltages are in a good range, and it's working so I'll just roll with it.

 

[stratele52]

yeah, it's reading properly.

I didn't write down the voltage before I changed it from the center tap wire to the resistors, so maybe it was 6.9 before I changed it, too.  I dunno.

All of the other voltages are in a good range, and it's working so I'll just roll with it.

 

If your transformer had a heater center tap , you don't need to make an artificial center tap with resistor . Remove them.

[John]

A nice way to lower the voltage to your heaters is to use 2 diodes in reverse parallel, like below
-->-
-<--

Just on one side of your filament wiring. It will drop the voltage about .7 volts.

Use 6A diodes on the heater winding

Tubes, meant to ask you earlier, why a 6 amp diode? Or, is there a "6A" diode (like the 1n4007)? I've used the 4007s so far with no issues. My math is always suspect, but it seems if you're pulling at most 2 amps, and dropping ~10% across the diode, a 1 amp should be fine? I don't work on those 50 watters (yet).

[sluckey]

but it seems if you're pulling at most 2 amps, and dropping ~10% across the diode, a 1 amp should be fine?

Diodes still need to be rated for the full amp draw of the entire filament string.

The current does not split, sending half thru one diode and the other half thru the other diode. The diodes are connected parallel, but back to back. Only one diode will be conducting at a time and that's why it must be able to carry the full filament load current.

[John]

Ah hah. Thank you! I'd best switch 'em out then.