What type resistors on heater false ground?

[wattsup]

What type and wattage resistor should I be using on my heater wires false ground?

[Bob S]

It's been a while since I did this but, if I recall correctly, I used 100 ohm 2 W flame proof.
If an output tube goes bad these can see a big surge until something pops.

[Stevem]

X2
Be sure to land that other end at the chassis point where the PT secondary center tap is landed, or at the point where the first power supply filter gets its ground!

[wattsup]

Thanks guys. Would a 2W metal oxide be ok?

[tele_player]

Yes.

[wattsup]

Oh good. Thanks!

[tubeswell]

Each 100R sees 3.15VAC from winding to ground

3.15VAC/100R = 0.0315A.

0.0315A x 3.15VAC = 0.1W (tops)

1/2W resistors will be fine. Even 1/4W resistors will do the job adequately.

[martin manning]

1/4 watt resistors will do the job as far as a ground reference goes, but in the event of a tube short or arcing across socket pins you don't want them to open up before a fuse blows. Otherwise the filament circuit will be elevated to B+, potentially damaging the h-k insulation on the tubes. There was recent discussion on this in another thread.

[Leo_Gnardo]

Would a 2W metal oxide be ok?

2W would be a never-fail rating in a circuit like this. I generally use half or one watt carbon films and metal-oxide films. Occasionally half-watt carbon comps if I'm in a vintage mood.

No matter what the wattage rating or composition, all bets are off if you have a hi voltage short to filament line. Even a 5W part can become a smoldering heap, fast.

[vibratoking]

Would a 2W metal oxide be ok?

No matter what the wattage rating or composition, all bets are off if you have a hi voltage short to filament line. Even a 5W part can become a smoldering heap, fast.

^^^ YUP!!!

[pdf64]

Agree, but all else being equal, wirewound resistors rated for several watts should be more resilient than 1/2 watters of whatever type, and have more likelihood of sustaining a fault current for long enough for a fuse to blow.
By that, they may maintain a ground reference and thereby prevent all the pre-amp tubes being ruined.
Pete

[wattsup]

Thanks for all the help guys. I'm glad this forum is kind to guys like me who have to ask really basic questions.

[wattsup]

Another question and totally unrelated so let me know if I should have started a new thread... If I have a transformer with wires for the rectifier tube and I'm not using them, would it be ok to power an LED on lamp (the pilot?) or would it be better to use the power from the heaters? Or does it matter?

[tubeswell]

Another question and totally unrelated so let me know if I should have started a new thread... If I have a transformer with wires for the rectifier tube and I'm not using them, would it be ok to power an LED on lamp (the pilot?) or would it be better to use the power from the heaters? Or does it matter?

Perfectly fine to run a LED off AC. Remember to calculate the series resistance needed to provide appropriate forward voltage and current to the LED. (Different LEDs have different forward voltage drops)

[David Root]

I've been using 100R 1W PRP metal films as I had some but I'll be changing to 100R metal oxide 2W as I've just run out of the 1W MFs.

On the pilot lite, look at how Dumble did it off the heaters, one side to a 1N4xxx diode, the other to a 330R dropping resistor, thence to the LED red wire from the diode. Sure you can also use the 5V winding if you have one.

While you CAN use the heaters to power the pilot lite this way, DO NOT use the heaters to power a 5V relay power supply, noisy & unreliable. Use the 5V winding if you have one, or buy a Hammond 166F5 transformer and tie off the CT.

Here's how I do it. The PS diode bridge uses Schottkey diodes so 5V power works fine with a red LED.