What size resistor to use for an artificial filament winding tap?

[mfreqmaster]

I have a PT with no heater center tap. I see where Fender used 2 100 ohm resistors. I have 2 10 ohm, 1 watt resistors available. Will that achieve the same effect as the 100s? Thanks.

[g1]

Lower values will put an excessive load on the heater winding. Better to go higher than lower. I have seen 220 ohm in some Peaveys.

[loudthud]

Calculate the power of a 10 ohm resistor with 3.2V across it. Hint: the resistors will be hot enough to cause serious burns.

[Leo_Gnardo]

I'm with loudthud and g-one. You'll achieve the same effect with 10 ohms all right, and likely have a nice smoke machine besides.

Smallest value I've used 75 ohms. Largest I've seen used, 680 ohms but I busted those back to 150 because I was out of 100's. I'd say up to 220 or 270 is OK. Close enough for jazz. IOW it isn't critical, as long as you don't sizzle your resistors or transformer. Also nice to select your R's so that they're closely matched.

[mikepukmel]

I'm with loudthud and g-one. You'll achieve the same effect with 10 ohms all right, and likely have a nice smoke machine besides.

Smallest value I've used 75 ohms. Largest I've seen used, 680 ohms but I busted those back to 150 because I was out of 100's. I'd say up to 220 or 270 is OK. Close enough for jazz. IOW it isn't critical, as long as you don't sizzle your resistors or transformer. Also nice to select your R's so that they're closely matched.

So, 3.2V/100R -> 0.032A -> 0.102watts. So, the old Fender 100's are 'close enough for folk music', Leo? If we used a 1 watt resistor, is this enough headroom? For the rest of the amp, I weakly understand that voltages and currents vary by load, how loud you're playing,etc. But shouldn't the heater current be fairly stable across most playing conditions? (Or I could be completely wrong, which is often the case).

[Enzo]

It is a steady 6 volts. The two resistors form a virtual center tap, 3v 100 ohms 100mw. 1/2 are fine, 1w are fine, anything larger is fine. there is no stress on them.

[Pdavis68]

No reason not to create a humdinger with a 500R pot...

[mikepukmel]

It is a steady 6 volts. The two resistors form a virtual center tap, 3v 100 ohms 100mw. 1/2 are fine, 1w are fine, anything larger is fine. there is no stress on them.

Thanks Enzo.

[Pdavis68]

"Also nice to select your R's so that they're closely matched."

Leo said that above. It's an important point. This is the disadvantage of a transformer with a filament CT, because the windings aren't always balanced and if they're unbalanced, they can introduce hum. If you have matched resistors (or a humdinger), you'll get better hum suppression.

[Malcolm Irving]

... If you have matched resistors (or a humdinger), you'll get better hum suppression.
...

But the funny thing is that when you do have a humdinger it rarely gives minimum hum at the 50:50 position.

[Enzo]

Never forget, there are many sources of hum in an amp, and the real balanced point of a heater line may not be the best. Sometimes getting a little off center compensates for some other hum source and they cancel.

[mikepukmel]

I'm with loudthud and g-one. You'll achieve the same effect with 10 ohms all right, and likely have a nice smoke machine besides.

... Also nice to select your R's so that they're closely matched.

Leo, do you just mean picking resistors with better tolerance, i.e. 1% rather than 5%? or should I measure them?

[Pdavis68]

You should measure them. Generally speaking you want them as close as possible.

[Chuck H]

You should measure them. Generally speaking you want them as close as possible.

There's never a good reason to NOT check something, but I wouldn't get my undies all twisted up about the matching. To measure 1% resistors specifically for matching in this circuit implies that a match closer than that is necessary.?. As noted, a humdinger pot usually results in the lowest hum somewhere other than centered. Of course you wouldn't want to be off center in the wrong direction either. That's why we balance the resistance for this circuit when using fixed resistors. I measure only to see that resistors aren't off spec but I don't trouble beyond that. I buy 1% resistors for these circuits and test them before putting them in the bin. They're all within 1% so no two can be more than 2% different. In fact it's usually much closer. I just grab two resistors and solder them in and I've never been surprised by excessive hum to find that a 1% difference in the false CT resistors was the cause.

[cjenrick]

i would not go over 1/2 watt. in the rare instance that a tube shorts plate to heater, a 1/2 watt will act as a fuse and maybe save a transformer. i have probably opened up about ten amps with a cooked 100 ohm resistor or two,

matching is not important, in fact if you install a pot, you will notice that the setting that provides the lowest hum will sometimes be off center.