Artificial center tap for heaters.

[pinkphiloyd]

I'm about to start my first build using a PT with a non center tapped heater winding.  This will be my first build constructing an artificial center tap.  I'm not nervous about the procedure; I understand the principle and I understand how to wire it.  What I'm wondering is...Radio Shack has a pack of five 1/2 watt 5% 100 ohm resistors for 1.49.  Do resistors usually drop the 60 cycle hum to an acceptable level, or should I spend ~6 bucks for a 100 ohm pot?  I'd stick to resistors combined with DC elevation but it's fixed bias and I don't really want to mess with setting up a voltage divider just to elevate the heaters.

[shooter]

Radio Shack has a pack of five

I'd do that, then you have a spare set......just in case murphy is helping on the build

[eleventeen]

The 100 ohm resistors will do it. It is possible (though not a given) that you might be able to measure a slight improvement 1-2-3% on a scope with the pot versus the resistors, but you'll never hear it.

[kagliostro]

When we were talking about artificial CT, Sluckey told me that the pot is "reserved" for boutique amps

If you want to build a "special feature" amp, use the pot, if you want to build an amp, use the resistors

the most part of artificial CT are build using resistors

Ciao

K

[sluckey]

but you'll never hear it.

Even with my poor ears, I can hear a difference between either extreme end of the pot and somewhere near the center, although I can no longer hear the exact minimum point in hum level. But with just an AC voltmeter across the speaker terminals you can accurately set the pot for a minimum of 60Hz hum.
 

[pinkphiloyd]

Sounds like resistors are the way to go.  I don't have super ears.  I'll start there and if the results aren't adequate I'll move on to other measures.  Which feels kind of stupid now that I type it out.  "Should I start simple and then get more complicated only if necessary?" 

[kagliostro]

or should I spend ~6 bucks for a 100 ohm pot?

I don't know well prices on the USA market, but here in europe they are usually higher

and for those humdinger pot they ask you 3.20€ not 6.00$

http://www.tube-town.net/ttstore/Potentiometer/Fender-Potentiometer/Fender-Potentiometer-Hum-Control-100R::5687.html

may be you can find the pot at a cheaper price also in the USA

Franco

[shooter]

Should I start simple

There's a principle called; K.I.S.S.    works very well when you start out in any new venture

[John]

The last 2 builds I used a humdinger on I can't tell any difference; there's no heater hum at all actually. BUT, on one of my older builds it's very apparent where the quiet spot is on the pot. I think but cannot prove that elevation is more useful?. I do it on every one.

[pinkphiloyd]

or should I spend ~6 bucks for a 100 ohm pot?

I don't know well prices on the USA market, but here in europe they are usually higher

and for those humdinger pot they ask you 3.20€ not 6.00$

http://www.tube-town.net/ttstore/Potentiometer/Fender-Potentiometer/Fender-Potentiometer-Hum-Control-100R::5687.html

may be you can find the pot at a cheaper price also in the USA

Franco

Six U.S. dollars on Amazon.  I'm gonna try the resistors first and go from there.

[Willabe]

It only takes 4 parts, 2R's, 1 cap, 1 pot to do this, see below.

Digikey sells the pot.

 

[kagliostro]

I was wondering if a special pot is to be used

the value of the pot can be 220R and also 500R as seen in old amps

Merlin on his site says that on a 500R pot there will be 80mW of dissipation

so ...... is really required a special pot or a normal pot can afford the job ?

Franco

[2deaf]

You don't want, nor do you need, a 100 ohm pot.  6.3V across 100 ohms is almost 0.4W and I wouldn't put that on a 1/2W pot.

A 1K linear pot with a 100 ohm resister from wiper to one end and another 100 ohm resistor from wiper to the other end works fine.  It even makes a finer control around center point than just a pot alone.  All kinds of other combinations work fine, also.

[eleventeen]

"6.3V across 100 ohms is almost 0.4W and I wouldn't put that on a 1/2W pot."


Yeah, but really, you have 3.15 volts across 100 ohms (or, 6.3 / 200 ohms) So half that wattage.


A teeny pot is not a good idea, of course. My point was that yes, a pot can be useful, but you can get into some overcurrent/overwattage issues when you have the pot thrown all the way (or near) to one end or the other. It has been rare, for me, that any rotation of a "hum" pot did anything noticeable. To my ears. I suppose a bench test could find an optimum setting. And if that setting is all the way to one side, then your pot is a problem.

The 2 resistors do the deed in the great majority of cases.

[trobbins]

What 'overcurrent/overwattage' conditions are a cause of concern??

I somehow got a lot of 220 ohm 10T long-style bourns pots - they work fine.  Some amps exhibit no difference - others I've needed the pot near one end - imbalances in capacitive coupling to the grid occur for many reasons, hence the variety of outcomes people experience.

If you don't normally change your input tube, and your keen on hum reduction, then temporarily put a pot in, and see what setting is best, then solder in fixed resistors of that pot setting.

[2deaf]

Yeah, but really, you have 3.15 volts across 100 ohms (or, 6.3 / 200 ohms) So half that wattage.

They're talking about a single 100 ohm pot in place of the usual two 100 ohm resistors.  A 100 ohm pot hooked-up to 6.3V works out to 0.3969W.
 

you can get into some overcurrent/overwattage issues when you have the pot thrown all the way (or near) to one end or the other.

Where the wiper is positioned makes no difference as far as current and wattage goes.  The wiper and whatever is attached to it are not part of the 6.3V circuit.

[sluckey]

My point was that yes, a pot can be useful, but you can get into some overcurrent/overwattage issues when you have the pot thrown all the way (or near) to one end or the other.

Turning the pot from end to end does not change the current thru the pot. At max CCW one end of the filament winding is connected to ground (or whatever reference you choose). Turning max CW will ground the other end of the filament winding. Somewhere in the middle of the pot travel you will have exactly the same resistance from each end of the filament winding to ground. No current flows into the wiper of the pot under noemal operating conditions.

[pinkphiloyd]

In the interest of reporting back on what we've learned and questions asked, etc...

I bought a cheap pack of two 1 watt 100 ohm resistors from Radio Shack for my center tap.  Finished this build a few days ago and...dead quiet.  I can tell no difference between this and having a 6.3 V center tap.  This is good to know.  In the past I've always limited my transformer searches to those with center taps, so this should expand my options a bit.