Heater voltage and Ohm's Law

[xtian]

I have a Champ 5c1 circuit with just two tubes (6SJ7 and 6V6GT). Heater current draw total is 0.75 amps, according to the sheets.

My heater voltage is 7.0 volts AC (old iron!). I want to drop 0.7 volts (to get back to 6.3vac). Ohms law suggests I can use a 10ohm resistor that will dissipate 0.53 watts.

Sound right? Can I just use a 10ohm, 1 watt resister in series with one of the heater legs?

[matt h]

(deleted)

[xtian]

Yes! Decimal error. Duh.

[matt h]

(deleted)

[xtian]

Well, weirdness.

I measure 7.1vac across pins 2 and 7 without, and 5.3vac with the 1R resistor in series with one leg of the heater winding.

I confirmed, 1R, 1%, 1W resistor. Measures right.

WHAT?

[shoggoth]

Transformer impedance messing with you? Tube filament resistance changes at higher temperatures due to higher voltages & current draw?

Try adding 1 ohm resistors in parallel with the first til its in the ballpark, do the math, and that's the value you really need

[matt h]

(deleted)

[tubeswell]

A pair of nose-to-tail 6A diodes (or 3A diodes) in series with one side of the heater winding will drop the voltage by 0.6V.

(I presume you took the 7V measurement with the PT loaded?)

[xtian]

Yes, measurements taken warm and under full load. Maybe will try a pair of resistors.

[xtian]

Cool. One pair of 1R in series with each leg (0.5R) gives me 6.3vac.

[tubeswell]

Cool. One pair of 1R in series with each leg (0.5R) gives me 6.3vac.

Each pair of 1R||1R losing 0.7A and dissipating 0.25W. By my reckoning = (0.35V/0.5R)x0.35V = 0.245W

(I take you meant you had 1R||1R in series with each side of the heater winding?)

[pdf64]

Consider a line bucker to address the root cause http://www.geofex.com/Article_Folders/v ... ntvolt.htm
Pete

[sunnydaze]

Just curious, would the result be the same if the two 1 ohm resistors were Paralleled on just one leg of the transformer?

thanks
mike

[Phil_S]

They make sandblock resistors in 1/4 and 1/2 ohm values (or maybe .26 and .51, I forget). The math is good at getting you into the right range, but there must be other forces at play. In my experience you've got to tweak this in circuit -- as you did -- to get the desired result. When I do this, I try to make sure the dropping resistor has space around it because I expect it to get pretty hot.

[Zippy]

10% is an acceptable tolerance for a Champ.

How does it sound?