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What are some good ways to lower the AC filament voltage without adding resistance to the secondary of the filament transformer. I'm
using a hammond 166L2 and getting 2.89 vac on a 45 tube with a cathode resistor on the center tap.
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Replace with Hammond 166K2 for the 45 tube, and keep the 166L2 for a future 2A3 project.
The bitterness of poor quality remains long after the sweetness of low price is forgotten
Alternatives to primary/secondary resistors?
* a variac can be set to give the right voltage
* a step-down autoformer, or a spare filament winding off the HV transformer to modify the filament primary
* "hidden resistors" - small gauge wiring over long distances, common-mode choke, etc.
* replace with a regulated supply - too many kinds to mention all
* ...
You can add resistance in series with the primary. Try a 50-100 Ohm rheostat first, and dial in the voltage you want, then replace it with a fixed resistor.
Is there a formula to determine resistor wattage and value for a resistor in the primary? Is it ok to just ust one resistor or do you need one in each half primary?
Lets try to use Ohms law and ballpark resistor dissipation.You are at 2.89 VAC and you want 2.45 VAC, about 0.44 VAC lower, times 1.5 A. of Type 45 filament's current, is 0.66 Watts continuous dissipation. You should use a minumum of a three Watt power R, in ONE leg is OK, its how I do it.
I experimented in 5-2015, by substituting Rs with my Hammond 166M2 ( has a 3 Ampere secondary ) and ended up settling on 300 Ohm 5 Watt power resistors.
YES, put them on the primary side, very good. One in one leg, to each filament transformer, is fine.
------------------------------------------
I found out in 2015 that the quality, and quantity of my SECONDARY lead outs with a DHT filament transformer are CRITICAL towards obtaining the best sound.
I modified the Hammond's 166M2's lead outs, on the secondary. 'Used mil spec 12 AWG copper/silver lead outs for all three of the DHT filament transformer's secondary leads. Employed absolutely NO signal degrading hum pot, ( of course ), and secondary lead lengths are ALL UNDER 4.2 inches.
I eventually re-modded the Filament transformer, ( 6-2015 ) to use TWO 12 AWG runs on its center tap, ( 9 AWG ) and .... I liked that EVEN BETTER, more dynamics, slam, "you-are-there-ness". OMG.
This center tap lead length ( to triple Mills MRA-12 Rks -its a DC amp's Finals' Rk ) is minimal in length to Star Ground - very IMPORTANT on Rk returns. Under three inches from the filament transformer center tap to the Rk, and under 0.9 inch on the other end ( of the triple Mills Rk ), to Star Ground.
Have fun and pay close attention to what you are doing.
Jeff Medwin
Edits: 08/15/15 08/15/15 08/15/15
Using secondary volts 2.5 x 1.5 amps = 3.75 VA. Isn't the primary current different, and much lower?
Yes, much lower.
So 120vac at 3.75va is how many amps?
3.75 / 120 = .03125amps or 31.25ma
You need to drop .39 volts at the secondary.
2.89 / 2.5 = 120 / 103.8
So you need to drop 16.2 volts at the primary and you're drawing 31.25ma.
16.2 / .03125 = 518.4 ohms
A 520 ohms resistor in series with the primary and it doesn't matter which side, the primary is just one winding.
16.2 X .03125 = 1/2 watt. I would use at least a 2 watt resistor.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Thanks to everyone for shedding some light on this subject. I think I'll try the 520 ohm resistor in the primary first.
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