# Help needed reducing heater voltage

Discussion in 'Shock Brother's DIY Amps' started by jvande7, Oct 14, 2020.

1. ### jvande7TDPRI Member

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Would anyone be so kind as to help me calculate the required resistors to drop my heater voltage?

I built a 5F1 Champ circuit and the heater voltage is 6.8v instead of 6.3v, and as I understand it this will shorten the life of my tubes.

6V6GT

2: 3.4v
3: 356.5v
4: 330v
7: 3.4v
8: 20.3v

B+1 383v

I'm using the Classic Tone 40-18019 PT. I took a crack at the calculations and I think I might need to use two 0.5 ohm resistors, but that could be wrong. I'm also not sure what wattage they should be rated for.

Thanks!

2. ### ThermionicScottPoster Extraordinaire

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I assume your 12AX7 and panel lamp are in, too? What's your wall voltage?

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3. ### NickflFriend of Leo's

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Don't worry about 0.1v, that is well within spec. Heater voltage tolerance is +/- 10%, though Merlin recommends shooting for 5%, you are within 1.3%. Even of you got it to exactly 6.3, you mains power voltage fluctuates depending on demand on the grid by +/-5% so you can't do better than that anyway.

Edit: sorry, read your numbers wrong. Still, your off by 0.5v, I'd just let it be. Still only 8%. Check to see if your wall voltage is high today too.

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4. ### printer2Poster Extraordinaire

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It is 8% high. Add up the current, use Ohm's Law to figure out how much resistance you need to drop the voltage, cut that in half and put each resistor inline from the transformer.

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5. ### jvande7TDPRI Member

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120v right on the money, and yes the lamp and preamp tube are installed. Same heater voltage on the 12AX7

6. ### jvande7TDPRI Member

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Thanks yeah this is what I believe I did, but I'm still just a rookie amp builder and not confident in my calculations. This is only my 2nd amp kit.

I was hoping someone could do the calculations so I could compare the results to my own. I believe I need to use two 0.5 ohm resistors, with a rating of somewhere between 1-5 watts haha, so yeah, I'm unsure at best.

The Google tells me 6V6GT current is 0.45 amps, 12AX7 is .15 (series) or .30 (parallel) ??
My transformer spec sheet says the 6.3v heater is putting out 2.25 amps

Which of these am I adding up?

Last edited: Oct 14, 2020
7. ### LowerleftcoastFriend of Leo's

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6.8VAC heaters? frrnt.

If you reduce the voltage with resistors you will create an unacceptable heat source IMO.

Your tubes will be fine at 6.8VAC. Maybe start to be concerned well into the 7V range.
Then start thinking *Bucking transformer*.

Ignore the 6.8 and play yer guitar!

Last edited: Oct 14, 2020
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8. ### radiocasterPoster Extraordinaire

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Who told you that? 6.6V is well within the specs of most tubes.

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9. ### jvande7TDPRI Member

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A fellow by the name of Rob.

"Most tubes list the required heater voltage as 6.3 volts +/- 10% or 5.7 to 6.9v. Running the heater voltage above 6.3 volts can shorten the life of your preamp and power tubes. Your amp will also run hotter and consume more electricity."

https://robrobinette.com/5F6A_Modifications.htm#6.3V

But okay, fair enough. I'm happy to leave it alone. I just thought it would be worth the minor trouble to throw a couple of resistors onto the lamp and maybe increase the life of my NOS tubes.

Last edited: Oct 14, 2020
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10. ### ThermionicScottPoster Extraordinaire

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And the panel lamp adds another 0.15 amps, so you're only using about 40% of the heater current rating. If you were drawing a full 2.25 amps, I bet the heater voltage would get pulled all the way down to spec. There was a time when I used to fret about heater voltages, but your tubes might well die of some other cause first.

11. ### LowerleftcoastFriend of Leo's

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Technically true... in practice the heaters run high without tube life complaints. 50yrs vs 46yrs ftw. As @ThermionicScott says something else will dust them first.

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12. ### Tom KamphuysTele-Holic

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Merlin actually mentions to go towards the lower limit for lower hum.

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13. ### tele_savalesTele-HolicPlatinum Supporter

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I've just been happy to have heater voltage in any of the amps I've built.

14. ### jvande7TDPRI Member

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Well, it looks like I would need 0.55 ohms of resistance to drop 0.5v since I have about 0.9 amps being drawn.

So I would need two 0.25 ohm 1w resistors, but I don't even know where to find such small value resistors, so maybe I will just leave it alone. I have a huge assortment of 1w but nothing lower than 1R.

15. ### LowerleftcoastFriend of Leo's

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Been there done that... IMO, don't bother. I even tried 0.1 5W to reduce *some* voltage. The result is too much heat within the chassis. Totally not worth the effort. You can go down the road if you want, heck I did. I won't take that road again. YMMV.

Last edited: Oct 14, 2020
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16. ### jvande7TDPRI Member

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Thanks y'all. That seals it for me. The amp stays as is.

I appreciate all of your input. Definitely put me at ease.

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17. ### tubegeekFriend of Leo's

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If you really want to try it, 4, 1R's in parallel for each .25 is an option.

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18. ### glenlivetTele-Afflicted

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put 4 1W in parallel...

edit: @tubegeek beat me to it.....

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19. ### printer2Poster Extraordinaire

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5% I am fine with, approaching 10% I think about it. I do use old preamp tubes though, power tubes I have enough NOS.

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