New to this and I'm trying to reduce the secondary heater voltage on an amplifier I just finished...
The amp has a 6072 and 12AU7 that need 6.3V. Currently, the tubes are running a little high at 6.8V, so I am trying to bring it down a touch. These tubes are the only tubes on the secondary, it is not center-tapped and I would like to put a resistor in series to achieve this.
I've poked around online and found some info at http://www.valvewizard.co.uk/heater.html that seemed to be applicable to my situation. I've gone through the calculations and would love a second opinion(s) on my math.
The 12AU7 and 6072 draw .3A and .35A respectively and each needs 6.3V. I need to lower the heater voltage by .5V and have the following worked out.
Resistance: .5V/.65A = .769 Ohm
Dissipation: .5V x .65A = .325 W
Does this approach make sense? Haven't seen many resistors lower than 1 Ohm, so unsure how to move forward.
Thanks in advance.
The amp has a 6072 and 12AU7 that need 6.3V. Currently, the tubes are running a little high at 6.8V, so I am trying to bring it down a touch. These tubes are the only tubes on the secondary, it is not center-tapped and I would like to put a resistor in series to achieve this.
I've poked around online and found some info at http://www.valvewizard.co.uk/heater.html that seemed to be applicable to my situation. I've gone through the calculations and would love a second opinion(s) on my math.
The 12AU7 and 6072 draw .3A and .35A respectively and each needs 6.3V. I need to lower the heater voltage by .5V and have the following worked out.
Resistance: .5V/.65A = .769 Ohm
Dissipation: .5V x .65A = .325 W
Does this approach make sense? Haven't seen many resistors lower than 1 Ohm, so unsure how to move forward.
Thanks in advance.
There are resistors that low in value, but you may find yourself putting some larger values in parallel as you tweak them.
DigiKey, Resistors, Through Hole Resistors
under 1 Ohm (shift-click the Value field)
Apply-- Results: 16,517
Click In-Stock!! Digikey lists more parts than they have in the warehouse (they are happy to order 100,000 of anything, but you may not need that many...)
Heaters are often wired balanced, which suggests two 0.3845 ohm parts. I selected mOhms, range 300 to 400.
Your 0.325 total Watts should be at least doubled for good life. However there are many 1W parts in this range and they are easier to handle and more robust.
KNP100JR-73-0R39
RES 0.39 OHM 5% 1W AXIAL YAGEO
7,182 In Stock $0.47000
https://www.digikey.com/en/products/detail/yageo/KNP100JR-73-0R39/2058759
If you do plan 100,000 units you can shave the power and maybe shaves some pennies. In DIY it's not worth it.
The proposed 2*0.39r at 0.65A is 0.507V drop, and I know tubes won't mind the 0.007V error. (Back in the day we couldn't even measure 0.007V.) The 5% tolerance in dropping half-Volt off six Volts is a 0.5% possible error which is moot even in precision tube heating (thermocouple amps, analog computers..)
{quote is missing attribution because every time I clicked Quote it put the quote two lines above the cursor. The small image may be confusing the line-count.}
Even the slightest variation in mains voltage will blow that 7 mV error out of the water anyway. 🙂
I would derate the power rating of the resistor by 3-5x. So if you calculate 0.33 W dissipated, I'd use a 1-1.5 W type or 2 W if available. A 2 W type will cost a few cents more than a 0.5-1 W type. No biggie in the DIY world.
Many modern power resistors will run at 250-300 ºC at the full power dissipation (assuming 25 ºC ambient). That's way, way, WAY too hot for my taste and will lower the lifespan of the resistor and anything around it. Even at 3-5x derating I would expect the resistor to sit at 80-100 ºC, which is still quite toasty.
0.75 Ω is an E-24 standard value. You should be able to find those. Otherwise two 1.5 Ω resistors in parallel will get you there.
Tom
I would derate the power rating of the resistor by 3-5x. So if you calculate 0.33 W dissipated, I'd use a 1-1.5 W type or 2 W if available. A 2 W type will cost a few cents more than a 0.5-1 W type. No biggie in the DIY world.
Many modern power resistors will run at 250-300 ºC at the full power dissipation (assuming 25 ºC ambient). That's way, way, WAY too hot for my taste and will lower the lifespan of the resistor and anything around it. Even at 3-5x derating I would expect the resistor to sit at 80-100 ºC, which is still quite toasty.
0.75 Ω is an E-24 standard value. You should be able to find those. Otherwise two 1.5 Ω resistors in parallel will get you there.
Tom
Thank you for your insight. Yes, seems like .75 is more common than I thought! Will go with 2 or 3 watt resistors for sure. This is probably a can of worms but had planned to order wire wounds... any benefit or otherwise in using metal film resistors in this application?
Um... You're trying to reduce the voltage on a heater. The resistor is not in the signal path. Get the cheapest kind, which is probably a metal oxide type.
Tom
Tom
That's an interesting idea. You'd probably have to experiment quite a bit to find an NTC that has the right resistance when hot, but the soft start feature of that setup is nice. Then again, inserting 0.75 Ω in series with the filament provides some current limiting on start-up as well.
Tom
Tom
i use the 10ohm NTC's and when filaments have all warmed up, i find the drop is around less than a volts which is where i wanted it to be....those NTC's were harvested from discarded boards...a nominal 6.3 volt filament can be 5.9 to 6.9 vac right as the sockets, i tend to favor the low end of 5.9, i once serviced a 6H8 line preamp running on 5.2 volts filament and did not sound bad at all....
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