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Calculate Needed Filament Resistor - dconant - 01-31-2026

Hello, I have a radio with a resistance cord (long gone now). I need to determine what the proper resistor would be needed for this tube line up.

25Z5, 43, 75, 6D6 and 6A7. All are .3 amps for total of 1.5 amps. 

Currently I am running the radio at 120 volts with a 25 watt 250 ohm resistor. Been on 5 minutes and the resistor is at 425 degrees and still climbing. The 25Z5 filament is at 19 volts so 250 ohms is probably too much.  Probably need higher wattage too. The 25Z5 and 43 are 25 volt tubes where the rest are 6.3. The chassis is very small so there is not much room for a power resistor. I certainly would not want it on the back of the chassis where someone would get burned. I am not 100 percent sure what the radio is. All I know it is Model 5DK (maybe not that) and no manufacture.

Thanks,
Dan

RE: Calculate Needed Filament Resistor - RodB - 01-31-2026

Dan, you would need a resistor of about 180 ohms 20 watts to drop the 50 volts. The current is 0.3 A if the heaters are in series. That's a lot of heat.

RE: Calculate Needed Filament Resistor - dconant - 01-31-2026

Sure is Rod. Thank you.

RE: Calculate Needed Filament Resistor - morzh - 01-31-2026

OK. This is a simple matter and I always prefer teaching fishing vs giving a fish.

And I am not sure why you are saying why they are for total of 1.5 Amp when these are usually in series.
Do you want to quickly learn how to do it?

If yes, please give the radio model.

Meantime, the first attempt.

Sum up all the voltages of your tubes.

25 +25+6.3*3 = 68.9V

Now you have 120V.

You thus need to drop 120 - 68.9 = 51.1V

Your current is 0.3A.

Therefore your resistor is the voltage you need to drop divided by the current which is:

51.1/0.3 = 170.3 Ohm
Which is pretty much what Rod got for you.





Now. I suggest you calculate the power.  Which is either 51.1V-squared divided by 170.3 Ohm which is 15.3W, or, which is the same,  51.1V times 0.3A. Also 15.3W. Pick the one you like.

In order to be able to dissipate this much and not to smoke anything you will need at least 30W resistor, which, especially under the chassis, will generate a lot of heat.
I suggest you use a capacitor instead.
Again, if interested. Just yesterday someone gave links to those calculators (I do it by hand).

RE: Calculate Needed Filament Resistor - RodB - 01-31-2026

Mike, is that a right-hand or left-hand calculation? Icon_crazy

He doesn't know what model or make the radio is. 15 watts is enough heat to make toast. The capacitor would be the best solution but that too will be a large one. Maybe a motor starting cap at 16mfd.

RE: Calculate Needed Filament Resistor - morzh - 01-31-2026

Rod

Which one do you refer to, the resistor or the capacitor? The resistor is right hand. This is the one I used to scratch my head, so gotta be it.

The capacitor....it depends if I calculate using the voltage or the current. And you have to gate it by the side of the bed I wake up on.

Oh...it is 8.1uF.

RE: Calculate Needed Filament Resistor - dconant - 01-31-2026

Mike, A crude schematic came with the radio so that is how I know it is a 5DK of some kind. I have heard of using a capacitor and I am interested in figuring out what cap I would need. The underside is very crowded so not going to want to put a 30 watt resistor under there but a cap might be doable top side.

I gave total amps as 1.5 because I though you added amps in series. Guess not. I learned this kind of stuff 40 years ago and have not used it since last couple of years. Programmers don't deal with series or parallel circuits.

Thanks Rod for the input.

Thanks,
Dan

RE: Calculate Needed Filament Resistor - morzh - 01-31-2026

OK. When things are in series, it is like you put the tubes in series. Same flow of water goes through each one. So is the current. Same current traverses each serialized part in turn.

So you add the voltages.

In parallel is the opposite: the voltage is the same across the parts, but the currents add up.
I gave the cap value in the previous post.

The voltage rating of 150-200V will suffice, so it does not have to be a very large cap.

RE: Calculate Needed Filament Resistor - dconant - 01-31-2026

Ah I didn't catch that Mike. Thank you.

RE: Calculate Needed Filament Resistor - morzh - 01-31-2026

Do you want to know how it's done?

RE: Calculate Needed Filament Resistor - Joe Rossi - 02-01-2026

Here is link for the schematic at Nostalgia Air:

https://www.nostalgiaair.org/pdfs/Zephyr-Radio-Co/Zephyr-Radio-Co-5DK.pdf

Good luck!

RE: Calculate Needed Filament Resistor - dconant - 02-01-2026

Thanks Joe.

Mike I'm thinking cap is in series with the filaments right off the hot AC line right?

Dan

RE: Calculate Needed Filament Resistor - morzh - 02-01-2026

Since it is in series, it could be anywhere. As long as it is only in series with the filaments and not therest of the circuitry.

I can't makse sense of what this sch shows without seeing it myself and matching it to the sch, but I think "Brown" is defunct as this probably was the cord's resistor.
So it could be in series with the "White (before the 25Z5 filament)".
Or it could be before the 6A7 filament (as long as the Common wire going to cathodes' feedback resistors still goes to the same pin it was going to before). However, the fact it goes to Common might help pwith hum.

Sp, I suggest the "White" and 25Z5 filament as the point of the 8uF cap insertion.

The value of the cap is approximate: 8.2uF is very common and is being sold at Tubesandmore (Amplified parts today).

This is Solen, @$4.95, 8.2uF 630V
https://www.amplifiedparts.com/products/capacitor-solen-630v-metalized-polypropylene-fast

This is Audiofiler, $3.75, 8uF 400V
https://www.amplifiedparts.com/products/capacitor-audiophiler-400v-mkp-metallized-polypropylene

For your purposes, I'd go with the latter. It is also smaller.


PS. When you need an AC rating for a cap that gives only DC rating, divide your DC voltage rating by 1.414.
In your case you need to odrop 98V AC, which is 139VDC, so for good measure your cap should start from 200VDC rating. 250V is a good value to not go below.

RE: Calculate Needed Filament Resistor - dconant - 02-01-2026

Ok Mike I've got it. I'll order up the 8uF cap.

Thanks,
Dan

RE: Calculate Needed Filament Resistor - radiorich - 02-01-2026

Hello Dan,
That capacitor should work a treat !!
Sincerely Richard