[JohnSchw]

I am working on a Philco 90 (1-47 tube) and the large 3-section WW resistor is bad.  I have found the values needed (180 ohm, 60 ohm, 3500 ohm), but am not sure of what wattage resistors I should replace them with.  I found on another thread that they should be replaced with wirewound resistors again for reliability reasons.  It also cautioned against using too high of wattage ratings?

How do I determine the rating?  The tube manual lists the 80 rectifier as having a max plate current of 125ma.  The spec for this radio lists the 80 plate current at 32ma.  Can I use the 32ma to calculate the wattage of these resistors, or are there other factors to consider?  I have yet to find a parts list that shows the wattage rating.

Thanks for any help.
John

[Radioroslyn]

It also cautioned against using too high of wattage ratings?

The higher the wattage the larger the physical size so some times it's a matter of will it fit. The larger the wattage the better it can dissipate heat. In circuits like power supplies higher wattage resistor are common. In low level signal circuits like rf amps, mixers, IF amps, and such they aren't necessary. Pretty much 1/2w is the standard.

How do I determine the rating?

What I do is figure the current flow thru the resistor. Do rough calculation of the tubes vs the current they draw. Came up with 45ma and the high voltage at 250v. If you know the voltage on both sides of the field coil you can you use that to figure the current of the set.
I came up with 180 ohms @ 1w 3500 ohms @20w and 60 ohms @ 5w

[JohnSchw]

radioroslyn: I was going over your results and it seemed that a 20 watt resistor would much more than needed, and physical size also.  I attempted to do some calculations to see what I came up with.  Unfortunately, I am working on a chassis that I have not powered up.  The speaker I have has an open field coil, so could not measure resistance, but I found that the DC resistance should be around 3000 ohms.
  
Adding the total tube current from the data sheet, it comes to 42.15ma, not counting the power dissipation of the field coil.  I used your estimate of 45ma for the calculations.  If this is the total current throughout the B+ loop, I came up with much lower power ratings.  Using the current and resistance values, I calculated the voltage drop across each resistor, then calculated wattage.

The 180 ohm = 8.1 volt = .36 watt.  The 60 ohm = 2.7 volt = .122 watt.  The 3500 ohm = 157.5 volt = 7.1 watt.  Based on my calculations, I should be able to use 2 watt resistors for the 180 ohm and 60 ohm resistors and a 10 watt resistor for the 3500 ohm?  Does that sound reasonable?

The other variable is a resistor to replace the field coil, possibly 3000 ohm at 10 watt?

If my calculations are wrong, please let me know.
Thanks,
johnschw    

[Radioroslyn]

Hi John,

I didn't scroll down to see the voltage/current chart. In my head I did a rough calculation per tube. Rf, mixer, osc, IF, and pre audio equal about 3-5ma. Diodes and triodes configured as a diode 0ma. Audio output tube(s) 20+ma each. I guessed at the HV and used 250V. The only way the field coil would come in to play is if it was in parallel with the HV. Then it draws current.

As for math it all looks good. There is no harm in having the wattage higher than the actual dissipation. General rule of thumb is at least 2-3X. If you measure the physical size of the 20w vs the size of the bracket that's welded to the chassis It might be a good fit. A 20w power resistor isn't all that big. The other two could be wired in the circuit or attached to a terminal strip. I did see these https://www.ebay.com/itm/253830523764 I think they are 2 1/4" long but I'd check w/the seller if interested.

Replace the field coil with a resistor is doable situation with a 3k. In most sets the field coil is used to filter out the 60cy ripple from the dc HV. In this set the filtering is done by #'s 55,56, and 57. The coil is more or less as volt drop to supply the screen grid voltage to the rf and IF tubes. If there any instability in the circuits it can usually be quenched be adding a bypass cap to the screen line. And of course you will have to replace the speaker and add the an output transformer. As you've probably read already the Philco models 20B, 21B, 70B, and it's alliterations, and 90B's are the kinda the same well the coils are. The wiring and output transformers aren't.

There are some here on the Phorum in yrs past that have rewound K speaker coils. Early ones have a large nut on the back that makes it easier the remove the pole. The pole is a steel shaft that runs thru the center of the coil. Later K speakers the pole the pressed in. It can be removed with a large vise and a few sockets.

Here's a copy of the diagram w/the parts values. Makes it a bit easier to follow https://philcoradio.com/library/images/schem/90b.jpg