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 Post subject: Tube Saver Resistor Replacement - Philco 53-561
PostPosted: Dec Wed 06, 2017 2:54 am 
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For reasons I'm too embarrassed to mention, I snipped out of a Philco 53-561 a "tube saver" resistor and I need to find a proper replacement.

Photos below of the resistor and the section of the schematic with the appropriate info.

Can anyone advise what I should replace this with? Thanks!

Attachment:
IMG_2146.jpg
IMG_2146.jpg [ 84.12 KiB | Viewed 348 times ]


Attachment:
IMG_2147.jpg
IMG_2147.jpg [ 148.1 KiB | Viewed 348 times ]


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 Post subject: Re: Tube Saver Resistor Replacement - Philco 53-561
PostPosted: Dec Wed 06, 2017 3:07 am 
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Proposed solution:

The tube filaments are in a series string. That means the current through the entire string is the same. One of the tubes is 12BA6. Data sheet shows current is 0.15A (150mA).

http://rtellason.com/tubedata/6ba6.pdf

Ohm's Law equations:
Voltage drop across the resistor is: V = IR, so V = (0.150)(100) = 15V
Power dissipated is: P = IV, so P = (0.150)(15) = 2.25W

Apply a safety factor...you should replace it with a resistor of the same value (100 ohm) rated at 4W or more.

Next step: This is a "tube save" resistor where it has a higher resistance when radio is cold. Designed to suppress in-rush current when you turn the radio on, then resistance decreases as it warms up. Most older radios don't have this. If you still want this function, you can use a modern in-rush limiter. Place this in series with the new resistor; initial resistance doesn't need to be as high as ~800ohm (per schematic).

Example: Here's one rated for 200mA with initial resistance at 400ohms. (Cantherm MF72-400D9)

https://www.digikey.com/products/en/cir ... ageSize=25

In-rush limiters have a small resistance at steady state, so you may have to adjust the value of that 5W resistor.

There are other solutions, including not using an in-rush limiter at all. I'm sure you'll get other inputs.

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 Post subject: Re: Tube Saver Resistor Replacement - Philco 53-561
PostPosted: Dec Wed 06, 2017 5:15 am 
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If you didn't loose the thing after snapping the photo, why not just wrap a wire around each end, solder it to what's left of the existing wire, then put the part back in the radio.

Even a regular resistor will reduce the surge current a little, since it will drop more voltage until the tubes warm up. You can adjust the value of the resistor to give normal heater current at your usual line voltage.

Ted


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 Post subject: Re: Tube Saver Resistor Replacement - Philco 53-561
PostPosted: Dec Wed 06, 2017 5:29 am 
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I agree with Ted. If this were my radio, an ordinary 100ohm 5W resistor would be good enough replacement.

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 Post subject: Re: Tube Saver Resistor Replacement - Philco 53-561
PostPosted: Dec Wed 06, 2017 6:52 pm 
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Thanks, so just to confirm, a 100 ohm 5 watt resistor by itself should do the trick?

If I can get away without the in-rush limiter I may do so and save not having to order.

Thanks for all the input!


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 Post subject: Re: Tube Saver Resistor Replacement - Philco 53-561
PostPosted: Dec Wed 06, 2017 7:16 pm 
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If you install a 50C5 in place of 35C5 you can jumper it out completely...

The surge suppression can be retained by installing one of these(have same in a couple AA5) along with 50C5, or use resistor and thermistor with 35C5...

https://www.ebay.com/itm/8pcs-AMETHERM- ... Ciid%253A1


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 Post subject: Re: Tube Saver Resistor Replacement - Philco 53-561
PostPosted: Dec Wed 06, 2017 7:39 pm 
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hartleymanages wrote:
Thanks, so just to confirm, a 100 ohm 5 watt resistor by itself should do the trick?

If I can get away without the in-rush limiter I may do so and save not having to order.

Thanks for all the input!


I think all the recommendations here are valid. The way it works is that all the tube filaments are in series and divides AC line voltage (~117VAC) among them. The 100 ohm resister picks up 15 volts so voltage drop across each tube is correct. It doesn't need to be exact; anything between 100-120 ohm would work.

As already mentioned, the 100 ohm resistor would handle some of the surge already. Using a 50C5 could work too, but using a resistor is probably the most cost effective solution.

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 Post subject: Re: Tube Saver Resistor Replacement - Philco 53-561
PostPosted: Dec Wed 06, 2017 8:21 pm 
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I agree. Use the 50C5 since 35C5's seem to love burning out much sooner. I see this a LOT in radios from this era


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 Post subject: Re: Tube Saver Resistor Replacement - Philco 53-561
PostPosted: Dec Wed 06, 2017 8:31 pm 
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Should I just install the new resistor and swap the 35C5 with a 50C5 as well?


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 Post subject: Re: Tube Saver Resistor Replacement - Philco 53-561
PostPosted: Dec Wed 06, 2017 8:52 pm 
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hartleymanages wrote:
Should I just install the new resistor and swap the 35C5 with a 50C5 as well?


No. The "35" in 35C5 means 35 volts across the filament. So 50C5 (a similar audio output tube) would be ~50 volts on the filament (50C5 picks up that "extra" 15 volts).

If you use 50C5, don't install a new resistor at all (just put a piece of wire there). Ensure you put some type of marking on the chassis so you or someone else would remember/know there's a permanent mod on the radio.

. . . . . . .

Just to be clear, that original "tube saver" served two functions. One was to pick-up/absorb 15 volts on the filament string (necessary); the other function was to limit in-rush current when tubes are cold (optional).

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 Post subject: Re: Tube Saver Resistor Replacement - Philco 53-561
PostPosted: Dec Thu 07, 2017 3:06 am 
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The third function is to burn the resistor above it to a crisp. (Note, the chassis is upside down here.) They get very hot. Soldering wire to it might cause the solder to melt.
Attachment:
BurntUp.jpg
BurntUp.jpg [ 248.92 KiB | Viewed 233 times ]


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 Post subject: Re: Tube Saver Resistor Replacement - Philco 53-561
PostPosted: Dec Thu 07, 2017 3:49 am 
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I have a few little Firestone Mercury bakelite radios.

they all have the standard AA5 octal 150mA tube lineup, however one has a 35L6 in the output with an 82 ohm resistor in series with the fil string. the others have a 50L6 for the output and no resistor in the series string.

all are of factory original design. the tubes are verified on the tube layout chart as well.

I guess there was a great deal on cheap 35L6 tubes and 82 ohm resistors over the 50L6 design.

I would just do as said above and eliminate the resistor, install a 50C5, then make note of the change on the tube chart.

steve

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 Post subject: Re: Tube Saver Resistor Replacement - Philco 53-561
PostPosted: Dec Thu 07, 2017 3:58 am 
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Use a thermister CL-90
It starts out 120 ohms cold and drops to 2 ohms hot in about 15 seconds.
https://www.mouser.com/ProductDetail/Am ... faLQ%3D%3D

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 Post subject: Re: Tube Saver Resistor Replacement - Philco 53-561
PostPosted: Dec Thu 07, 2017 5:26 am 
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The "back in the day solution" was a Workman FRT1 - the universal Globar (Carborundum) Thermistor for inrush protection. Good luck finding one now.

I still snip everyone I see in a junk chassis and save it, but it's been a while. The last I saw was on eBay - in a MidAmco Globar lot from a seller in Canada. Can't find it now.

Similar (and not mine..):

Image

https://www.ebay.com/itm/Globar-Workman ... SwXwBZngBq

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 Post subject: Re: Tube Saver Resistor Replacement - Philco 53-561
PostPosted: Dec Thu 07, 2017 5:01 pm 
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AGAIN, this is probably the easiest & cheapest option...

https://www.ebay.com/itm/8pcs-AMETHERM- ... Ciid%253A1


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 Post subject: Re: Tube Saver Resistor Replacement - Philco 53-561
PostPosted: Dec Thu 07, 2017 5:15 pm 
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SkyKing wrote:
The third function is to burn the resistor above it to a crisp. (Note, the chassis is upside down here.) They get very hot. Soldering wire to it might cause the solder to melt.
Attachment:
BurntUp.jpg


The tube saver resistor had absolutely nothing to do with the failure of the other resistor underneath it in the photo, that one burned open due to excessive current in the circuit it's a part of. The tube saver does overheat solder connections like the one we can see at the tube socket.

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 Post subject: Re: Tube Saver Resistor Replacement - Philco 53-561
PostPosted: Dec Thu 07, 2017 6:51 pm 
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Mr. Detrola wrote:
The tube saver resistor had absolutely nothing to do with the failure of the other resistor underneath it in the photo...


Here's another one, but the small resistor hasn't failed yet.
It isn't failing from excessive current, it's being cooked to death.
Attachment:
TubeSaver.jpg
TubeSaver.jpg [ 124.6 KiB | Viewed 166 times ]

Attachment:
SolderJoint.jpg
SolderJoint.jpg [ 163.87 KiB | Viewed 166 times ]

These are both Philco sets. I moved the small resistors out of the "oven" when I replaced them.


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 Post subject: Re: Tube Saver Resistor Replacement - Philco 53-561
PostPosted: Dec Thu 07, 2017 7:07 pm 
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If you do try soldering new wires on one of these, you may need to use high melting point solder.

Also, the high temperature activates any remaining solder flux and it will eat through the wires. You want to wash off all of the flux after soldering. This may apply to the connections to a replacement thermistor, as well. I think you want to leave the leads pretty long, and locate the device away from other things that might tend to get toasted.

Ted


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 Post subject: Re: Tube Saver Resistor Replacement - Philco 53-561
PostPosted: Dec Thu 07, 2017 8:37 pm 
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While the thermistor may have not been the direct cause of those resistor failures, it was no doubt a contributing factor...
A power handling resistor should be in "free air", not just above a device that's probably operates near 150*C...


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