[Phlogiston]

Have you checked the current consumption in your output stage? You might be able to adjust the bias to lessen it, or it might be too high. I have seen a radio that seemed to work fine run a great amount of current through a 42 tube. The tube did not red-plate but the transformer got hot.

I got the power co to turn the voltage down at the shop/museum to 119V from 127. It is the only building on that transformer.

As I recall it is 13 turns and 13 turns and 13 turns - 3 taps (5 wires) on that band 2 osc coil.

[Mondial]

Ron, what you say regarding the ceramic resistor is true, but keep in mind the temperature of the ceramic will rise much higher, to several hundred degrees.

Since the heat is not being conducted to the chassis, the resistor temperature will rise to the point where the heat generated can be dissipated by convection and radiation to the air and surrounding components. Therefore, the underchassis components will be subjected to much higher temperatures than if the heat was conducted directly to the chassis. Since there is very little ventilation under the chassis, the heat will be trapped there until it warms the chassis surface where it will be eventually dissipated.

This was the reason Candohms were used in the first place. Instead of cooking the under chassis components and wiring, the heat could be transferred at a much lower temperature directly to the metal chassis which acts as a heat sink, where it could be dissipated to the surrounding air.

[morzh]

Ron

Here's my thoughts:

1. No, zeners, even if theoretically possible, will bot buy you ANYTHING. Reason - think of it, you conducting the same currents (as some of these are used as cathode loads) at the same voltage drops. Hence same powers. Unless separating "voltage" generators from "current conductors" ... then maybe but it will be an involved effort. And resistors are inherently better at handling heat than semionductors.

2. Who's afraid of big bad heat.
If by touching this chassis you do not immediately sustain bad burn and simply feel very uncomfy, this means your temp is quite within normal and below incendiary point.
The chasss can take it, the lytics are on top and the air is below 105C.
And uing the cassis (a big massive metal piece) as a heatspreader is in fact good.

Unless you see things being 150C and up....I would not worry too much.

[y2kbruce]

I try to run my 'warm' sets on variacs set to about 105 or 110V. It seems to keep the transformers a lot cooler to the touch. My RCA D-22 used to get very warm but now I can run it all day and still hold my hand on its trans.

I have never thought about the resisters temperatures before seeing this very good discussion thread.

[Ron Ramirez]

Bruce - The transformers do not seem to be running hot. I'll be puling it out again over the weekend and will move the 3K resistor to a more "open" part of the chassis, and will recheck the transformers. That is an excellent idea you have, to run your "warm" sets at 105-110V. In my experience, all of the old radios seem to prefer 110V: any more than that and the majority start running warmer than they probably should.

Mike (Morzh) - No, my fingers did not immediately get burned upon touching the chassis where the 3K and 2.2K are currently mounted, but I think they would have been burned had I held my hand there for more than a few seconds. My main concern is the wood cabinet. If splitting the two resistors up will help - thus dividing the heat between two places - I'm all for it. My friend likes to be able to play his radios for hours. I just want to be sure this one is safe to do that before I release it. I will probably do a test run of my own with everything in the cabinet, a good "burn in" (no pun intended) of 3-4 hours, to see how it does, once I move the 3K resistor.

Russ (Phlog) - I have not. Perhaps I should.

Mondial - Great info, thanks.

And if anyone reading this thread has a working 37-690, preferably with the original 3K/2240 ohm Candohm in place, I would like to know if the chassis gets red hot where the Candohm is mounted after the set plays for an hour or two - just to satisfy my curiosity. As Mondial and Morzh have stated, it probably does get hot. Dry, brittle wiring in the vicinity of the Candohm in this 37-690 seems to bear that out - which is why I replaced all of the speaker wires (the originals were very dry, brittle and cracking, since they all go in to the chassis just above where the Candohm was mounted).

[morzh]

In my 111 wiring in that thick cable running around the board (contains power mostly) got brittle without any help from any heat.
All in all, the cooler the better.

[Ron Ramirez]

Agreed, Mike; I once had to make a complete new wiring harness (the thick cable as you mentioned) for one of those large chassis Philcos - I do not remember which model it was now, maybe a 96? - as the wires were very brittle and crumbling. I think on that one, it was due mainly to age and probably poor storage, rather than heat.

[Ron Ramirez]

I moved the 3K resistor to a more "open" area under the chassis this morning.

Had to go out of town this afternoon, so I haven't had time to try it out again yet.

Will try to do so tomorrow after work.

[Ron Ramirez]

Okay!

I put everything (upper and lower chassis, woofer, tweeters) back on the bench this evening and tried it out. Plugged it into my Variac and set it to 110 V.

After running for about half an hour, the 3K resistor is hot, but I can keep my fingers on the chassis where it is mounted, so it isn't hot enough to burn.

The 2.2K resistor is lukewarm.

The power transformers are fine, barely warm (if that).

Moral of this story: The 37-690, like any vintage radio, prefers 110 V to today's 125+ V line voltage. So this one, I believe, is good to go and ready to launch.