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38-690 Tweeter Temperature - Printable Version

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38-690 Tweeter Temperature - RGK20m3 - 04-21-2022

I know that this has been discussed before, and may be normal- the tweeter field coil that is in the audio amp power supply runs considerably hotter than the other tweeter; voltage drop across the audio side tweeter field is about 85v, while the other is about 45v.  This equates to 125ma current flowing, of which 60ma is the woofer field.  There is about 25ma through the 6F6 plate and screen, which leaves 40 ma for all the screens on the upper chassis, which seems reasonable. But 10.5 watts seems a lot for the tweeter field to dissipate.......
Why would it be designed (or desirable) to operate this way? I am thinking of shunting the field to reduce the current through the field, which would raise some of the upper chassis voltages. Or, connect the woofer field before the tweeter field with a resistor in series.
What do you think?

RE: 38-690 Tweeter Temperature - Brad Winder - 04-21-2022

I'm not sure if you know this, and it may or may not be related to this possible issue you are seeing...but the tweeters were wired "out of phase" or backwards on most sets from the factory. Changing this makes the tweeters work much better, and increases the high notes. And maybe it will "correct" the voltage difference? Not sure...

http://www.philcoradio.com/tech/tweeters.htm

RE: 38-690 Tweeter Temperature - Ron Ramirez - 04-21-2022

It won't correct the voltage difference/heat issue.

RE: 38-690 Tweeter Temperature - RGK20m3 - 04-21-2022

As the magnetic field is a function of current flow, connecting the woofer field upstream of the tweeter (with the appropriate resistance in series) should not affect its performance. But it will still result in the same higher voltages on the upper chassis as if the tweeter field were shunted.
Odd that Philco has the upper chassis plate voltages from one power supply and the screens on the pentodes from the other power supply.

RE: 38-690 Tweeter Temperature - morzh - 04-21-2022

Brad

Those are two different issues, entirely unrelated.


RGK20m3

I do not think you shoud mess with the setup. And putting the woofer field before (I take it, still as a load, not as a filter choke), will expose it to more ripple.

I would re-phase them as required (there was a factory mistake) and left them be otherwidse.

RE: 38-690 Tweeter Temperature - RGK20m3 - 04-21-2022

I already have corrected the tweeter phasing and will probably leave alone. I did temporarily shunt the tweeter with 1.2 K and 2.4 K resistors; it did affect the gain (which is what I expected), but I didn’t want screen voltage to exceed plates, which would have to be checked on each pentode.

RE: 38-690 Tweeter Temperature - Brad Winder - 04-22-2022

The next time my friend comes over, I'll have him use his infrared camera to see what the temperatures are on my 38-690. I'd like to see if this is a common issue.

RE: 38-690 Tweeter Temperature - RGK20m3 - 04-22-2022

Based on my measurements and the schematic, you most certainly will have the same issue (if your 38-690 is not modified). You don’t need the IR thermometer, just measure the voltage across the two field terminals (smaller diameter pins) on each tweeter- one will be 40ish volts, and the other 80ish volts. After operating for a few minutes, the 80v one will be warmer to the touch, and hot after 15 minutes.
Voltage across the woofer field will be about 250v, yet cool.

RE: 38-690 Tweeter Temperature - RickC - 11-23-2023

Does the 37-690 and the 38-690 use the same tweeters and woofer?