Thanks for having me in the forum. I collect radios and have restored several Philcos over the years.
I inherited a Philco 42-380 from my Dad. It had never worked while he had it. I know it is a pretty common one. I am in the process of restoring it and am awaiting some parts at the present.
While in the process of replacing the original rotten rubber wiring I noticed that pin 5 (plate) of the 6X5 rectifier is tied to the ground connections of the two 71 audio output tubes. There are no ground connections currently on pins 5 and 6 of the 71s which is in direct conflict with the schematic. I know there is a history of 6X5 rectifiers destroying the main transformer. I plan to replace it with two diodes to prevent that but wonder why the factory would have done this? I assume it had to run with this configuration since there is no sign of it being done by a previous service. The audio output transformer and speaker field coil were destroyed long ago by capacitor failures.
I looked at the schematic and it seems something is out of whack with your explanation. First, the only wire on pin 5 of the 6X5 should be to the hv secondary of the power transformer. Second, the output tubes in the schematic are 41s not 71s. Third, from your photo, it appears someone has made some modifications. So maybe you can fill us in on these issues. I may have the wrong schematic.
Sorry when I wrote 71 in stead of 41. (brain fart I am 73 years old). It is most certainly a 42-380 Code 121. I merely replaced the original wiring on the tube until I can replace it but it had the rotten rubber wire in those locations. The 41s never had any ground wires and were wired directly to the 6X5 on pin 5. The schematic definitely says 6X5. I am just as stumped as why this was done as anyone but have to say that the radio had very few repairs over the years. The main electolytics were replaced at some point.
More on this. The original wiring was undisturbed. All the tubes are correct. The octal socket is the original riveted to the chassis. Pins 5 and 6 of one of the 41 output tubes can be seen in the previous photo. This had to have worked out of the factory but I am at a loss as to why a secondary plate wire would be wired this way to the rectifier plate on pin 5 and thus to the grounds of the audio output tubes. Hopefully someone here has seen this done before. I have not. Photo is of chassis before much was done to it. I have already replaced much of the wiring, many caps and resistors, the negative voltage divider and removed to audio output transformer. At this point I am going to put in rectifier diodes and wire the 41's to ground as the schematic shows.
I merged the two threads. We do try to keep a thread together. That way the flow is easier to follow if someone needs to back track to earlier parts of the thread for understanding and for future folks to find information that is in the thread that might help them with their restoration. Thanks.
Actually in thinking about all of it believe I figured it out on my own. Without the benefit of Philco service bulletins during WWII years of course there is no way to tell what they may have put into modifications. The radio was built in late 41 and the schematic would have come out with it. At any rate what they were apparently doing was putting the grounds of the 41 outputs about 200 ohms above ground (plus the transformer winding) by passing back through the plate via the transformer and through the negative bias dividers to chassis ground. I am not sure what this would accomplish with no information available. The only thing I see that could happen is it might stabilize the negative bias by the load of the output tubes. If so directly grounding pins 5-6 on the 41 outputs might change the bias voltages on the divider. Nothing is stated on the schematic as to what these voltages are supposed to be. Removal of the feed through the plate of the rectifier would cause those to be off. The original divider failed mechanically on the first resistance. I have replaced it with 3 resistors. Interestingly one schematic version I have shows 3 resistors of lower values penciled in. This might indicate something. I was hopeful when I posted this that someone would have already encountered the modification. All I can say for sure is it apparently worked this way for decades as the wiring was undisturbed other than some filter caps being replaced.
And sorry for posting 2 threads. I didn't see how to put a photo directly on a reply. Not much else to say at this point.
I believe I saw those before pertaining to the line voltage and transformers. However I measured the tube filament voltage with the rectifier out and it was in normal range.
I was just going to mention this, Philco had a habit of building multiple variants of the same chassis, according to what tubes, and other components were available. There is typically a code number, but also a production run number, fortunately I have the 1942 U.S Philco service manual, but it may not show every variation in the 42-380. These are extremely common sets so it's likely someone has posted a thread on every version by now. Audio output transformer failures are very common in the 1939-42 Philco AC models for some reason, I wondered if it may have something to do with the screen grid inversion scheme used in many of these sets but the single ended sets seem to have almost as many failures.
Regards
Arran
(This post was last modified: 05-19-2024, 05:01 PM by Arran.)
This particular radio is a code 121. The audio transformer and speaker field coil are toast. I previously used a Hammond transformer from tuibesandmore.com that worked well on another set and is rated for several output tubes including the 41 pair. I will compensate for the field coil resistance.
Speaking of the 6X5 controversy, I think it's much ado about nothing, at least when it comes to Philcos. Early 40s Philcos that use them are not really prone to power transformer failures, many used the old style equivalent of a 6X5, an #84. The reason brand Z had problems with them is because they used them outside of spec, they did silly things like tried to use two 6X5Gs in parallel in 10 tube sets, with 6V6s in push pull, just so they could use a transformer without a rectifier tube winding. A 6X5 is rated for a maximum of 70 ma, a 5Y3/5Y4/80 is rated for 120 ma, most of these Philcos used either #41s or 6K6s, and only ran them at 193 volts on the B+.
Regards
Arran
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