[exray]

I've been waiting for the concise answer on this question! My thoughts are that the 25cycle xfmr is simply more efficient at 60cycle and has less voltage drop under load. I don't have much experience with these things. My only 25 cycle radio has a non-original 25 cycle replacement xfmr so I have nothing to compare to.

[Arran]

Have you tried swapping out the Rectifier tube? Internal intermittent rectifier tube plate shorts possible? Not usually the prob tho, without the tubes giving a spectacular light-show.The 25-50 cycle power- trans will work on 60 cycles, but generally always run hot in my past experiences. Whenever I run across these sets, I always replace the 25-50 cycle ac trans with similar type 60 cycle trans I rob from other cheap parts sets as they come avail. I would suspect the power trans in your case(s)?
I remember I bought a couple nice TV sets many yrs ago for cheap at a garage sale ( without close inspection) of the mains, that never worked right. After closer inspection, I had purchased 2 TVs that had 50 cycle ac trans mfd in Mexico,for Mexico 50 cycle pwr mains, and although they worked, the "picture" was never clear & audio was distorted.Their lifespan was short also on 60 hz mains! Both ended up with smoked power trans & blown fuses. I have run across several old Canadian model Philcos with the 25-40 cycle trans that were previously totally-fried, windings internally by someone previous plugging them in using modern day 60 cycles mains due to internal excess heat inside the trans. Limiting incoming ac line voltage keeps these 25-50 cycle trans from smoking, for awhile, but its better to rid them if possible and replace them with other vintage 60 cycle types when possible.

That makes no sense whatsoever, I'm sorry but there are hundreds of sets in Canada still with their original 25 cycle transformers that still operate perfectly with proper secondary voltages, most even state on the build plate, 25/60 cycle. Given that 25 cycle transformers have a larger core, and use a heavier gauge of wire, this means less core saturation and cooler operation, the physics say the opposite. We have not had 25 cycle power for domestic use since the late 1950s, only 60 cycle, the only sets I have personally seen with bad transformers were sets that experience a dead short or other high load condition in the power supply, most of those were 60 cycle sets by the way. If running 25 cycle sets on 60 cycle was a problem then we would see many, many more radios up here with smoked power transformers. If there is excessive B+ there has to be another cause, shorted primary or just a higher line voltage.
Best Regards
Arran

[Texasrocker]

Hello Aaran. I have to disagree. I have had several Canadian Philcos cross my workbench over the yrs that had orig 25-40-50 cycle ac trans on them.Many were already "smoked". The still working 25-40 cycle trans heated up after running awhile on 60 cycles & produced higher B+ voltages in ALL cases, unless a suitable size dropping resistor had been added previously by other techs somewhere down the road. Yes, perhaps this particular orig 25 hz trans ( we are discussing) had internal shorts ( wonder why), nevertheless, I always replace them. To each his own I guess. And if the rectifier tubes plates are getting near short-status, it will in fact make the trans work harder also indeed just before the fireworks show inside the rect. tube.Even the best tube testers can lie, the best test is tube under-load in the set it is working in.
I didnt say a 25 cycle trans "wouldnt work" on 60 cycles!, but in many of my past experiences servicing these sets, I found all them running higher-voltages in B+ here on 60 cycles unless someone previous had added a pwr dropping resistor either at the rectifier socket feeding 1st B+ filter, or by previously lifting & installing a suitable power resistor at the 25 cycle trans CT to chassis ground. Many of these vintage 25-50 cycle sets have had these mods already done many- yrs ago by other techs for the same reasons as I have discussed here, & I found their reasons for "adding" the dropping resistors in the 1st place to work on 60 cycles for a good reason.Those previous "mods" were to drop B+ voltage probs many yrs ago in the US. You bet, vintage 25 cycle trans will work in vintage radios on 60 cycles without any noticable audio probs because vintage tube radios are very-forgiving in "audio". As I stated in my last post, try the same with any solid-state vintage items ( IE tape decks,etc.) designed for 50 cycles vs running on 60 cycles, & you will definately hear the audio distortion due to the cps differences. The last Canadian Philco I had here was a 38-9C, ( equil to US 38-8) I purchased as a parts set on ebay. The B+ dropping-resistor idea had "not" been previously added to that set that had the 25-40 cycle notation plate,and larger size trans & the pwr trans was already "fried" many yrs ago! Wonder why, coulda been the filter-caps also? I installed a good used 60 hz trans on it and saved it.
Im not here to argue at all, Im here to help explain why B+ runs higher here on any vintage 25-50 cycle trans in my previous experiences in servicing vintage radios for years. My local incoming ac line voltage is running around 123-124 volts at 60 cps here depending on time of day usage. And yes,all vintage trannys will heat-up abit, but the 25-40 cycle types really take awhile to build their heat, then "pop",.. your done in many cases if you dont monitor the B+ carefully. I still say using a 25-50 hz power trans on 60 hz, your still " livin on borrowed time"? Just my.02 only.

[Arran]

I think we need to go back to the original post, one was a Zenith (Zenette) that definitely had a 25 Cycle transformer, the other was a Sparton where it is not clear exactly what it had, the only thing that they definitely had in common was a high B+ voltage. My understanding of power transformers and line frequency is that they are similar to resistors with regard to wattage, but the inverse. A higher frequency transformer does not like to operate at a lower frequency, but the lower frequency ones can operate at a higher frequency. I have owned several radios with 25 cycle transformers, it was the standard in my area and many others until the late 1950s in Canada, and not one has had a defective power transformer, I have repaired many for other people and never encountered a 25 cycle set with a smoked transformer. I have a collection of spare transformers, but have yet to have needed any for my sets, the last one needed for a radio was sold to someone else, a 60 cycle post war RCA Victor. Most manufactures offered both 25/60 and 50/60 cycle power supply sets in Canada except for one of the largest companies, Rogers-Majestic, every set they made had a 25/60 cycle transformer regardless of where it was sold.(I should mention that sets usable on 25 cycle cost more, hence why most companies offered 60 cycle sets as well) This is not a case of a ticking time bomb that has not gone off, it has been fifty years since 25 cycle power was offered for domestic use so if it were in inevitable almost all such devices would have developed this problem, much like the late 1920s RCA Radiola sets with type 50 tubes. I have also known several retired servicemen who were around in the era of the changeover, plus I have known probably a dozen long time radio collectors, and I have much of the factory service data from the same, the issue has never been mentioned with anything aside from electric motors.
I can't comment on the Mexican TV sets, although I wasn't aware that they used 50 cycle mains in Mexico, 50 cycle usually goes along with 220-240 volt power systems like they have in Europe, and the last radio I saw from there was a 110-125 volt Hotpoint tube set. But TV sets are a much more complex device then a radio, a number of component failures could cause these problems, without an autopsy it's hard to say. Maybe they were defective to begin with? It isn't unknown for people to sell broken down electronics at a garage sale just to save a trip to the dump. Even just the length of time the TVs were out of service can be a factor, electrolytic condensers don't age well and poor storage conditions do not help.
I think perhaps the higher line voltage may be more of a factor then line frequency, it may be worth carrying out a few tests on my own 25 cycle sets to see what is going on. Many were originally rated for 115 volts, it only takes a voltmeter to see what is going on, if anything. Strange enough I have never run across any Philco with a bad power transformer, unlike Zenith whoever manufactured them for Philco never cut any corners with their 1930s models.
Best Regards
Arran

[Texasrocker]

You bet Aaron, I know you have alot more experience in dealing with both 25- vs 60 cycle ac trans than I in your area. Ive ran across a few of the 25 cycle trans over the yrs also, that run higher B+ in cases where someone had added the dropping resistors,as they werent orig to the chassis.And most probably, as you say, because of all our higher line-voltages these days. It took awhile for the trans to get hotter than the 60hz types, but whoa, they did here.
In any case, I sure appreciate your input on this topic as Ive never had the opportunity to "A-B" the ( hz) differences as much as you have in the past. You bet, Mexicos used garage-sale finds 50hz TV ( garage-sale-specials) I purchased only cost me $5. each, so I didnt lose much, but learned alot in the interim.I used one of them in my workshop for about 3 months before it gave-up the ghost! Im sure all your orig 25-50hz vintage tube Radios will be fine on 60hz, or they would have already shown-signs of troubles by now.
I just missed a "sweet" Truetone D-723 on ebay (25 hz canadian model) tonite. I really like those sets for their beautiful dials! I have another (USA same 60hz model) here already fully restored & they are great DXers also! I also have another (spare) good-used 60hz tran from a exact parts D-723 US chassis to install on one if necessary. I got outbid, and missed it, darn ebayers with more money that I care to spend sometimes! The seller said it was working. I wanted to recap it, & see how long the 25 cycle tranny would last here since I have a 60 hz identical ac tran repl here to use as a A-B comparison.
Seems were all at odds from set-to-set these days due to higher line voltages. Vintage radios chassis are rugged & forgiving indeed! Im currently restoring a very nice un-molested Radiola 80 for a customer. All the tube filaments are still good, thats a +, especially since the tubes are all orig style "globe types"! The orig #45s radiotron (p-p) pair still test good, & I bounced them into another of my fully restored TRF sets. They still sound very good. The 80 rectifier orig clear globe tube was replaced with a blue-glass Arcturus 80 type & still works also.All the orig RCA chassis trans all test good as well as the choke, so once I get it re-capped, cant wait for it to belch-back to life! Gotta love all these old Radios indeed!!