Incorrect Tubes Installed? - Page 1
Forum section: electronics / beginners

author: Wil_Bloodworth
author_rank: Regular Contributor
timestamp: November 10, 2023, 08:37:32 pm
content: Hello!I'm restoring my first radio which is a 1946 Mason model 45-4. I could not find a schematic but I was able to find a schematic for model 45-1A which looks like should very closely match the radio in my possession.I'm confused by the Power output tube that is currently installed in the radio which is a 50L6-GT. The schematic for the 45-1A model has a 35L6-GT in it. If I add up all the voltages for all of the tubes for model 45-1A, I come up with 118V which makes sense. When I add up all the voltages in the radio in my possession, it comes to 133V which does not make sense. See image below.QUESTION: Is it possible/reasonable that someone else tried to substitute a 50L6 for the 35L6 (thus increasing the voltage 15 volts)... which is why this radio stopped working in the first place? Or (more likely) am I just not understanding the schematics (and probably a lot of other things)?Thank you,- Wil

author: retiredfeline
author_rank: Frequent Contributor
timestamp: November 10, 2023, 09:05:47 pm
content: Those tubes also have a rated heater current, but in the case of the 50L6-GT and 35L6-GT they are both 0.15 A. So the 35 may be a more efficient version of the 50. Or may be a slightly different tube and you need to get better schematics. If the voltage of the chain is above that of the supply, then the heater current will be less but it may work after a fashion.I'd get better schematics or trace out that of the specimen you have first before speculating too much.

author: Wil_Bloodworth
author_rank: Regular Contributor
timestamp: November 10, 2023, 09:18:55 pm
content: I don't have an option for better schematics... that I am aware of.- Wil

author: floobydust
author_rank: Super Contributor
timestamp: November 10, 2023, 09:47:02 pm
content: Filament inrush current is too high so there will be a series power resistor, and I can see the dial-light is part of the filament string and gives an extra voltage drop.

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: November 10, 2023, 09:57:24 pm
content: 35L6 has 35 volts on filament recommended50L6 has 50 voltsAccording to RadiomuseumBut that is voltage drop in you case across the filaments since your radio uses almost the whole line voltage to heat the tubes in series. Your real voltage is the sum of the real filament voltages, not the sum of the rated filament voltageRemember the filament is just really a resistive wire.Line voltage has changed since your radio was made and instead of 110 volts it is now little more than 120.Maybe the 50 tube was put into compensate for the higher voltage. Probably unnecessary.If the filaments are not burning out then I would assume the filament volts are just fine. They do not have to be exact.Just measure the AC voltage across each filament, to be sure.

author: Wil_Bloodworth
author_rank: Regular Contributor
timestamp: November 10, 2023, 10:00:51 pm
content: Yes Wallace... I already posted that information at the top.

author: TimFox
author_rank: Super Contributor
timestamp: November 10, 2023, 10:21:37 pm
content: The 35L6 and 50L6 are both rated for series-string operation at 0.15 A, and are otherwise equivalent.The 25L6 and 12L6 are rated for 0.3A and 0.6 A, respectively.The 35L6 was not so common as the 50L6: usually, the first 12SK7 in your diagram was not used, making the "All-American Five (AA5)" complement, which required the 50L6.(None of the above is remotely like a 6L6).With a 50L6 installed in place of the 35L6, the nominal resistance of the string would be (12.6 + 12.6 + 12.6 + 12.6 + 50 + 35)V / (0.15 A) = 903 ohms (warm). At 120 V (now), the current is 120 V/903 = 0.133 A (10% low).With the 35L6 installed, the resistance would be (12.6 + 12.6 + 12.6 + 12.6 + 35 + 35)V / (0.15 A) = 803 ohms (warm), for 110 V/803 = 0.137 A (when US had 110 V), or 117 V/803 = 0.146 A (at 117 V), or 120 V/803 = 0.149 A (now).The heater current with 50L6 is only 10% low: that should still work (the data sheet does not specify tolerance on that).In the pictorial diagram of the tube sockets, the dial light should be across a part of the 35Z5 heater (between pins 2 and 3), not in series with the heater string (2 and 7 for the 35Z5).Otherwise, the inrush current would pop the dial light quickly: here, it is across a voltage divider.

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: November 10, 2023, 10:30:51 pm
content: Sorry I was too quick to post and did not finish before posting, I modified my post.

author: Wil_Bloodworth
author_rank: Regular Contributor
timestamp: November 11, 2023, 12:28:53 am
content: That's quite a lot of information...99% of which I don't understand since I know nothing about any tubes that aren't in this radio and isn't on the data sheets I have. I don't see how your first four sentences have anything to do with my radio so forgive me on that.I cannot really test anything since the entire contents of the radio are spread out on my desk and are awaiting a rewire. The voltage imbalance of the 50L6 vs. a 35L6 is what is throwing me off but if that is not an issue then I guess I won't be concerned with it until it becomes an issue.I'm not sure what you're trying to tell me with the sentence that starts, "With the 35L6 installed,...". Are you saying that the 35L6 would be suitable based on the math?I don't see (in either photo) where the bulb comes in contact with pin 7 of the 35Z5. I have them going from 2 to 3... which still confuses me but that is a whole different topic.Thank you!- Wil

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: November 11, 2023, 01:02:53 am
content: You can check the filaments for continuity with a multimeter. You can also check the other tube elements for a short .At least this is something..But if they did light up, the filaments are OK.

author: TimFox
author_rank: Super Contributor
timestamp: November 11, 2023, 05:36:00 pm
content: Shorter answer:Series-string heaters obviously have the same current in each heater along the string.The datasheet specs give the expected voltage for each tube's heater at a given current: 150 mA for the tubes in your radio.The 35L6 and 50L6 are both specified for 150 mA, but will have 35 V and 50 V across the heater at that current.The actual current in the string depends on all of the tubes in series: I gave the math for each case (35L6 or 50L6 in output stage).With the less-common 6-tube consist of your radio, the 35L6 is a better choice for 110 to 120 V power, but changing to a 50L6 (in series with the other five tubes) changes the actual current by roughly 10%, which is not ideal but should work.The 50L6 was more common at the time of your radio's design, in a 5-tube consist.

author: CaptDon
author_rank: Super Contributor
timestamp: November 11, 2023, 06:49:46 pm
content: Those first 4 sentences were relevant, Tim explained that 6 tube versions used the 35L6 and 5 tube versions used the 50L6. He also explained to you that either tube would work properly in a string of .15a filament current tubes. In your case with the 50L6 installed ALL of the tubes would be seeing about 10% less filament voltage and I have seen this exact scenario before!! The only downside I have seen is it takes longer for the radio to begin playing and it possibly may not play as loud although I haven't determined that to be true. The upside is the tubes may last an extra 40 years. His reference to the 25L6 and 6L6 was more 'matter of fact' to inform you that other versions of the 'L6' exist and none are even close to resembling the legendary 6L6. Are you able to measure leakage of coupling capacitors? Do you have a decent multimeter with a high impedance input like 1 megaohm or higher? A Simpson 260 just won't cut it for troubleshooting grid leak bias and in most cases will radically disturb any grid voltage measurements.

author: Wil_Bloodworth
author_rank: Regular Contributor
timestamp: November 12, 2023, 01:01:53 am
content: Tim & Don,Sorry... I have zero history with electronics and tubes so what is obvious to you/most, isn't clear to me at all... yet. I'm working on it.I did not see it before, but each of the tube sockets have the recommended tube written on them. The 50L6-GT is the correct tube according to the socket.As for continuity, neither the one that came in the radio nor the one I ordered from eBay have continuity across any of the pins; including the heater pins. I've ordered another 50L6-GT from a reputable source (recommended by another member on this forum) so when that one comes in, I will be able to proceed there.I do not have the ability to test for leaking capacitors so I'm just replacing all of them with the exception of the one mica capacitor. I do have a couple of good DMMs but I'm not sure about the impedance; I would have to look in the user manual. I don't know what a Simpson 260 is but it sounds like it isn't important yet since I'm probably a very long way from trying to test any grid voltage measurements. I'm still trying to figure out what "B+" is and why it's called that and where it comes from.I now have a Variac, am in the middle of building my dim-bulb tester, and am waiting on an isolation transformer to arrive... so I can attempt to test some things without killing myself or burning the house down.My apologies for the ignorance... I'm absorbing all of this as quickly as I can. Lots of mental light bulbs come on daily and I'm hoping a lot of this will all come together by the end of the year.Thank you!- Wil

author: retiredfeline
author_rank: Frequent Contributor
timestamp: November 12, 2023, 02:19:48 am
content: In early radios the filaments were supplied by the A battery and the higher voltage by the B battery. The terms stuck even after radios were mains supplied.You have a lot of fascinating history to catch up on.

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: November 12, 2023, 12:37:55 pm
content: And the "C' battery was grid bias, which is a neg voltage.Old multimeters like the Simpson, which was a standard good meter in it's time, have lower impedance and will give you a lower volts reading than modern ones, don't worry about this but keep it in mind. So your volt readings with a modern meter might be a little higher than that stated in schematics or repair documents. Sometimes repair instructions call for VTVM, Vacuum Tube Volt Meter, referring to the the tubes in the meter (not the radio). A modern multimeter is probably the equivalent of a VTVM in most applications, so if you see this don;t worry, a VTVM was just a much better meter in the "old days"I recently purchased a cheap BSIDE component tester, I am amazed at how good it is and use it all the time. You may wish to look into one Tests caps, resistors, inductors and diodes and transistors.....BSIDE ESR02 PRO Digital Transistor TesterReplacing all the old caps is kinda standard in these old radios...Mica caps usually do not fail though so you are doing the right thing.You may wish to read a bit about tubes, maybe start with concept of "self biasing" for the grid bias of a triodeDont give up, eventually you will start having some fun.

author: Wil_Bloodworth
author_rank: Regular Contributor
timestamp: November 13, 2023, 02:54:52 pm
content: I found an old (1942) book, 'Principles of Radio" by Keith Henney which has this perfect explanation of A & B power. I probably should read the rest of the book!- Wil

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: November 13, 2023, 09:02:22 pm
content: The book sounds like a good start from what I can see. I used to have a radio book WWII era, by the US Army training that was an introduction to Radio... It included stuff like Ohms law and onward to amplifiers and receivers etc.. My mother still has it. I believe. These things were meant to start training for the farm boys, so they started training with the basics and were simply written, but if you understood the whole book, you could probably design a radio.