35A5 tube substitution - Printable Version
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35A5 tube substitution - wally73 - 03-13-2017
Greetings to all ,
I Recently acquired a Philco 41-220 and someone installed a 50A5 tube in place of the 35A5 that is supposed to be there. Is this an acceptable replacement? I can't any info that approves it as an acceptable substitution. The radio is not working but I think it has other problems too. Thanks for your help.
walt
RE: 35A5 tube substitution - sam - 03-13-2017
walt
I believe they are the same
here is the data
http://tdsl.duncanamps.com/show.php?des=35A5
http://tdsl.duncanamps.com/show.php?des=50A5
sam
RE: 35A5 tube substitution - David - 03-13-2017
Walt both the 50A5 and 35A5 draw .150 amp. With todays higher line voltage should be no problems. Will add some info as PDF. David
RE: 35A5 tube substitution - TV MAN - 03-13-2017
No, they are NOT the same. 35A5 has a 35v heater and the 50A5 has a 50v heater. Other than THAT, they are the same. They probably changed or eliminated the series resistor 18, which looks like a candohm. Those resistors were known to be troublesome and may be the reason for the change. Don't try to stick a 35A5 back in there without checking that or it may be pretty short lived, along with some other tubes.
RE: 35A5 tube substitution - Radioroslyn - 03-13-2017
Hi Walt,
Here's how it works.
The tubes that start w/#7 equal 6.3v you have four of them = 25.2v
The tubes that start w/#35 equal 35v you have two of them =70v
This makes a total of 95.2v
The AC line voltage is about 120v. So if we hooked the heater directly across the AC line we would have about 25v too much.
Resistor #18 is used to drop the "extra" 25v. It equals about 180 ohms.
All the tubes are happy!!!!
When you substitute the 35A5 w/a 50A5 ( the 50A5 equals 50v instead of 35) it increase the needed voltage by 15v (50-35=15) This means instead of needing 120v now you need 135v.
If you put a jumper across #18 two things are going to happen. One dial lamp won't work any more. And w/the 50v tube in the circuit now the total is about 109v still a bit too low to power w/120v
When you recalculate #18 to only drop the "extra 15v" is total resistance equals 100 ohms. You'll want to leave the value of the left half of #18 the same as it is used to balance the current though the dial lamp.
Sorry for all the math!
RE: 35A5 tube substitution - wally73 - 03-13-2017
(03-13-2017, 10:47 AM)wally73 Wrote: Greetings to all ,
I Recently acquired a Philco 41-220 and someone installed a 50A5 tube in place of the 35A5 that is supposed to be there. Is this an acceptable replacement? I can't any info that approves it as an acceptable substitution. The radio is not working but I think it has other problems too. Thanks for your help.
walt
RE: 35A5 tube substitution - wally73 - 03-13-2017
Thank all of you for the reply. I know a lot more about these tubes than I did and I think I can figure it out now.
walt
RE: 35A5 tube substitution - Arran - 03-13-2017
Terry is quite right about the filament voltages, with a 50 volt tube in there and the original dropper, the series string voltage will probably be too low. This is why I usually wince when people suggested using a 50L6 in place of a 35L6 in an AC/DC set on the alternative forum, engineers in the 1930s and 40s were not stupid, if a five tube set has a 35L6 it has to have a dropping resistor as they typically used 117 volts as a "working voltage". The stories about a 110 volts as a standard power line voltage, is a myth, the only power grids that put out 110 volts were the DC ones, the AC ones were all over the place depending on where it was, what time of day, as well as the power line frequency. The 25 cycle power grids were nominally rated at 115 volts, the 60 cycle ones were nominally rated anywhere between 115 and 120, I can't remember what the 40 cycle grids were rated at but those were rare.
If it were my set, and the candohm was shot, I would probably replace the resistors with proper sized ones and use a 35A5 again just so things aren't confused somewhere down the line. Knowing my luck if I converted it to using a 50A5, and changed the resistor values accordingly, I would forget in a few years and plug a 35A5 in there again and cook the tubes.
Regards
Arran
RE: 35A5 tube substitution - wally73 - 03-14-2017
Arran,
You are probably right. I should order the proper tube for the set. That way there will be no guess work. Looks like the last guy that worked on this set was a real bungler, using black tape and such. I am still trying to figure out what he did and why. I doubt seriously that he did any resistor changes ; probably just plugged in what he had handy and hoped for the best. I am waiting for a readable schematic to come in the mail so I can try to figure this out . All I know at this point is that the oscillator is working. I am an amateur at this myself so I will probably be asking more questions . Thanks to all for the tube insight.
walt (wally)
RE: 35A5 tube substitution - wally73 - 03-15-2017
Ok I have ordered a new 35A5 tube . Meanwhile I measured the heater voltages on all the tubes.
Here are the results: on all the "7 tubes" I got 5.3 volts. On the 35Z3 I got 32 v, on the 50A5 I got 45 v .
Does this mean that the circuit has been modified to provide more voltage for the 50A5 or does it just naturally have more voltage on it by virtue of the tube structure, in other words it demands more so it gets more voltage.?? thanks
walt
RE: 35A5 tube substitution - Mondial - 03-15-2017
The voltage readings you are getting are exactly in line with what would be expected with the 50A5 installed.
If you add up the rated filament voltages of all the tubes you have now in the circuit, it comes to 135 V. Since you are actually only applying about 120 V, each tube will receive a bit less than its rated voltage, which is what you are measuring.
When you install the proper 35A5, all the filament voltages should be at or near their rated values.
RE: 35A5 tube substitution - wally73 - 03-15-2017
OK but I have a further question. I only have 4 # "7'" tubes : 7a8 ,7c7 ,7b7 ,7c6. Would that not be 28v? Plus the 35v plus the 50v = 113v Sorry to be a bother but I guess missing something.
walt
RE: 35A5 tube substitution - Mondial - 03-15-2017
Yes, you are correct in that the tubes themselves add up to about 110 V, but there is also the additional voltage drop of resistor 18 which is 25 V. So in total you have the expected voltage drop of 135 V.
If you apply less than 135 V then the voltages reduce proportionally across each tube according to its individual voltage rating. So if you apply 10% less voltage across the series string, each tube will have its filament voltage reduced by the same percentage of its voltage rating. So for the 50A5, it would have 10% less or 45 V, for the 35Z3 it would be 3.5 V less or 31.5 V, the 7B7 6.3 V less .63 V or 5.67 V, etc.
RE: 35A5 tube substitution - wally73 - 03-15-2017
AAAHHHH FANTASTIC!! NOW I get it. I was not considering the # 18 resistor which is 25 volt.
Thanks for the explanation. Learn something new everyday.
walt
RE: 35A5 tube substitution - Radioroslyn - 03-15-2017
It may be a little confusing but tubes that start with a 7 or 14 tell us that they are Loctal tubes (99% of the time) The 7 also indicates the it has a 6.3v heater and 14 has a 12.6v heater (99% of the time) There are a few odd ball tubes that don't fit in to this scheme. Philco is the early 30's made a #14 tube but it was used in their 110vdc sets. It's an old standard base tube. In the late 50's some tv's had some odd ball 7v tubes like a 7AU7 which is a 9pin miniature. So there are a few exception but not a lot.
So to be picky about it it's 6.3vX4=25.2.