Testing tubes

[Philcofan42]

What's the best way to test vacuum tubes without an actual tube tester?

Thanks,
Dan
(Learning all the time)

[radio63]

Substitution. Remove one tube at a time and replace with another of the same type. Try the set and if performance is greatly improved, likely the tube you have just swapped out was weak or bad. Turn the power to the set off and unplug each time you swap tubes. If no difference was noted, put the old tube back. Do only one tube at a time or else you might get mixed up. This works best for tubes in the RF, IF, and audio sections of a radio, and power suppiles where the rectifier has died due to old age. One caution, if your set has B+ problems such as shorted filter caps, no matter how many rectifiers you install, they will keep blowing until the cause of the circuit problems are resolved. Beware.

Gilbert

[radiotvnut]

I'll second that. I'll add that if your radio uses a power transformer and has shorted caps, you could take out the rectifier AND the transformer. That's one reason just to go ahead and replace all the paper and electrolytic caps in a radio.

[Philcofan42]

Thanks! Don't happen to have any spare tubes although both IF tubes are the same. I've replaced all the caps except the micas and am now thinking it's a tube or two that's the problem.

Dan

[radiotvnut]

Don't forget to check the resistors. Those old carbon resistors can drift way up in value over the decades. Also, spray all the controls and tube sockets with deoxit or some other good contact cleaner.

[leadlike]

I agree with the folks above-I don't pull out the tester unless I am at a loss to what problem a radio could have. I would try applying signal to a few areas to see what stage is giving the trouble. If you find you need some help, give us the model number and we'll be happy to help you.

[Fisherdude]

Well, tube substitution is certainly one thing you can try.

Keep in mind that tubes don't always fail outright. In most cases they get weaker and weaker until they're so far away from their original specifications that the circuit that they're in doesn't work properly any more, and you can then tell that something's not right.

If you're going to test tubes by substitution, you really need a set of known good, tested good, tubes to use as your reference.

If you substitute one untested tube for the questionable tube in your radio, there are three possible results, it'll get better, it'll get worse, or you won't be able to tell the difference.

If it gets better, that doesn't necessarily mean the the substituted tube is "good", it might just be that it's just not as bad as the original one. At least you'll be able to pinpoint the original tube as questionable.

[Philcofan42]

Originally Posted by leadlike

I agree with the folks above-I don't pull out the tester unless I am at a loss to what problem a radio could have. I would try applying signal to a few areas to see what stage is giving the trouble. If you find you need some help, give us the model number and we'll be happy to help you.

I'm pretty new at this and am trying to learn as I go. I have done one successfull repair to a Philco console. The current project is 1940 Silvertone R1171-D that I am mostly doing for the experience since it's in pretty bad shape. I'll have to go after the resistors now since I didn't realize they tend to go bad too. I did replace most of them in my Philco, but wasn't going to bother with the Silvertone. Before I replaced the caps, the Silvertone was making that constant hum unaffected by the volume control. Now it makes a sort of fast oscillating noise that does change with the volume. Oh, and I noticed that one of the IF tubes has something rattling around inside it so I'm pretty sure that one is bad.

Thanks for the help!
Dan

[gadget73]

I wouldn't blanket replace the resistors, but checking them is a good idea. You may find a few out of spec, but likely they won't all be bad. I usually give 20% tolerance as an ok range, even though originally its probably 10% per the schematic. Almost all circuits will work fine even at 20%.It might not be optimal, but it ought to receive.

The high pitch whine sounds like oscillation. I'd check the wire routing, and ensure that the volume control itself is good. If its open, it could cause the detector to be unloaded, which will make it oscillate. Also make sure that all tube shields are in place. I don't know anything about this particular radio, but radios designed to use shields or metal tubes will get funny with unshielded glass or missing shields.

[Philcofan42]

Originally Posted by gadget73

The high pitch whine sounds like oscillation. I'd check the wire routing, and ensure that the volume control itself is good. If its open, it could cause the detector to be unloaded, which will make it oscillate. Also make sure that all tube shields are in place. I don't know anything about this particular radio, but radios designed to use shields or metal tubes will get funny with unshielded glass or missing shields.

Well, I'm sorry to say I didn't take a good enough look at the tubes until now. Some were in the wrong sockets, one of the tube shields was in the wrong place and some are just plain wrong. I know nothing about which substitutions are allowed so here is the list of what's going on with my tubes now:

1st & 2nd I.F. - both correct at 6U7G
Det-AVG - correct at 6J5G
Rect - should be a 35Z6G but is an 2576GT/G
Output - Should be a 25C6G but is an 25L6GT
A.F. - correct at 6F5G
OSC-Trans - no longer has any markings, but should be an 6K8G

Thanks again!
Dan

[Dan Starnes]

I have a tube tester, but in the end I always just end up doing substitution anyways. Dan

[Philcofan42]

I'm also confused about the e-caps. The schematic symbol for the capacitors looks something like this: -[ - and I'm having a hard time not only figuring out the polarity, but figuring out which e-cap goes where. I disconnected the original can, but it didn't provide a color code like my Philco did so I'm not sure I have the right caps in the right place. e-cap #26 (not one of the three in the can) which is a 50 mfd running between the output tube and the rectifier had been replaced by two 20's wired together to make a 40 mfd. I followed the polarity when I put in the new 50, but I'm still not sure it's correct since this radio was modified quite a bit over the years. The schematic can be found here:

http://www.nostalgiaair.org/PagesByM...8/M0016688.pdf

One of the (canned) e-caps goes to the on/off switch, one goes to a terminal, and one goes to the output tube. I'm assuming the black wire running to the rectifier is the common ground, but I'm learning not to assume anything. I'm reading up so I can better read schematics, but it's hard when some of the symbols are out of date.

Thanks,
Dan

[gadget73]

Originally Posted by Philcofan42

Well, I'm sorry to say I didn't take a good enough look at the tubes until now. Some were in the wrong sockets, one of the tube shields was in the wrong place and some are just plain wrong. I know nothing about which substitutions are allowed so here is the list of what's going on with my tubes now:

1st & 2nd I.F. - both correct at 6U7G
Det-AVG - correct at 6J5G
Rect - should be a 35Z6G but is an 2576GT/G
Output - Should be a 25C6G but is an 25L6GT
A.F. - correct at 6F5G
OSC-Trans - no longer has any markings, but should be an 6K8G

Thanks again!
Dan

Thats probably a 25Z6, which is more or less compatible with a 35Z6. Its a rectifier tube with a 25v heater instead of a 35v. With todays higher line voltage, I'd suggest locating a 35Z6 to put in there so the heaters in the other tubes aren't stressed to take the excess voltage. The 25L6 will sub for a 25C6, so thats not a problem.