What is this thing?

[Kamakiri]

Working on this old Silvertone ac/dc farm set, and I ran across something I've never seen. Here's a picture of what I found.....I've been told they are a battery of some kind. One is complete with the little discs, and one is missing them.

I've also attached a schematic of the radio.....

[Reece]

Haven't seen it applied in this position. For a few years at the end of the thirties/early forties, little cells like this were used to provide negative grid bias, usually to the first audio tube. This looks like it works back to the IF grid. I see another one down by the voltage changeover switch. A bit headachey trying to trace what they do!

[old_coot88]

It's a bias cell. Well two cells actually. See..
http://aqcollection.com/antique-radio/article-3119.html
If you use watch batteries for replacement, be sure to observe polarity.

[Kamakiri]

Two questions:

1. With the radio in AC mode, is it necessary?

2. Can these be replaced with capacitors?

[Reece]

You can use watch batteries, not caps. When used to bias a first audio tube, it's possible to replace the bias cell with a high value (several megohm) grid resistor, but not sure how they work in your radio.

[Electronic M]

I have an AC Grunow teledial table set that uses them. I have not checked if the cells are still good, but after a recap it still works with the originals.

[Kamakiri]

I recapped the radio, and everything seems to work, yet I get no stations whatsoever. Problem is that a couple of the tubes I can't test on my tester, there are no roll chart listings for them. I'm just going to end up replacing them, but I'm not sure if those cells could be the issue.

[DavGoodlin]

Try swapping the two 1P5s and see what happens.

Did you try connecting signal generator at IF frequency to the 1A7 grid and IF 1P5?

[Reece]

This set has the filaments connected in series. In a dark room if you can see the dim glow of any of the 1-volt tubes, then all the filaments are OK, which is where to start. Another check would be to do a continuity test on the filament pins of the tubes that won't go in the tube tester.

[Jeffhs]

One note of caution: If you use an ohmmeter to check continuity of tube filaments in battery radios, many of which used 1- or three-volt tubes, you are taking a very real risk of burning out the filaments in the blink of an eye. Many older VOMs/VTVMs (vacuum tube voltmeters) had one or more batteries used only on the resistance ranges; if there is more than one such battery in the meter you are using, it may well blow the filaments long before you realize any damage has been done. I don't know whether or not today's digital multimeters use these batteries, but if they do, the same thing can happen if the meter is used to test tube filaments. Six- and 12-volt tubes in AC-powered radios, however, can be tested using a modern DMM if you don't have a tube tester.

I'm not sure why the battery radio tubes are so sensitive to burnout if tested using a standard ohmmeter. My best guess, however, is that these filaments are made of very thin wire which can be burned open before you know it if subjected to higher than normal voltages; this could go at least double for battery radio tubes that used the filament as the cathode, for almost instantaneous operation as soon as the radio is switched on.

[wa2ise]

You could use a DVM to see if the voltages on the filament string decrease as you progress from one tube to the next. And see if the voltage drops are consistent with the specs of the tubes.

[jr_tech]

I really doubt that any decent modern meter presents a danger to battery tube filaments. Most of these require a filament current of 150 milliamps to operate, although some tiny hearing aid tubes require only 20 or 50 milliamps. I spot checked a couple of modern DMMs and found that they only supply less than 1/2 milliamp to measure ohms... not a problem! Although many DMMs are *powered * by a 9 Volt battery, the current supplied for resistance measurement is indeed quite small.

I have no doubt that somebody at some time, using a very primitive ohm meter did, indeed blow a filament of a battery tube (I have read the same warnings before in old electronics magazines), but it is highly unlikely with decent modern meters.

If concerned, simply *measure* the amperage delivered by your ohm meter in various ohm ranges.

jr

[electroking]

Originally Posted by jr_tech

I really doubt that any decent modern meter presents a danger to battery tube filaments. Most of these require a filament current of 150 milliamps to operate, although some tiny hearing aid tubes require only 20 or 50 milliamps. I spot checked a couple of modern DMMs and found that they only supply less than 1/2 milliamp to measure ohms... not a problem! Although many DMMs are *powered * by a 9 Volt battery, the current supplied for resistance measurement is indeed quite small.

I have no doubt that somebody at some time, using a very primitive ohm meter did, indeed blow a filament of a battery tube (I have read the same warnings before in old electronics magazines), but it is highly unlikely with decent modern meters.

If concerned, simply *measure* the amperage delivered by your ohm meter in various ohm ranges.

jr

Most of the battery type tubes (such as 1R5, 1T4, 1U4, 1U5, etc.)
have 50-mA heaters. Just connect your ohmmeter to a milliammeter
of adequate range to check the current supplied.

[jr_tech]

Originally Posted by electroking

Most of the battery type tubes (such as 1R5, 1T4, 1U4, 1U5, etc.)
have 50-mA heaters.

OOPS! I grabbed a common portable radio tube, a 1R5 and looked it up, knowing that all of these common tubes draw the same current. Unfortunately, I read the current for the tube above in the manual, a 1R4 which does indeed draw 150 ma heater current.

The conclusion is still the same, these battery radio tubes draw far more current than a a modern DMM can deliver when measuring ohms.

jr

[Kamakiri]

I really have to get back on this radio