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 Post subject: Tubes too old to test
PostPosted: May Fri 28, 2021 2:58 pm 
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I have a radio that uses tubes with two digit numbers (#30, #38) and have four pins. My tube tester can't test these tubes. Any ideas out there?


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 Post subject: Re: Tubes too old to test
PostPosted: May Fri 28, 2021 3:12 pm 
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Find someone with a tester that is capable of testing those types. Such testers were still made well up into the 1960's, so it won't be too difficult.

The alternative is to make an adapter having the old style tube sockets, which can plug into the octal socket on a tester not having those sockets. Then all you have to do is figure out which octal types those roughly correspond to, so you have some idea of what settings to use to test them.

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 Post subject: Re: Tubes too old to test
PostPosted: May Fri 28, 2021 3:12 pm 
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If you want to use a tube tester for 4 pin tubes, then you need a older tube tester with 4 pin socket. You can check the filament with a voltmeter.

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 Post subject: Re: Tubes too old to test
PostPosted: May Fri 28, 2021 3:40 pm 
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Just use an ohmmeter and test continuity between the two fat pins on 4 pin tubes and the two close together pins on 5 pin tubes. The odds of them working if read OK, are as good as a tube tester giving an OK on the meter.

Dale


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 Post subject: Re: Tubes too old to test
PostPosted: May Fri 28, 2021 4:37 pm 
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If the filaments are good, you can test tubes like that in the set. If the tubes are good and working properly they will draw their nominal plate currents. This can be measured directly with a milliammeter (this was the basis of early set testers), or indirectly by looking at the voltage drop across a resistor in series with the cathode or plate. If a tube is weak or worn out, it will be drawing far less than its nominal current; if it is gassy or leaky it will draw more. The nominal currents can be determined from the RCA Tube Manual or others, using the numbers that correspond to the plate voltage closest to what the set applies. What many techs are (were) in the habit of doing is checking the plate and grid voltages with a suitable VTVM or other meter and comparing them to the schematic. If the voltages on the electrodes of a tube are in the right ballpark, the tube is almost certainly good.

Of course this assumes that the rest of the radio is in good working condition and applying the proper voltages to the tube electrodes. If the radio has problems or is in unknown working condition is unknown then it'll have to be fixed and checked out first. Having spare tubes to substitute and try comes in very handy at times like this.

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 Post subject: Re: Tubes too old to test
PostPosted: May Fri 28, 2021 5:07 pm 
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I want to know what kind of radio uses both type 30 and type 38. The use cases for those tubes are not very similar.


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 Post subject: Re: Tubes too old to test
PostPosted: May Fri 28, 2021 9:23 pm 
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The EICO 625 tube tester will test them. It can test both #30 & #38 and many other four pin tubes. But as already said.. generally if filament tests good with your meter it likely will work fine and can be tested further in radio itself.


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 Post subject: Re: Tubes too old to test
PostPosted: May Fri 28, 2021 9:36 pm 
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As Paul mentioned, it would be very strange to see a 30 and a 38 in the same radio. The 30 has a 2 volt filament, and is intended for battery operation, while the 38 has a 6 volt filament and is intended for a mains supply. If this is a radio that you just bought, it's not uncommon for a seller to stuff it with random tubes just to fill the sockets. So it would help if you would tell us the make and model of this radio.

Another possibility is that part of the number has worn off, and the 30 is actually an 80.


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 Post subject: Re: Tubes too old to test
PostPosted: May Sat 29, 2021 12:35 am 
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It might be an idea to post a circuit of the set, as it seems to be odd; Notwithstanding it may be home made. Whist I have tube testers that can handle UX4. These are either diodes, or triodes and an adapter easily made to conform with a newer triode.

I have put UX-201-A prior to them being 01, back into service. Gassy tubes will tend to have that shiny silver "getter" go to a distinct "bluish" pastel white. Some with a bit of white will work.

Marc


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 Post subject: Re: Tubes too old to test
PostPosted: May Sat 29, 2021 2:16 am 
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The Triplett 3444 has settings for both those tubes as well.

But as the others have indicated, the two have completely different filament voltages according to those settings. The plate voltages are completely different too, but I don't know how significant that is.


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 Post subject: Re: Tubes too old to test
PostPosted: May Sat 29, 2021 4:17 am 
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Tube testers do have provision for applying an array of voltages to filaments & heaters.

Some testers recommend applying "B" for only a few second and there is enough room in the socket for a voltage divider, even if that requires a separate "Fly lead" to plug into an earth socket. Some adaptors on the 1938 tester here require that.

Marc


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 Post subject: Re: Tubes too old to test
PostPosted: Jun Tue 08, 2021 3:24 am 
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You need a tester older than yours to test these old 4/5-pin tubes. :)


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 Post subject: Re: Tubes too old to test
PostPosted: Jun Tue 08, 2021 4:18 am 
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Not one to totally subscribe to that. I have as noted often a 1938 Tube & Circuit Tester and a more recently acquired Knight 600 series. During WWII the early one had described an adapter box to accommodate newer tubes and Military versions were built with it in the lid and stand alone adaptors were available & described. As before, I therefore see no reason if voltages are available for the heaters and filaments, why an adaptor cannot be made using old sockets.

Photo is of two stand alone "described" homemade adaptors for the old machine for newer & differently wired tubes, to the testers wiring.

Marcc


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 Post subject: Re: Tubes too old to test
PostPosted: Jun Tue 08, 2021 4:45 am 
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A sometimes overlooked fact is that those old tubes are internally identical to more modern types. If I remember correctly, a 75 is like a 6SQ7 or something like that.

The AA5 that was ubiquitous in the 1940s and later was preceded by a sort of AA4, using types 36, 37, 38 and 39/44 I think. The 37 triode was connected as a diode for the rectifier.


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 Post subject: Re: Tubes too old to test
PostPosted: Jun Tue 08, 2021 4:57 am 
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There were indeed official adapters released by tester manufacturers to accommodate older/newer tubes.

The key difference between these official adapters and DIY adapters though is that the official adapters came with a manual that tells you how to set up the adapter and the tester to test the older/newer tubes.

For DIY adapters, it might be possible to figure out the pin connections, however, other than the heater voltage, what other voltages to apply and what results to expect can be a big problem. :mrgreen:


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 Post subject: Re: Tubes too old to test
PostPosted: Jun Tue 08, 2021 5:06 am 
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Well that was my point, that the older two-digit tubes were the same as the more modern ones. A comparison by using a tube manual will indicate which is the same as which. Check transconductance and mu and plate dissipation etc. and you will see what I mean.


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 Post subject: Re: Tubes too old to test
PostPosted: Jun Tue 08, 2021 7:16 am 
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The manual of my early one does describe a way of establishing settings for new tubes. However, it is in a lot of cases just a matter of cross referencing the bases. The Philips tube data a books do often give a cross refence to the earlier tubes. I seem to think so does Radio Museum & others.

There were several popular tubes that had base & heater changes & were re-numbered; 2A7, became 6A8 with heater & base change; Same with 2A5 it became 6F6; 2A6 is 6SQ7. More base & heater changes 6D6 is a 6U7 is a #58; #30 reappeared as 1H4G. In several triode pentodes that pentode looks like 6M5 as does the 6CK6 only the suppressor is on pin6 not internally connected like 6M5. So there was not a lot of re-inventing of the wheel. The ones that disappeared are likely those that could be out performed.

This is why I see scope for adapting.

Marcc


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 Post subject: Re: Tubes too old to test
PostPosted: Jun Tue 08, 2021 4:07 pm 
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Not always so much outperforming, but standards changed. A lot of performance improvements happened even before the octal tube base did. In the US Philco forced the change away from 2.5V tubes when it didn't see any good reason to maintain that somewhat random heater voltage for AC radios that could run just as well with the universal 6.3V tubes that auto radios needed. And RCA forced the move to octal bases, obliging the popular pre-octals to migrate to the new base with no electrical changes. The 2A5, 42, and 6F6 are all the exact same tube and all perform equally in the same circuit. But if your tester doesn't have a 6-pin socket, you have a problem testing the first two even though the tester is electrically capable of doing it.


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 Post subject: Re: Tubes too old to test
PostPosted: Jun Tue 08, 2021 4:29 pm 
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Paul Dietenberger wrote:
The 2A5, 42, and 6F6 are all the exact same tube and all perform equally in the same circuit.


is there a chart somewhere that says X=Y=Z or do you guys just know this from long experience?


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 Post subject: Re: Tubes too old to test
PostPosted: Jun Tue 08, 2021 4:37 pm 
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torch wrote:
Paul Dietenberger wrote:
The 2A5, 42, and 6F6 are all the exact same tube and all perform equally in the same circuit.


is there a chart somewhere that says X=Y=Z or do you guys just know this from long experience?


Both. Tube substitution guides are definitely a thing and can be downloaded from many places on the net. But interpreting them properly and knowing what actually works to replace something else takes some practice, and an understanding of a tube's ratings and what it does in a circuit. And possibly a little history, too. 2A5 and 42 were both descended separately from type 47; the 2A5 made by RCA to replace the 47's filament with a heater and cathode. The 42 made by Sylvania did that and also switched to a 6.3V heater so that Philco could use it in their home radios. 6F6 is an octal 42. The 2.5V tubes never came out as octals because RCA gave up on the 2.5V heater for their 1935 radio line and switched to 6.3V tubes, one year before they came up with octal metal tubes for their 1936 line.


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