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 Testing AA/5 tubes without a Tube tester

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Cliff Jones
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Join date : 2010-11-22

PostSubject: Testing AA/5 tubes without a Tube tester   2012-01-05, 4:14 pm

Testing tubes in ac/dc 5 tube sets without a Tube tester.
This is paraphrased from Television and Radio Repairing by Markus 1953

Don't know which tube filament is open? Rather than remove each tube and test with a Tester try this approach. you could test in place using an ohmmeter and save time.

Be sure the set is unplugged

Turn the set switch on

remove chassis and invert to see wiring

Trace the line cord to where it connects to the set wiring, one wire usually goes to the on/off switch, the other wire will normally go to one of the tube sockets (generally one of the rectifier filament terminals)

Using the Ohms dial setting on the Ohmmeter

Clip one of the ohmmeter leads to tube socket filament that the wire coming from line cord is attached to.

Test by touching each socket that is a filament connection with the other probe, in succession, working away from the first probe, until all filaments are checked.

Actual ohmmeter readings don't mater as each set can vary (and what you are looking for is an infinity reading to indicate an open) as you progress down the line.
The last tube in the series circuit string will generally have one of the filaments connected to the chassis and the return path to the other side of the line will be through the chassis.

The general makeup of tubes are as follows
1- 35Z5GT
2- 50L6
3- 12SK7
4- 12SA7
5- 12SQ7

the filament connections (pins) are as follows: Note each has a letter for future reference
---------------
tube 1- 2(clip) 3(a) 7(b)
tube 2- 2(c) 7(d)
tube 3- 7(e) 2(f)
tube 4- 2(g) 7(h)
tube 5- 7(i) 8(j)
------------------------
Position of probe---------reading in ohms-----------meaning of reading
-----------------------------------------------------------------------------------------------------
a-------------------- 35 -------------------section 2-3 of 35z5GT and/or pilot lamp(s) OK
aa ** -------------- 50 -------------------section 2-3 of 35z5GT OK
b-------------------- 235 ------------------section 3-7 of 35z5GT OK
c-------------------- 235--------------------wire from b to c OK
d-------------------- 235--------------------Filament of 50L6 OK
e-------------------- 565--------------------wire from d to e OK
f-------------------- 645--------------------Filament of 12SK7 OK
g-------------------- 645--------------------wire from f to g OK
h-------------------- 725--------------------Filament of 12SA7 OK
i-------------------- 725--------------------wire from h to i OK
j-------------------- 805-------------------- Filament of 12SQ7 OK
k-------------------- 805 --------------------on/off switch and chassis grounds OK
note k is after closing on/off switch
** aa---Same position as a, but with one pilot lamp removed

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FrankB
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PostSubject: Re: Testing AA/5 tubes without a Tube tester   2014-02-25, 9:55 am

Or you can simply plug in the radio, turn it on, and measure across the filament pins. THe tube that has 117V across it, is the one with the open filament.
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MEZLAW

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Join date : 2012-10-28

PostSubject: Re: Testing AA/5 tubes without a Tube tester   2014-08-24, 8:03 pm

[You must be registered and logged in to see this link.] wrote:
Or you can simply plug in the radio, turn it on, and measure across the filament pins. THe tube that has 117V across it, is the one with the open filament.

This is the methode I've always used, source voltage will always be read across the open.
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