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 Post subject: Weak 45 triode tester
PostPosted: Aug Thu 01, 2019 5:44 pm 
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Joined: Nov Wed 27, 2013 5:59 am
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Location: Metzger Oregon
I have about 8 or 9 45s that I have stashed away that are supposed to be weak, and I would like to put together some sort of in-circuit tester to see if they are actually functional. I would like to find a few to use in a couple old radios that have been in the family for a while. They have some sentimental value to me, but really are probably worth less than the value of good tubes. I would need just enough output to get moderate volume in a quiet room. I understand that weak tubes will often work okay in circuit, and I also have learned not to trust a tube tester as much as a true in-circuit test. While these weren’t tested by me, I wouldn’t trust my results any more than someone else’s. What I am thinking of doing is trying these tubes in a simple single ended amplifier, just to ensure they are functional. I have an old amp that was picked up from the local radio club, it has a 27 driving a transformer, and that transformer feeding a pair of 45s in push-pull. There is no output transformer as it would have been mounted to the speaker. I can use this for testing if it is useful, I would probably connect it as single ended to allow testing of one tube at a time. I am wondering if anyone has any suggestions on a simple test circuit, and if there is anything I should be concerned about other than just checking for audio output.


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 Post subject: Re: Weak 45 triode tester
PostPosted: Aug Thu 01, 2019 6:40 pm 
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Weak 45's often operate ok in a radio. A lot depends on actual filament voltage.

What voltage do you have on tube filament pins while in the tester. Should be 2.5 volts. Since tubes draw high current voltage may be less due to poor contacts? Increase filament voltage a little. This will cause tubes to read higher on a tester. Doing this you can evaluate best of the weak tubes.

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 Post subject: Re: Weak 45 triode tester
PostPosted: Aug Fri 02, 2019 12:38 am 
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Joined: Jan Fri 27, 2017 8:41 pm
Posts: 171
Location: Springwater, NY
If you have a socket, some meters and two variable supplies, for the grid and plate, you could measure Gm by tracing characteristic curves with reference to published data. You could then get a sense of remaining lifetime by measuring Gm at a reduced filament voltage, say -10%.


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 Post subject: Re: Weak 45 triode tester
PostPosted: Aug Fri 02, 2019 3:51 pm 
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Joined: Nov Wed 27, 2013 5:59 am
Posts: 722
Location: Metzger Oregon
Thanks for the tips, I might have to investigate a little filament boost if needed. I suppose I could piece together a test setup with 2 meters and 2 power supplies, I'm just not sure I would know what to do with the data.


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 Post subject: Re: Weak 45 triode tester
PostPosted: Aug Fri 02, 2019 11:02 pm 
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Joined: Jan Fri 27, 2017 8:41 pm
Posts: 171
Location: Springwater, NY
Borrowing from Tomer, "Below ~10MHz, transconductance, Gm, gives a very good estimate of amplification. Transconductance is usually permitted to vary about +/-40% of the published bogey value. Plate current is usually controlled within limits of +/-20%." See the datasheet and characteristics for the bogey information. Then you can compare your pre-owned tubes to the bogey. If your tubes fall outside of the ranges on the low side, then they're less than great...but probably still very usable in your amplifier. These targets are somewhat arbitrary, but that's all we have.

Borrowing from Alan Douglas, "The "grid shift" technique used during the 1920s (to measure Gm). For that, all you need are some power supplies and a milliammeter in the plate circuit. Measure the grid bias at whatever operating point you like, and its corresponding plate current. Then shift the grid bias by a tiny amount (digital meters are good for this) and note the new plate current. Gm is the difference in plate current divided by the difference in grid voltage. The smaller the shift, the more accurate the result."

So, you can measure Ip and compare to the datasheet. Think of your apparatus as a kind of curve tracer. How does your tube's Ip compare to the bogey tube near your operating point? And you can measure Gm using the grid-shift method for comparison to the published value of Gm at your operating point. This gives you a direct measure of what to expect in a similar circuit.

Now, if you have the ability to lower the filament voltage by say 10%, you can make these measurements again. If Gm falls off rapidly, the tube is on the weak side. If Gm hangs in there with a reduction in filament voltage, then the tube is likely to perform better/longer.

73 Dean


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 Post subject: Re: Weak 45 triode tester
PostPosted: Aug Sat 03, 2019 5:24 am 
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Joined: Nov Wed 27, 2013 5:59 am
Posts: 722
Location: Metzger Oregon
But I know they are less than great, I just want to know if they are bairly functional or totally dead.


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 Post subject: Re: Weak 45 triode tester
PostPosted: Aug Sat 03, 2019 11:28 am 
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Joined: Jan Fri 27, 2017 8:41 pm
Posts: 171
Location: Springwater, NY
Is it this?

classicelectronicsguy wrote:
I have about 8 or 9 45s that are supposed to be weak, and I would like to put together some sort of in-circuit tester to see if they are actually functional....I also have learned not to trust a tube tester as much as a true in-circuit test....I am wondering if anyone has any suggestions on a simple test circuit...

or this?

But I know they are less than great, I just want to know if they are bairly functional or totally dead.


The test I mentioned is simple and might take a few minutes per tube, but if you only want to know if they're "totally dead," a tube tester might be sufficient. 73 Dean


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 Post subject: Re: Weak 45 triode tester
PostPosted: Aug Mon 05, 2019 8:03 pm 
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Joined: Nov Wed 27, 2013 5:59 am
Posts: 722
Location: Metzger Oregon
Yes, I just want to know if they still have enough life left to drive a speaker. Knowing the percentage on a tube tester or any other numbers doesn't really translate into anything real world for me, unless someone can point me to some method to convert between the two.

In light of all of that, I am thinking of testing them in a single ended amplifier, and just seeing what I get. Does a 4k load from the output transformer sound about right? Based on Norm's reply, I will keep a close eye on filament voltage, to insure I am testing at a full 2.5V.


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 Post subject: Re: Weak 45 triode tester
PostPosted: Aug Tue 06, 2019 12:58 am 
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Joined: Jan Fri 27, 2017 8:41 pm
Posts: 171
Location: Springwater, NY
To meet your requirement, 4Kohms should be fine at 250v or so. Here is the datasheet:

https://frank.pocnet.net/sheets/021/4/45.pdf

73, Dean


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 Post subject: Re: Weak 45 triode tester
PostPosted: Aug Tue 06, 2019 3:43 am 
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Location: Vancouver Island, Canada
I would just test them in a radio, like you say, with a single ended output. If you get a reasonable volume, you've got a usable tube.
For a matched pair, you'd need to be a bit more scientific, set up something that can meter the output. Maybe measure cathode current.

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 Post subject: Re: Weak 45 triode tester
PostPosted: Aug Sun 11, 2019 3:01 am 
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Location: Aurora, CO 80013
westcoastjohn wrote:
I would just test them in a radio, like you say, with a single ended output. If you get a reasonable volume, you've got a usable tube.
For a matched pair, you'd need to be a bit more scientific, set up something that can meter the output. Maybe measure cathode current.

I've had them test 40% on an emission tester and work fine in little cathedrals and such.

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