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 Post subject: GE J-105 Project Tube Set
PostPosted: May Sun 19, 2019 1:12 am 
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Background: This radio has an unknown history but seems to be in good shape once I cleaned a LOT of dust off and out of it.
In another post, I was able to test the transformer (dim bulb) and it checked out OK, so I decided to dive in an redo the chassis which had a lot of cracked insulation on wires (replaced), and replaced all the caps and resistors. I'm getting close to getting this setup for a first "light", have some soldering left to do but am encouraged by how things look after rewiring.

I tested the original tubes on my old Jackson 715 and they all failed, no shorts but not a good one in the bunch. I suspected my tester might be funky as these are all on the same socket. But, eternally optimistic, I bought a new set of tubes and which arrived today, the tester works fine, all the tubes tested 100%.

If you were faced with the unknown and a complete set of bad tubes, would you suspect there was a single cause? Line spike? Age? Barn storage? Transformer problems? Act of God? Nuttin' to worry about?

The line up is: 5Y3G, 4 x 6J5GT, 6SA7, 2 x 6SK7, 2 x 6V6G and I'm replacing the two 6SK7 with 6SK7GT and 6V6G with 6V6GT.

Thanks
John
corrected tube count

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 Post subject: Re: GE J-105 Project Tube Set
PostPosted: May Sun 19, 2019 2:05 am 
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nebbermind......... I was guessing too much lol

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 Post subject: Re: GE J-105 Project Tube Set
PostPosted: May Sun 19, 2019 3:16 am 
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I think the most common reason for a complete set of bad tubes, is that someone switched out the good ones with duds before they put it up for sale.


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 Post subject: Re: GE J-105 Project Tube Set
PostPosted: May Sun 19, 2019 4:15 am 
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BobWeaver wrote:
I think the most common reason for a complete set of bad tubes, is that someone switched out the good ones with duds before they put it up for sale.

Never thought of that - shows what experience can teach. If the originals didn't have the 2mm of dust that matched the rest of the chassis, I'd agree!

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 Post subject: Re: GE J-105 Project Tube Set
PostPosted: May Sun 19, 2019 11:10 am 
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Maybe the previous owner just used it to death? Basically, natural wear and tear?

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 Post subject: Re: GE J-105 Project Tube Set
PostPosted: May Sun 19, 2019 5:09 pm 
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It's really difficult to wear out a complete set of tubes all at once. About as easy as making all the popcorn kernels pop at once.
Looks to me like your radio was stuffed with duds. It's more common to see them swapped.

I have seen ALL the tubes burned out. One was a farm radio with five octal 1-volt tubes (1A7, 1N5 etc.). Every filament was vaporized, as if 120 volts was applied where 1.5 volts was required.
The other was a Zenith porthole set. Every one of the tubes in the 6.3V string was blown out with little blobs of metal at the ends of the heater wires. The separate power supply was missing. "Here's a lamp cord. Hold my beer - let's try these two pins"

Bob

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 Post subject: Re: GE J-105 Project Tube Set
PostPosted: May Sun 19, 2019 6:56 pm 
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I did a closer exam of the original tubes, looking for some tell tail sign, but didn't come up with anything. I popped the 6J5's back in the tester and they lit up but tested bad as they did before. The rectifier didn't show any visual signs of damage compared to the new one except for discoloration along the bottom glass ridge that holds the wires for the filaments. The others are metal cased.
Considering that there is a common thread on the older posts I read where the tube might be "good enough" for radio A but fail in radio B, I'll run the old set first and if nothing goes up in smoke, I'll put in the new rectifier first, then work through the circuits to the amps.
Thanks again for the posts, it helps!
Cheers,
John

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 Post subject: Re: GE J-105 Project Tube Set
PostPosted: May Sun 19, 2019 9:42 pm 
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If you're certain that someone didn't put in duds, then I suspect that this radio just had a very long and productive life. When it was finally retired, the tubes all had low emission, as might be expected. But, testing bad on a tube tester doesn't necessarily mean that they wouldn't still work in the radio.


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 Post subject: Re: GE J-105 Project Tube Set
PostPosted: May Mon 20, 2019 5:26 pm 
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How "bad" do they all test?

Tube testing is not an exact science. Many tubes will test bad in a tester, but still work fine in a radio.
Same the other way around, but not very often.

I knew of a guy who never tested the tubes in the radios he fixed up. Only changed them if he had a problem with one.

As they say, Your Milage May Vary, so it all depends on the situation :D

-Steve

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 Post subject: Re: GE J-105 Project Tube Set
PostPosted: May Tue 21, 2019 12:35 am 
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azenithnut wrote:
How "bad" do they all test?

Tube testing is not an exact science. Many tubes will test bad in a tester, but still work fine in a radio.
Same the other way around, but not very often.

I knew of a guy who never tested the tubes in the radios he fixed up. Only changed them if he had a problem with one.

As they say, Your Milage May Vary, so it all depends on the situation :D

-Steve

Not an exact science seems to be true from what I've been reading! The set of originals went from kinda bad to very bad. The rectifier has a loose base, you can swivel it back and forth too much for comfort, considering what it does and most of the others didn't plant the meter to the far side, but wouldn't come up at all after cleaning the pins, for example.

Speaking of which, I've got one of those RShack brass sponges in a weighted base into which I push most tubes and component wires for cleaning, seems to brighten things up quickly and with not much effort. Push the tube in, give it a gentle twist a couple of times and the pins are nice and clean. If a resistor or something has a little crud on the leads, same thing, push them in and wiggle them around a couple of times and they come out bright.

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 Post subject: Re: GE J-105 Project Tube Set
PostPosted: May Thu 23, 2019 8:52 pm 
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Filling a set with "duds" is not uncommon; I have seen that before.

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 Post subject: Re: GE J-105 Project Tube Set
PostPosted: May Thu 23, 2019 9:20 pm 
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Be careful swiveling around old tubes like that ... you can easily make the base come loose if it isn't already. BTW ... you can re-glue those if they do pull loose.

If they were already loose, and you twist them around too much, you can snap off wires inside the base. Sometimes those can still be repaired, but not worth the effort on common tubes. Easy enough to avoid the problem by not twisting them too much if at all.

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 Post subject: Re: GE J-105 Project Tube Set
PostPosted: May Thu 23, 2019 10:29 pm 
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Some tube testers are the 'drugstore' variety, they may intentionally test lower than the norm. A guy walks in (60 years ago), 'cause the wife can't see/hear her soaps on the TV/radio. He brings in a bag of tubes, half of them check bad, he buys new tubes and goes home and installs them. Odds are, one of them indeed was bad, and now the TV/radio works; wife's happy, drugstore owner's happy.
My two cents worth.
Dan


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 Post subject: Re: GE J-105 Project Tube Set
PostPosted: May Thu 23, 2019 10:37 pm 
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Tim Tress wrote:
Filling a set with "duds" is not uncommon; I have seen that before.


Agreed, it does happen.
In that case it does help to have a tube tester on hand.
I do test tubes in a radio before I work on it, but unless the tube is especially lacking emission and looking tired (getter looks scorched or rainbow-ish) or has shorts, I let them be until the radio is working.
If I'm having issues, I refer to my notes to see if a tube could be the possibility of the trouble and proceed from there.
More often than not, tubes with emission in the red work just fine. Even down to like 35% on my Jackson 648.

-Steve

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 Post subject: Re: GE J-105 Project Tube Set
PostPosted: Jun Fri 28, 2019 8:11 pm 
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I have another question - with the new set of tubes the radio's audio stage is working very well, phono input is loud and clear but I couldn't get any reception on BC.

Earlier this week, I used my signal gen and was able to tune in the 455 kc signal all up and down the band via tuning he IF transformers and I thought I was on my way, I had one dead spot on the low end and some crackling when tuning. I went back to the tuner gang and tried a few things to clean it up (for the dead spot) and went over the wiring which cleared up a few problems.

Then I wired up the "Beam-O-Scope", this radio's internal antenna (I was using long wire to the antenna leads and ground.)

However, things didn't go so well, still no BC signals and now nothing from the 455 kc signal generator and a large hum. Disconnecting the antenna ground stopped the hum, but didn't fix the 455 kc reception.

The 6SK7 R.F. AMP tube which I replaced is now testing bad which might explain some of these problems but I'm curious if I did something to fry that tube? The Jackson 715 shows it as about 20% in the bad zone.

Last voltage measurement I recorded showed pin 6 (screen?) at 128 VDC vs the 95 per the docs. Would running that tube at the higher voltage cause it to burn out after a few weeks? I don't recall reading about voltage tolerances for tubes vs discrete components, I was thinking 20% over may be OK but I see now those figures are more like 33% over, oops.

COV. OSC. 6SA7 Pin 4 = 116 (should be 95)
IF Amp 6SK7 Pin 6 = 123 (should be 95)

Naturally, after writing this out, it seems obvious I have to get the screen voltage down within range before I replace that tube but I'd still like some input.
Thanks,
John

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