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 Post subject: How did this even work? No smoke genies
PostPosted: Feb Thu 08, 2018 10:53 pm 
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Location: Wichita, Ks.
Buddy of mine brought me over a GE 202 to restore. While he was here I powered it up slowly with a variac to see if it was even worth working on. Played fine for the few seconds that I had it on. I scrolled through the stations and shut it down. Good enough I told him, I will restore it for you. Today I start digging into it and after blowing the dust off and pulling the tubes I discovered that someone had put a 50L6 beam power tube in where the 35Z5 rectifier tube belongs. How is it possible that it worked at all? Obviously I am going to check the socket pin outs and voltages, but geesh. How did it work at all and yes, I learned a lesson. I could tell that someone had been into it at some point by the mismatched screws on the back cover. I have never seen that before. I know that some people think that if the tube fits it will work. It obviously worked as far as passing voltage but un rectified and those two tube have different pin outs and purposes
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 Post subject: Re: How did this even work? No smoke genies
PostPosted: Feb Thu 08, 2018 11:03 pm 
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ignorance is bliss... :)

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 Post subject: Re: How did this even work? No smoke genies
PostPosted: Feb Thu 08, 2018 11:22 pm 
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Location: Near Brandon, Iowa
A pentode (50L6) can function as a diode, but a diode (35Z5) cannot function as a pentode.
Heater and cathode pinouts between the two types are the same. The plate pins don't match (unless someone rewired the 35Z5 socket to move pin 5 to pin 3) so apparently the signal grid of the 50L6 was functioning as the diode plate.

It would be interesting to know how much output current the tube was good for when used in this manner.


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 Post subject: Re: How did this even work? No smoke genies
PostPosted: Feb Thu 08, 2018 11:24 pm 
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I looked at the data sheets for both and the pin out is different. I can’t see how it could possibly work either. The only common pins are the heaters (2&7), the cathode (8), and that’s it.

On the 50L6, pin 3 is the plate, which is the pilot light tap on the 35Z5.
On the 50L6, pin 5 is the first grid, which is the real plate for the 35Z5.

I can’t imagine how you’re getting B+ through G1 of the 50L6...


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 Post subject: Re: How did this even work? No smoke genies
PostPosted: Feb Thu 08, 2018 11:29 pm 
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Rereading the original post it wasn’t mentioned that the 35Z5 was in the 50L6 spot so I think it means there were two 50L6 tubes in there. One of them was in the right spot and someone put a second one in the rectifier’s spot.

I guess it would be interesting to try it out on a set I didn’t care about wrecking...


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 Post subject: Re: How did this even work? No smoke genies
PostPosted: Feb Thu 08, 2018 11:37 pm 
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It will definitely work. How well, or for how long, is another story. But given the pin connections of the two tubes, the 50L6 in the rectifier socket will definitely function as a rectifier for at least a little while.

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Experience is what you gain when the results aren't what you were expecting.


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 Post subject: Re: How did this even work? No smoke genies
PostPosted: Feb Thu 08, 2018 11:47 pm 
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I looked up the schematic for the GE 202 at it doesn't even show a 50L6... shows a 35L6. So.....


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 Post subject: Re: How did this even work? No smoke genies
PostPosted: Feb Fri 09, 2018 12:52 am 
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Whether it has a 50L6 vs 35L6 is splitting hairs...

Since this radio apparently doesn't use the dial light tap on 35Z5, I can see where 50L6 could work... No doubt the control grid would be stressed from load.. BUT if someone wired pins 3, 4, & 5 together on 35Z5 socket there's no doubt a 50L6(or 35L6)will work & probably just fine... Something I'll have to try... Curious how much voltage drop a 50L6 would have as rectifier...

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 Post subject: Re: How did this even work? No smoke genies
PostPosted: Feb Fri 09, 2018 2:09 am 
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Well I've tried operation using a somewhat tired 50L6...

With only cathode & control grid connections, B+ in a std AA5 creeped up to 90v... With control & suppresor grids plus plate in parallel, it made 101v but after a point voltage begins to drift downward... For short time I had it powered up, played fine...

BTW with a 35Z5(actually 45Z5) it has 110v B+...

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 Post subject: Re: How did this even work? No smoke genies
PostPosted: Feb Fri 09, 2018 3:23 am 
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Yes.. of course 50L6 vs. 35L6 is splitting hairs. But an important issue that should have been known/accounted for before powering up and possibly damaging a radio. Hopefully that is the lesson that was learned. I'm always mystified when I read someone just powers up a decades old radio with absolutely no internal inspection, testing or replacement of likely defective components beforehand. But hey.. to each his own.. including using a 50L6 as a pseudo rectifier tube.


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 Post subject: Re: How did this even work? No smoke genies
PostPosted: Feb Fri 09, 2018 3:42 am 
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That would not happen here: I have fixed too many to show such exuberance, as to power an unknown set, to see if it goes. One inspects it first to see if it is actually safe to power: That one failed: It wasn't.

You look for blatant tampering and bad wire minimum I have had one in the last 12months where I spotted a different substituted, tube in the set than original, & it was wired wrong.

Marc


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 Post subject: Re: How did this even work? No smoke genies
PostPosted: Feb Fri 09, 2018 3:49 am 
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On a AA5???

I'll power 'em up and wait for hum, smoke, music or sometimes nothing at all... Last year I got a nice BANG out of a busted clock radio destined to be parts, bumblebee across line blew apart, after that it played...

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 Post subject: Re: How did this even work? No smoke genies
PostPosted: Feb Fri 09, 2018 4:11 am 
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Hot chassis & AC / DC radios are not that common here, even so if I do get a hot chassis set. The rules don't change. There are GFCI equivalent devices on the power here & the the isolation transformer has its own circuit breaker on the secondary side, plus a kill switch & CB on the primary (3A)... Plus filters.

I only get payed when I fix them: Not after I destroy them.

Marc


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 Post subject: Re: How did this even work? No smoke genies
PostPosted: Feb Fri 09, 2018 4:32 am 
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Location: Aurora, IL
So solid technique and troubleshooting can be situational... as in this is only an AA5 so I'll just power it on and see what happens? Not for me and never will be. If nothing else for those that do this... it's good to remember they ain't making too many original parts anymore. Every original tube, transformer lost is a part lost forever.


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 Post subject: Re: How did this even work? No smoke genies
PostPosted: Feb Sat 10, 2018 3:07 am 
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Every radio or amplifier that crosses my bench gets inspection for obvious defects, and signs of previous servicing. I have seen some real hack jobs, over the years.

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 Post subject: Re: How did this even work? No smoke genies
PostPosted: Feb Sat 10, 2018 9:07 pm 
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All too often lately I've found someone has plugged tubes in higgeldy pig with no concern for the consequences. So, please take nothing for granted.

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 Post subject: Re: How did this even work? No smoke genies
PostPosted: Feb Sun 11, 2018 2:19 am 
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Location: Long Island
At one time it was okay to take an unknown radio and plug it in to see what the story was. If the radio was still working when it was set aside, it might still work. Or there'd be at most one or two problems, like a bad filter capacitor, that would be recognized by the characteristic hum. Pull the plug, change the cap, back in business again.

Nowadays you are dealing with radios that have been repaired numerous times, "restored" in various ways by various collectors with tubes and parts swapped with other sets, maybe used as the subject of a u-tube video or two, and possibly full of hacks, mods, and incorrect parts. It's always worth taking a minute or two and checking an old piece of equipment over before plugging it in.

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