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 Post subject: 35Z5GT - a few questions
PostPosted: Feb Fri 08, 2019 11:17 pm 
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Location: SE PA 19335
I've restored several AA5 radios which use this tube. A common problem has to do with the dial light ( #47) failure followed by an open circuit in the filament tap. I have several 35Z5's that have this issue. To clarify, there is continuity between pin 2 and pin 7, but not pin 2 and 3, or 3 and 7.

Can the 35Z5 tube without the filament tap be used , by removing the dial light modifying the original circuit? What mods are required?

Can the 35Z5 tube be replaced with a diode? Other than a series resistor to mimic the 35Z5 filament resistance, what other mods would be required.

Please note attached typical schematic.

Thanks, Frank


Attachments:
35Z5 connections -r1.jpg
35Z5 connections -r1.jpg [ 44.02 KiB | Viewed 646 times ]
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 Post subject: Re: 35Z5GT - a few questions
PostPosted: Feb Fri 08, 2019 11:29 pm 
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Location: Detroit, MI USA
The 35Z5, 35W4, and several other 35 volt rectifiers can easily be replaced with diodes. One 1N4007 is adequate for dropping the required amount of filament voltage in the string, another serves the function of rectifying B+. You need to address the voltage drop across the dial lamp with a resistor, or another pair of zener diodes but it can be done, and I've had sets running with that kind of homemade solid state 35Z5 replacement for decades.

If you want to use the original tube with a bad filament and don't care about having that tube operating with part of it's filament shorted out, then you could simply place a jumper wire across the filament section for the dial lamp and go without the lamp. Or you can put a resistor or pair of zener diodes across the open section to make the dial lamp functional. Either way the rest of the tube will function almost like normal in most situations, although it doesn't get the cathode quite as hot with one section of filament not working. But if you make the modification below the socket and someone who doesn't know that later on puts in a good 35Z5, then the dial lamp isn't going to get as bright as it should.

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 Post subject: Re: 35Z5GT - a few questions
PostPosted: Feb Sat 09, 2019 1:11 am 
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I've never seen a 35Z5 with continuity between 2 & 7 but not to pin 3. AFAIK pin 3 is the tie point so 2 & 7 have continuity. The heater section(lamp tap) between 2 & 3 is 7.5v, while the main section between 3 & 7 is 27.5v. Only way I know pin 3 could be open, is if there is a defect in wiring between press(glass seal) and solder connection inside pin 3. Possible but not a very likely failure.

It's easy enough to jump a 39 ohm, 2 watt resistor across pins 2 & 3, this restores operation of both tube and dial lamp(common repair when tubes were in short supply during WW-II). Instead of resistor, can use a pair of 8v zeners connected cathode to cathode across 2 & 3. This also restores operation, plus will protect a good tube and dial lamp from surge in the event of B+ short. This of course nullifies the highly controversial "fuse" action of tube, that was never intended to be a fuse when tube was designed. If main heater section between pins 3 & 7 is open, the tube is junk.

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 Post subject: Re: 35Z5GT - a few questions
PostPosted: Feb Sat 09, 2019 7:39 pm 
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Location: Northeast Florida
Why reinvent things? 35Z5's are cheap and readily available, why not just buy a new tube?

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 Post subject: Re: 35Z5GT - a few questions
PostPosted: Feb Sat 09, 2019 8:10 pm 
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tubes4life wrote:
Why reinvent things? 35Z5's are cheap and readily available, why not just buy a new tube?

Cuz it draws comments from the group that'll never understand hot rodding, plus there's a minority of us that will never admit the wheel was ever perfected.

As far a 35Z5 it's more like a triangle.

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 Post subject: Re: 35Z5GT - a few questions
PostPosted: Feb Tue 12, 2019 9:32 pm 
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Location: Pennsville, NJ 08070
The other argument for working around the damaged heater section of 35Z5/W4 tubes is neither are being made anymore, so why hasten the day when we run out of them?


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 Post subject: Re: 35Z5GT - a few questions
PostPosted: Feb Tue 12, 2019 10:30 pm 
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Location: 253 Blanche St. Plymouth, MI USA
tubes4life wrote:
Why reinvent things? 35Z5's are cheap and readily available, why not just buy a new tube?



not so cheap anymore and its the one tube that is being burned up more regularly than any other. If you go thru a box lot of tubes, about 70% of the 35Z5's have the pilot tap burned out as it was always the weak spot.

Always good to add an inrush limiter, I forget... is it the CL-90 that is preferred?

I like to use the 45Z5 GT tubes as subs today. Cheaper and work great with today's higher AC line voltage.
Mark Oppat


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 Post subject: Re: 35Z5GT - a few questions
PostPosted: Feb Wed 13, 2019 2:09 am 
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NOS 35Z5GT is $5.00 at ESRC tubes in Florida. But 45Z5GT is $3.00.

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 Post subject: Re: 35Z5GT - a few questions
PostPosted: Feb Wed 13, 2019 2:39 am 
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Pin 3 can be open if the pin numbers are counted backwards . :D
Attachment:
RR 35Z5.jpg
RR 35Z5.jpg [ 34.39 KiB | Viewed 417 times ]

Attachment:
RR 35Z5  eh.jpg
RR 35Z5 eh.jpg [ 22.99 KiB | Viewed 417 times ]

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 Post subject: Re: 35Z5GT - a few questions
PostPosted: Feb Wed 13, 2019 5:29 am 
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I've got at least a dozen spare 35Z5's, but if I ever had to buy again, I'd go for the 45Z5. I like the fact that it drops 10v with today's higher line voltages

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 Post subject: Re: 35Z5GT - a few questions
PostPosted: Feb Wed 13, 2019 2:03 pm 
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Just keep in mind that the “fuse action” of the 35Z5 and 35W4 is not highly controversial at all. If there is a short or even an excess current draw on the B+ line, the bulb will burn very brightly until it fails, and the heater tap will open shortly thereafter. It’s a sad fact of life that has been proven over and over again millions of times! If you get a radio in this condition, it pays to check for shorts on the B+ line, leaky or shorted filters or bypass caps, a leaky coupling cap to the output tube, or a gassy or leaky output tube.

If you just plug in a new bulb and tube without checking the rest of the set, it’s your own fault if they burn out in short order too. As for repair, I like the idea of the 39-ohm, 2-watt resistor between pins 2 and 3. It allows you to get some more use out of a tube that is still for the most part good. Just use a wirewound or metal oxide film resistor, not a carbon comp. (The resistance of low value carbon resistors tends to go down with heat, which forces more voltage on the rest of the heater string.)

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 Post subject: Re: 35Z5GT - a few questions
PostPosted: Feb Wed 13, 2019 4:08 pm 
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A lot of good info here.Been working on a set that uses a 35Z5gt. I have 2 35Zgts both are good. :D

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 Post subject: Re: 35Z5GT - a few questions
PostPosted: Feb Wed 13, 2019 8:40 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 5565
Location: 253 Blanche St. Plymouth, MI USA
If you just plug in a new bulb and tube without checking the rest of the set, it’s your own fault if they burn out in short order too. As for repair, I like the idea of the 39-ohm, 2-watt resistor between pins 2 and 3. It allows you to get some more use out of a tube that is still for the most part good. Just use a wirewound or metal oxide film resistor, not a carbon comp. (The resistance of low value carbon resistors tends to go down with heat, which forces more voltage on the rest of the heater string.)[/quote]


The resistance of ANY carbon comp resistor goes down with too much heat as the particles weld together. If you stay within rating of the part it should be OK
Mark Oppat


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 Post subject: Re: 35Z5GT - a few questions
PostPosted: Feb Wed 13, 2019 9:38 pm 
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Joined: Jun Fri 19, 2009 6:34 pm
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Location: Long Island
A carbon comp resistor consists of mixed carbon granules and filler material. Generally, the lower the resistance, the more carbon. Since carbon has a negative temperature coefficient, a low value resistor can drop a few percent in resistance between cold turn-on and full operating temperature. Higher value resistors are more likely to go up in value a few percent as they heat up, as the filler material may have a larger positive TC than the carbon.

This is normal. It’s not due to overheating and the resistance goes back to what it was when it cools back to room temperature. Overheating would cause the resistance to change permanently.

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 Post subject: Re: 35Z5GT - a few questions
PostPosted: Feb Thu 14, 2019 12:03 am 
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Joined: Sep Thu 23, 2010 6:37 am
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Location: Powell River BC Canada
Quote :

I have several 35Z5's that have this issue. To clarify, there is continuity between pin 2 and pin 7, but not pin 2 and 3, or 3 and 7.

Interesting.

Attachment:
R  35Z4.jpg
R 35Z4.jpg [ 74.73 KiB | Viewed 329 times ]

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 Post subject: Re: 35Z5GT - a few questions
PostPosted: Feb Thu 14, 2019 12:24 am 
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Location: 253 Blanche St. Plymouth, MI USA
Chris108 said "Overheating would cause the resistance to change permanently."

yes exactly. Few servicers know this. And, it can be a snowball effect usually until total failure, since typically something like a shorted capacitor or gassy tube is causing the issue.
Mark Oppat


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 Post subject: Re: 35Z5GT - a few questions
PostPosted: Feb Thu 14, 2019 1:36 am 
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radiotechnician wrote:
Quote :

I have several 35Z5's that have this issue. To clarify, there is continuity between pin 2 and pin 7, but not pin 2 and 3, or 3 and 7.

Interesting.

Attachment:
R 35Z4.jpg


Is posting pinout for 35Z4 a test? Of course pin three is open, it has one 35v heater while the 35Z5 has a 7.5v & 27.5v tied together by pin 3.

I'm going to stand on my statement that 2 & 3 or 3 & 7 can be open but not 2 & 7 with continuity and pin 3 open.

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 Post subject: Re: 35Z5GT - a few questions
PostPosted: Feb Thu 14, 2019 4:57 am 
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No, in a box of old tubes, a 35Z4 with numbers worn off may be mistaken for
a 35Z5. Both outline 17 . Also any 35Z5 may sub for a 35Z4.

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 Post subject: Re: 35Z5GT - a few questions
PostPosted: Feb Thu 14, 2019 2:42 pm 
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Joined: Jun Thu 30, 2011 9:03 pm
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Location: Pennsville, NJ 08070
I have also been using 35Z4 in place of the 35Z5 on radios that don’t use a pilot lamp. Not sure if those sets still do anything with the pilot lamp tap to do any kind of fusing.


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 Post subject: Re: 35Z5GT - a few questions
PostPosted: Feb Thu 14, 2019 6:53 pm 
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radiotechnician wrote:
No, in a box of old tubes, a 35Z4 with numbers worn off may be mistaken for
a 35Z5. Both outline 17 . Also any 35Z5 may sub for a 35Z4.

Assuming pin 3 isn't a tie point. There may be a shunt resistor that operates pilot, was method used in period between introduction of 35Z4 & 35Z5. Even possible AC line is attached.

I have a AA5 Admiral without pilot that specs 35Z5 but it's just as happy using 35Z4.

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