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I have an old RCA rectifier tube with no printing left on the glass. Is there a way to determine if it is a GZ34/5AR4 or a 5UR4, 5Y3 etc? I desperately need a GZ34 for a '67 Marshall JMP 50 watt on my bench that I cracked the rectifier tube. It was a Zaerix branded tube, which I gather was a British re-branding outfit, probably using Russian tubes. I don't want to give this holy grail amp back with a modern GZ34 in it.
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It's weird, because it WAS working fine..... -
Breathe on the glass and while it's steamed up check it for the etched numbers/letters. If you're lucky they will show up in a bright light. If that doesn't work, use an LED flashlight held at an angle while turning the tube, I've been able to read most tubes this way when the printing is gone.--Jim
He's like a new set of strings... he just needs to be stretched a bit. -
No luck with that.It's weird, because it WAS working fine.....Comment
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Post pics? Description? See how much voltage it drops in an amp?
Justin"Wow it's red! That doesn't look like the standard Marshall red. It's more like hooker lipstick/clown nose/poodle pecker red." - Chuck H. -
"Of course that means playing **LOUD** , best but useless solution to modern sissy snowflake players." - J.M. Fahey -
"All I ever managed to do with that amp was... kill small rodents within a 50 yard radius of my practice building." - Tone Meister -Comment
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Yup.
The output voltage will tell all.Comment
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I.D. Tube
I have about a half a bushel basket of 5u4's (mostly used)
and 1 vintage RCA GZ34 (well used). The GZ34 has an extra ribbon wire attached to one side of the heater that connects to the separate cathode. You can observe this by look at the pin wiring with a magnifier. The heater wires are wired in parallel for each plate. The 5u4's "usually" have the heater wired in series between the plates. None of the 5U4's have an extra connection for a separate cathode. I "imagine" the 5Y3's are similar.Comment
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If the choice is between a 5AR4, 5Y3 and a 5U4, it's very easy. The 5U4 is about an inch taller, 3 1/8" tall glass vs. 2 1/8''. Then the difference between 5AR4 and 5Y3 is about 40-50 volts out and that connection Shadrock mentions.
The only one of these that has an indirectly heated cathode is the 5AR4. If you get almost immediate voltage output, it's not a 5AR4. That one will take the usual 10-12 seconds to develop output. 5Y3 and 5U4 put out voltage almost immediately upon power up because of the directly heated cathode.Turn it up so that everything is louder than everything else.Comment
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The cathode of a GZ34 looks like a tube that extends below the bottom mica spacer. The heater is tightly stuffed inside. The heater in a 5U4 or 5Y3 is usually a wider strip of metal that only makes one inverted U shaped path from the bottom mica, up to the top mica and back again.
Edit: There are two plates in these type rectifier tubes, each one has it's own heater and cathode if so equipped.Last edited by loudthud; 12-28-2015, 04:14 AM.WARNING! Musical Instrument amplifiers contain lethal voltages and can retain them even when unplugged. Refer service to qualified personnel.
REMEMBER: Everybody knows that smokin' ain't allowed in school !Comment
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