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Mullard 5AR4 looks good but won't test at all...

EastPoint

EastPoint

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I picked this up at a swap meet recently. It is unlabelled except that it has "5AR4" scratched into the base, and it reads "F31 B9G" and part of "Great Britain." I looked up the F31 B9G and it looks like that's the code for a Mullard Blackburn 5AR4.

It looks kind of odd--the base appears to have been super-glued to the bottle at one point, and the pins look like they were scraped clean or something. The bottle is a bit tilted as well.

I put it in my tester, an Eico 747B, and it will not read at all. It doesn't light up, and none of the tests do anything. I tried wiggling it, pushing it side-to-side, etc. Nothing. The tube itself has tons of silvering and looks perfectly fine. No rattling or anything. All 5 pins are there, and the key is as well--I know I'm putting it in the socket correctly.

I can dig up an amp tomorrow to try it in, but any ideas otherwise? I don't think it's the tester--I tested a 5AR4 in it last week and it worked great.
 

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Use a DMM to check for continuity between the pins. Use another 5AR4 as a reference.
If you're concerned it's a counterfeit because the guide pin has no hole in the bottom as do "all" Mullard tubes, this is the one exception to that "rule" that I am aware of. The proof is in the base, there are no rays between the pins. All the other Mullard 5AR4's with the hole in the guide pin have rays between the pins.
I'm holding in my hands a Fat Base Mullard 5AR4/GZ34 Made in Gt Britain f31 B9f. There is no hole in the guide pin, there are no rays between the pins, there are no notches in the folded and crimped plates and it has two halo getters.

As I said, use a DMM for the time being to test for continuity between the pins using another tube as a reference. You should also google the pin out for that tube as a guide.

5AR4 was probably scratched into the base because the fragile print rubbed off. "Huff" your moist breath across the glass (as if you were cleaning eyeglasses) and look for any evidence of the missing print. It takes a little practice and sometimes you have to catch the images ghost at an angle, but you'll be surprised how often it works. My tube has the ghost image of Dynaco and Mullard when I huff on it.

If the tube is good...it's one of their best and very valuable.
 
With a meter set on ohms, check for continuity between pins 2 & 8. If nothing reads there, the heater is open which means the tube is bad and is garbage.
 
You may just need to reflow the pins. Get some good solder and heat each pin up and flow the solder down the hole at the top of the pin. I've saved several octal tubes which otherwise would have ended up in the garbage this way (5AR4, 7591, KT77).

-D
 
I have found more than one 5ar4 that has no emission with a good filament , doubling the filament voltage did get week reading from them .
 
Possible someone unplugged the tube by pulling on the bottle and it broke the wires inside. You may or may not be able to make go. Worst case, unsolder the base and pull it loose. If one of the wires is broken, but not so short that you can't splice a piece in there give it a go. You've really got nothing to lose at this point if its not working now.
 
I used the "breathe on it" trick and could not get anything to show up. I tested for continuity and got "open line" across anything I tried. Yep. Everything is open. I am going to try reflowing the solder and I will report back.
 
Well, I reflowed the solder in each pin, but still no continuity and no testing. The base is glued on pretty well, and I don't have a solder-sucker, so I think I'm out of luck on this one.
 
I suspect someone tried for a "fix" and failed.

See if you can get the base off after clearing all the pins of solder.

Then try your continuity test on the wires from the tube bottom, if you get heater continuity, try and use extra wire and reconnect the base and re-solder. :scratch2:

Mark T. :music:
 
battradio said:
I have found more than one 5ar4 that has no emission with a good filament , doubling the filament voltage did get week reading from them .
Click to expand...

I've seen the same - ZERO emission on a 5AR4. Probably died when a filter cap shorted (maybe it saved the transformer by opening up!). And it was a metal base one from the '50s...

Saw something similar in a Hoffman amp - the transformer was thoroughly cooked - winding was black and burnt smelling, but still measured good. And the 6CA4 was dead as a doornail.
 
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