[R-390] 26Z5W Failure

[Bob Camp]

Hi

The problem with the pair of tubes both going out at once is that there 
needs to be a reason. The tubes run for a good long time and rarely 
blow out. That makes the random chance of them both blowing at once 
pretty darn remote. That doesn't mean impossible, one of the list 
members might win the lottery this week.  If we have a statistics 
professor still hanging around I'll let him explain that part of it.

When one of the 26Z5's goes the B+ drops but not by as much as you 
might think. The radio will work ok with only one tube lit up. You can 
verify this fairly easily by simply pulling one tube. Part of this is 
because the radio is designed to work down to about  90 or 100 VAC and 
we normally run them on 120 VAC. The filaments are more of an issue at 
low line voltage than the B+ ....

There have been numerous postings on radios with "a bit more hum than 
normal" that turned out to be one of the rectifiers (tube or solid 
state) being blown. Since the hum drops from 120 Hz to 60 Hz it's not 
as noticeable as you might think due to the radio's fast roll off in 
the bass region.

Since these are filament + cathode (indirectly heated) rectifiers there 
should be no interaction between the filament circuit and the B+ part 
of the circuit. Pulling lots of current on the anode or no current at 
all should have virtually no affect on the filament.  That will hold 
true right up to the point you get a monster voltage surge down your 
house power line. At that point the cathode *could* short to the 
filament. This *could* pull enough current to pop the filament. .

According to the spec sheet I found the 26Z5W is rated for 1.3 KV 
cathode to anode, but only 450 volts from cathode to filament. Roughly 
a 2:1 over voltage on the AC input line could put the cathode / 
filament junction into trouble. A 2:1 surge isn't all that uncommon. 
The problem with this theory is that one tube should break over first 
and pull down the transformer output  before the other one broke over. 
It's a whole lot more likely to pop the fuse or destroy the power 
transformer first. It probably would be a bit hard on the first 
electrolytic filter capacitor as well unless you have a later build 
radio with a B+ fuse in it. You might want to check the value of the 
line fuse in your radio ...

The two filaments are wired in parallel. The voltage on one will not go 
up or down when you pull the other one. Unless we are dealing with 
space aliens here it's kind of tough to find a mechanism in the 
filament circuit it's self that would cause them both to to at once 
without taking out other tube filaments in the radio.

I in no way am disputing that both of the tubes were dead when you 
checked the radio. I'm simply questioning how they both got that way. 
If it was from a major surge then the radio may need a few other checks 
...

	Take Care!

		Bob Camp
		KB8TQ


On Feb 21, 2005, at 12:08 PM, Gene Dathe wrote:

> Steve wrote:
>
> It is possible to loose one 26z5 at an earlier time and the radio 
> operate
> normally. Each tube has two cathodes and two plates (four plates and 
> four
> cathodes total). Both high tension leads from the power transformer are
> shared by each tube. If one tube goes dead, there is still full wave
> rectification.
> ...73..Steve...N8YE
>
> Yikes, that's not the radio I have..!  The plates of each tube are 
> connected in parallel (pins 1 and 6).  Each tube is connected 
> separately to either side of the center-tapped secondary.  So each 
> tube is a half-wave rectifier, fed through L601 and fiter cap C606A.  
> Lose one tube and you have a half wave rectifier...
>
> The responses I have gotten (Thanks, all,) seem to indicate that both 
> tubes go at once.  Anyone blow just one? Or just too much current in 
> the remaining tube?
> 73 de NA0G  Gene
>
>
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