When you run across mention of a 6X5 on antique radio forums, chances are that it's being cursed by Zenith owners who are (or once were) faced with the nightmare of replacing a power transformer blown by a shorted 6X5. It is possibly the most universally-reviled type in all of tubedom.

So what is this little beast and why is it so generally despised?

In contrast to ordinary filament-type full-wave rectifiers (80, 5Y3, 5U4 etc), the 6X5 is one of a family of heater-cathode full-wave rectifiers. Unlike some large members of the family (5AR4, 5V4), there is no connection between heater and cathode in the 6X5 family.



Several styles of 6X5GT are shown here. They vary in plate and cathode arrangement. The tube on the left displays "offset" plates surrounding a single cathode sleeve. The next tube rightward shows two independent plate-cathode structures. The third tube rightward (the most modern) shows a figure-X plate arrangement with a single cathode sleeve. The tube on the right, a 6X5WGT, shows a ruggedized figure-X plate arrangement. I do not currently have a 6X5GT having a "stacked" plate arrangement such as the 6X5G and its forerunner, the 84/6Z4, commonly had.



These are three 84/6Z4 tubes. On the left is the oldest one, showing the "stacked" plate arrangement. The one in the center has the offset plates, and so does the late-vintage tubular-bulb 84 on the right.

Apart from filament/heater burnout, any vacuum rectifier tube has two major vulnerabilities. One is excessive peak inverse voltage, and the other, excessive peak plate current. Both may cause arcs and shorts in any vacuum rectifier.

Heater-cathode rectifiers like the 6X5, with no connection between heater and cathode, have a third vulnerability, which is heater-cathode voltage at or near redline.

No heater-cathode potential can appear unless the heater is referenced to ground, or worse yet, operated at ground potential (the cathode carries the full B+ potential above ground). Unfortunately, this is the case in almost every application of the 6X5, as it is operated on the heater circuit carrying all the other tubes, a circuit almost invariably tied to ground in one way or another.

The alternative, of course, is a separate rectifier filament winding that could be "floated" without reference to ground, but that separate winding is what designers were trying to get away from in the first place...

The 84/6Z4, first of the family, was originally meant for use in the auto radios of its day (early-mid 1930's). It wasn't long, however, until someone realized that in AC sets, the 84 might be used on the same heater circuit as the other tubes, thus saving a rectifier winding on the power transformer, and thus saving a few nickels per transformer. First the 84, then the 6X5, found its way into AC set designs.

It was common, unfortunately, for designers to redline many parts (especially tubes) to get the most "bang" out of their products for their invested buck (many "Golden Age" hi-fi receivers and amplifiers are notorious for this). With the 6X5 being operated at redline (or beyond) in certain radios, there were a lot of bangs (and still are) .

When you redline any tube, you are simply asking for trouble. Redline output and rectifier tubes, and you're asking for BIG trouble. The 6X5 was apparently pushed to redline on heater-cathode potential and/or peak plate current in sets where it is notorious for shorting out, but this was a designer's fault, not the fault of the tube itself (it may also be possible that the tube was originally rated beyond its actual ability to stand up to punishment).

On the other hand, the 6X5 and any of its kin will last a long time if run comfortably within their ratings, and especially on a floating heater winding. If the heater has no reference to ground, the major cause of a heater-cathode short is removed. The only caveat is that the floating heater winding should be able to withstand full B+ potential to ground in case there is such a short (rare without heater reference to ground, but not unheard of).

As time went on and the vulnerabilities of the 6X5 became apparent, manufacturers took steps to address the issue, not only for the 6X5, but for its loctal equivalent, the 7Y4. These types were improved over time, and tougher replacements began to appear.



Two 7Y4 tubes. The older one on the left has the stacked-plate arrangement, and the one on the right (which has lost its vacuum and getter) has the figure-X plate arrangement, which the 6X4 miniature equivalent of the 6X5 seems to have pioneered. As shown above, 7Y4 and 6X5 tubes both migrated to this arrangement over time.



The 6X4 and 6X4W.

These four tubes (84, 6X5, 7Y4, and 6X4) are fairly closely equivalent in performance. Tougher tubes include the 7Z4, which uses a pair of plate and cathode structures (between its aquadag inner coating and getter, however, you hardly ever see a 7Z4 with visible elements).

Later came the 6BW4, a 9-pin miniature with a pair of plate and cathode structures, and the 6CA4, another 9-pin miniature, with a figure-X plate structure. It is the toughest of the lot and is often found in hi-fi components.



7Z4 to left, 6BW4 in middle, 6CA4 to the right.

Other improved types include the 6AX5GT (haven't one available for pix ).

NOTE: Although not shown here, the various damper tubes (6AX4, 6W4 etc) and the AC/DC cousins of the family, full-wave and half-wave, are subject to the same vulnerabilities as the 6X5 tribe, and tubes such as the 35W4 and 35Z5GT, with their pilot-light taps, are subject to particular vulnerabilities of their own.

Hope this helps. Now let the horror stories begin....

Larry

Larry - Excellent writeup, Thank You ! Could you possibly post it somewhere in either html or pdf format for downloading and saving ?
(or how about in the Antique Radio Archives ?)
--Chuck

I second that !

And its even more poignant, in my case, since I'm using a 6X5 in a project right now. The power supply is already built. On the plus side its only 200V, 40mA out. Hopefully, that margin will save my butt from roasting my power xfmr.

Great job Larry... Guess I've been shielded, can't say I remember seeing a 7Z4 with the coating, ones I have are clear...

Being there were almost always locktal/octal equiv, seems odd there was never one for the 7Z4... Guessing there would be enough safety margin that it would have lived in most applications that caused failure in a 6X5...

Tom

Thank you for the writeup, the pictures are a very useful reference. Fortunately I have not had to deal with 6X5 tubes in radios, but found them in instrumentation (HP VTVMs) where I suspect they were not value-engineered to their limits. BTW, a number of damper diodes (e.g. the 25AX4 and 6CQ4 that I found in a quick search) have the heater wrapped in what appears to be an insulating alumina-coated spiral that separates it from the cathode. Likely the maximum heater-cathode voltage is quite high for these. Warm-up time is a bit longer, though.

Mark

Many thanks to all . I'm glad I was able to do some good. When I get some photosamples of the 6AX5 at least, I'll add them to the exposition here.

Larry

I think this was (is) over the top. Within it's limits the 6X5GT, and certainly 6X4 has performed well over the decades but needs to be retired now. To keep old equpment going we have to face the fact that some organ transplants arre needed. Starts with the power supply.

I want to fine-tune the writeup a little more first. Then, if it's OK for the archives, it'll be fine with me.
Many thanks!

Larry

Thank you, TN. That power supply sounds fine to me. In the late 1980's, I built a power supply very much like that one (about 230v, 50ma out). It uses an 84/6Z4 on a floated heater winding. The supply was built for a 4-channel 6J7 mixer, and I ran the daylights out of that combo all through the nineties, day in, day out (estimate well over a thousand hours all told, maybe over two thousand). The original 84 is still in it; never an issue.

Good luck on your project.

Larry

Hi, Tom, and many thanks. Yes, there was no octal equivalent for the 7Z4. The Sylvania tube manual of that day indicates that they produced it specifically to handle loads that were causing problems for earlier heater-cath rectifiers (84, 6X5, 7Y4). Heater-cath rectifiers that outperformed it appeared over the next decade or so, but nobody seemed interested in matching the 7Z4 directly.

The 7Z4 and 7Y4 were the first of these heater-cath tubes that I saw. We had a Motorola 99FM21R in the family, and it eventually passed to me. It's a 9-tube receiver with PP 6V6's out, and uses a 7Z4. It had been worked on before it came to us, and a 7Y4 had been substituted for the 7Z4. By the time it was mine exclusively, the 7Y4 was still there, but it went up like a Roman candle not long thereafter.

I'd just started apprenticing at the local radio/TV shop at that time, so I took the patient in, and the boss took one look, shook his head, sighed, and pulled a 7Z4 off the shelf. He showed me how to check the power transformer and the filter caps, then plugged in the 7Z4, and everything clicked.

"This is the kind of thing that happens when someone gets in a hurry."

That's all he said (and probably all he had to say?). But it's also the day he gave me my first tube manual.

Larry

Take a look at 6W5G. Although close to 7Z4 I've never seen one.

Believe most of the 6X5 problem is with Zenith design. Zenith use two in some high end radios. The first section to heat (1 out of 4) had to supply all the radio current. That along with having high plate voltage destroyed tubes and power transformers.

Thanks for the tip, Norm, will follow up. I've never seen a 6W5G either.

Agree 100% on the Zenith design issue. The 6X5 (and 7Y4 and 84) failures often involve heater-cathode shorts, but I've wondered if some of them started as cathode-plate arcs due to excessive peak plate current, arcs that burned through the cathode sleeve and involved the heater within seconds.

On the other hand, high plate voltage means high cathode potential that might insure the failure with no assistance...

Larry

OK, it wasn't in the RCA manual and wasn't on Frank's TDS, but I finally located it in the tube manual section of my 1954 ARRL handbook, where I usually find the obscure types I can't find anywhere else.

A new one on me, and it DOES look close to a 7Z4. I stand corrected . But who manufactured it? Was it someone's proprietary type that didn't catch on in general usage?

Larry

Larry

I found 6W5G in Ratheon tube manuals. Tube Lore lists it as Tung Sol . Release date 1-4-37.

Many thanks, Norm, I'll keep digging...

Larry

Hi Norm, I found it in my Raytheon manual too, but it's not in my Tung-Sol manual (oldest pages in that are copyright 1940).

I understand that when we landed in WWII, tube manufacturers were ordered to discontinue a large number of types in order to make room for war production. Is that true? And if so, could this 6W5 have been one of the casualties?

Always curious!

Larry

Here's a 6AX5



Although exposed, it's very difficult to get a good shot of the cathode, sandwiched in a gap between the two large plates

Many thanks, David, nice shots, especially that beam-on shot that shows the entire cathode. I appreciate it . That tube must look unique when lit up, if you're viewing it from the right angle.

Thanks again ,

Larry

Yeah I forgot about the 6W5G, I've never seen one either... The registration was by Ken-Rad, in their release it's specifically mentioned it's for car radio use and that it will repl 6X5... With the Jan '37 registration, it was released almost four years prior to the 7Z4... If you ever wondered what EIA tube registration #100 was well now you know, 6W5G...

Another fairly obscure rectifier if the era is the 6ZY5G, that one was Sylvania's baby... Stated it can supply 40 ma current, IF the output is running class B but only 35ma if class A... Never seen it used in a set but I do have one in stock...

Tom

I have a Zenith that has a separate 6 volt winding for the 6X5 filaments and has them tied to the cathodes. Since they had to use a new transformer for this, wonder why they didn't just go to 5 volts and a 5Y3?

RRM