The 6X5 — a problematic tube

[Eric Strasen]

This message is for those new to the vintage radio field and those unfamiliar with the 6X5G and 6X5GT rectifier.
This tube — especially the earlier shoulder-type “G” versions — has a bad reputation for unexpectedly shorting out and taking power transformers along with it.
The tube was used extensively by Zenith in their late 1930’s and early 1940’s table radios.
I have a 1942 Zenith 7S633, one of their last table sets which could qualify as furniture. It has been painstakingly restored, with hours devoted to replacing all the rotted rubber wiring. It uses a 6X5 rectifier.
I put in a late production RCA 6X5GT and for sake of originality probably will leave it there rather than resorting to solid state rectification, but I don’t play the set much in spite of its amazing sensitivity on the broadcast band (not so much on short wave, where it could really use a long wire antenna and better bandspread).
Pictured below is a 6X5G being offered on an auction site. I wouldn’t put it in any radio, no matter how well it tests.
Eric S.

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[Bob Masse]

Hi! Eric,
I’m curious as to what part of the transformer gets damaged from a shorted 6X5 tube. Is it the filament section or the plate section? I’m wondering why a person could not add a fuse or use a 0 or 1 ohm carbon resistor, of low wattage, as a fuse.
Bob

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[Eric Strasen]

Howdy Bob,
The 6X5 problem was especially bad in Zenith radios, known for having somewhat puny power transformers — no matter what other virtues their sets may have had.
Over a period of time, the 6X5 would first develop a heater to cathode short. As one side of the heater circuit is grounded, that means you will eventually have a B+ short to ground. This can happen so gradually that a line fuse might not blow until it is too late.
In the early 1940’s, Zenith sent a service bulletin to their dealers recommending that a pair of #44 bulbs be wired in series with the B+ leads from the transformer, supposedly saving it in case of 6X5 failure. I have no idea whether or not this worked.
Zenith did send replacement power transformers to dealers who were servicing multi-tube consoles, which put an extra strain on 6X5’s, and these replacements had a 5-volt winding for a decent rectifier. The late production 6X5GT’s had some design improvements, but I still don’t trust them. There is no way I would put a 6X5G in ANYTHING.
Eric S.

[John Greenland]

Eric;
I am a long ways from an expert on tubes but an RCA Tbe manual shows that the ratings of a 6X5 are virtually identical to a 6X4 except of course the “4” is a 7 pin miniature. Why not fabricate an adapter from an octal base and a 7 pin socket?
This makes two assumptions;
1: that you have the physical space and
2: that the 6X4 is a more reliable tube. (Is it?)
I have made a few of these adapters because I have some octal relay sockets that I use for breadboarding. The adapter can even be made for 9 pin miniatures like the 12AX7 if I limit it to a 6 volt heater connection. It would allow you to use and enjoy your radio and still not modify it.
Regards; John G.

[Eric Strasen]

John,
I have had enough experience with 6X5’s to be leery of them, but next to no experience with 6X4’s.
I checked several vintage radio sites and discovered the 6X4 also was subject to the same type of failure as the 6X5.
You will find pro and anti camps on 6X rectifiers, usually on the same threads.
The pro 6X camp usually blames Zenith for cheaping-out on transformer design and manufacture. The anti camp will quote sets other than Zenith in which the transformer failed due to a shorted 6X.
You might want to check out https://www.antiqueradios.com/forums/viewtopic.php?f=g&t=66501. This is a thread which started out being a discussion of the methods of cutting the glass envelope on a 6X5G, so the innards could be replaced with a silicon diode, resistor and pilot lamp (or LED) in order to maintain the look of a tube, sort of. It rapidly changed to a sometimes heated discussion of the 6X family of rectifiers.
I quote one poster from that thread:
“6X5’s and 6X4’s are transformer killers, period. I have seen a number of transformer failures in various kinds of gear related to these tubes. They are a cheapskate solution for those manufacturers who did not want to bear the cost of a separate 5 volt winding in their transformers. Even in communications gear which is usually fused, the transformers fry. What happens is that the heater to cathode oxide insulation becomes leaky or intermittent. This increases the current drain on the B+ winding above the rated capability of the transformer. Often, the increase is not sufficient to blow a line fuse, just enough to cook the transformer.
“I invariably pull all such rectifiers from any gear that I get that uses them and substitute silicone diodes. As a result, I have quite a few lying around. I will be happy to send them to anyone who wants them if they will pay shipping costs…just don’t come crying to me when your power transformer melts down…I warned you!”
Eric S.

• This reply was modified 1 year, 2 months ago by Eric Strasen.

[Eric Strasen]

John (continued)
It should be fairly easy to fabricate a silicon diode rectifier, with a suitable resistor, in an octal base as a substitute for a 6X5. This has the additional advantage of taking some load off the filament winding on your power transformer.
And if, for some reason (like show-and-tell at a radio gathering) you want to use the real 6X5 tube, all you have to do is unplug the diode base and plug in the tube.
At one point or another, commercially made diodes-in-a-base were available, and may still be as far as I know.
I think you are better off making your own as you have more choice as to the value of a resistor you want to use.
Best wishes and happy holidays,
Eric S.

[Eric Strasen]

Well, it appears there are commercial plug-in solid state rectifiers still available.
The Weber Speaker Co. of Kokomo, Indiana makes a line of “Copper Cap” rectifiers for a wide variety of rectifiers, from OA2’s to 80’s and 83’s. Prices range from $12 to $27 each (U.S.) plus postage, from the factory. Replacements for 6X4’s and 6X5’s are $22 each.
These are rather interesting. In addition to diode(s), they contain a resistor to mimic the voltage drop in a typical tube, plus linear in-rush current-limiting circuitry to simulate warm-up time of a vacuum tube, except it lasts 1 to 2 seconds rather than the 7 to 9 seconds of a typical rectifier tube.
These are direct plug-ins, with no circuit mods needed. Since heater voltage is no longer needed for the rectifier, the power transformer should have an easier life. According to Weber, the average savings in a 5U4GB application is 15 watts.
I have ordered one of these to replace the 6X5GT my Zenith 7S633.
I should be able to play the set all day long with no worries.
Their site is https://www.webervst.com.
I am not related in any way to this outfit or anyone connected with it. I have bunches of relatives in Terre Haute and Evansville, but don’t know a soul in Kokomo.
Eric S.

[Bob Masse]

Eric,
That Weber site is quite interesting. Definitely worth browsing.
https://www.antiqueradios.com/forums/viewtopic.php?f=19&t=239430
Here is a post on ARF about a 6C5S Russian tube as a possible sub for a 6X5. This should make for an interesting thread.
Bob

[Eric Strasen]

Bob,
Thanks for the referral to that site. If you continued to follow their thread, you already know I issued a dire warning to the fellow who wondered what to do about the 6X5 in his Zenith console.
The radio has eight tubes, which is really pushing a 6X5 to its limits.
In defense of the tube, I have to admit it was used in a lot of 1950’s car radios, rather than an OZ4, with no apparent problems.
Eric

[Bob Masse]

Hi! Eric,
I checked my radios and found that I have a Zenith 5-R-312 with the 6X5G rectifier. I coudn’t find anything online about blowing transformers on this model. I’m wondering if the problem was confined to certain models like the 8 tube console you mention.

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[Eric Strasen]

Hello Bob,
Actually, the Zenith with the biggest (but not the only) 6X5 problem was their 10-tube console, which used two rectifiers. Invariably, one rectifier would warm up and begin operating quicker than the second and briefly would carry the full supply load for the radio.
Cathode to heater leakage was the result, cooking the power transformer’s B+ winding. Apparently this happened often enough during a set’s brief warranty period that Zenith was forced to send dealers replacement transformers with an added 5-volt winding.
This additional expense must have kept Commander McDonald awake at nights.
Nevertheless, the company continued to use 6X5G’s in new production sets through at least the 1942 model year.
Your 5-tuber should be quite safe with a 1960’s production 6X5GT.
But as I have said before and will say again, I wouldn’t put a 6X5G in ANYTHING. Why take the chance?
The GT’s are widely available and are not expensive.
NOTE: Although there are a number of Zenith detractors, and even haters, around the web, I am not one of them. I have gobs of their sets and generally am pleased with their design and construction.
I just don’t understand why they stubbornly stuck with the 6X5 as long as they did.
I will close this out with some pictures of my 7S633.
Happy New Year,
Eric

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[Bob Masse]

Thanks for the info Eric. That is a beautifully restored radio. Like you, I don’t hate certain brands of radio. I figure they all had their place in the market. Some of the originally cheap ones have commanded high prices.

Today is nice and warm here. I’m making a roll up curtain for my radio bench. It is located in my garage and needed protection, from the byproducts of other jobs, that I do in it. I spent the afternoon ripping wood for this project. I collected too much radio stuff last year and the radio bench is a disaster area. My New Years resolution is to declutter and organize my life.

Have a Happy and Prosperous New Year.
Bob

[Eric Strasen]

The Weber “Copper Cap” diode replacement for the 6X5GT arrived today. Works fine, and I notice the dial lights are a little brighter now that the 6-volt winding on the transformer is less loaded.
I tested the 6X5 for cathode-to-heater leakage and emission, and it passed with flying colors. Even so, I’m glad I spent the money on the diode.
Attached are a couple blurry shots of the diode and tube and the back of the radio with Copper Cap installed.
Eric S.

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[Jean Marcotte]

Hello Eric,

as I was scanning a some RCC schematics today, I ran across a few car radios where the rectifier could be either a 6×5 or a 0Z4. I was just wondering if the 0Z4 could be an acceptable substitute for a 6X5 in home radios. Has anybody tried that ?

Jean Marcotte

[Eric Strasen]

Hi Jean,
Let me get back to you on that.
Eric Strasen

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