Dual 6X5 set - Kaboom...

[JohnOBrien]

I was working on a Zenith 11s474 which has dual 6X5's. Recapped almost completely, found a short to the grid lead on the first amplifier(fixed), got a great signal booming through (finally) then pop pop divider resistors smoking....then nothing. I cannot detect any HV AC coming out of the transformer. One 6X5 now tests 0 on a scale of 1 -10. We are talking about no more than 15 seconds while the set was on with bad symptoms. Though before I recapped and powered up, there was some visible damage to some of the 2 and 3 watt resistors.

Can a bad 6X5 do that much damage to the power transformer in 15 seconds?
If I replace the transformer do you suggest going to a single stage rectifier in place of the 6X5's?

Any suggestions would be greatly appreciated.

[Mr. Detrola]

Yes, a bad 6X5 can destroy the power transformer in just a few seconds. They were notorious for doing that. In particular 1940 Zeniths have the reputation for that happening, sometimes on sets only a few months old.

I have had good success with using a single 6AX5 (has twice the heater current of a 6X5 so you can only use one with an original power transformer) which does not have the catastrophic failure history. It's necessary to rewire one of the 6X5 sockets so that both sides of the HV AC winding are on one socket, and blank off the other one so that no one can plug a tube into it. That's a good preventive measure on sets where the original transformer hasn't failed yet. Will also work well after you replace the burned up transformer.

Others completely disconnect all wires from both of the 6X5's except for heaters. Then add a terminal strip below chassis with 1N4007's and a dropping resistor to bring B+ back to correct level.

I also always add fuses when working on one of those sets. It's impossible to protect against everything but you can at least make it less likely that a catastrophic failure will happen.

Another fix is to get a replacement transformer having a 5 volt winding and use a single 5Y3 or 5Y4 rectifier. Those were supplied as repair parts by Zenith in later years as the "fix" for the problem. If you can find a scrapped out power supply chassis from a 1946 Zenith model in the 12H09x series, that has a transformer similar to the one which Zenith was using as a replacement for the 1940 models.
_________________
Dennis

Whether you believe you can do a thing or not, you are right.
Henry Ford

[Peter Bertini]

The usual 6X5 failure mode is a breakdown between the cathode and the filament insulation, taking the raw B+ right to ground!

I've added a separate 6-volt filament transformer just for the 6X5 tubes in many sets. That will keep the time delay on warmup, and reduce the power transformer heating due to less filament current load.

But, using 1N4007s for the real rectification and keeping the 6X5 tubes lite (with the plate and cathodes disconnected!!) like Dennis suggested is probably easier and less bother..

Pete

[Mr. Detrola]

I used to do that too, with the separate transformer and using only the latest 1950's production of 6X5's with the "X" plate structure. It works well, but also takes a bit more trouble as Pete noted.

I haven't had any repeat failures with the "X" plate 6X5's, but John Goller and others have reported that they did see that style of tube break down and take out a power transformer.

I think if it's your own set, and you fuse it and use the separate transformer and "X" plate tubes, *maybe* you are OK, and maybe not.

Makes more sense to use one of the other fixes.

Overkill = using a separate transformer for the rectifier tube heaters (need at least a 2.5 amp unit for this) AND using two 6AX5's.

BTW the old timers used to install a 6 volt lamp in series with each HV AC lead to the 6X5 plates to act as a fuse. Apparently they will burn out faster than a slow blow fuse will.

Some have also mentioned using a currently made replacement for the rectifier tubes called a "Copper Cap Rectifier". This is solid state and has no heater.
_________________
Dennis

Whether you believe you can do a thing or not, you are right.
Henry Ford

[JohnOBrien]

Guys I REALLY appreciate the replies. I thought that I had done something unusual to cause this problem. And I had tested the 6X5's before I even started to do the recap. This is a set for somebody else. I guess I need to explain what happened and the likely hood in the future with the 6X5s.

I will leave it to them if they want a rework to a non-6X5 solution. I am runing into this more and more. This being- after I recap a receiver -other marginal items blow out. Outside of letting the re-invigorated power supply stress test the rest of the components after recap - is there a better way to do the intial "turn on"? I am not sure using a Variac would lead to the identification of marginal items.. (I do not have one anyway.)

[Crosley]

[BDM]

Since this subject comes up a 100 times a year. Here is a pic of the Weber Copper Cap against an "X" plate 6X5. I've used mine for the last 3 or 4 years(who's counting?) in all my 6X5 equipped sets.

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[Don Meyers]

Excuse my ignorance, but when you talk about an X plate 6X5, are you refering to looking down at the top of the tube?

Most 6X5 tubes just have a rectangular top section and some have an X shaped top section.

Thanks,

Don

[Leigh]

Hi Don,

There's another active thread that addresses that question.

http://www.antiqueradios.com/f ... highlight=

Scroll down to the photo that BDM posted today at 13:14. It shows both types of 6X5.

Alternatively, this http://www.1970gtx.net/6X51.JPG __should__ be a direct link to the photo.
_________________
73 de Leigh W3NLB || Leigh@AtwaterKent.Info
http://www.AtwaterKent.info || http://www.Synchrophase.info

[Don Meyers]

Hi Leigh,

Okay, I see the difference. I looked through my stash and found some like the one on the left and some like the one on the right (the two staggered plates one on top of the other).

I also found some with a single wide plate top to bottom and some with 2 wide parallel plates not touching top to bottom. Are these 2 type examples of the non X type also?

Don

[bowfin]

The webber solidstate replacement isn't near as nice as the offering from radiodaze. It has a heat up "curve"

[cliff_marsland]

I'm glad I read this thread - I didn't know about the copper core replacement. I don't have any radios that take 6X5's, but it'll come in VERY handy if I do.

[blacksmith]

[Dave Doughty]

[Norm Leal]

Dave

Those are older style 6X5's. Do you see a date code?

Each diode having it's own filament can be a problem. Over time one section will heat before the other. Fastest heating section will carry the initial surge current.

Seems like Zenith radios using 2 - 6X5's have the most problems. I believe part of the problem being the first section to start conducting carries all the current. This can cause arc over.

Another problem B+ voltage is fairly high in dual 6X5 Zenith radios. The filament is grounded while cathode is B+. Tubes arc over and short, burning out the power transformer.

With the X shaped filament both diodes share the same cathode over the entire length. Less chance of uneven heating.
_________________
Norm

[BDM]

[Randy Flyckt]

This is an interesting thread to say the least, even for a die hard who has elected to keep my 1940 Zeniths origional with the 6X5s. (I do, of course, use the newer X plate style tubes)

I think one important lesson that can be taken from this thread is that a tube tester cannot be relied upon to be a fool proof method of showing every problem that might exist in a particualr tube. This is especially true when it comes to cathode to heater leakage & shorts.

My guess is that your shorted 6X5 already had serious cathode to heater leakage before you even tried the set. It just didn't show up on the tube tester. Many tube testers use a small neon bulb to indicate shorts between internal elements. It often takes a major short for it to show up in the bulb. Furthermore, the tester itself may not replicate the voltage the tube may see when in use. As Norm pointed out, the 6X5 cathodes on your Zenith run with about 350 volts, while the heater circiut is tied to ground. That white insulation on the fillament has to stand of to that voltage differential.

I have found the following test to be helpfull in finding cathode to heater leakage that won't show up on many tube testers: First, connect a VTVM (set to the highest resistance scale) to the fillament and cathode pins of a 6X5. If the measurment shows anything less than a complete open (even 100 megs) don't use the tube. Next, connect the fillament and cathode pins to the test leads of an old style capacitor checker. Set the checker for a leakage test, and set the test voltage at 450 volts. (just like you would test a capacitor for leakage) If the checker shows no leakage, then your tube should be free of cathode to heater leakege. If, however, the checker shows any leakage at all under the full test voltage of 450, don't use the tube. Although the tube may show good in a conventional tester, and seem to operate fine in the circuit for a while, the leakage will eventually grow into a full blown short over time.

Incidentally, I have also found the above proceedure to be helpful in locating cathode to heater leakage in AC/DC sets, where such leakage can sometimes result in hum that is difficult to isolate.

While we are on this thread, I have also read and heard about another fix from the 40s when it became necessary to replace the transformer in the 1940 Zeniths using the dual 6X5s. This was simply to use a conventional transformer with a seperate 5 volt rectifier winding, and connecting the 5 volt winding to the fillament pins of the 6X5s. It's not as crazy as it sounds! It provides the same protection as using a seperate fillament transformer for the 6X5s. The only drawback is slightly less than the 6.3 volt rated fillament voltage, but if you use new tubes it won't make much difference; minus slightly increased warm-up time, and slightly lower B+. With today's higher line voltages, it actually doesn't seem like that bad of an arrangement. Plus, since the 5 volt winding won't be loaded with the full 2 amps from a 5Y3 type rectifier, it will actually develop slightly more than 5 volts anyway. It might actually be a little easier on the 6X5s too.

[Curt Reed]

One more thing to add to Randy's posting is that often a tube when cold will test ok and have no shorts or leakage. Then as a tube warms up, these things develop. I have seen it many times where simply popping a tube in a tube tester will show it as being fine, but if you let it cook in the tube tester for a half hour while you are called away to the phone or some other interruption, when you come back to it, it will show bad things.
Curt
_________________
Curt, N7AH
(Connoisseur of the cold 807) CW forever!

[Mr. Detrola]

According to the old timers, Zenith eventually got wise and began supplying transformers having a 5 volt winding as replacements and instructing to rewire the set to use a single 5 volt rectifier tube such as a 5Y3 or 5Y4. I have seen them, it's the same power transformer used in the 1946 12H09X series. Repairmen using aftermarket transformers instead of genuine Zenith parts were often doing the same thing.

Also interesting to note they figured out right away something was wrong, as the designs using dual 6X5's went away quickly.
_________________
Dennis

Whether you believe you can do a thing or not, you are right.
Henry Ford

[FStephenMasek]

What if the power transformer still works after the 6X5 fails? I recently powered up my Cascade radio-phono/cutter after replacing the capacitors. At first, nothing happened, but the filaments on the tubes I could see lit. I fiddled with the function selector knob, and heard a boiling sound from the power transformer area, so shut it down quickly. The transformer was quite warm.

I removed the power transformer, and opened the ends. It appears that some of the wax / oil from the insulation was cooked and flowed down to the ends of some windings, but it could have been like that since new.

The 6X5GT shows an open heater circuit, but no shorts from either heater pin to the cathode.

I just bought a variac (shielded) and other things from Action Electronics on Friday, so decided to test the transformer before throwing it away. It works just fine, at least unloaded, producing 280 volts on each high voltage lead at 110 volts input, and 310 volts with 122 volts input (normal voltage in my house). Should I try to test it under load by using a resistor or do you think it will be OK with the Weber Copper Cap Rectifier I have ordered?