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 Post subject: Zenith 6X5 rectifier question
PostPosted: Nov Fri 27, 2020 8:19 pm 
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I have a dual 6X5 tube Zenith 10S566 .. I see tons of info on how bad the 6X5's are (heater to cathode shorts) and suggested fixes (solid state, #47 bulbs, etc,etc.. ) Problem is, on the 10S566 schematic, the heater and cathode are tied together!! How can short cause an issue????

http://www.nostalgiaair.org/Resources/download.asp?FN=\M0024677.pdf

Thanks
Rich


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 Post subject: Re: Zenith 6X5 rectifier question
PostPosted: Nov Fri 27, 2020 8:30 pm 
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Only one side of filament is common with the cathode, not both.

In case of a cathode/heater short within the tube, that could create a secondary short across the filament. If you follow that pair of wires back to the transformer, you would see that it's a short across that secondary winding.


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Last edited by AJJ on Nov Fri 27, 2020 8:51 pm, edited 2 times in total.
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 Post subject: Re: Zenith 6X5 rectifier question
PostPosted: Nov Fri 27, 2020 8:40 pm 
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Ok - I understand now.. SO ------- What's your preferred fix?? (1) Replace with diodes?? (2) 1 6X5 is bad (not shorted) - order new one and pray?? (3) Other??

Rich


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 Post subject: Re: Zenith 6X5 rectifier question
PostPosted: Nov Fri 27, 2020 8:49 pm 
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My Zenith has the 1N4007 mod.


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 Post subject: Re: Zenith 6X5 rectifier question
PostPosted: Nov Fri 27, 2020 9:22 pm 
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I used a 6X5WGT to replace the 6X5 in my 8S-463. It was a later designed tube from the original. I also added a wirewound resistor on the AC return line to drop the incoming line voltage down to about 110 VAC, so the set would run cooler.
https://www.mouser.com/ProductDetail/Vi ... esLacR8%3D

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 Post subject: Re: Zenith 6X5 rectifier question
PostPosted: Nov Fri 27, 2020 9:27 pm 
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Thanks tons guys.. I have considered the WGT with the Xplates - just not sure my tube supplier has them (2).. (BOB D).. I have tons of the diodes so may try that first.. Have heard horror stories about voltage surge using diodes - but will see what happens.. Thanks again!!

Rich


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 Post subject: Re: Zenith 6X5 rectifier question
PostPosted: Nov Fri 27, 2020 9:35 pm 
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If you've heard horror stories about the Diodes, why would you chance using them? Not worth risking the PT, IMHO.

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 Post subject: Re: Zenith 6X5 rectifier question
PostPosted: Nov Fri 27, 2020 9:50 pm 
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Well - thanks for nothing "fifties" , just when I make up my mind, you go and change it!! :) :) I'LL CHECK WITH BOB AND SEE IF HE HAS A COUPLE W's, THEN, DEPEENDING ON PRICE - MAY RECONSIDER..!! Thanks again.. :)


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 Post subject: Re: Zenith 6X5 rectifier question
PostPosted: Nov Fri 27, 2020 10:27 pm 
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...what "horror stories"?


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 Post subject: Re: Zenith 6X5 rectifier question
PostPosted: Nov Fri 27, 2020 11:07 pm 
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I think my main concern with the diodes was the surge/ instantaneous b+ prior to other tubes being warmed up - plus adjusting for the voltage differences.. Not a biggie - and still have not finally decided.. Checking first on cost of 2 6X5WGT tubes.. Still on the fence about what direction to go..


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 Post subject: Re: Zenith 6X5 rectifier question
PostPosted: Nov Fri 27, 2020 11:30 pm 
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You are worrying for almost nothing: after the disaster of their 1940 model year rollout with 6X5s that had heaters powered by the same ground-referenced winding that powered all of the other tubes, Zenith backed off and supplied their '41 models (of which the 566 is one) with power transformers that had a SEPARATE, DEDICATED 6.3 VAC winding for the heaters of the 6X5 tubes. So, if the deadly 6X5 cathode-heater short occurred (very common with first-generation tubes) there would be no total meltdown of the power transformer as in the 1940 models (-4XX series)... and as you noted, the cathode and one side of the heater of the 6X5 tubes are literally wired together.

Of course, this totally throws away a big anticipated cost-advantage of the 6X5: a simpler, less costly single-heater-circuit power transformer (the power transformer was probably a major cost-driver in these sets). But Zenith was on the ropes and had to do something lest they alienate even more consumers. No doubt that they lost orders of magnitude more money on in-warranty service claims (and bad press) than they ever realized from the 6X5 cost-reduction scheme.

The next phase ('42 model year) of the disaster mitigation plan was abandonment of the 6X5 tube entirely for their flagship radios, returning to rectifier tubes with dedicated 5V heater windings.


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 Post subject: Re: Zenith 6X5 rectifier question
PostPosted: Nov Fri 27, 2020 11:53 pm 
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If you go the diode route, measure the DC voltage at turn on. If it goes above the voltage rating of the electrolytics, replace them with a higher voltage version.

It’s not uncommon to have 500 volt or more Filter caps in a 5Volt rectifier set.

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 Post subject: Re: Zenith 6X5 rectifier question
PostPosted: Nov Fri 27, 2020 11:57 pm 
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lorenz200w wrote:
You are worrying for almost nothing: after the disaster of their 1940 model year rollout with 6X5s that had heaters powered by the same ground-referenced winding that powered all of the other tubes, Zenith backed off and supplied their '41 models (of which the 566 is one) with power transformers that had a SEPARATE, DEDICATED 6.3 VAC winding for the heaters of the 6X5 tubes. So, if the deadly 6X5 cathode-heater short occurred (very common with first-generation tubes) there would be no total meltdown of the power transformer as in the 1940 models (-4XX series)...

Oh terrific; he just potentially fries part of the transformer!

lorenz200w wrote:
(the power transformer was probably a major cost-driver in these sets).

And it still would be; add "availability" to the situation as well.

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 Post subject: Re: Zenith 6X5 rectifier question
PostPosted: Nov Sat 28, 2020 12:54 am 
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If you own a 1940 Zenith, use 1N4007 diodes. If 1941 with improved transformer 6X5WGT is the heavy duty version of the 6X5 thus can be replaced with a tube.

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 Post subject: Re: Zenith 6X5 rectifier question
PostPosted: Nov Sat 28, 2020 1:05 am 
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[quote="fifties"]
Oh terrific; he just potentially fries part of the transformer![end quote]

Nope; the failure mode of the first-gen 6X5 was a cathode-heater short. This was rectified (pun not intended, but I'll take it) with the dedicated heater winding described above. To expand on that statement:

The cathode-heater short was fatal in the '40 sets because these sets' power transformer had a single heater winding for all tubes in the set... including the 6X5s. And one side of the heater winding was tied to chassis ground. So, when one of the 6X5s manifested a typical cathode-heater short, it had the net effect of dead-shorting the B+ source (or, at least, one phase of it) to ground. This generally fried one side of the high voltage winding instantly.

With the '41 redesign, this 6X5 failure mode becomes a "don't care" since the 6X5 heater winding is never tied to ground... and to drive home this point, the Zenith engineers tied one side of the rectifier heater winding to the tube cathode, putting the whole winding at B+ source potential- just as if the tubes had a factory-installed heater-cathode short! So the main failure vector of the first-gen 6X5 tubes was neutralized.

Still, the need for a unique power transformer was probably a factor in Zenith reverting to the 5V dedicated-heater rectifier scheme that had been used for many years prior to their flirtation with the 6X5. Most likely they just flipped back to a transformer part number that was in use in '39 or before.

Bad filter caps can still kill any 6X5 ever made and probably would also take out the power transformer HV winding. But the same is true for any other transformer-operated set as well.

I've been running a '41 10S567 for several years now with the original (as far as I can tell) 6X5 tubes without incident. Of course, it has been fully recapped and rewired to eliminate the possibility of a bare-wire short. I own several copies of restored '40 model sets and have a mix of plug-in silicon diode ersatz tubes and "real" second- or third-generation 6X5 tubes.


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 Post subject: Re: Zenith 6X5 rectifier question
PostPosted: Nov Sat 28, 2020 1:53 am 
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I think 6x5 was developed from V1. Some designs had the plates on on top of the other & the slightest warping caused real issues. This flies in the face of robust tube initially for car radios.

There were design changes by most manufacturers to address the issue. Not only for that reason AWA Australia often (& I have copied) 100 Ohm resistors of the lowest wattage they could get away with. They like many back bias resistors (that should also never be "heavied up") actually offer greater protection than a primary fuse to the HT winding, as it is better they fry than the transformer.

+1 on running the heaters on a floating winding like a filament rectifier. My bench PSU does that to stop heater cathode flashover on the regulator tubes.

Marcc


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 Post subject: Re: Zenith 6X5 rectifier question
PostPosted: Nov Sat 28, 2020 2:01 am 
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Your transformer runs a bit cooler with silicon diodes.
If there is a surge it might damage a filter cap, no terrible loss. Don't forget the resistor to keep the B+ normal.

There are hundreds of these Zeniths that were refurbished with SS diodes years ago that have not gone up in smoke.

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 Post subject: Re: Zenith 6X5 rectifier question
PostPosted: Nov Sat 28, 2020 2:19 am 
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Silicon diodes seem to be rather robust & seem to handle surges quite well.

I have added a circuit which I used on a Thorn stereo radiogram. This was a metal rectifier being replaced on a new recycled transformer. The original transformer was full wave and a black mass (have pic). Fuse did not blow.

Transformer, Diodes, & 100 Ohm resistors are the only variance from original. Listed B+ 250VDC, actual 250VDC.

Marcc


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 Post subject: Re: Zenith 6X5 rectifier question
PostPosted: Nov Sat 28, 2020 2:37 am 
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fifties wrote:
....... Oh terrific; he just potentially fries part of the transformer. ......


Connecting the heater to the cathode put them at the same potential so breakdown would not occur. Even if the heater did short to the cathode nothing would happen.

Zenith's recommended fix for the 6X5 problem in their sets was to add a pilot light bulb in each of the HV connections to the transformer.

The 6X5 was used in zillions of places without problems. Zenith managed to find a way that would overload them and cause failures.

RRM


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 Post subject: Re: Zenith 6X5 rectifier question
PostPosted: Nov Sat 28, 2020 4:35 am 
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Retired Radio Man wrote:
The 6X5 was used in zillions of places without problems. Zenith managed to find a way that would overload them and cause failures.
RRM


Zillions of places didn't push the limits of the tube. From what I see the RCA tube manual says Capacitor input max 4 Mfd current limit 70 mA and max Voltages AC plate to plate 650 volts AC Max heater to cathode max voltage 450vdc. So Zenith pushed the 70 mA limit. And in the 10s464 they used first filter cap of 20 mfd. In the 8s463 they used 16 mfd. The AC plate to plate voltage is over 700 vac. And then Zenith wondered why the tube shorted out. All of this because they saved a few pennies per radio by going from the time tested transformers with separate 6.3v & 5v winding over to this single 6.3v filament transformer. i can almost hear the "Bean Counters" yelling cut the cost per unit. And the fight with the engineers who had to know better. Bean Counters won that round. In the end they lost due to all the warranty claims and additional lost business due to this mistake.

I as a restorer have been putting 1N4007 diodes in place of 6X5 tubes for over 25 years. Anybody want to buy at least 100 used maybe good 6X5 tubes? didn't think so. I also lower the incoming LINE voltage to these Zenith chassis. pay attention to your power transformer temperature. too hot for a solid hand hold for 30 seconds? Line Voltage needs to be reduced.

John k9uwa

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Last edited by k9uwa on Nov Sat 28, 2020 5:51 am, edited 1 time in total.

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