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 Post subject: Re: Question on subbing a 60FX5 for a 50C5.
PostPosted: Dec Mon 08, 2014 8:52 pm 
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I've got a series string Japanese set rated at 117V(heater string total) to 85V on the back label. Works fine at 85V as well.

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 Post subject: Re: Question on subbing a 60FX5 for a 50C5.
PostPosted: Dec Tue 09, 2014 5:06 am 
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Location: Pennsville, NJ 08070
35Z5 wrote:
This is so funny, one month the virtues of increasing total heater string voltage is in vogue, the next it's a Cardinal sin...

It's no doubt as Rob's example, other than a slower warm up 10, 15, even 20v isn't likely to create any noticeable issue...

After two NOS 35Z5 failures in approx 24 months(9 & 14mo), and three well used that lasted maybe 12 months total, I stuck a 45Z5 in the little Admiral daily player that's only turned off maybe five or six hours a day(and sometimes never)... After approx six months, if there's any difference in performance I can't tell it, even warm up occurs within what would be considered a normal amount of time...

I'll update in a year or so...

Tom


That was my reasoning, too. With the 1.2K shunt, the 60FX5tube works and sounds normal, all heaters appear normal and warmup time is about the same, too...


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 Post subject: Re: Question on subbing a 60FX5 for a 50C5.
PostPosted: Dec Tue 09, 2014 5:32 am 
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Location: Florida
quote="Chris108"]..............Most of the 410Bs I've worked on were older than I am, or at least contemporaries, so I did not know their whole histories. But very rarely have I ever found an original HP branded tube in one, suggesting that most were already on their second or third re-tubing by the time they found their way to my bench. Based on comparisons of the dead or dying tubes to the ones which still worked, I'd have to say that the first preamp tube only lasts about 50% as long as the rest. How many years that takes would of course depend on how the meter is treated: if left powered on all the time, one would probably be changing that tube about once a year. ...................

I just don't think the OP's idea of using a 60FX5 and a shunt resistor in place of a 50C5 is really advisable. Especially considering that while neither tube is scarce or hard to get, on average the 60FX5 sells for about twice the price of the 50C5 on eBay![/quote]

Seems as if HP would have had a lot of complaints if the meter had to have a tube replaced every year, no matter how it was used. As you are aware, replacing a tube in a 410B is not just a matter of pulling one out and plugging another one in. You may have to try several to get things to balance properly and this may even require replacing both tubes to get there. Then you have to let the tubes burn in before you recalibrate. This means the meter is out of the lab and money is not being made, not to mention tests that may have to be repeated before someone realized the meter was getting flaky. Maybe it did fail that often, but I never saw anyone throw one across the lab.

I used a 410B for a short time when I first went to work. That's one of the reasons I bought mine when it turned up in the local junk place (and it was just too cool to pass up).

There is another HP instrument I know of that ran a tube with reduced heater voltage. The 6SQ7 in the preamp of the 400C Audio VTVM has a 5 Ohm resistor in series with he filament. I haven't measured the voltage on my unit but I suspect the tube runs on about 5 volts.

What I'd like to see is some real data on lifetime effects of low heater voltage. I've seen charts for over voltage and lifetime goes down fast as the voltage goes up. I suspect the effects vary drastically from one tube type to another and as to how the tube is used.

The OP has a 60FX5 and wanted to try it. It's a klooge and he knows it. That's what we do here, and it's fun.

RRM


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 Post subject: Re: Question on subbing a 60FX5 for a 50C5.
PostPosted: Dec Tue 09, 2014 2:21 pm 
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Location: Long Island
Putting a firecracker under the 60FX5 and launching it into the neighbor's yard would be fun too, but you probably won't find that in "Radiotron Designers Handbook" or "Getting the Most out of Vacuum Tubes." Keeping heater voltages within 5% of spec for best overall tube life is in both.

Now if somebody wants to try an experiment with some odds and ends to see if they can get it to work, that's great. But getting something to work is all too often taken as the end of the experiment, when it really should be the beginning. Is it really working properly? Or as well as it can?

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 Post subject: Re: Question on subbing a 60FX5 for a 50C5.
PostPosted: Dec Tue 09, 2014 3:19 pm 
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Location: Pennsville, NJ 08070
It was never my intenion to light a firecracker under a tube and I certainly never expected so much spirited and educational debate. All this started because I was the lone bidder and won a lot of five tubes for 0.99. In the lot are two 60FX5 tubes. I have read literature such as Sylvania's publication from WWII, Aids to Wartime Servicing, which gave an excellent tutorial how to substitute into series string sets tubes of different heater current ratings. Looking over the RCA Tube characterstics manual, the characteristics between the 50C5 and the 60FX5 were very close except for the the current drawn by the heaters and of course the voltage.

I wanted to use the opportunity to learn a little more by applying some of the theory I read, perhaps follow in the footsteps of those techs who made those subs for real back then. I had also read the book, Getting the Most Out of Vacuum Tubes, which clearly noted that tube life could be extended by running slightly below the rated heater voltage by about ten percent. So it seemed like a good experiment at the time.


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 Post subject: Re: Question on subbing a 60FX5 for a 50C5.
PostPosted: Dec Tue 09, 2014 3:31 pm 
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Location: Florida
I can't find anything in the fourth edition Radiotron book except allowing for 10% variation with long term average about nominal for best life (not a surprise). There is also the mention of lower voltage being ok as long as the cathode current is low enough not to exhaust the space charge. How this is determined is not clear.

The only thing I've found in the Tomer book is that over voltage drastically shortens life and that nominal is best, again not a surprise.

I thought I had seen references to 5% (or10%) somewhere but now I can't find either. I think I have a Terman book somewhere, maybe it's in that.

I suspect there is little real data except maybe for some particular use of a particular tube.

The OP will let us know if his 60FX5 goes up like a rocket.

RRM


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 Post subject: Re: Question on subbing a 60FX5 for a 50C5.
PostPosted: Dec Tue 09, 2014 3:47 pm 
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jdivito wrote:
............ So it seemed like a good experiment at the time.


It is a good experiment. All sorts of things that are a little out of the norm have been done over the years. For example, filament transformers have sometimes been used as output transformers. Not what they were intended for but ok for some uses. This is obviously not the kind of thing you'd do for a new design but fine for the kind of things we do here.

As has been pointed out by some of our members before, we aren't going to the moon here.

RRM


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 Post subject: Re: Question on subbing a 60FX5 for a 50C5.
PostPosted: Dec Tue 09, 2014 5:10 pm 
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I'm at work now, so don't have my books handy, but there is in the Radiotron Handbook a brief, one-sentence mention of 5% being the best tolerance for oxide coated cathodes in receiving tubes. Tungsten filaments are a whole different story, with them, best life is usually obtained by running them 10% low until the emission falls off, then pushing the voltage up.

Anyway, the reason for extending the thread a little longer is to encourage experimentation of this sort, though that may seem a little counter-intuitive. You now have an idea of some of the more subtle issues that were involved, the design choices that were originally made. How close can you come with substitute parts? I think you could make a 60FX5 do a perfect stand-in for a 50C5 with a couple other changes. And while a 50C5 is not a rare or expensive tube, you could someday be faced with a radio for which exact replacement tubes are unobtainable--just as happened in WW-2--and you have to make available tubes serve. It gets even hairier in solid state equipment, where older semiconductors are superseded on a regular basis and there are often no exact replacements in existence.

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 Post subject: Re: Question on subbing a 60FX5 for a 50C5.
PostPosted: Dec Tue 09, 2014 5:24 pm 
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Location: Pennsville, NJ 08070
Retired Radio Man wrote:
I can't find anything in the fourth edition Radiotron book except allowing for 10% variation with long term average about nominal for best life (not a surprise). There is also the mention of lower voltage being ok as long as the cathode current is low enough not to exhaust the space charge. How this is determined is not clear.

The only thing I've found in the Tomer book is that over voltage drastically shortens life and that nominal is best, again not a surprise.

I thought I had seen references to 5% (or10%) somewhere but now I can't find either. I think I have a Terman book somewhere, maybe it's in that.

I suspect there is little real data except maybe for some particular use of a particular tube.

The OP will let us know if his 60FX5 goes up like a rocket.

RRM


I pulled up the book (Tomer's) and mostly remembered it right. What it does say is voltage regulation for tube heaters is key to the longest life, and goes on to give a chart showing increased voltage stability using a zener diode as an example. It also noted the trend to increased line voltages, and even in the early 1960s mentions house voltages as high as 130 volts were being reported in some parts of the country. It then noted that of the two extremes (over or under voltage on the tube heater), it was better to be slightly under than over. Being up to 10% below the design voltage was always better than above the rated voltage as far as deviation is concerned. Although they may not make it to the extreme end of their possible service life by under voltage, Tomer noted that tubes are apt to not suffer any complications or be prone to catastrophic failures when run up to 10% below rated heater voltage.

So my takeaway from re-reading that section in Tomer's book is that for a set such as the Z510 Zenith I tried the shunted 60FX5 in, the set was originally designed for 117 volts AC and is now operating at 125 under modern conditions, possibly more. The ten extra volts from the 60FX5 in theory should offset the higher house current. I think that explains one of the other posts (Tom I believe) who uses a 45Z5 in place of a 35Z5 and has seen improved tube life.

Finally, the tube most definitely did NOT go off like a rocket. If anything, it worked and the radio sounded just like it always does. There's plenty of room for ventilate the resistor, and as I noted in an earlier post, the set heated up and ran exactly as if it had a 50C5 in it.



Joe


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 Post subject: Re: Question on subbing a 60FX5 for a 50C5.
PostPosted: Dec Tue 09, 2014 6:46 pm 
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Location: Florida
This kind of exercise is a lot of fun and can be instructive, especially to guys who are just discovering the hobby. I had many years of having to worry about exact specifications during my engineering career. The kind of things we do here is a welcome mix of doing some low-level analysis from time to time just for the fun of it and playing with old junk.

Chris, let me know if you find that 5% comment. I'll keep looking. Likewise, if you know of any other definitive references about tube life at reduced filaments voltages I'd be interested in them also. This kind of thing keeps your brain alive.

RRM


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 Post subject: Re: Question on subbing a 60FX5 for a 50C5.
PostPosted: Dec Tue 09, 2014 7:00 pm 
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jdivito wrote:
..............
Finally, the tube most definitely did NOT go off like a rocket. If anything, it worked and the radio sounded just like it always does. There's plenty of room for ventilate the resistor, and as I noted in an earlier post, the set heated up and ran exactly as if it had a 50C5 in it. ....Joe


The only real concern I had was that the increased gain might cause the audio stages to oscillate or have more hum. The lower voltage wasn't a concern due to the experiences of myself and others with sets that ran that way for many years. The post about sets running routinely on 100 volts was a little surprising, the one about 85 more so.

Keep having fun.

RRM


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 Post subject: Re: Question on subbing a 60FX5 for a 50C5.
PostPosted: Dec Tue 09, 2014 7:27 pm 
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Retired Radio Man wrote:
jdivito wrote:
..............
Finally, the tube most definitely did NOT go off like a rocket. If anything, it worked and the radio sounded just like it always does. There's plenty of room for ventilate the resistor, and as I noted in an earlier post, the set heated up and ran exactly as if it had a 50C5 in it. ....Joe


The only real concern I had was that the increased gain might cause the audio stages to oscillate or have more hum. The lower voltage wasn't a concern due to the experiences of myself and others with sets that ran that way for many years. The post about sets running routinely on 100 volts was a little surprising, the one about 85 more so.

Keep having fun.

RRM



Excess gain can be controlled by adding a bit of cathode degeneration... a resistor in series with the cathode bypass cap could do this.

pete

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 Post subject: Re: Question on subbing a 60FX5 for a 50C5.
PostPosted: Dec Wed 10, 2014 1:49 am 
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No one seems to remember that std receiving tubes were used in car radios, heaters had to regularly perform from under 12v to 14v and higher... Specs for the 12AB5(9 pin 6AQ5 with 200 ma heater) says it can be operated from 10 to 15.9v but does recommend 11-14v for longest life... Nothing in the tube manuals I saw for 12BA6, 12BE6 etc mentions anything but 12.6v(would have been used with 12AB5 or 12AQ5 and maybe 12X4) while the low volt types like 12AC6, 12AD6 etc also list operation at 10-15.9v, with 11 to 14v prefered...

JEDEC release 446E(12/7/59) mentions several types 12AU6, 12BA6, even 6BE6 etc as having a permissible heater operating range of +/- 10%

In RC25 the 6X4 says when operated from three cell battery(automobile) permissible operation is between 5 to 8v no mention of 12X4 heater operation... BUT the JEDEC release 1103C(1/27/58) says 12X4 has a +/- 10% range of operation...

JEDEC release 636F(12/14/59) for 35C5, 50C5 and 6AS5 changed specs for the 35C5 & 50C5 to rated voltage +/- 10%... No mention of any change to 6AS5 heater ratings(6AS5 is a 6v 35C5)...


Tom


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 Post subject: Re: Question on subbing a 60FX5 for a 50C5.
PostPosted: Dec Wed 10, 2014 4:45 am 
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With a resistor across the filament a 60FX5 will sub for a 50C5. A 60FX5 is closer to 50EH5 which has higher gain that 50C5 but is interchangeable. These came about to be used in record players with single stages and high voltage cartridges.

As discussed, 50C5 is a common tube while 60FX5 isn't...

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 Post subject: Re: Question on subbing a 60FX5 for a 50C5.
PostPosted: Dec Wed 10, 2014 5:55 am 
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35Z5 wrote:
No one seems to remember that std receiving tubes were used in car radios, heaters had to regularly perform from under 12v to 14v and higher... Specs for the 12AB5(9 pin 6AQ5 with 200 ma heater) says it can be operated from 10 to 15.9v but does recommend 11-14v for longest life... Nothing in the tube manuals I saw for 12BA6, 12BE6 etc mentions anything but 12.6v(would have been used with 12AB5 or 12AQ5 and maybe 12X4) while the low volt types like 12AC6, 12AD6 etc also list operation at 10-15.9v, with 11 to 14v prefered...

JEDEC release 446E(12/7/59) mentions several types 12AU6, 12BA6, even 6BE6 etc as having a permissible heater operating range of +/- 10%

In RC25 the 6X4 says when operated from three cell battery(automobile) permissible operation is between 5 to 8v no mention of 12X4 heater operation... BUT the JEDEC release 1103C(1/27/58) says 12X4 has a +/- 10% range of operation...

JEDEC release 636F(12/14/59) for 35C5, 50C5 and 6AS5 changed specs for the 35C5 & 50C5 to rated voltage +/- 10%... No mention of any change to 6AS5 heater ratings(6AS5 is a 6v 35C5)...


Tom
Radiotron Designers Handbook 4'th says, unless otherwise specified, all tubes (Design Center) are tested for +- 10% operation, which incudes the heaters. That was based on line voltage varying +-10% around 117 V. As a side note, modern line voltage is higher but the upper end tolerance it tighter (normally + 5%), so it stays within the previous maximums.


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 Post subject: Re: Question on subbing a 60FX5 for a 50C5.
PostPosted: Dec Wed 10, 2014 9:12 am 
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I skimmed through some of the vacuum tube texts available at Technical Books Online. It seems that part of the explanation for very long tube life from radios operated on a lower line voltage (ie, 100VAC in Japan) is that the B+ (and consequently the current drawn) are lowered along with the heater voltage. Also, the B+ on AA5 and other simple receivers is not very high to begin with, so there's a large, safe operating zone in regard to heater voltage (isn't that one of the design objectives of the AA5?). Otoh, the safe operating zone is much smaller with tubes running near their maximum B+ and drawing a lot of current.

Rob


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 Post subject: Re: Question on subbing a 60FX5 for a 50C5.
PostPosted: Dec Wed 10, 2014 3:57 pm 
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An AA5 can beat up tubes as well, pretty easily in fact. The 50C5 is rated to 50mA plate with up to 8.5mA on grid 2. That gives you 58.5 of the 60mA rating of the 35W4 with just the final audio. Not sure what B+ is in an AA5 but in current capacity alone it can get ugly. Granted, they may not have run that hot in practice but they're capable of running each other out of the house.

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 Post subject: Re: Question on subbing a 60FX5 for a 50C5.
PostPosted: Dec Wed 10, 2014 6:57 pm 
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Design Center for the 50C5 is 135 V plate and 117 V screen. Same for the 60FX5 except its rated Design Maximum. So they are operating 'near max'.


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 Post subject: Re: Question on subbing a 60FX5 for a 50C5.
PostPosted: Dec Wed 10, 2014 7:40 pm 
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Design Maximum for 50C5 is 150v plate & 130v for screen, seven watt plate dissipation... Info from JEDEC release 636E, same one that lists heater as +/- 10%...

BTW 636E is the last update for the 50C5, update 636F lists Design Max specs for 35C5(I missed fact ratings changed in separate updates)... 636G addresses heater cathode voltage increase for 6AS5(from 90 to 100v), nothing mentioned about Design Center/Max ratings...


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 Post subject: Re: Question on subbing a 60FX5 for a 50C5.
PostPosted: Dec Wed 10, 2014 8:21 pm 
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35Z5 wrote:
Design Maximum for 50C5 is 150v plate & 130v for screen, seven watt plate dissipation... Info from JEDEC release 636E, same one that lists heater as +/- 10%...

BTW 636E is the last update for the 50C5, update 636F lists Design Max specs for 35C5(I missed fact ratings changed in separate updates)... 636G addresses heater cathode voltage increase for 6AS5(from 90 to 100v), nothing mentioned about Design Center/Max ratings...
135 V is Design Center and 150 V is Design Maximum, for the same tube. The difference is that Design Center includes line voltage variation and Design Maximum does not (you have to), hence the higher rating.


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