Extending Tube Life - Reforming

[RF Man]

I seem to remember reading an article quite some time ago regarding extending tube life on the 3CX and 4CX transmitter tubes by applying voltage from a 110v source to burn up some of the carbon. I believe they called it "reforming" the tube.

Anyone ever hear of this? Any comments appreciated.

 

[BobOnTheJob]

Never heard that one, but it makes me wonder where the 110V would be applied? Across the filament, it would almost certainly pop the breaker. Those tubes draw what...30 amps at 6 volts? The current draw would be in the hundreds of amps at 110V...for about 100 milliseconds until the filament opened up. 110V across any other tube elements would probably have no effect. But I look forward to hearing about this "technique"...

 

[radionekkid]

This process will revive the tube because you will definitely then have to get a replacement for it when you are finished. When all the free electrons are freed, its over.

 

[littlejohn]

The 4CX15000A filament is rated to draw 160 amps at 6.3 volts.
Greatly overdriving CRT filaments and receiving tube filaments (Called 'rejuvenating' by the tube tester manufacturers) sometimes added substantial life to the device. And soimetimes opened the filament... but if the tube wasn't emitting anyhow, what had you lost? Proper filament voltage management of power tubes will do the best job of extending their life. EINMAC published a comprehensive bulletin on doing this. My record in VHF service was something over 35 thousand hours on a 4CX15000A, in a Contineltal 817R series radio. TPO was 20,500 Watts. The tube in question was a build, I believe it may have been an EEV built by Econco. Whatever it was, the station still has it on the shelf and it will still work, we field modified the radio to a 35KW and it now sports a 4CX20,000. It's got the dual voltage filment iron though, and we kept the blocker, so it could presumeably be put back in and run some more.

 

[amfmsw]

Yes, tube rejuvination does work in many cases, not always, not on all types. The added voltage to the filaments for a few minutes would accellerate the electron flow, restoring about a 10-20% effeciency gain. It might buy a year of extra life for the tube.

In the case of CRTs, the filament(s) were run up from 6.3 to 9v for two minutes, then, at least in the case of B&K checkers, the filament would be cut off entirely, and several thousand volts at low current would be shot through the cathode. The sparking would clean off the oxidation build-up on the cathode, restoring gain. It could be done for B&W, and seperately for RGB guns. There was a seperate method for repairing shorted grids. The problem would arise once in a while when a customer failed to mention this had been done by someone else about a year earlier. The result was stripping the cathode to a nub.

Ever change a 21BAP22 picture tube in a Curtis Mathis? Or worse, a Magnavox hi-fi combo? Oh, for the simplicity of Muntz TV's.

 

[amfmsw]

Oh, I forgot. Reforming is used for filter capacitors. Ever buy a used radio with a hum or tv with hum bars in the picture? Put the unit on a Variac and start at 50 volts or less, slowly cranking it up to 117v over a few days. Many times, the now hard to find (and expensive) filter caps will reform and work as new. If they're leaking, they're toast.

 

[Tom Wells]

How effective this reforming may be depends on the cathode type. As regards receiving types:

Indirectly heated cathodes, with rare earth oxide coating (not Xmttr types) benefit nothing from such treatment. (6SN7)

Tungsten filament directly heated( operates at ORANGE heat) benefits very little if at all. (UX201)

But Thoriated tungsten filament cathodes, ( operates at dull RED heat) (UX201A) become depleted of thorium on surface.
There were many tube rejuvenators sold in the late 1920's after thoriated tungsten tubes were developed.
These required FAR less current than the original '01 triodes, but in storage battery service, required almost contiunous gain
adjustment in an evening's listening especially as the surface thorium became depleted.
The cost of tubes made replenishment desirable. I have one 1920's unit.
Think in terms of 45 dollars, todays money for one tube replacement!

The process involves using no plate or grid currents. The tube is operated a higher filament overvoltage for some short time to "boil"
the tungsten out of the deeper filament, and a longer "low overvoltage" for some longer time to fix the thorium in the surface of
filament-cathode.

Transmitter tubes run >real< hard on cathode current, and I would expect this to be only useful to get by for a lean month or two until one could afford a new tube. But it really does work.
Unless the thermal stress on the filament takes it out.

It is somewhat like a Schroedinger's Cat quantum physics unknown state experiment while you're waitng.
You'll either have live tube, or a dead tube when you're done.
But during the time the process is in efffect, the quantum state is that of a tube half dead and half alive.
This is one of the many reasons I love electronics engineering.

 

[amfmsw]

Tom, on your background description, you mention "Hot-Rodding radios". Explain.

 

[BobOnTheJob]

While we are talking about power tubes, I recall a story from a well respected CCA engineer (not sure if it was Jerry Meyer or not) that a grounded grid PA tube would "diesel" if you suddenly removed the filament volatge & would continue to run at full power until the transmitter was shut down. Anyone ever try this? It's one of the crazier stories I've heard, but 40 years in this biz has taught me to never underestimate the impossible.

 

[littlejohn]

Given the amount of drive needed, you might in some circumstances have enough RF driving the cathode (Directly heated ones anyway, which are most beam power tubes)to heat it into emission... in which case the transmitter would keep running. Plus, a certaon amount of the drive will couple throiugh the tube even with out voltage on it.
I got a 25KW CCA in a backup situation, I'll try this next time I'm out there wth a few minutes to kill.

 

[Boulder Engineer]

Let me add to Tom Wells excellent posting.

The only tubes that can possibly be 'reactivated' are the thoriated tungsten filament types, ie, the old glass 833s, 4-1000s, etc and the modern 4CX5000-10000-20000 series etc. You *cannot* effectively do it to heater-cathode tubes like receiving tubes, or transmitting tubes like 807, 813, 4CX250, 4CX300, etc.

In general, you apply ~ 150% of rated filament voltage to the tube for a half hour with no other electrode voltages applied. This boils a fresh layer of strontium or thorium oxide to the surface of the filament. Then test the tube. If it's still not good, repeat the process for another hour. If the tube is still not good, it's too far gone.

In a real pinch you can run a totally depleted tube at maybe 3X rated filament voltage or straw yellow to white-hot.
It'll behave just like one of the old early tungsten filament jobs, but the filament will burn out in a relatively short time- a matter of a few hours.

In the earlier days of TV they used to sell heater boosters to use with expensive picture tubes that had reached the end of their life. It was a small transformer that plugged into the tube base that increased its nominal 6.3 volt heater voltage to 8 or 9 volts. They worked- But with the tradeoff that the picture tube wouldn't be lasting very much longer, at least postponing an expensive repair or replacement of the set.

 

[BobOnTheJob]

Given the amount of drive needed, you might in some circumstances have enough RF driving the cathode (Directly heated ones anyway, which are most beam power tubes)to heat it into emission... in which case the transmitter would keep running. Plus, a certaon amount of the drive will couple throiugh the tube even with out voltage on it.
I got a 25KW CCA in a backup situation, I'll try this next time I'm out there wth a few minutes to kill.

I have a 25KW CCA for sale in case your backup needs a backup.

 

[Tom Wells]

Hot rodding radios is just the same as it is with old cars.
Sometimes new parts, sometimes modifying circuits, adding things like switchable AVC and/or different AVC time constants,
adding better RF decoupling, changing IF xformer coil coupling, maybe putting in Collins magnetorestrictive IF filters.
Sometimes a good alignment is all they need.
I've added -42 db 10kc notch filters to all the AMs I use regularly.
I've added a lot of capacitance to audio cathode bypasses and bigger interstage coupling caps for deeper bass.
Many table top 5 tube AMs used common RF bypass back to the electrolytic power filter cap with nothing else.
These benefit from a handful of .01s at the screen and supressor grids.
Modern caps being so much smaller, it is feasible to throw new filter caps in
and completely eliminate hum.

I found the instructions for the Jefferson Tube rejuvenator.
Sez here.. that prior to March 1st 1923, tubes were pure tungstens.
They state that thoriated types will become depleted too soon if run over rated voltage, but even proper fil voltage will still
run a tube down.

They called it paralyzed at the time......
In the days when most radios' local gain was a filament rheostat, darn near everybody ran them too high in the quest for more audio.

It recommends rejuvenating once every thirty days, and says that in testing, even after 60 to 70 cycles, the tubes were restored to full efficiency.

Their suggested reforming is exactly 45 seconds on the boiling voltage "H", then 10 minutes of fixing voltage at "L".

Once tetrodes and pentodes came out, the ample reserve of gain and good AVC circuits made such devices instantly obsolete.
The last consumer home radios with front filament controls must have been sold around '28-'29.
And they were the close-out models, as the more expensive sets had eliminated this additional dial to fiddle with, the new higher power stations made a true AF gain control necessary.

I have a few old CRT boosters around somewhere. I remember many times these gave 1 to 2 years more service in a color set, and
B&W sets could then keep on going on and on....


Dieseling?
I would believe by the time you have enough current off the cathode for 25kw, the cathode would be disappating enough heat
to sustain emission. The cathode would of course still need a DC path to RF ground, even though one side of the filament circuit
might be opened.

 

[littlejohn]

>25 kw CCA for sale

I got one boat anchor, I don't need another. The entire plant is being rebuilt shortly, the CCA will go far away.

 

[BobOnTheJob]

>25 kw CCA for sale

I got one boat anchor, I don't need another. The entire plant is being rebuilt shortly, the CCA will go far away.

Have you had a bad experience with a CCA? I did, but I solved it. The plate voltage was 8KV and it popped plate blockers once a year. Dropping the plate voltage to 7100 solved it...I have 23 years on this blocker & counting. I cut my teeth on a CCA 40 years ago & am rather fond of them myself...guess that's why I bought the thing for resale...although it appears I can do OK parting it out...something I've resisted so far.. The #1 station in the market I live in has a 1971 CCA 20KW that I've taken care of since 1981...and no backup. Hopefully I don't jinx myself, but they let me take it down once a year for a bath & for tube replacement as needed...and I kid you not, it's been many years since it cost the station any lost airtime. Frankly, I'd feel better with a 36 year old CCA full of relays for the control ladder than a new Harris full of microprocessor based circuit boards. I can almost fix the CCA with my eyes closed...with the Harris, I'm on the phone to tech support & hoping there's a way to make it limp along until the parts arrive.

 

[Tom Wells]

See, just like with old Hot Rods.

>25 kw CCA for sale

I got one boat anchor, I don't need another. The entire plant is being rebuilt shortly, the CCA will go far away.

Have you had a bad experience with a CCA? I did, but I solved it. The plate voltage was 8KV and it popped plate blockers once a year. Dropping the plate voltage to 7100 solved it...I have 23 years on this blocker & counting. I cut my teeth on a CCA 40 years ago & am rather fond of them myself...guess that's why I bought the thing for resale...although it appears I can do OK parting it out...something I've resisted so far.. The #1 station in the market I live in has a 1971 CCA 20KW that I've taken care of since 1981...and no backup. Hopefully I don't jinx myself, but they let me take it down once a year for a bath & for tube replacement as needed...and I kid you not, it's been many years since it cost the station any lost airtime. Frankly, I'd feel better with a 36 year old CCA full of relays for the control ladder than a new Harris full of microprocessor based circuit boards. I can almost fix the CCA with my eyes closed...with the Harris, I'm on the phone to tech support & hoping there's a way to make it limp along until the parts arrive.

There's only a few things that go wrong in the old ones.

Such things work because they are obeying the laws of physics, which can't be broken.

The old stuff does break, but never, ever gets confused.

I worked for the Web Printing Press division Harris spun off in the early '80s, for 15 years.
The same technology / age /dependability issues arose...
Someone would decide a design was good to go on production, and we'd get creamed in the field.
But back in the day when the Harris stuff was designed and built in Melbourne Florida (actual rocket scientists!) it was
never a stupid concept that we had to repair, just a failed part.
[/quote]

 

[littlejohn]

Did not Harris get into the press business by acquiring Intertype? The radios had harris intertype on them for years when Gates fell off of them. During the transition, they had Gates in large letters and Harris Intertype Corporation besaide in smaller as a logo.
And, I don't believe they ever built any Broadcast stuff in Melbourne did they? That was all put together in Quincy.
The guys in the newspaper business - the ones my age anyhow - claim in The Day there was Intertype and Gestetner and everybody else folowing them. Not being a printed word type, I can't speak to the accuracy of that, perhaps you can.

 

[Tom Wells]

Did not Harris get into the press business by acquiring Intertype? The radios had harris intertype on them for years when Gates fell off of them. During the transition, they had Gates in large letters and Harris Intertype Corporation besaide in smaller as a logo.
And, I don't believe they ever built any Broadcast stuff in Melbourne did they? That was all put together in Quincy.
The guys in the newspaper business - the ones my age anyhow - claim in The Day there was Intertype and Gestetner and everybody else folowing them. Not being a printed word type, I can't speak to the accuracy of that, perhaps you can.

Harris acquired the Cleaveland-based Intertype sheetfed press company sometime in the 60's.
They also acquired the Westerly Rhode Island Cottrell press company, which at one time made sheet fed presses, but by that time was building the workhorse M1000 web press (paper off a roll).

They acquired Radiation, Inc. in Melbourne, Florida to build the electronics for the web presses.

Radiation had developed and built microwave link radio systems for NASA and the military.
Harris also bought AdressoGraph MultiGraph Intl, ( the little old stick-on address labels for subscription magazines),
and a bindery equipment company in Dayton, Ohio.
Along with the Gates division in Quincy, Ill, they were quite the media "iron" company.
Then they began manufacturing semiconductor devices in the 70's.
They sold data entry terminals for mainframe systems in the early 1980s.
I believe they just plain shut down the Intertype press division.

Somewhere around the late 70s they decided printing presses were not where they wanted to be, so they sold off
the web press company to a private group who held it till 1988, when it was bought by Heidelberg of Germany.
The Harris Graphics name was used from '82 to '90 and after that there was a gradual fading away of the Harris
part of the name, to give customers some continuity. The customers still referred to the machines as "Harris".
Heidelberg held the company till 2004, when they spun the web press division off to Goss, a newspaper press
manufacturer.

I got out before the sell-off and went with Komori, a Japanese press company.

The old days and the old tech electronics were a joy to work on.

 

[thebeaver32]

While we are talking about power tubes, I recall a story from a well respected CCA engineer (not sure if it was Jerry Meyer or not) that a grounded grid PA tube would "diesel" if you suddenly removed the filament volatge & would continue to run at full power until the transmitter was shut down. Anyone ever try this? It's one of the crazier stories I've heard, but 40 years in this biz has taught me to never underestimate the impossible.

I doubt this very seriously. Filament currents in large power tubes draw upwards of a hundred amps. Tube plate and grid currents combined don't begin to equal this amount of current. 3 or 4 amps at best. That's not enough to keep a filament even warm.

 

[littlejohn]

Troubles with CCAs? Not really, the only problems which occurred with them were due to either low quality or underrated components. Most of which were fitted in the Bohannon Road days. Non-concentric plate blockers will burn up now and again. Coupling caps onto the final cathode are often running close to the edge, and can experience 'rapid disassembly' as the euphamists say. Meters which weren't very accurate... cheeeep movements... and the occasional blower failure. That last was often due to a replacement or even a new unit which wasn't Rotron's 'Select' deries. I forget what they called them, but they had a premium blower which was better balanced and lasted longer.
That being said, if I was going to do FM in Uganda or Zaire or somesuch out in the no - foolin' boonies, the CCA would be an excellent choice. There isn't much in there to break, and absolutely no magic in the way they work. And in such an environment, the wveform distortion and lower efficiency isn'y a concern anyway.