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Author Topic: 4-125A Filament Inrush  (Read 5770 times)
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W4AMV
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« on: August 24, 2020, 09:05:14 PM »

Good day,

Looking at Eimac service bulletins and several threads and prior design posts, it appears the 4-xxxA series does not require inrush current protection. I was given an amplifier which supports a pair of 4-400A's and the on board filament transformer is rated at 30A. Yes, much more than required for the smaller 4-125A. So, should I be concerned before starting the design and be prepared to add inrush protection.

Thanks!
Alan
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K1JJ
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« Reply #1 on: August 24, 2020, 11:30:58 PM »

Good day,

Looking at Eimac service bulletins and several threads and prior design posts, it appears the 4-xxxA series does not require inrush current protection. I was given an amplifier which supports a pair of 4-400A's and the on board filament transformer is rated at 30A. Yes, much more than required for the smaller 4-125A. So, should I be concerned before starting the design and be prepared to add inrush protection.
Thanks!
Alan


Hi Alan,

In a word, yes.... a bigger fil xfmr can mean a bigger risk of mechanical shock - fil start-up damage.

However, in the real world I've run all kinds of tubes since the 70's in finals. 4-1000As, 4-400As, 3-500Zs, 813s, 4D32s,  8877s, 3CXXXXX, etc.   All but one of these gets turned on with a switch, full power. I've never had one fail like turning on a lightbulb and the filament blows. Never.   But the transformers I used were always near rated fil specs.  Mostly because of space limitations for practicality.

That said, in your case, you could kill two birds with one stone and add a small Variac to slowly bring up the filament voltage AND at the same time, have a marked preset to accurately set the fil voltage to its exact requirement. I'll bet the correct voltage will add more longevity to the tube than anything else.    

A simple resistor step start could be used for a soft start if desired. And yes, a bigger transformer will have less internal resistance, thus a bigger current surge on start up that can damage the filament. This is one application where a bigger xfmr is not a good thing without surge protection.

The only very fragile exception I've seen is WWII tubes like the 750TL when there was a tungsten shortage. I had six of these beauties and one by one the filaments fell apart, just sitting in storage.

Bottom line is that most of the big tubes will outlive us all even with full fil power turn ons..   But be nice and use a Variac and you will feel better about it every time you fire that baby up...   Grin

Tom, K1JJ


* Coming soon:  - Baby Blue -    New homebrew project:  Self-contained single 3-500Z  linear with pics. Still in testing and fine tuning mode.

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« Reply #2 on: August 25, 2020, 09:00:51 AM »

Excellent Tom,

Thank you,
Alan
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« Reply #3 on: August 25, 2020, 11:03:53 AM »

More transmitter designers used rheostats rather than Variacs for filament control. 

A Variac will pass a surge easily into a low resistance, whereas a rheo won't.

73DG
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W4AMV
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« Reply #4 on: August 25, 2020, 12:26:42 PM »

Thanks Dennis for the input.

Is this a line, 120 V, transient or one introduced by the variac itself.
I have never scoped out the AC line output of the variac I have used.
I would assume the auto transformer in for example my GENERAL RADIO is well
behaved, that the rise in voltage would be spike free. Perhaps not,  due to the design
of a variac.

Thanks,
Alan 
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« Reply #5 on: August 25, 2020, 12:35:04 PM »

Any variable transformer is going to have a high inrush current, due to the low impedance at whatever frequency it is designed for.  It will have that inrush available to the output, proportionally, to wherever the output is set.

The unit's quality is not the question.

For filament control, they are more efficient, power wise (less heat loss), than a rheo, but the latter doesn't induce any surges, aka a type of soft start.

73DG
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« Reply #6 on: August 25, 2020, 12:42:19 PM »

Good point about the Variac passing a surge while in operation, Dennis.   Though it may be a wash.

A rheostat is cheaper than a Variac so commonly used.   But if the rheo is left in line, there is additional heat waste. But maybe no more than a Variac.   A Variac can start at zero volts and SLOWLY be brought up for virtually very little cold-start stress.  But a panel-mounted rheo can do that too...

If the rheo is not left in line like a step start, then the operating surge protection is not there.  

For some reason I have always loved Variacs for most of my power supplies  and disliked power resistors like screen dropping, heavy bleeders, fil supplies... but that's just me.  But sometimes they are the best solution.


T
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W4AMV
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« Reply #7 on: August 25, 2020, 12:43:30 PM »

Understood Dennis.

There is, like any transformer a finite DC R, however, depending on the units VA rating I suppose it could be quite small.

I will look at adding a soft start relay. I have some with the built in timer. Or... I will see if the junk box will yield a proper sized xmfr and save the big one for another day!

Thanks for the clarification.
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« Reply #8 on: August 26, 2020, 01:21:34 AM »


Line voltage here is variable, so it's worthwhile to regulate the filament voltage on expensive tubes with a ferroresonant transformer which can be had second-hand on the cheap, then fine-adjust after that with a rheostat in series with the primary (taps if avail) of the filament transformer.

Most ferroresonant transformers made over the last 20-30 years are the 'low harmonic distortion' design. The usual auction web site offers them in all sizes even less than 100VA. Best to use one >60% of its VA rating for good efficiency because they draw some current even if not loaded. 4-125 uses 33W for the filament. There's a 60VA one there right now looks good.. someon painted blue? LOL. The case ends come off easily to dispense with the cord and sockets.

Soft starting should be put after the ferroresonant transformer. This is set-and-forget 8-) but always use a filament meter, right?
A ferroresonant transformer also by its nature resists transients and limits the current in case of an overload as in page 2 of this:
(https://solahevidutysales.com/pdf/powerconditioners/opchars.pdf)

Henry used a 10 Ohm 50W resistor placed continuously in series with a tap on its 3CX3000A filament transformer primary. That's not the whole story since the transformer impedance and saturation current is not known. funky soft start? Those machines would be powered up 24x7 anyway as part of semiconductor manufacturing.

Eimac? somewhere published guidance on inrush limiting, and also put inrush current limits in their data sheets where it differed from their guidance for specific tubes (e.g. 4CX25,000). The point of it was that the smaller the tube, the lower the % of overcurrent allowed for inrush. Some tubes can take 300%, some 150%. I can't find this right now.

VOA published this useful paper with much about getting the longest filament life.
(https://www.photonis.com/system/files/2019-03/Voice-of-America-Paper.pdf)

The attached chart shows tungsten microhms per cm resistivity (and also 3CX3000 filament resistance) vs temperature. It also contains a reference to a chapter on the exact subject friom which the chart was made. Maybe it will be interesting for future use.
The reference is "tungsten, thoriated tungsten, and thoria emitters" by W. E. Harbaugh.
(https://electrontubestore.com/thoriated-tungsten.pdf)

The whole book is "Electron Tube Design" from RCA, 1962, if 88MB is not too large for 900 pages.
(http://www.tubebooks.org/Books/Atwood/RCA%201962%20Electron%20Tube%20Design.pdf)



* 3cx3000 fil inrush.png (33.83 KB, 532x392 - viewed 205 times.)

* fil reg-adj-inrush.png (24.39 KB, 728x428 - viewed 269 times.)

* Henry fil resistor.JPG (91.44 KB, 800x724 - viewed 226 times.)
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« Reply #9 on: August 26, 2020, 11:29:34 PM »

Here is an inrush circuit that I use on my Henry Amp and is similar to Pat's circuit.

Phil

* General Inrush Circuit.pdf (46.42 KB - downloaded 129 times.)
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« Reply #10 on: August 26, 2020, 11:42:50 PM »

Ah, General Inrush.

The instigator of many the electrical skirmish... Wink

73DG
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« Reply #11 on: August 29, 2020, 02:21:52 AM »

Ah, General Inrush.

The instigator of many the electrical skirmish... Wink

73DG
Not forgetting his adjutant Major Malfunction..
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W7TFO
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« Reply #12 on: August 29, 2020, 02:27:23 AM »

And his buddy, Major Disaster!

------or-------

A guy walks into a bar and orders two Hurricanes and a Corona.

The barkeep says "That'll be $20.20

Ta-dum.

73DG
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« Reply #13 on: August 29, 2020, 10:56:18 AM »

In the restaurant next door a guy orders atifasto, with bananas foster flambé for dessert.
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« Reply #14 on: September 01, 2020, 10:36:27 PM »


Henry used a 10 Ohm 50W resistor placed continuously in series with a tap on its 3CX3000A filament transformer primary. That's not the whole story since the transformer impedance and saturation current is not known.

When I built my 4x4-125A amp some years ago (just starting construction, in my profile pic) I used two surplus Triad 5 volt, 14 amp filament transformers (each supplying two tubes) with the primaries paralleled and fed through an adjustable power resistor of a few ohms, so I had 5.0 volts at the tube pins. Line voltage tends to be high here, 122-123 volts. Anyhow I've never popped a filament either, although admittedly I don't use it much  Wink
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« Reply #15 on: September 02, 2020, 12:58:56 AM »

Good design!

73DG
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« Reply #16 on: September 07, 2020, 07:26:27 PM »

Here is a soft start delay that I have used in the past. I recently ran some more tests in spice and it behaves as the bench unit. The delay is probably to long at about 7 seconds. Easy to speed up. The main advantage compared to the circuits I have seen in the handbooks is the shut off time is quick. The simple R-C across the relays I have tried takes way to long to shut off... that is remove the short across the inrush resistor. If not careful, the inrush is not in play when switched back on!

I have set the inrush resistor to 100 ohms and that should work out to about 3 V filament prior to full on. I am not sure what a good pre-heat V might be or the desired T delay. I assume 7 seconds and ~ 1/2 full filament is ok, but open to recommendations.

I found a 5 ohm 4.4 A ohmite rheostat and will place it in the primary line. Set the V to 5.0 V when the series R, ~ 100 ohms is out of circuit. The Stancor 5V/30A transformer needs tamed. It provides 5 V at 107.6 V in. At my line, 125 V,  that would place my filament at 5.8 V unloaded. Probably not good!

Alan


* filament_soft_start.jpg (170.1 KB, 1365x604 - viewed 234 times.)
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