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Author Topic: Dentron Clipperton L QRO Filament Voltage  (Read 16771 times)
KC5NGX
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« on: December 26, 2005, 06:54:45 PM »

Hello All,

 How much voltage should there be on the filaments (4x572B)in standby mode in a Dentron Clipperton L(QRO)?

Happy New Year,

John S.
KC5NGX
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WA9SVD
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« Reply #1 on: December 27, 2005, 01:20:16 AM »

I'd surmise the filament voltage should be fairly constant, as those type tubes can be damaged when operated  (i.e., transmitting) with low filament voltage.  They are rated 6.3 Volts @ 4.0 Amperes each.
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W8JI
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« Reply #2 on: December 27, 2005, 01:51:15 AM »

Dentron never was careful about filament voltages. My recollection is it was high, about 7 volts, with a cold transformer and 120V.

Fortunately very few if any amateur service tubes show deteriorated life from excessive fialment voltage. The accumulated service hours are too low. You also have the advantage the thoriated tungsten filament is very non-critical compared to MOX catode tubes.

You'll find 99% of the failures in amateur service are related to things other than loss of filament emission, it's only in commercial service where the tube is operated very conservatively with very long filament hours that filament voltage becomes critical for total emission hours. Outgassing of elements inside the tube, gas ingress through leaking seals, and over dissipation of anode or grid are the major worries, not emission life. Still, some people worry as if they were running the filaments 24/7 and otherwise babying the tube.

If you do opt to lower the filament, remember there is a peak emission current limit. Try to adjust voltage so under key-down load the filament is just slightly on the low side of normal, say 6.1 volts or so, AT THE TUBE PINS. Remember the filament choke and wiring has a few tenths of a volt drop, and that drop is higher  when components (including transformer) are hot.

You'll have to live with the "no load and cold" voltage you get.

Again this worry has been greatly exaggerated for amateur service. It is a very valid concern for commercial applications where ten's of thousands of hours of filament time accumulate and the tube is otherwise babied, but it nearly a non-issue for amateur type service. Out of thousands of tube failures, you would be very lucky to find one from emission loss or filament damage due to inrush or excessive voltage in amateur service.

73 Tom  

 

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N3ZKP
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« Reply #3 on: December 27, 2005, 01:50:40 PM »

Tom,

<< Out of thousands of tube failures, you would be very lucky to find one from emission loss or filament damage due to inrush or excessive voltage in amateur service. >>

Can I take this to mean that the various inrush suppressors for sale by Ameritron and others are not really necessary for glass tube amps?

73,

Lon - N3ZKP
Baltimore, Maryland
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KC5NGX
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« Reply #4 on: December 27, 2005, 03:49:30 PM »

Hello,

  I have measured 9.5v in standbymode with a cold transformer. Does this seem like something to be concerned with?


Thanks Again,

John S.
kc5ngx

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W9GB
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« Reply #5 on: December 27, 2005, 05:17:04 PM »

John -

Very Good question! W9NUP restored a butchered Dentron a couple of years ago and he found no center tap for the filament transformer (a Dentron save a penny decision). He found that the filament voltage was too high.  He used the  same trick that Roy Norgard, KL7FM outlines in his Heathkit SB-200 Rebuild Notes (which also uses 572B tubes).
http://home.gci.net/~rnorgard/ampnotes.html

Here's what KL7FM did: The 572-B tubes recommended filament voltage is 6.3 VAC +/- .3 VAC. This means that the tubes should put out full rated power with 6.0 volts on the filaments.
When Bob measured his, they had 6.76 volts on the filaments. This was more than the maximum recommended voltage.

EIMAC did a study and found that for every 3% above the minimum filament voltage required for full output reduced tube life by HALF.
That could, in fact, be why I'm not getting as much power out of mine as yours. The heaters are getting weak. You probably don't have much time on your tubes. That's why you've got the extra power.

It's hard to find resistors to do the job. Each tube draws 4 amps filament current. That's 8 amps total. Each of the two coils of wire is about 30 inches long. They're made of 22 ga. Teflon wire. The resistive coils drop the extra voltage for me. I started with two 36 inch lengths and kept shortening them until I measured about 6.05 to 6.1 volts at the tube sockets. When I got the length right, I just coiled them up and hold them down in the corner with a stick-on and a ty-rap.

Now, how much wire to add?  
The wire I supplied will drop about 10 milli-volts per inch.
So, take the voltage reading you just got and subtract 6.00 from it.

Here is an example:
You read: 6.76 Subtract: 6.00 You get: 0.76
Divide this by 0.01: .76 / .01 = 76
You need 76" of wire. But, half goes in each leg, so that's two pieces 38" long.

So... for the above example, you need two coils of wire 38" long. The wire will heat slightly when in use, and in so doing will increase in resistance, dropping more voltage. To compensate for this, use about 8% (4") less wire. Use two pieces about 34" long. Of course, you will have to figure this out with the voltage reading YOU take.
==================================

Give your Dentron a longer life!

w9gb
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W8JI
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« Reply #6 on: December 28, 2005, 06:03:40 AM »

by N3ZKP on December 27, 2005
<< Out of thousands of tube failures, you would be very lucky to find one from emission loss or filament damage due to inrush or excessive voltage in amateur service. >>

Can I take this to mean that the various inrush suppressors for sale by Ameritron and others are not really necessary for glass tube amps?>>

Right. They are largely a waste of money.

I worked closely with Eimac over the years. While they make filament inrush recommendations, they never were able to record a define change by limiting inrush.

The senior engineer at Salt Lake strapped a 200 amp 5V transformer to the pins of 3-500Z tubes and cycled it thousands of times in tests. His conclusion was in the typical real-world environment of a proper sized transformer no one would even be able to tell the difference.

But if it makes you feel good, use one!!

73 Tom
 
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W8JI
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« Reply #7 on: December 28, 2005, 06:38:37 AM »

It's OK to worry about being at 6.89V when the rating is 6.3 +- .3V, and to be paranoid about the half life for 3% over voltage claims, but in the real world it probably isn't that big a deal with thoriated tunsten filaments.

Very few tubes in amateur service fail from low emission, and the only ones that typically fail from low emission are indirectly heated tubes that have had the cathodes stripped through operation with low cathode temperature.

The vast majority of tube failures are due to outgassing of internmal elements, seal leakage, poor internal element spacing control or alignment, and just plain old crappy materials.

If you ran the filament 24 hours a day seven days a week, and ran conservative power levels, then you would be in the class of operation where emission life would be a concern. As a matter of fact the often parroted emission life reduction comes from BC operation data, where the tube is operated at conservative power in uninterrupted service.

24 hours a day times 7 days a week for 365 days in a  year is 8800 hours every year. If you follow guidelines and stay on the low side of voltage you can easily get 25,000 hours or three years out of a tube like a 4CX5000. In that case a 3% needless increase would reduce life and you might only see 15,000 hours from the same tube averaged over many samples.

The average very active amateur probably operates 10 hours a week, 50 weeks a year. He puts on 500 filament hours a year, if that much.

If the tube was a well-constructed tube and the only concern was emission life, he might accumulate 20,000 hours on the filaments in about 40 years. In that case, if the voltage was 3% high, he might only have the same emission life for 20 years.

This is why the real major source of failures in amateur service is:

1.)  gassy tubes or manufacturing defects

2.) hammering the tubes until something inside overheats and fails

3.) Hardly using the tubes, so they fail from sitting and not being operated as required to maintain vacuum

This shows how silly it is to worry about emission life.

Now if you ran 7 volts or more steadily on a 6.3v filament, that would be unwise. You MIGHT get into a mode where the emission life was reduced enough to compare to other failure modes.

The exception to this is indirectly heated tubes like the 8877 or 3cx800. They can be ruined in no time by operating with low filament voltage. I can poision a cathode in a matter of minutes with 4 volts on an 8877 5V filament. I'd certainly keep a MOX cathode tube at or slightly above the design voltage rating.

Directly heated tubes don't have that problem.

The 3% rule is a good one, but we have to be very careful how we apply that rule to services other than the one where data was collected and the tube types that were studied.

We can worry this stuff to death and what we will find is we have about the same tube life on average no matter what we do, as long as we don't get really wild with voltages or melt the tubes down from grossly exceeding power ratings.

It is NOT true to say "tube life is half". What is true is to say EMISSION life is half in a large thoriated tungsten tube **if** the tube is operated in steady conservative long operating time service like broadcasting.

So if you use a 4CX5000 at 2KW output and run it 24/7/365, you better watch that voltage close. If you are a Ham and hammer the tubes at full rating, it only needs to be in the ballpark.

73 Tom
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N3ZKP
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« Reply #8 on: December 28, 2005, 12:39:11 PM »

Tom,

Thanks for the information. Your mini-dissertations are always most informative!

<< But if it makes you feel good, use one!! >>

Don't have one. My AL-80A has a three-digit S/N and spent most of its life in a Navy MARS Station. I have had it for six years. It has what appears to be the original tube in it and it still puts out 800+ watts with about 75w drive and is used on a regular basis.

73,

Lon - N3ZKP
Baltimore, Maryland
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W8JI
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« Reply #9 on: December 28, 2005, 04:15:49 PM »

RE: Dentron Clipperton L QRO Filament Voltage  Reply  
by KC5NGX on December 27, 2005

I have measured 9.5v in standbymode with a cold transformer. Does this seem like something to be concerned with?>>

John,

I'd check your meter, or double check how you are making measurements. I have a difficult time believing your amp has 9.5 volts on the filaments.

73 Tom
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KC5NGX
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« Reply #10 on: December 29, 2005, 07:12:34 PM »

Ok, I will check again. I will use a different meter this time. Seems odd to me also.

Thanks Again,

John S.
kc5ngx
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W9GB
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« Reply #11 on: December 30, 2005, 08:16:23 AM »

John -

When you measure the filament (AC) voltage - also measure your local line voltage.  Tom didn't ask - but I will - what meter (e.g. brand, type) are you using for measure the AC voltages??

[I use my Fluke 87 - which is a true RMS multimeter]

Greg
w9gb
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W8JI
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« Reply #12 on: December 30, 2005, 03:29:47 PM »

you might check to see if the meter is set to read peak voltage, and not RMS.

AC meters should read RMS as a default, and most should be very very close.

Reading over nine volts means the meter is either reading about 1.4 times the normal RMS in that amp, or perhaps cockpit error.

There isn't any way possible your line voltage is high enough to cause the fialments to be at over 9 volts, and there isn't any transformer failure mode that could do that either.

73 Tom
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KC5NGX
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« Reply #13 on: January 01, 2006, 09:43:32 PM »



Looks like I have a faulty Wavetek 27XT DMM (Darn!). Have measured 6.6Vac with a different meter. Now I guess I can start on problem #2. I have found the RF IN
wire (shielded coax maybe RG-174?)coming from K1 has been unsoldered from the printed circuit board. The schematic indicates that it goes to the primary side of K4. Am I reading this correctly?  


Thanks Again,

kc5ngx
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K9AUB
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« Reply #14 on: January 07, 2008, 03:59:51 PM »

Well, I picked up a used Dentron Clipperton-L with flat tubes and MANY problems (I think a CB operator had butchered it; it was a mess), and proceeded to completely overhaul it.  In the process, I discovered that there was 7.4 VAC at the tube filament pins!  No wonder the tubes were flat!  I don't claim that this is an original fix, but I don't remember who originally came up with it.  Anyway, I cut two lengths of #20 Teflon coated silver wire, each 46" in length.  I originally thought that this length would be excessive and would provide too much voltage drop and I would have to trim some wire, but to my pleasant surprise, it was exactly on the money!  The tube pins now show exactly 6.3 VAC, as measured with a good Fluke digital VOM.  I wound the wire up in a pair of little coils and added a terminal strip to provide a mechanical anchor for the coils, then tied them with tie wraps to keep them neat and tidy.  Problem: the wire runs very warm!  However, I did not observe any coil deformation or bubbling/blisering of the Teflon insulation.  Out of curiousity, I worked up a little thermal probe and stuck it down in the middle of the wire, to measure the real temperature.  The wire insulation is rated for 200C (about 390F).  The coils are dissipating 8 watts per coil, which is a significant amount of heat.  However, the measured temperature turned out to be 145F - well below the maximum rating for the insulation.  It will run slightly warmer in the summer and when the interior of the cabinet heats up, but I think temperatures will remain well within the ratings of the wire.  My only other concern was the added heat in the power supply compartment; 16 watts of heat is not insignificant.  However, most of the heat seems to be rising vertically and dissipating through the metal cabinet cover.  (A shame Dentron didn't put a few ventilation holes in the center divider, to suck cooling air through the power supply compartment.)  I suspect these new 572B tubes will give me long life and reliable service, particularly since I do not attempt to run them beyond the amplifier's original rating of 1000 watts DC input.  At that level, I get about 650 watts out into the antenna with about 60 watts of drive.  For my purposes, that's more than enough power to do anything I want.  I VERY briefly tuned it up once to 1800 watts DC input, but of course neither the tubes nor the transformer are rated for that power, so it was merely a brief experiment to see what the amplifier was capable of.  I think this Clipperton-L will be good to go for many years into the future, and so will the tubes.
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