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Author Topic: Tubes going soft - Again  (Read 18811 times)
KH6AQ
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« Reply #75 on: April 16, 2010, 05:07:21 PM »

K7ZRZ the 0.04 resistor goes in series with transformer secondary feeding all four tubes.

I wonder though if reducing the amplifier primary voltage would be a better solution. This would reduce both the filament voltage and the high voltage. Being that modern 572Bs are said to have a lower hold-off voltage this could help. The cost is 10% lower RF output.

If the Clipperton has the transformer taps to do this it is no problem. If not a primary bucking transformer, as suggested by another, would work. The Triad VPT12-20800 sold by Digikey for $76 would work. It has secondaries to put out 12 V at 20 A or 6 V at 40 A. This would allow the 120 VAC input to be set to 114 or 108 VAC.
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K7ZRZ
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« Reply #76 on: April 17, 2010, 08:55:36 AM »

The Clipperton does not have the transformer primary taps available at the voltage-setting terminal strip. Perhaps I could dig a little deeper and find that the windings exist... and that would be GREAT.

A couple of thoughts over all this have come to mind. Tom has many times made the statements (in a nut shell here) that tube life in casual amateur service would normally be far greater, time wise, than in continuous commercial service... from which inferences and generalizations are often made. However, Tom, aren't some of your discourses on the subject taking into account the nature of tubes which had been produced by American manufacturers and of much better quality stock and manufacturing processes than the currently-available tubes?  Meaning - perhaps you can not apply some of the same logic about functional stability of tubes we are able to purchase these days... including the resilience of filaments and cathodes to somewhat increased filament voltage and other operating parameters.  (Just what came to mind).

Another question I have:  If a tube or tubes are going to deteriorate under a given set of operating parameters, will the deterioration get to a certain point - after which a leveling off of performance degradation is reached... or will they just keep going down to zero in a straight line?  If I've wrecked tubes as much as they are going to be wrecked now by the voltages in my amp., and it still can crank out a 1000 watt carrier, then it's not so bad overall.

Thanks for continuing all this fellows. I do have the resistors up and on the screen now. The .04 w/heatsink really looks like the one to use, and can be mounted to the deck below the transformer right where the filament leads come out.  Thanks for the numbers.

Brian K7ZRZ
« Last Edit: April 17, 2010, 11:27:19 AM by Brian J. Ingoldsby » Logged

Brian K7ZRZ
KF6QEX
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« Reply #77 on: April 17, 2010, 04:13:45 PM »

As I was looking around  to find out more about this amp, so this thread will make more sense to me, I stumbled on these two links which I decided might be a good place to paste here since they discuss the tubes and the amp of this thread.

http://www.isp.ca/ve3nh/clip.htm
http://kx5jt.net/572B.html

Now I can't say "I've never even seen a Clipperton L amplifier" Smiley
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W8JI
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« Reply #78 on: April 17, 2010, 05:59:14 PM »

You would never want to place a resistance in the primary.

The Clipperton does not have the transformer primary taps available at the voltage-setting terminal strip. Perhaps I could dig a little deeper and find that the windings exist... and that would be GREAT.


You might just do what I suggested and add wire to the filament leads between the transformer and choke.


Quote
A couple of thoughts over all this have come to mind. Tom has many times made the statements (in a nut shell here) that tube life in casual amateur service would normally be far greater, time wise, than in continuous commercial service... from which inferences and generalizations are often made. However, Tom, aren't some of your discourses on the subject taking into account the nature of tubes which had been produced by American manufacturers and of much better quality stock and manufacturing processes than the currently-available tubes?  Meaning - perhaps you can not apply some of the same logic about functional stability of tubes we are able to purchase these days... including the resilience of filaments and cathodes to somewhat increased filament voltage and other operating parameters.  (Just what came to mind).


Since I'm not sure why the emission is going south, and I'm not even sure how many bad tubes out of four you have.

I would make a measurement of filament current at a known voltage and then compare the results to other tubes, and even the same tubes later.

The reason I keep asking about filament current is if the filament is NOT carburized enough, the current will be high.  As the tube ages and loses carburization as the ditungsten carbide layer goes away over time. Filament current would start high and increase over time if they did not carburize the filaments enough.

It is quite possible, knowing how it works, that you have too much initial current because the filaments are under-carburized. Current would go higher and higher as the small carburized layer depletes, and this would greatly accelerate loss of emission.

Quote
Another question I have:  If a tube or tubes are going to deteriorate under a given set of operating parameters, will the deterioration get to a certain point - after which a leveling off of performance degradation is reached... or will they just keep going down to zero in a straight line?  If I've wrecked tubes as much as they are going to be wrecked now by the voltages in my amp., and it still can crank out a 1000 watt carrier, then it's not so bad overall.

Measure the filament current and see if it is to high. As the filament ages if the problem is under-carburization the current will keep going higher and higher and emission will keep going lower. I don't think it would be a linear progression. I think it should be an accelerating process up to the point where the tube levels off when the tube is nearly useless.


Quote
Thanks for continuing all this fellows. I do have the resistors up and on the screen now. The .04 w/heatsink really looks like the one to use, and can be mounted to the deck below the transformer right where the filament leads come out.  Thanks for the numbers.

Buy a roll of Teflon wire. Make your own resistor.

« Last Edit: April 17, 2010, 06:00:56 PM by Tom Rauch » Logged
K7ZRZ
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« Reply #79 on: April 17, 2010, 06:20:16 PM »

Tom,

You've provided me with a wealth of information - some of which I actually understand. But you are asking me to accomplish tasks (measuring filament current, etc.) that I'm not equipped to do or have any knowledge about doing. Perhaps if I didn't live in a motorhome out here in a state park somewhere, I'd be better off in that respect.

I know your suggestion about the teflon wire was a good one... however the .04 ohm / 25W mounted power resistor also seemed like a good one and somewhat more tidy. I had to make a choice of which to get... either of which would have to be orderd online and sent to me.  I opted for the resistor and have indeed already made that order - earlier today - from Digikey. I got two of those (two amplifiers that need them) for half the price of ordering a roll of teflon coated wire.

All the suggestions made here have been good ones and well worth consideration by anyone, and me, but again, I'm limited in what I can do without driving a long, long way to get somewhere that has the test gear you intend for me to use doing what you suggest.

Brian K7ZRZ
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Brian K7ZRZ
KH6AQ
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« Reply #80 on: April 17, 2010, 06:56:00 PM »

As Tom says the filament current may be high because the filament was not processed correctly. High filament current will cause the filament to run hot. The 'old good tube side by side in the amp with a known bad tube' experiment will tell the tale. Compare the filament colors. No instrumentation is needed other than your eyes.

The bucking transformer can be mounted externally in a box. Some fuses, a line cord, and plug for the amp to plug into. The purpose of the transformer is to drop the AC line by 5% or 10% as needed.
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K7ZRZ
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« Reply #81 on: April 17, 2010, 07:45:40 PM »

Again, I need to fetch the original Dentron tubes back here to make that comparitive eveluation. The only other tubes I have now are the new set of Taylors. I'll work on that.

Yes, the buck/boost transformer is indeed a good idea. I don't quite understand the specification yet.... in that there's a 120V winding and  6/14 V windings (or something like that). Doesn't really made sense.  It would seem to me that there would be a 120 V winding and windings on the other side of 114 or 108 - or the like.

(QUOTE)  "If the Clipperton has the transformer taps to do this it is no problem. If not a primary bucking transformer, as suggested by another, would work. The Triad VPT12-20800 sold by Digikey for $76 would work. It has secondaries to put out 12 V at 20 A or 6 V at 40 A. This would allow the 120 VAC input to be set to 114 or 108 VAC."

It's that part above that doesn't quite compute to me. I'm sure you will help me get a grasp.

Brian

« Last Edit: April 17, 2010, 07:47:22 PM by Brian J. Ingoldsby » Logged

Brian K7ZRZ
KH6AQ
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« Reply #82 on: April 18, 2010, 03:52:12 AM »

For the bucking transformer I did assume you are running 120 VAC. If 220 VAC a different transformer is needed.

If 120 VAC:
The 12 volt transformer output is placed in series with the 120 volts to the amp. It is phased such that is subtracts from 120 volts and the amp sees 108 volts. The bucking transformer handles 10% of the load power and a 200 VA transformer will work.

As far as measuring filament current:
You can remove two 572Bs from the amp. Connect the filaments (fat pins) of two tubes in series using clip leads. Connect this 12.6 volt string of tubes to the 12 volt power supply that runs the transceiver or to your car battery (engine off). Measure the DC current and the voltage across one tube. Radio Shack stocks the clip leads and a suitable digital multimeter. Tom R may be able to judge if the measured current means there is a manufacturing defect with the filament.
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N2EY
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« Reply #83 on: April 18, 2010, 04:56:32 AM »

Yes, the buck/boost transformer is indeed a good idea. I don't quite understand the specification yet.... in that there's a 120V winding and  6/14 V windings (or something like that). Doesn't really made sense.  It would seem to me that there would be a 120 V winding and windings on the other side of 114 or 108 - or the like.

Do you understand the concept of the autotransformer?

The buck-boost idea consists of connecting an available transformer as an autotransformer to reduce the voltage to the amp. The low voltage secondary is connected in series with the primary in the proper phase, and then to the line. The load (the amp) is connected to just the primary, so it gets less voltage.

The advantage of using an autotransformer to reduce line voltage is that less transformer is needed than a full isolating transformer.

Here's why:

If you used a transformer with a 120 volt winding on one side and a 108 volt winding on the other, it would have to be rated to handle the full amplifier load (1500 VA?)

But if you use an autotransformer to do the job, it would only have to be rated to handle the difference in voltage times the current (150 VA?)

In practice you'd want to run either under ratings, so the difference is even greater. A 250 VA autotransformer is a lot smaller and less expensive than a 2500 VA transformer!

Is your amp powered by 120 or 240 volts?

btw, as others have mentioned, some AC voltmeters aren't all that accurate, despite having digital readouts.

73 de Jim, N2EY
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W8JI
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« Reply #84 on: April 18, 2010, 09:00:58 AM »


But if you use an autotransformer to do the job, it would only have to be rated to handle the difference in voltage times the current (150 VA?)

In practice you'd want to run either under ratings, so the difference is even greater. A 250 VA autotransformer is a lot smaller and less expensive than a 2500 VA transformer!

Is your amp powered by 120 or 240 volts?

btw, as others have mentioned, some AC voltmeters aren't all that accurate, despite having digital readouts.

73 de Jim, N2EY

Not exactly.

The Clipperton is a capacitor input supply. While the transformer is not huge, it is still large enough to cause a high apparent power factor. This is because the rectifiers pull all the power of a small fraction of the sine wave.

The buck-boost transformer has to be sized to handle the peak current and VA power without overheating or loss of regulation.

A good rule of thumb for supplies like the Dentron on a fairly stiff mains is the peak primary current is about two-three times the actual power required.

If the amp is wired for 120, since the load might be 2500 watts, the buck-boost would need to be about a 40 ampere transformer.

If it is on 240, then it is a more comfortable 20 amperes.

Also, the ESR of the buck boost is in series with the supply line. If the ESR is high, the supply will lose regulation. The buck-boost transformer's ESR also has to be considered. ESR would have to be a fraction of an ohm, so that means a pretty good transformer.

The only reason I got away with that at Ameritron is the buck-boost is on the same core as the main windings. At one point Schumacher Electric wound some transformers (100 of them) wrong and wanted to send us a buck-boost to correct the error. When I tried the normal sized buck-boost they sent, the HV sag was more than twice the normal value. A large enough buck boost to not overheat and not sag excessively would not fit the AL80A.

Thinking the buck boost is handling a resistive load instead of a capacitor input doubler is going to trip everyone up.

While we are being helpful, correcting his problem will involve some metering.

http://www.mcmelectronics.com/product/TENMA-72-7222-/72-7222

$38, you know the current and voltage close enough to compare tubes.

For $80, you can get really accurate.

everyone should have a good basic meter.

http://www.mcmelectronics.com/search.aspx?C=3829457&K=DMM&T=R&T=R







 

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K7ZRZ
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« Reply #85 on: April 18, 2010, 11:20:07 AM »

Jim... I actually DIDN'T understand the concept of the autotransformer until now (looked up some info). Thank you for bringing that up.

So, for me, that comes back to the Variac line of thinking.  Because I will encounter voltages too high AND too low at various places, that variability seems the most sensible to me. My amplifier (here in the Motorhome) is powered by a dedicated line which I run from the power pedistal. If the pedistal has both 50 amp AND a 30 amp outlets, I can rewire the plug out there at the pedistal for 240 volts and adjust the amp for that too.  However, there is not always that voltage availible, and there is ALWAYS 120v available. The amplifier runs just as well on 120V.. and the voltage sag is minimal through my #10 cord that goes back there. I have plenty of room to set a variac under the gear and wire it in permanently.

My amplifier is labled 22 amps & 30 amp fuse. My understanding is that maximum current draw would occur at maximum boost - trying to get 120 volts from a 108 volt supply, lets say. That's not my primary interest, of course... so if bucking voltage downward, would a 15 or 20 amp unit be sufficient?  Such as this one?  http://cgi.ebay.com/Staco-3PN-Variable-AutoTransformer-Variac-Transformer_W0QQitemZ400115303569QQcmdZViewItemQQptZLH_DefaultDomain_0?hash=item5d28bb0491  (I can't seem to get the link active, but that's the correct line) ... or something similar. I'm not trying to buy something really cheap, and eBay may or may not be a good source for something like that, but there are lots of examples of variable transformers on there to look at.

It seems like getting the ratio of proper line voltage (120 VAC) to proper filament voltage (6.3 V) would be a good start, then use a variac to maintain that no matter what the voltage situation.

Does any of this sound right?

As far as measuring filament current... I don't know yet how to respond to that. If that's the best way to determine the health of a tube or tubes, then I'll have to look into that further. If tubes keep coming that are going to be having that as an issue for me, then it seems that lowering voltage over-all (or filament voltage to 6.3 volts) is only going to help - if the 6.9 as I now measure is actually the biggest contributing factor to my tube troubles.  Still trying to digest a lot of these suggestions and writings.

Brian

P.S.  Here's another variac of 22 amps...   http://cgi.ebay.com/Powerstat-variable-AutoTransformer-Type-136BU-22A-3-1KV_W0QQitemZ160356096791QQcmdZViewItemQQptZBI_Circuit_Breakers_Transformers?hash=item2555f7db17
« Last Edit: April 18, 2010, 11:37:05 AM by Brian J. Ingoldsby » Logged

Brian K7ZRZ
KH6AQ
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« Reply #86 on: April 18, 2010, 03:46:22 PM »

Brian,

have you considered selling the amp and buting an amp that doesn't use 572B tubes? Something with 3-500Z tubes, a 3-1200 or maybe even MOSFETs.

I sold the AL-572 amp and today I'm running an ALS-500. The ALS-600 will be plugged in later this week when it arrives.

Once you go solid state tubes you will hate.
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KH6AQ
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« Reply #87 on: April 18, 2010, 07:59:15 PM »

The bucking transformer is not connected as an autotransformer.

It is sized to handle CW and SSB operation. Regulation? The bucking transformer is responsible for 10% of the voltage. If it sags 25% the line voltage to the amp sags 2.5%. No problem.
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K7ZRZ
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« Reply #88 on: April 19, 2010, 04:36:35 PM »

Giving up on this particular amplifier might solve some issues... but after 2 sets of new tubes, and new Harbach power supply, new Ameritron plate choke and Suppressor board assby, soft key/fan controller, 2 fans, meter lamps, new zener, etc., it doesn't seem like a good idea just now.

I'm getting good experience with this thing, and the most important part... it perfectly matches the other stuff I have set up there. The motorhome (limited space) makes it difficult to fit together a lot of dis-similar units.

So, with a power resistor in the filament supply to corral the filament voltage, I'll be closer to home, I think.

Thanks, and hope this can end sometime.

Brian K7ZRZ
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Brian K7ZRZ
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« Reply #89 on: April 19, 2010, 04:52:30 PM »

The bucking transformer is not connected as an autotransformer.

It is sized to handle CW and SSB operation. Regulation? The bucking transformer is responsible for 10% of the voltage. If it sags 25% the line voltage to the amp sags 2.5%. No problem.

Dave,

The problem is not voltage sag or heat into a resistive load.

The problems are:

1.) The apparent power factor of a capacitor input doubler causes very high peak currents

2.) The ESR of the buck boost system can have a much larger effect than we might expect, since it is in series with the power mains

It takes a bit bigger transformer than we might expect if we want to preserve the voltage regulation.

73 Tom





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