Variacs

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Variacs

Post by neali on Wed Jul 08, 2015 10:33 am

Check out post 15 on this forum. Note that filaments should be run 95-105 per cent of the rated value.

http://www.diyaudio.com/forums/tubes-valves/211145-another-tube-fact-fallacy.html




I was always of the opinion that a little lower line voltage made it easier on everyone and would frequently run my line at 110V.

Take a look at that graph!! Running your heaters at 85% of rated voltage drops expected life from 5000 hours to 3 hours.  affraid

No more slow warm ups for me unless the tubes are out!

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Re: Variacs

Post by Bill Cahill on Wed Jul 08, 2015 11:49 am

Hog wash!! That is false! It does not shorten the life of tubes!!

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Re: Variacs

Post by CrazyCanuck on Wed Jul 08, 2015 1:07 pm

Given that the figures quoted and graphed are apparently sourced from RCA, I'd tend to believe them until a more authoritative opinion is presented ..... and I'm thinking that being more authoritative than RCA on the subject of RCA tubes is going to be a bit difficult.....

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Re: Variacs

Post by Guest on Wed Jul 08, 2015 1:48 pm

When voltage is decreased current rises to maintain power.

Higher current can be very destructive!  I've seen this with DC in the automotive world.


I'm not sure the effect on vacuum tubes is this severe.

I also believe in some circuits when voltage decreases so does current.  It all depends on what we are looking at and how it is wired.


Last edited by MEZLAW on Wed Jul 08, 2015 1:58 pm; edited 2 times in total (Reason for editing : corrections)

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Re: Variacs

Post by Guest on Wed Jul 08, 2015 1:54 pm

Another point in that thread;

In audioxpress april 2004 Graham Dicker suggests with tubes that were out of service for an extended time should run just their filaments for a couple of days before testing. He says "often a tube tested new from the box will test low, but after a few days of filaments only, it will often re-test 100% and be perfectly okay" Can anyone confirm this practice to be effective? And what about used tubes, will this technique work also.

I know the Bell System, back in the day when vacuum tubes were used, would "cook" them for a few days before placing into service. This was to maintain adjustments made to the circuit with the new tube.

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Re: Variacs

Post by neali on Thu Jul 09, 2015 11:37 am

Bill Cahill wrote:Hog wash!! That is false! It does not shorten the life of tubes!!

Excellent scientific argument there, Bill. Rolling Eyes

Read this.

http://www.w8ji.com/filament_voltage_life.htm

And I quote,

Low filament voltage can result in faster emission loss than excessive voltage.

While generally not mentioned on amateur radio forums and web pages, excessively low filament or heater voltages can actually be more deleterious than needlessly high filament or heater voltages.

Apparently low heater voltage causes a phenomenon called "splatter" which ruins the cathode. I know it is counter intuitive at first, but imagine trying to preserve the life of your car by running the engine on 70 octane gasoline. It would be a very unhappy engine.

I brought this to the attention of the forum because of the widely internet dispersed advised to bring your decades cold radio, amplifier, whatever up to voltage, often starting at 20% and raising it 20% on an hourly basis. While this is a useful procedure for reforming electrolytic capacitors - it realigns the dipoles in the electrolyte if it is still present, and mitigates the possible destruction of the power transformer should the electrolytics fail short from an abrupt power up at full B+, it should NOT be done with any tubes present. Or with power applied to any phonograph motor.

In addition, we are all paranoid about today's increased line voltage. Because of this, I have often run my prized gear with a Sencore powermite at reduced line voltage. Running it at 115V, I notice that the power transformer is noticeably cooler. So, I said to myself, self, if 115V is good, then 105V might be even better! While I am sure this would make the power transformer run even cooler, the resultant tube damage from lower filament/heater voltage is not worth the price.

Remember why power companies run their voltages a little high to begin with. To mitigate the damage at the end of the run done to electric motors from LOW ac voltage. Pretty much everyone is aware of that phenomenon. Even though that too is counter intuitive.

For me, if I adjust the line voltage at all, it will be to tweak those filament voltages to 6.3VAC or 12.6 VAC measured by a good true RMS meter right at the tube socket.

Excellent discussion by Cannuck and Mez. I will follow up.


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Re: Variacs

Post by neali on Thu Jul 09, 2015 11:46 am

Mez,

I have also read about cooking tubes. I think there are two things happening. Running the heaters hot can sometimes strip off a layer of contaminants from the cathode that were restricting emissions. The second is that increased heat can dramatically increase the action of the getter improving a tube that has gone a little gassy. Some have even suggested doing so in an oven for a few hours with the tube unpowered.

On an amusing note, I once had this conversation.

Wife: "Honey, what is in the oven? It smells funny"

Me: "Oh that? It is a tar filled capacitor bathtub from 1928. Don't worry, it is on a cookie sheet."

Wife: "%$%&$^&(*^*)&*(%((!!!!"

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Re: Variacs

Post by neali on Thu Jul 09, 2015 11:51 am

Oh, and one more thing. I do not reform electrolytics. I replace them. And they go back where they belong, in the can, above the chassis, shielded by metal keeping their high 60 or 120 Hz AC currents away from my pre-amplification small signals. That is just me, to each their own.

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Re: Variacs

Post by willy3486 on Thu Jul 09, 2015 1:28 pm

Hopefully this won't stir up the mess but here is my experience. I have been in electronics for a long time and about 25 years ago I bought one of the B &K 470 picture tube testers. It has a setting that you could run the filiment at a higher voltage to "clean the tube" as they called it the best I remember. Its been years since I used it but I still have it. I had an old TV that after a few years the picture tube would dim. It was about a 15 year old set when I got it and run constantly. I tried that and "cleaned" it. The picture was really brighter after that. I used that TV set for about another 20 years I guess and every so often I "cleaned" the tube. The tube never gave me issues and it was the chassis that finally died completely.

As far as this issue goes I can see all sides. When I work on a radio most of the time I replace all caps then plug it into my Sencore  mighty mite and slowly turn up  the voltage. Most of the time the radio will play fine , if not I go to real troubleshooting of it. I have repaired hundreds if not thousands of tube radios and amps and during that time I have never had a issue of blowing tubes by slowing bringing the voltage up. But I have blown tubes by plugging a radio I replaced the caps into 120 volts without using a variac. After it is restored and working I usually plug them directly with no issues. I wonder if the slowly bring up tubes that have not been used in a long time somehow does similar to the picture tube tester in that it cleans or burns off the contaminants.

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Re: Variacs

Post by Guest on Thu Jul 09, 2015 1:34 pm

I may have been miss-leading with the term "cook."    Although "cook" was the term we used, it was really inside slang for running them at operating voltage for a few days.  Tube were used in "N" and "ON" carrier and when new tubes were install are frequencies would drift out of spec unless we "aged"  the tubes for a few days.  I think "age" would have been a better word.

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Re: Variacs

Post by 75X11 on Thu Jul 09, 2015 6:35 pm

Add me to the short list of those who use a variac to run up a unit after repairs or restoration have been performed. It is a habit I have had since testing high voltage power supplies and monitoring their consumption and output during initial testing. I have never had any reason to continuously operate a unit at a reduced voltage, nor would I ramp up a unit that has compensation features for variations in applied voltages. I do think the info supplied by Neali is good to remember, although my burn ins are at the nominal operating voltages.
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Re: Variacs

Post by Ragwire on Thu Jul 09, 2015 7:47 pm

Tubes that run a fairly high plate current should not be run with reduced filament power for long. It strips the cathodes of their emissive coating. It depends on the circuit/application though. If a tube is normally run to the cool side in a given circuit, running lower filament power isn't too bad.
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