[Amps] "babying" radios and tubes, can do or not?

[Carl]

----- Original Message ----- 
From: "Charles Harpole" <k4vud at hotmail.com>
To: <amps at contesting.com>
Sent: Thursday, June 14, 2012 3:17 AM
Subject: [Amps] "babying" radios and tubes, can do or not?


> 
> I want to know if there are any REAL benefits from babying a radio or an amp?

** Yes but you have to seperate facts from myths


> 
> 
> 
> I hear lots of talk about running lower RF power to "save the finals" or "save the tubes" and even hear people talk about turning radios off for taking a one hour (or less) break.


** Ive managed to keep the original Eimac 3-500's in my 1964 LK-500ZC by running at 1200W and regularly gettering them. At the same time the TS-940/TS-950SD are run at 80W which cools down the PS a bit plus has better IMD. Only one PS board failure since 1987 in the 940 and they are notorious for that.
I also have a 3 holer Alpha 76PA that also runs 1200W as its not worth the ~1db difference based upon replacement cost of 8874's.

On the shelf is a Dentron MLA-2500 and DTR-2000L both with original tubes, 2X 8875 and 8877 respectively. They get used once in awhile to keep the replacement caps reformed and the tubes gettered.

 
> 
> 
> We do know, probably, that high inrush current may stress parts inside tubes.  But, there must be some smart folks who test things to destruction who can speak with scientific instead of parental certainty about this.

** For most ham style amps that is a myth as the cold resistance of the filament causes a properly designed transformer to be self limiting. This is especially important with directly heated tubes such as the 3-500 where way too many go into a panic and get poorly designed step starts. Consider that the 3-400/3-500 filament is the same as used in the 4-250A/400A since the 40's with no such silliness required.
The only benefit of a step start is to save undersized power on switches and then the step start fails and takes out the resistor when least wanted.In the SB-220 the switch is almost unobtanium but there are work arounds.  In an Ameritron the usual step start failure blows the resistor and line fuse and if the owner keeps trying fuses the transformer complains or the damage is already done. Its a very poor design.

Unless overly high C caps (totally unnecessary) have been stuffed in a commercial amp the PS doesnt need a step start, the transfomer clunk at turn on is non destructive and the 1N5408 and larger diodes can easily take the surge. The 270uF caps available from Ameritron are more sufficient for the normal user; forget about the obsessive ones. If buying elsewhere the 330uF are OK but 470uF is pushing it. I use 330uF Snap-Ins in SB-220's since they fit the blocks perfectly, always turn on in the CW position and the switch survives if it hasnt already been beat to hell and back over 30-40 years.

> 
> It seems to be true that one can run any of the usual amp tubes at rated specs for hundreds of thousands of hours before they really go bad?


** Pure myth. The average tube is good for 10K hours in proper use and running at proper full spec before emission starts to noticably fall off. Many do more and others less, especially the latter for Chinese tubes. If I average 1 hour a day over the 26 years on my LK-500ZC it hasnt reached 10K yet. At 2hrs/day thats a hair under 19K.


> 
> I guess tiny impurities in the metal and not a perfect vacuum does lead to failure eventually, but what are the real reasons?


** GAS and not to be confused with seal leage to outside air. Ive gone over the gettering process on here enough times and Im not going to keep repeating myself.
All glass tubes outgass and destructive internal arcs do happen that are preventable. Most ceramic-metal tubes do it a lot less and the gettering process is simply normal use. Seal leakage is almost non existant altho certain tubes with glass seals in the base are prone, ask anyone with PL-172's.


> 
> What is the real physics condition of a so called "soft" tube?  Why?


** Loss of filament/cathode emission due to
.. hours
.. under/over filament voltage
.. excessive dissipation, this can also be due to dirty cooling systems and nicotine covered glass tubes. Heat is a killer.
 
 
> 
> What is really the cause of transistor failure if always operated within specs?  
> 

** Gremlins
.. parasitics
.. voltage spikes
.. poor VSWR/intermittent relays coupled to insufficient protection circuits
.. Other component failures

The average power transistor is made up of many small transistors in parallel under the cover. If say 24 are on the wafer and ony 18 are needed to meet spec they get shipped as there is no way to know how many are good to start with. Each wafer is not inspected and its a rare case that any are ever perfect. 
As each subsequent one fails the others carry the load until there is total failure.



> 
> 
> And, if you run your 100 watt radio at 80 watts, are you "saving" the radio or just your electric bill?

** I dont pay attention to my electric bill, if I cant afford it I should get rid of the 5 PC's and 3 TV's first.
To me it only means less stress and better IMD. The sweet point varies in a curve and 50W may be horrible but 80 is just about a given with a nominal 100W rig.

> Inquiring minds want to know.  Please.
> 73
> > 
> Charles Harpole
> k4vud at hotmail.com      
>


** Hope Ive answered a few questions before proceeding to Part Deux

Carl
KM1H



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