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Onelasttime
Junior Member
Username: Onelasttime
Post Number: 29
Registered: 8-2011
| | Posted on Wednesday, August 17, 2011 - 1:39 am: | 
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Ok I hear all kinds of Ham's talking about using variac for testing their finished amps. My question is this. The power control board I am going to build is all digital. It will take the output from what ever transformer cap and diode set up I use and will feed the tube the power it needs and keep track of bias and plate voltage etc based on load. So with a system like this would it not be better to just toss a 50 ohm to 100ohm resistor in the line going out to each tube connection to cut the power in 1/2 roughly and feed the amp regular un-molested power???? I was thinking that the control board would not work with the output cut in half from the transformer. Am I correct in thinking that a variac would only work on a very crude amp that does not have a digital control board??? I am just thinking that things like relays, digital circuits etc.... usual will not work at all with less then specified voltage and current. Most analog stuff will function just at greatly reduced output.
I find that most of these Amateur guys leave out important details when giving out advice or commenting on how they do something.
The above was about keeping the power at half the required out put and slowling checking for arcing inside the tube as you slowly bring the power up with a variac. It was again mentioned with regard to put NOS 30+ year old tubes back into service by slowly heating them up over a 5 day period starting with 50% of rated power and all grids grounded and using a variac to bring the power up in increments over a 5 day period to drive off any gas that has leached in from the ceramics over time. I was thinking that this would not work in a modern design and that resistors inline would work better?
Opinions????? I have never used a variac I have turned low power as in under 50 volt power supplies down then brought power up for testing and I have used resistors inline to bring down power for testing so never really used a variac so while I understand what they do I have questions about how well relays and digital circuits would work under powered????? |
Funtimebob
Intermediate Member
Username: Funtimebob
Post Number: 293
Registered: 5-2002
| | Posted on Wednesday, August 17, 2011 - 10:26 am: |
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Hams usually use a variac to limit voltage going into just about any old tube gear. especially if it has been sitting around for a while in order to re form the capacitors in the power supplies and to check for bad parts before full power is applied......
with tubes, over the years tiny amounts of air can enter through the microscopic spaces around the pins etc.
heater voltage is applied to activate the getter material inside the tube (grey looking stuff) used to absorb the tiny amount of air trapped in during the manufacturing process.....
Dyslexics of the world UNTIE!
73 De Bubba Honorary O.F.
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Tech833
Moderator
Username: Tech833
Post Number: 2031
Registered: 8-2002
| | Posted on Wednesday, August 17, 2011 - 11:59 am: |
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Using a variac for tube type equipment is helpful. However, if you have gear with digital circuitry, it will not be much help.
The idea behind the variac is to bring things up slowly so that capacitors have a chance to re-form so that as high voltage is slowly applied, it won't punch though the dilectric and cause failure.
Your radio 'Mythbuster' since 1998
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