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I am interested in a Variac, if it does indeed prolong power amp tubes. Does it affect the sound, as I guess you can set it to full voltage (no effect) when warmed up.
Does it increase hum, and does it get hot after setting full voltage? I am wondering if you can get a Variac that literally contacts the mains on full setting, so you would only use the action
for say 5 minutes till the amps warm up.
Follow Ups:
Hi,The use of a variac to prolong tube liffe is a very good idea.
To keep the variac running cold, oversize the variac. If it can handle x5 the current required by the amp, then it will run ice cold, and wil not affect the sound. If the variac, or an isolation xformer is less than x5 oversized, then it will compress dynamics to a certain degree.I have a 3A variac, that I was hooking up to the stereo 70 - which consumes 1.5A the way it was biased. That's all right with the variac; in fact I run the old Tech scope from the variac, too, and that guzzles 2.5A, and that's fine with it, for hours on, too - and I live in Hawaii, no AC, so the ambient temperature is quite high in addition.
One benefit of the slow turn=on of the variac is the strain-free turn on of the electrlytic caps. with gradual turn-on (esp w ss diodes), the caps ccharge up in a mild manner, and their life will be prolonged significantly. Caps get old by stressing them out: they get cycled from no charged to full charge each time the amp is turned on. If this is a blast, they will suffer. If the amp is never turned off, the caps will not go bad for many times their life expectancy, unless there is a voltage surge. So, with variac turn-on they experience a limited reforming of the cap each time.
The power tubes also benefit. One reason why the power tubes die is the strain of the turn-on on the filaments. Cold flaments conduct many times the rated current. For many seconds, until the filament warms up, they are under terrific strain. This strain weakens the filament material, and any impurities in it will create hot spots, that will become worse and worse as the filament ages.
By turning the amp slowly on with the variac, you can avoid the inrush on the filaments. My variac has an amp meter on the front. When bringing up the st70, if I suddenly turn it to 30V from 0V, it shows a huge current draw of over 1A. That's the hungry cold filaments conducting. I heat up the st70 by montioring the amp meter. slowly turn it up from 0 volts, and when the amp meter wants to rocket up, I slow down. There will be one surge, that you can avoid with the variac when the filaments turn on, then a few seconds (or many seconds, depending on the rectifier) later the surge caused by the warmed up rectifier tube charging of the power supply caps.
The idea of using a variac is to prevent excessive current draw during these two events. This translates into approx 1 min of toying with the variac, no need to extend it longer.
And if you want to be super-gentle on the tubes, you can turn the amp off with the variac, in a matter of a few minutes. The idea for that is that the electrodes cool down gradually, which is not so stressful for them. Remember, the cathode has to heat up by 1000 degrees celsius when turned on (immense stress), and cool down by 1000 degrees c when the amp is turned off. If you studied metallurgy, you know that rapid heating or cooling of metals is the best way to break them. when the tube is brand new, the rapid cooling/heating acts to break the tube in, to anneal the cathode, but once it's broken it, operating at optimum emission, it will be weakened by each further stress cycle.
You can observe the inrush current on the cold filaments by eye, too: after turn-on, the tube lights up very bright, then the brightness slightly fades, and a few seconds later, when the B+ goes up, the tubes reach full brightness again. On some tubes this is very visible (1626), on others it's harder to see.
Good luck!
Janos
nt
all the best,
mrh
In general, a variac is used to initially bring up an amp that hasn't been used for a long period of time. It helps to re-rorm the caps and prevents dammage if there are shorted caps. I suggest that when bringing an amp up with a variac you should use a solid state subsitute in place of the tube rectifier so the voltage will build up slowly. A tube rectifier won't start conducting untill at least 50% of the voltage is present. Also when using a ss rectifier, don't go over 80% of the ac voltage. After 80% for a few hours, put the tube back in and start at 80% and then continue in 10% increments with an hour between each untill you hit the desired voltage.For everyday use I wouldn't reccommend it unless you have a very old amp that cant take the modern 120V AC. In this case you would need a volt meter so you can set the correct voltage (say 110V) to run the amp.
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