Hello,
I am a french student in EE, musician, and passionate about old stuff (well, in fact I'm also passionate in some new stuff, but that's not the point here). I've been working on tube amplifiers for not that long, in fact I'm still learning. EE studies no longer cover tube technology, so I have to find infos by myself. In this search, your particular website, Fun with tubes, has alleady been quite helpful. And before asking my question I would like to thank you for sharing your knowledge.
I would also like to apologize for the length of what I wrote just down there. I'm not asking something randomly, I have a precise problem, so I think it's important to give you all I have so far, in order to get an accurate answer.
What pushed me to email you is a question that troubles me and some people I work with. We searched a lot, we thought about it, we read, but we got nothing clear enough. We have hints, not really more.
The question we were trying to answer was the point of using a "Stand-By" switch on most of the tube amplifier for guitar. Usually, on guitar tube amplifiers, there is a "ON/OFF" switch which put line voltage on in the power transformer, powering the heaters, the rectifier (solid state or tube), and sometimes the grid bias. Then, there is a "Stand-By" switch which put on or off the high DC voltage after the rectifier.
A schematic being more helpful than a thousand words, you can find an example of this on this Fender Twin schematic (here, the ON/OFF switch is labelled "AC Switch") :
http://www.freeinfosociety.com/electron ... php?id=922
I don't know if you're familiar with the guitar world, but tubes are something "magical" for most of the guitarists. In two ways : the sound is wonderful, and most of the guitarists don't have any idea on how they work and why they sound wonderful. In this particular context, there is some kind of rituals all around tube amps, some rational, and some... quite mysterious. This Stand-By switch is at the heart of one of them. We're "supposed to" switch the ON switch first, then wait for some times, and then switch the Stand-By on. The main reason for this is believed to be : "let the tubes heat before applying high voltage".
But some amplifiers don't have a stand-by switch. The schematics you give on your site don't have any either. Hence the question, is this switch really usefull ??
So my problem is to discover if this is rational or not. If this standby switch is here just for some kind of "religious" belief, or for a good, logical, ie cartesian, reason.
Some guys answered their ways :
http://psg.com/~dlamkins/Articles/stand ... at=musings
This question emerged again as I was reading your "Restoring Dead Capacitors" page. Here is what made me react :
>Some people say...
>Some people say you can reform capacitors by connecting the equipment to a variable transformer
>and slowly bringing up the voltage. If the unit has solid state rectifiers this can be done. However,
>this can ruin a rectifier tube by damaging the oxide coating of its filament or cathode.
The "damage of the oxide coating" is the most recurrent reason we came across. Applying high voltage on the tubes when they are cold would damage them, by ruining their cathode, more precisely their oxide coating. This could be a correct explanation, but...
...but 2 questions remains :
1) If this is true, why do some (many, in fact) amplifiers not have this switch ?
2) If this is true, why do this happen ? What is the physical explanation of this damage ?
For the first point, some people claim that a rectifier tube induces a timing between heating and high DC voltage. So, the stand-by switch would be useful for solid-state rectifier amps only. But then again, some tube rectifier amps have it, some SS rectifier amps don't have it... Furthermore, the same problem would be deplaced on the rectifier itself : it is first cold when the high voltage is applied.
So there we are...
As you speak about this on your site, I hope you will be able to bring some light on this subject. Is this damage real ? Why would it occur only when we slowly raise the power ? And what do you think of the use of the stand-by switch ?
I thank you if you take the time to read all this, and to answer me. I'm an engineer, I like to understand what I'm doing, so I'm spending lots of time studying, and this particular point remains doubtful for me.
Cordialy,
