I thought you meant adding another capacitor and switching between it and the original.
I doubt the capacitor resonates because years ago someone mentioned a line of microwaves being identical except for power rating, cavity size, and the size of the high voltage capacitor, which ranged from about 0.6uF to 1.0uF.
They're not as expensive as some people implied.
Yes, that's what I had in mind. All you need is a switch that can handle that much volts safely...and maybe some means to make sure that switching a charged cap doesn't explode the diode or the switch.
Some months ago, I had to replace the diode in my old one. Hooked a resistor to a couple of clip leads and hooked them to the cap. I never did find any of the resistor pieces. May have gotten swept up when I cleaned up the puddle I made. ;-)
It's all relative. Paid $80 for mine. I did visit a few stores and the inverter ones seemed to be almost 3x that and they don't go on sale often. Knowing what I know now, I might have gone with the inverter. Spilt milk now...
In general, with indirectly heated types, no, it may not be ok. It has been said that it can lead to cathode poisoning and sometimes cathode stripping. In the good old days, HT standby switches often used to have a resistor across them to keep a small bleed current running through the tubes, however, I must admit that most modern amps that I see have no such arrangement, so perhaps modern tubes are better at standing up to this sort of abuse, or else it has become just a forgotten technique? Or maybe, it was one of those 'myth' things that some manufacturers believed in, but wasn't actually the case ? Eimac have published booklets about applying voltages in the correct order and with the correct delays, to their RF Power transmitting tubes, so I guess that there must be something in it, somewhere. Are magnetrons indirectly heated ? I had a feeling that I had seen somewhere that they weren't, and that the modulation introduced by the AC on the filament, was insignificant compared to the output power at the frequency of interest, so was ignored.
Arfa
You exceeded the maximum voltage rating of that resistor.
On 1/20/2011 4:44 AM Michael A. Terrell spake thus:
Gee, ya think?
Another incredibly useful 1-line post from the shoot-from-the-hip master ...
And yet another snide, useless post from you.
Not sure about the ones used in uwave ovens, but with some magnetrons, once they get going, the filament power has to be reduced (sometimes to zero) because the filament is heated by the impact of accelerated electrons banging into it.
Isaac
The filament is a minor current, the main HV voltage is the issue. To cycle off the transformer rapidly with a relay would likely result in a relay lifetime of a few weeks at best.
I don't believe there ever was an issue with the filaments.
I agree, a triac could possibly be used, though you are talking about swtiching as much as 1500 watts, inductive (because of the transformer). Might be too expensive to find a triac that will take that kind of abuse and survive!
Neat trick since the filament emitts teh electrons...
Actually, I think that it is quite a useful comment, as in my experience in the electronics service business, many techs - particularly modern ones - have no understanding that a resistor has a maximum working voltage rating, and have even laughed at the very idea when I have suggested it to them, until I have shown them in a component catalogue. I'm willing to bet that prior to this exchange, there were more than a few reading what Michael said, and doing just that ...
Arfa
On 1/21/2011 6:02 PM Arfa Daily spake thus:
Well, I have to admit that it (maximum resistor voltage ratings) doesn't often occur to me, simply because I don't work with HV. But the person Mr. Terrell responded to above was certainly aware of why his resistor got blown to smithereens. Hence my annoyance.
Absolutely they do, but I suspect he also exceeded the power rating of that resistor by a tiny bit as well...
You can exceed the voltage rating, without exceeding the power rating. Pulse applications can be well below the rated wattage, yet arc over internally.
Well yeah. That too, I agree ! :-)
Arfa
I see that, but some other less experienced people reading the thread, may well have learnt something from the comment. Still, all sorted now, and everyone's up to speed on resistor voltage ratings !
Arfa
On 1/22/2011 7:56 AM Arfa Daily spake thus:
Well, point taken.
While we're on the subject, where would one get HV resistors, anyhow? Any US suppliers?
I haven't seen any *very* high voltage resistors for some time now in catalogues, although I expect if you went looking for them, they are still around from specialist manufacturers. Most that are readily available here in the UK are rated at up to 400v for some, rather less for others. If I am replacing a resistor that is going to have a lot of volts across it - for instance a bleed or voltage sharing resistor across a high voltage cap in a tube amplifier - I tend to make one up from two half-value resistors in series. Likewise, when replacing some anode load resistors in 12AX7 or whatever stages, as these are often 100k connected to a rail of up to 400v, with the actual anode voltage at the other end, two or even three hundred volts lower than that. Two 47k resistors rated at 250v each, and connected in series, ensure that the replacement is up to the job, long term.
Arfa
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