Wears the tube out prematurely.
I had read about plate voltage leading to stripping/poisoning of the cathode only applying to voltages higher than about 600V. Not the case with my phono stage. All tubes had 280V on them without heat. After about 6 months, the tubes were "worn out". Where I expected 2 or 3 volts (cathode resistor), I got only 0.8V.
Maybe there is something to keeping HV off the tubes unless they are hot or warming after all.
I had read about plate voltage leading to stripping/poisoning of the cathode only applying to voltages higher than about 600V. Not the case with my phono stage. All tubes had 280V on them without heat. After about 6 months, the tubes were "worn out". Where I expected 2 or 3 volts (cathode resistor), I got only 0.8V.
Maybe there is something to keeping HV off the tubes unless they are hot or warming after all.
I've seen some tube sheets give a resistance at which point the heaters have warmed up - then HT.
I don't think applying B+ voltage to cold tubes is the problem.
Else classic designs like the well known MC275, MC240, MC225 will suffer what you experienced, and it is not the case at all. Tubes last forever in these amps.
Maybe the tubes were unwell to begin with, or they are run too hard, unlikely on the phono preamp.
Else classic designs like the well known MC275, MC240, MC225 will suffer what you experienced, and it is not the case at all. Tubes last forever in these amps.
Maybe the tubes were unwell to begin with, or they are run too hard, unlikely on the phono preamp.
Strange, in Philips datasheets you usually see a higher design centre rating for the anode voltage with cold cathode than with hot cathode. Then again, maybe the implicit assumption is that this will only happen during the first 20 seconds after power on anyway.
I wrote an article about that for AX:
The Internal Life of Vacuum Tubes | audioXpress
One conclusion:
• High voltage should preferably be switched on only after the cathode has reached operating temperature, and this is especially important when using solid-state rectifiers.
Jan
The Internal Life of Vacuum Tubes | audioXpress
One conclusion:
• High voltage should preferably be switched on only after the cathode has reached operating temperature, and this is especially important when using solid-state rectifiers.
Jan
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I did until I saw the wear on the tubes that I didn't expect. I added a relay so now heater and HV are switched together.
I did the same thing back when I was a kid. I learned that turning the amp off by switching only the heater transformer will kill the tubes. A well worn 6L6GA may even spark out when doing this. Makes an evil sound in the speakers too.
I don't know of any of those amps that run HV all the time without heaters. Not talking about just while the tubes are warming up.
Really wondering why is the HV on all the time when literary it could be switched by the same single switch that does the filaments...dunno seems not logical to me who would design such a deadly amp.
It takes time for the heaters to heat up. During that time there is anode voltage with no current.
Also, because there is no current, anode voltage can be quite a lot higher than in operation, stressing both the tubes and the electrolytics in the power supply. As I explained in the linked article.
Jan
Also, because there is no current, anode voltage can be quite a lot higher than in operation, stressing both the tubes and the electrolytics in the power supply. As I explained in the linked article.
Jan
it is a common occurrence, when you flick the power switch on a tube amp, the filaments have not heated up, the psu B+ builds up, goes as high as it can be, then as the filaments warm up, that B+ starts going down to operating levels....
this does not harm the circuits....of course, if you want, you can soft start the B+ with a high value resistor and then short it out after a while...
this does not harm the circuits....of course, if you want, you can soft start the B+ with a high value resistor and then short it out after a while...
I remain unconvinced by this. It is well proven that preheating a rectifier tube is more likely to cause it to fail, and it is not clear to me why receiving valves would be any different in principle. The bulk of contemporary literature from the tube age also does not seem to support your argument, and I trust that more than I trust hearsay from bloggers and internet forum users who's main source seems to be Tomer's book, which is not a serious academic resource.
Sure, there are situations where preheating can be a band-aid to fix an unlucky set of circumstances in an amplifier circuit, such as DC-coupled valves, unfortunate coupling time constants, or underrated PSU caps. But those are system-based reasons; the tubes themselves are not inherently harmed by allowing heater and HT to turn on together.
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The phono stage was switched by lifting a ground on a relay using the source selector switch. I don't know why you think that makes an amp "deadly" since you can't get a shock even without the tubes in it while it's on... 🙄
Do you turn off the breakers when you have an unused outlet?
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This is true, and fine in practice since it happens for a few seconds. The HV was left on the plates at all times with cold heaters - As I posted before - I had heard about cathode stripping etc but most stuff I read says it only applied to 600+ volts. Now we know better 🙂
I knew something wasn't right when I had 700mV across 240R through 6N1P!
The worries and discussions about tubes, their heaters, cathode stripping, delayed B+ and such seem to go on endlessly in many forums across the internet.
I've got a 1963 RCA Victor console stereo, and it contains 18 tubes on two chassis.
I'm sure it was well-used since it was manufactured, judging by the thick carpet of dust on the chassis, but I aquired it in 2009 for a cheap ($30) price.
When I got into overhauling the chassis, I naturally tested all those (original RCA) tubes, which had Date Codes of 1962 on all of them.
And guess what?....... only one tube tested weak, a 12AX7, used in the FM multiplex circuit.
Even the original 6BQ5's, all 4 of them in the amp chassis, tested around 75% good.
The 5AS4A rectifier which powers both tuner and amp chassis, tested fine too.
I'm still using it as a matter of fact.
This console doesn't have any of that "soft start" stuff.... manufacturers didn't employ such things back in those days.
And I've restored and worked on countless hundreds of consoles and radios dating back as far as 1929, and rarely came across a dead tube.
Yes, tubes can and do "go bad" sometimes, but the fact is that most of the time it's because of the "under the chassis" components that are at fault, causing the tube to operate outside of its design parameters.
In rare cases, tubes fail due to impurities inside them from manufacture, along with gassy tubes.
The above is all "real world" facts that I've experienced over the years, long before the internet fertilized the growing worries and paranoia about tubes.
I've got a 1963 RCA Victor console stereo, and it contains 18 tubes on two chassis.
I'm sure it was well-used since it was manufactured, judging by the thick carpet of dust on the chassis, but I aquired it in 2009 for a cheap ($30) price.
When I got into overhauling the chassis, I naturally tested all those (original RCA) tubes, which had Date Codes of 1962 on all of them.
And guess what?....... only one tube tested weak, a 12AX7, used in the FM multiplex circuit.
Even the original 6BQ5's, all 4 of them in the amp chassis, tested around 75% good.
The 5AS4A rectifier which powers both tuner and amp chassis, tested fine too.
I'm still using it as a matter of fact.
This console doesn't have any of that "soft start" stuff.... manufacturers didn't employ such things back in those days.
And I've restored and worked on countless hundreds of consoles and radios dating back as far as 1929, and rarely came across a dead tube.
Yes, tubes can and do "go bad" sometimes, but the fact is that most of the time it's because of the "under the chassis" components that are at fault, causing the tube to operate outside of its design parameters.
In rare cases, tubes fail due to impurities inside them from manufacture, along with gassy tubes.
The above is all "real world" facts that I've experienced over the years, long before the internet fertilized the growing worries and paranoia about tubes.
Yes, but the B+ wasn't on without heat for years, was it? That's what I'm talking about in this thread. There's no issue turning on the B+ and heaters at the same time and waiting for the warm up, but leaving 300V on the plate all the time on a cold tube will damage the cathode on Soviet tubes and possible others.
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I'm skeptical. 300V is a low voltage, can you propose a mechanism which would cause the damage? It takes millions of volts-per-meter to tear the cathode material off the cathode, and I can't see what chemical processes would take place while cold? It's more likely your tubes wore out for some other reason.
What else would cause it? Tubes don't wear out by themselves, right? 🙂
If you're bored, build a gain stage, test it, disconnect the heater, leave it for months, and test it again.
The outcome is basically the same as running the heater with no current flowing for a long time - that "cathode interface" kind of thing. What I haven't tried (but I can since I didn't throw the tubes out yet) is to give the heaters 12V for a while to "burn" off the cathode coating to see if I can get emission back up.
If you're bored, build a gain stage, test it, disconnect the heater, leave it for months, and test it again.
The outcome is basically the same as running the heater with no current flowing for a long time - that "cathode interface" kind of thing. What I haven't tried (but I can since I didn't throw the tubes out yet) is to give the heaters 12V for a while to "burn" off the cathode coating to see if I can get emission back up.