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Vacuum tube x-ray question

  1. Feb 26, 2016 #1
    I recently got an older tube amplifier , since ti was dirty from being left for a long time in an old shed i took the covers off and cleaned it , now im listening it also with the covers off , everything works etc.
    Now I wonder do tubes at lower voltages and lower frequencies such as in the audio stuff emit some sort of x ray or other frequency radiation or are they completely safe in these applications?

    a crt for example does emit some radiation but it's plate voltage I assume is also much higher and so is the operating frequency , so does it depend on both the frequency and plate voltage or more on voltage?
  2. jcsd
  3. Feb 26, 2016 #2


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    yes it is an energy related thing
    I'm not sure of the specifics on what sort of voltages are required to produce x-rays
    the 20 - 25kV of the old TV CRT's were capable of some x-ray production

    will have to do some googling

  4. Feb 26, 2016 #3


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  5. Feb 26, 2016 #4


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    The B+ for audio amps was usually only a few hundred volts with 500vdc being a typical max plate voltage for beam power audio tubes. The danger from any type of radiation is pretty close to zero in the audio range
    Completely safe is asking a lot, I wouldn't eat them. :biggrin:
  6. Feb 26, 2016 #5


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    An electron accelerated across 500 volts will gain an energy of 500 electron volts.
    The photon emitted when it hits the anode (target) will obey the Planck–Einstein relation, E = h * u.

    In convenient units, that relation becomes; photon wavelength (nm) = 1239.8 / voltage.

    For 500 volt that gives a wavelength of 2.5 nm which is beyond ultraviolet and is classed as a soft x-ray.
    If soft x-rays manage to pass through the glass envelope of the vacuum tube they will be rapidly absorbed by the air.
    They are not a danger.
  7. Feb 26, 2016 #6
    Ok so I see that it doesn't matter so much on frequency as on the plate voltage , because above certain threshold plate voltage the electrons hitting the anode will have energy high enough to produce x rays.but then we are talking in terms of several Kv and higher I assume.

    It's an old soviet made amp called Priboj 50. the dual beam output tube plate voltage is above 400v.If I'm correct about 420v.So unless I get a shock from that , otherwise it should be harmless.

    P.S. Just while on the topic , one channel sounds great the other one has minor but audible distortion and also when observing the speaker cone visually on lower frequency drum kicks it moves more inward than outward and the sound definitely feels wrong.
    I got a capacitance meter and will measure the electrolytics if they will be ok then i will swap the tubes from the good channel to see maybe one of them is slowly dying, i haven't had that much experience with tubes so will have to learn some new " old" tricks.
  8. Feb 26, 2016 #7


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    The envelope of a drum beat may be a low frequency event, but the fundamental and harmonics that are enclosed by that envelope will be broad band since the impact of the drum stick is a step function excitation. Vacuum tube amplifiers can have a very good low frequency response because they have high grid impedance. You may be seeing a faithful DC reproduction of the drum stick impact.

    Alternatively, if it is a push-pull output stage, a fault may cause circuit asymmetry that will generate even harmonic distortion.
  9. Feb 27, 2016 #8
    that is a push pull output stage yes.But I doubt that's a what you called it "faithful" reproduction , it really sounds like distortion , and the other channel sounds much better more like normal I would say.Don't want to brag about it but I do have some decent listening skills as well as history.
    it's almoust like that channel is just either pushing or pulling but not both

    So I measured the caps , they are all up to spec even though since they are high voltage there may be something going on under voltage that I can't " see" So I will replace them after a while.
    the tubes themselves are good because I did the old tube trick, simply swapped the tubes one by one from the working channel into the one that distorts and nothing changed.
    the thing that helps is that the channels are both identical , and quite frankly except for the tubes and a few caps there arent much other parts on there to blame for anything.
  10. Feb 27, 2016 #9


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    Measure differences in screen grid voltages between channels and sides.
    Old carbon resistors eventually go open-circuit in HV circuits. Most new resistors are only rated to 100 volts. If you replace resistors use series chains for HV and power.

    Cross over the two signal paths between the channels to (bracket) where the problem changes sides. Coupling capacitors are convenient points of access.

    Swap components between channels and/or between push-pull sides to identify the faulty component.
  11. Feb 28, 2016 #10
    I repaired the priboj yesterday , the fault was quite simple , each channel has two output tetrodes with big anode pins on top of the tubes envelope , eahc tube has a 240ohm 1w resistor attached to it's driving grid or accelerating grid can't remember , one of those resistors was changed by someone some time ago , I could tell because the original resistor probably was dead due to heat which left a black spot on the pcb although the resistor on the board looked new and fresh but when i pulled it with my two fingers it came out like a knife from hot butter - easily.
    I resoldered the resistor and now it's tight in place with good contact and he channel works and the amp sounds very nice overall.

    although after some two hours of listening I started to get some weird popping and crackling noises in that same channel which progressed into louder bass type booms after a while , at first i thought that maybe one of the tubes are going bad due to heat, after all the soviet 6P3C-1 tube was a civilian tube and wasn't that stable as it's military counterpart the GU19 which I plan to get as an analog and simple replace , but after i turned the amp off and let it cool for a while I then plugged the amp back in and now that very channel had a mains 50hz type HUM which was stable and loud, almost like someone is driving the output transformer with AC.
    Now heres my analysis , the tubes were cold and when i plugged in the power cord the hum in that channel was immediate right at the moment the cord touched the socket , so the hum is not comming through the signal path because these tubes take atleast 10 secs to warm up before any music or input signal can be heard.
    since there are not many parts to go wrong in such an amp , I'm almost 100% sure that the old high voltage rated electrolytics have gone dead on that amp side of the psu and now due to the circuit architecture the mains ripple is literally driving the output transformer since the rectified high voltage goes right through the transformer primary center tap to both ends were the tetrodes are located.
    also the caps may fall short circuit because the mains transformer is taking excessive current i can tell because everytime i switch it on with this hum the mains 5A fuse blows after some secs.

    what do you think?

    also i assume i should make another thread were i would like to discuss more in depth the tube analogs that could be used and the further evolution of this tube thing that i'm into now.

    also P.S. Baluncore , even though we had some rough times in my smps thread im still happy to see you around and right now im in the final stages of finishing the same smps just on a new pcb were everything is put together with attention to detail and not using point to point wire connections like in my previous experiment case.which also worked very nicely given the rough construction it had.
    I am now trying to choose which filter caps should be better for my secondary output.Maybe if I gave some links to the caps i have in mind you could help me choose the better option for the money ?
  12. Feb 28, 2016 #11


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    Electrolytics are always prime suspects. Select high temperature rated electrolytics, similar to SMPS mains supply reservoir caps.

    Also, thermal cycling cracks solder joints over time and they probably all need to be redone after 20 years.
  13. Feb 28, 2016 #12
    it's just that both for the smps and the tube amp the electrolytics to choose from are so many , some have better ESR but lower rated lifetime some vice versa also the brands differ but basically i have choosen between panasonic, nichinon, rubycon, cornell dubilier etc I mean in the end does it matter that much to search out specific series etc or simply ake a good overall cap and put it in and use it ?

    for the smps secondaries i'm looking towards some 200v/1000uF, I need 4 pieces for one channel so that means 8 in total as I have two independent transformers and rectifiers.
  14. Feb 28, 2016 #13

    jim hardy

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  15. Feb 28, 2016 #14
    If the electrolytics are more than ten years old change all of them. They dry out and can cause big issues. They will work sometimes even if very old. I cranked up a 1934 RCA Radio that had the original caps and the radio worked...not very well but it worked. Don't try this at home as they say...lol Good way to start a fire or worse.

    The hum you get as soon as you turn on the power is most likely AC leakage from the filter caps. Old transformers will hum sometimes. Crackings and pops are almost always tubes, well if the speakers are working proper. Sometimes not only from a bad tube but from a bad tube socket which is arcing. Look for black spider web looking stuff on the tube sockets. Check the speaker with a ohm meter and if it is a 8 ohm speaker the read should be somewhere in the ballpark of 7.3 ohms.Move the speaker cone with you hand to see if it is sticking and moves smoothly. Careful and don't put to much pressure in the speaker cone as old speaker cones are dry and easy to break. Be sure the speaker leads are hooked up correctly. There is + sign and a - sign on most speakers pin connectors.

    The only real danger with tubes is the high voltage they run at. If you come in contact with 500 volts you may get lucky and it may not kill you but you will unlikely make the same mistake again. I have seen a few old know it all TV guys get knocked on their butt, and one die while I was 10 foot away. Many cool looking tube amps have the tubes exposed so you can see how cool they look..lol
    They get hot as hell and will burn you good and proper if you touch then...well the output tubes for sure.

    Not all tubes are safe to be around. Microwave for example will/ can burn you up. The only good news about that is people will get out of your way if you are on fire...lol...well at least that what Richard Prior said...lol

    Tubes in general do strange things as far a noise is concerned even if they test good. I don't understand for sure why that is true but it is.

    The best filter caps for "my" money are F&T from Germany, at least that is what I like and are easy to find. Yes I use Sprague Atom...good caps but cost a bit more than F&T. And yes there are all kind of high dollar filter caps to be had but the bottom line is if the ripple is gone what more do you want.

    While I am on my soap box I will comment about tubes. For about 99.99% of the people in the world in a blind test they can not pick out a NOS RCA 7025 tube from a new JJ 12AX7S, well... that is if both being the same type and good tubes. There is more smoke and mirror BS being kicked around about vacuum tubes than anything I have ever seen. I own a couple of RCA 7025 NOS tubes and I can assure you they are not worth the 150 bucks or so that I could sell them for as far as sound goes. Cool stuff to say you own, yes, much better sounding, not so much.

    Have fun with the amp..


    Last edited: Feb 28, 2016
  16. Feb 28, 2016 #15

    jim hardy

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    If you poke around with search engines you can find the 1960 or 1964 RCA Tube Manual online in pdf .
    First few chapters of 1960 edition were my text in high school electronics, and i still recommend it.

    Print those and the pages for whatever tubes ou have.

    Learn what are typical operating voltages . A lot of troubls are old carbon resistors drifted high and those you find by voltage measurements. Your open screen resistor would have showed up as low screen voltage which causes low and distorted signal. Easy to mistake for a bad tube.
  17. Feb 28, 2016 #16
    Hi Jim,

    I downloaded the RCA Tube Manual and someone gave me a really old beat up hard copy plus a sub book. I have looked at every data sheet I can get my hands on from as many MFG as I can find..

    How do I go about finding the true dissipation in watts for a 6l6GC running in my amp?

    I have seen data sheets that say everything from 25 watts to 40 watts.

    So...assuming a plate voltage of 450V just as an example it goes like this. 25 / 450 =0.05555555555555556 X .7 = 0.0388888888888889 or APX 39 ma plate current at 70% of max. For 40 watts at 450V the plate current at 70% of max would be 62 ma

    50% could be considered cool 60% perhaps normal 70% Hot and 80% really pushing the limit with for sure shorten tube life.

    For my own amp I don't care if I have to replace tubes every three months if I get the sound I want but I assume not many people would be willing to spend that money for tone.

    With these Fender Hot Rod Amps I been rebuilding, TP30 at 60 mV per Fender schematic should give the proper plate current. I have measured it but I forget just what is was, and again without knowing the exact dissipation in watts the number does not mean much.



    Perhaps I am missing something??
  18. Feb 28, 2016 #17
    This site has a ton of free books and tube manuals related to vacuum tubes.
    Some are really math heavy and some are easy to understand.
  19. Feb 28, 2016 #18

    jim hardy

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    What's wrong with measured (plate volts X plate amps ) ?
  20. Feb 28, 2016 #19
    So I run the amp into a say 250 watt 8 ohm resistor dummy load and turn it all the way up and measure the plate current?? I also assume some amount of drop in plate voltage at full volume...well I guess...never measured the plate voltage at full volume before.

    I guess my real question is the data sheet stated dissipation in watts is only correct if everything in the tube is up to specs.....correct???
    Last edited: Feb 28, 2016
  21. Feb 28, 2016 #20
    Sorry Jim....I have to get some sleep...I am really a bit worn out tonight and most likely not thinking well too begin with. Plus I have to go to Marathon in the morning to install a radar on a boat.

    I will look at your answer tomorrow.ZZZZZZZZZZZZZZZZZZZ


  22. Feb 28, 2016 #21


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    That sure sounds like the output transformer primary is shorted to ground. Remove the B+ wiring to it and use an ohmmeter to Gnd.
  23. Feb 29, 2016 #22

    jim hardy

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    i think the datasheet says 30 watts is maximum , presumably continuous.
    Have you noticed the plates glowing red? If not you're probably ok.
  24. Feb 29, 2016 #23
    today is a sad day for me , my cat got lost yesterday while falling out the window , since its only second floor and below is a garden he was ok after the fall which i saw but then he just went away, i went after him but couldn't find him.

    AS for the Priboi tube amp I think he output transformer of that channel just went bad.After all it looks like it's not the electrolytics , I measured them all and they show good capacitance , but then i measured the output transformer simply with an ohm meter and the primary winding which goes from the +420v line to the tube anodes shows a very low resistance to the secondary winding which is the output winding, about 20 Ohms.
    that is definately a short given that the primary runs at above 400v.
    i'll disconnect the transformer and see what can be done.

    P.S. Just wanted to say that I'm officially lost, i now disconnected both output t5ransformers and one still measures those 20ohms to speaker output from primary but the other one also measures from primary to speaker out only about 80 Kohms , but they shouldn't measure at all I think, or i'm I just dazed and confused
  25. Feb 29, 2016 #24

    jim hardy

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    Have they been in a moist place ?
    The 20 ohm one is bad you might as well take off its covers and see if you can find the short, it's probably burnt there. Maybe you can separate the shorted wires, i fixed my wife's dishwasher motor that way. Sometimes baking it in the oven around 120F overnight to dry it out then dipping in varnish will fix one. I use plain woodworker's varnish because i usually have a gallon can of it around and that's convenient for dipping. Oil based enamel works too, i've fixed fan motors from a spray can..

    The 80K one i'd definitely bake before applying voltage again.
  26. Feb 29, 2016 #25
    Hi Salvador,

    Very sorry to hear about your cat. I am working on a boat today but when I return tonight I will help you with the amp. I get "officially lost" about once a day!!! So don't let it get you down. Here is a site with the schematic...I think...I did not try to download it.


    Any way I will find one when I get back home today. If you have skype send me a PM and perhaps being able to look at things will make it a bit less of a issue and I can be better help.


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