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5U4 Rectifier tube

Discussion in 'General Electronics Discussion' started by Edwin Fitzpatrick, Apr 10, 2013.

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  1. Edwin Fitzpatrick

    Edwin Fitzpatrick

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    Dec 26, 2012
    In a tube Guitar amplifier the 5U4 rectifier tube warms up fast before the 6L6 's draw plate current this lets the DC plate supply voltage go higher than the filter cap's voltage .A bleeder resistor will work but will waste a lot of power. I could put a time/contact in the center tap of the high voltage winding. My friend said to put a Thermistor in series with the 5U4 Rectifier filament. This will slow down the warm up of the rectifier and let the 6L6's warm up first. The question is, will this be bad for the 5U4 rectifier tube ?
    Fitz
     
  2. brevor

    brevor

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    Apr 9, 2013
    I think it would be OK, It may also relieve some of the stress on the 5U4 filament at turn on.
     
  3. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    Instead of a bleeder resistor you could use a PTC. On startup, the PTC is cold, showing low resistance. The current through the PTC heats it up, increasing the resistance, lowering power consumption. It may be a bit fiddly to find the right type of PTC, though.
    This was used in tube TVs to demagnetize the tube at turn-on.
     
  4. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    Putting a thermistor in the rectifier filament supply may be workable if you can find a suitable component. I don't think it would damage the rectifier, but make sure the filament voltage is not much lower than it should be when the thermistor has warmed up fully.

    Some guitar amps handle this problem by switching the HT with a second switch. You would wait long enough for all the valves to reach operating temperature before you turn on the HT switch. This also allows the user to kill the amp without turning off the heaters, so it can be switched on instantly. I believe the switch was connected after the HT smoothing capacitors; in your case the problem is too much voltage across the capacitors so you'll need to connect it between the rectifier cathode and the capacitors.

    Or you could just use smoothing capacitors with a higher voltage rating.

    Electrolytics designed for HT smoothing often had a second, higher voltage rating (as well as their continuous voltage rating) for short term overvoltage, for this exact reason.
     
  5. Edwin Fitzpatrick

    Edwin Fitzpatrick

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    Dec 26, 2012
    5U4 Rectifier

    Hi Kris blue NZ






    Hi Kris Blue NZ,
    I ran this test on a good tube tester. First I ran the tube (5u4) at normal conditions and checked the warm up time as indicated by the tester's meter. Next I put in the thermistor and noted the warm up time, Much more time. I never measured the AC filament voltage. The thermistor go too warm to touch. After the tube reached full warm time, by the tester's meter, the thermistor was only warm to touch. I got a bunch of these thermistors and could put two in parallel. I will measure the AC filament voltage. This may take 50 0r 60 seconds to reach the normal tester's reading.
    I built an adapter socket to test this on the tube tester.
    Fitz
     
  6. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    You need to make sure that once the thermistor has warmed up, the filament voltage is not significantly lower than it should be. In other words, make sure that the thermistor isn't dropping much voltage. That is why it's very important to get the right thermistor.

    Actually I suspect that a suitable thermistor may not exist. You would need an NTC (negative temperature coefficient of resistance) thermistor which starts off at room temperature with its resistance being quite high, but low enough that the 5VAC filament voltage will make it start to heat up. Then it needs to heat up slowly, and when it reaches its final temperature, it needs to have a low resistance so that very little voltage is dropped across it. This means that it would dissipate relatively little power, so there wouldn't be much energy available to keep it hot.

    So a thermistor that meets those criteria may not exist. There might be a different type of device you could use, but I don't know of any. It's kind of like a reverse polyswitch!

    If you don't like the manual switch idea, you could make a delay circuit that operates a relay that switches the rectifier's filament supply. It could be powered from the filament supply for the rest of the amp.
     
  7. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    May 8, 2012
    What is your plate supply voltage after all tubes have warmed up?

    Chris

    Edit: Also, what's the voltage rating of the caps?
     
    Last edited: Apr 11, 2013
  8. john monks

    john monks

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    Mar 9, 2012
    I am operating from memory but I was trained as a broadcast engineer never to turn on the B+ supply (the high voltage) to a broadcast transmitter without having the tube filaments reach near full temperature. And this could take some minutes. This is because to much space charge takes place between the cathode and the surrounding space causing the emissive surface to physically strip away from the cathode leaving the tube with lower emission. If this relates to the 5U4 I am not sure. It's hard for me to back this up because I never tried it but I would be leery about running the filament lower than 5 volts. I have destroyed 5Y3 rectifiers and discovered that the filament was destroyed but this may be because I ran to much current through it. Maybe this link will help.
    http://www.w8ji.com/filament_voltage_life.htm
     
    Last edited: Apr 11, 2013
  9. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    The reason I requested this information is because I think you're approach to this problem is a bit back door. You said you don't want to use a bleeder because it's wasteful. Well, I agree but there are alternatives. One alternative would be a HV Zener in series with the bleeder. If you select a Zener with a sharp knee it will effectively switch the bleeder out of the circuit when the amplifier tubes are warm enough to load the B+ on there own. I don't know what the highest voltage Zener manufactured is but my Tina library lists them up to 200V, so a series connected stack will probably be required.

    Chris
     
  10. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    Good idea Chris.

    I suspect you would need several zeners in series anyway, to handle the required power dissipation.
     
  11. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    Thanks Kris. On the up side if it's designed right the Zeners and Bleeder Resistor will only be dissipating power for less than 30 seconds.

    I guess another alternative could be a 555 driving a relay coil. The contacts would close and supply power to the caps & B+ line after timeout. If Edwin wants to go this rout he could probably supply the 555 Vcc from a rectified and filtered filament line.

    Chris
     
  12. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    Yes, true.
    Yes, I suggested that earlier in the thread (post #6). Actually I suggested switching the rectifier's filament supply, but your suggestion is probably better.
     
  13. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

    4,960
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    May 8, 2012
    It's not surprising that I missed that. If not for my wife Marilynne I wouldn't know what day of the week it is. :confused: I guess it's not important any longer. ;)

    Chris
     
  14. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    LOL

    In another thread, I asked the OP what his AC mains voltage was... and it was stated in his first post, and in the thread subject!
     
  15. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    May 8, 2012
    OMG! This may be contagious? :D

    Chris
     
  16. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    Nov 28, 2011
    Oh no! I think it is! My friends are suffering from it too!

    Or it might be that we're all just getting old... :)
     
  17. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

    4,960
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    May 8, 2012
    Well in that case the OP better hurry back here. After all time may be running out! :p

    Chris
     
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