What happened to this tube?!

Discussion in 'Tube Gear' started by shoshin, May 14, 2018.

  1. Wntrmute2

    Wntrmute2 Junior Member

    Oh man how I appreciate the incredible help!
    The transformer feeds just the single 300B.
    It is a Hammond 166ms. Primary side is supposed to be 115 V and I typically have 121
    ( I'll get more measurements ).
    Your explanations as to the different calculations helps me understand what is going on. Much appreciated! Get some sleep.
  2. My pleasure.

    Given that the transformer is wound for 115V, the 121V from the wall may not directly correlate with the 5.5V at the filament: the calcs will have been off a bit. If you can, measure the filament at the same time you measure voltage from the wall.

    In the absence of measurements, and assuming the filament transformer primary is (accurately) wound for 115V, this should get you close enough:
    1. Secondary VA = 5.0*1.2 = 6.0VA
    2. Current on the primary: 6.0VA/115V = 0.05217A (i.e. 52mA)
    3. Check the calc: 0.05217*115v = 6.0VA. Good.
    So, 121V from the wall and a transformer wound for 115V means we should drop 6V while drawing 0.05217A, that is 6V/0.05217A = 115 Ohm. So, a 120 Ohm/5W resistor should get you there.

    While the calculations may look precise,the practice is less so! I'd pick up a few values from 100 to 200 Ohm and try them in circuit to see which one gets you closest most of the time. I tend to buy filament voltage trimming resistors as part of my standard build process these days.

    Edit: apparently, a low filament voltage is less likely to wear the tube out than high voltage and will be fine so long as the filament voltage comes up to correct voltage occasionally. So, if I had to drop too little versus too much, I'd drop too much, say targeting no less than 4.75V (better 4.8V) on the filament and I'd be sure that the filament came up to at least 5.0V when wall voltage is high.

    Last edited: Nov 5, 2018
  3. Wntrmute2

    Wntrmute2 Junior Member

    Thanks Shoshin. I'm going to place an order tonight with Mouser. Should I care about which type of resistors I buy? Metal Oxide, Wire Wound etc.? I need to narrow the field by something as thousands of choices come up even after selecting for at least 5 watt power handling and high temperature performance.
  4. I think Ohmite 5W wire wound should do the job. I use them myself.

  5. What he said - that is what I use too! I have been forced on occasion to use Mills MRA-5 (more expensive) to get some values. That said, these are not in the signal path and the white coffins etc. should be fine also.
  6. Wntrmute2

    Wntrmute2 Junior Member

    Thanks guys. On the search for them.
  7. Good luck! I'd go for 100, 120, 150, and maybe a 200 Ohm. Honestly though, a couple of values between 100 and 150 Ohm (or 200 Ohm) should get you there. If not, at least you will have more data points to work it out.

  8. Wntrmute2

    Wntrmute2 Junior Member

    So, this may be a weirdo idea. I see there are 2.5V 300Bs out there. Could I put a larger resistor, say 1.5Kohms and drop my transformer output to around 2.5V? I'd like to minimize the 390B hum. I know I can get 2.5 V transformers but would this be possible?
  9. Looking at the datasheet, the 166MS is rated for up to 3A on the secondary; a 2A3 draws 2.5A, so that is okay. But, you will be burning a lot of power with the dropping R. Very roughly working this, you'd want to drop circa 55V on the primary at a bit more than double the current draw we used for the 300B, say 0.11A = circa 500R and 5.5Watt. You'd be looking at series/parallel 12W resistors (this is what I'd do), or decently-mounted (to good heat conductor) 25W finned things, or possibly 50W finned things screwed the chassis... use thermal compound between the finned resistors and the mounting surface.

    It can be done, but would generate some heat - ventilate well. Maybe worth a try.

    You should also drop your hum pot from 100Ohms to 50Ohms.

    The JJ2A3-40 is a nice tube - get 'em from EuroTubes.
    Last edited: Nov 9, 2018
    Wntrmute2 likes this.
  10. you can just use half of the 166MS to get 2.5V 3A. Use one end and center tap.
    Wntrmute2 likes this.
  11. Wntrmute2

    Wntrmute2 Junior Member

    Thanks for those two suggestions. Since I need to buy another pair of Sophia mesh-plates, as I like the sound over the more robust seeming EH 300B, I may go ahead and buy their 2.5V 300B tubes. I was looking longingly at the center tap on the 166MS transformers. Do I need to do anything other than insulate the now other unused tap of the 5 volt transformer? I am going to lower the voltage on that transformer as I decide what to do as it seems a worthwhile exercise. The resistor assortment is on the way from Mouser. Thanks guys!
  12. upload_2018-11-9_20-28-7.png

    Attached Files:

    Wntrmute2 likes this.
  13. Not sure how this works, so asking the question: Does using only half the secondary support the full current rating... will it support the 2.5A current draw?
  14. Yep. You still get all 3A.
  15. Wntrmute2

    Wntrmute2 Junior Member

    I'm going to try wiring this with a switch. Be cool to try the Sophia 2.5V 300B as well as the usual 5V version. Probably screw it up but what the hell!
  16. you can wire it this way, But make sure you measure and label which way the toggle is 2.5v which way is 5v

  17. Wntrmute2

    Wntrmute2 Junior Member

    Will the fact there is 0 volts on one leg of the filaments screw things up? Seems too unbalanced.
  18. Sorry the 0V should say CT

    Attached Files:

    Wntrmute2 likes this.
  19. Wntrmute2

    Wntrmute2 Junior Member

    Just fooling around with a new program. Screen Shot 2018-11-11 at 6.22.20 PM.png
    Tleung likes this.

Share This Page