1962 RMS Double-Talk Tube Intercom

Discussion in 'Tube Audio' started by xXSHADoW93Xx, Jul 14, 2016.

  1. Chip Chester

    Chip Chester Super Member

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    Don't worry. As soon as the dry crumbly insulation wears thru on that power cord going thru chassis with only a metal grommet, you'll have your hot chassis soon enough... :yikes:
    Chip
     
  2. Retrovert

    Retrovert AK Subscriber Subscriber

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    As long as you're rewiring this, I suggest changing from a half-wave to a full-wave rectifier. Given the same filtering capacitors, a half-wave rectifier has greater ripple current than a full-wave. This will increase B+, of course, which can be adjusted through a dropping resistor, but it will provide far less stress on the filtering supply, will likely decrease tube noise since the bias point won't be moving, and it will decrease the ringing of the isolation transformer.
     
  3. battradio

    battradio Electron trainer Subscriber

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    Last edited: Oct 13, 2017
  4. Retrovert

    Retrovert AK Subscriber Subscriber

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    Just a note on the one previously described. OP wrote in Post No. 13, "Will be ordering a Triad N68X to isolate the chassis." And the followup poster wrote in No. 19, "Using a Triad N68x."

    Just looked up the specs. That transformer is 50 Watts, about 0.4 A. Not enough to do the job, I fear. The heaters alone on four tubes will draw 150 mA apiece, so figure 600 mA total @ 120 VAC since it is a series string. On startup that current will be many times that. Yes, that startup overcurrent is temporary, but it still overloads the transformer, and the steady-state current is an ongoing overload. The B+ is roughly tens of mA, so not significant compared to the heaters.

    Yes, this is least inexpensive way but one must have enough current capability and any voltage loss doesn't matter. A 6.3 VAC transformer at 5 A is only 30 Watts at the secondary, less than the isolation transformer. As far as voltage loss from the two transformers, this is likely a good thing as the original circuit was 110 VAC or 115 VAC, not 120 VAC. But it does need to be accounted for.

    I've heard of people scavenging 24 V transformers from old UPS units which people discard when the batteries fail. (I just replace mine. My APC units have been running for a decade with new batteries every two years.)

    Isolation transformers above 0.5 A become very expensive, as in more than $100, for some reason. Clearly best to purchase a used unit.
     
  5. battradio

    battradio Electron trainer Subscriber

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    He has 3 tubes in series at 150 Ma total 150 Ma . They are not parallel , the voltage adds in a series string , the current remains constant so , .150 A X 120 V =18 watts for the filaments and dropping resistor , figure other losses .double it 36 watts and + 3 watts output +39 watts , it should do for the intercom . If he wants to use it for something else he can use a 10 amp set of transfomers to be on the safe side , this antec http://www.antekinc.com/as-4458-400va-58v-transformer/ is what they sell as an isolation transfomer on ebay , 400 VA is over kill to the max .

    Yours has 4 tubes still ,150ma + 18 watts for the filament string .
     
    Last edited: Oct 13, 2017
  6. Retrovert

    Retrovert AK Subscriber Subscriber

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    Yes, that is absolutely correct! I totaled the load as parallel, not series, because that's what I normally do.

    The initial startup current, however, can easily be 2-3x that 150 mA, of course. The tens of seconds for that likely won't overheat the transformer.
     
  7. I LIKE MUSIC

    I LIKE MUSIC Super Member

    Not exactly. Just so those that are following understand the difference between a series circuit and a parallel circuit if we are explaining how things work. It is less confusing.

    The current in a series string is not the sum of the individual currents. It would be the sum of the individual currents if the filaments (heaters) were connected in parallel.

    The full warm power consumption would likely be in the range of less than 40 watts or so.

    As previously mentioned, inrush current might be an issue, although the power transformers in some of our gear are subjected to 5 to 10 times or more inrush current than their nominal constant current rating.

    Just some round off the top of my head numbers would indicate, based on the specifications of the transformer and the circuit, it would current self limit (surge current at turn on) at likely less than 2 amps, in the range of 1 amp or so. There is the 47 Ohm resistor (that by its self would limit the surge current to about 2.5 amps), the cold resistance of the tubes and the characteristics of the transformer.

    Surge current is not likely to be an issue.

    Edit: Battradio beat me to this. I left my computer and did not refresh the page before I posted.
     
  8. Retrovert

    Retrovert AK Subscriber Subscriber

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    Consider the wattage:

    120 VAC x 1.0 A = 120 W
    120 VAC x 1.5 A = 180 W
    120 VAC x 2.0 A = 240 W

    So that does exceed the 40 W transformer, just not for very long.

    A wirewound resistor to slow the inrush current, the way we used to do it on tube TVs, with a TRIAC to take the resistor out of circuit after warmup, would be a good idea.
     
  9. I LIKE MUSIC

    I LIKE MUSIC Super Member

    Given the integral of the power over time, that amount of power is not likely to be an issue as I said in my post. My reference to the 47 Ohm resistor is in reference to the 47 Ohm series resistor.

    That transformer is likely to be okay with a time limited surge current in the the range indicated. Again it is not uncommon for the transformers in our gear to be subjected to 5 to 10 times their nominal ratings at turn on.

    When it comes to turn on surge, it is usually current that we look at rather than wattage. The integral of the turn on surge wattage over time is likely not to be an issue, although it can be.

    For example lets look at fuses. Lets say that a 1 amp fuse will blow when the current reaches 1 amp, although that is not exactly correct it makes for a simple example.

    If that fuse is protecting a 120 volt circuit that is 120 watts, but if it is protecting a 12 volt circuit that is only 12 watts, but the fuse will still blow if the current exceeds 1 amp (again this is not exactly correct but it makes for simple example).

    In the this application we are not worried about the integral of the wattage over time at turn. We are concerned about one of the windings of the transformer acting as a fuse if its maximum current is exceeded.

    I understand why you might do this to build in a numerical fudge factor, but this can be confusing for those that are following this thread and trying to learn how things work. You indicated that the total current draw would be 0.6 amps and would exceed the rating of the transformer. Again this could be confusing for those trying to learn how things work.

    To be clear, all of this is just my OCDness attacking the keyboard on my laptop computer.:D


    BTW, if one is really worried, the inclusion of the correct CL-XX type NTC (negative temperature coefficient) current limiter would be cheap and easy.
     
  10. Retrovert

    Retrovert AK Subscriber Subscriber

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    That heating spike when cold is why I advocate current limiting heaters to prolong their lifespan, but that's a separate issue. (Light bulbs fail in this fashion. Temporary thermal runaway why heater flash occurs.)

    The important issue here is that the isolation transformer, like any transformer, must be designed to survive overloads; I do not know if an isolation transformer is designed for overloads of five times operating current lasting for tens of seconds, maybe even ten times the rated current. This is not a very robust transformer, it appears to be designed to power small analog circuits.

    A filament transformer, to contrast, would, of inherent necessity, be designed to for that startup load; how could it not, given its intended application?

    Triad is closed for the weekend or I would call and ask.

    It's not purely the fuse aspect that's the problem. High current causes spot heating. If the winding has any weak spot in the wire, that can fail over time, not in the same way as a fuse, but because it gradually is eroded and the maximum current load drops to the point that it fails. The junction of the two pieces can be higher resistance which has current forced through it and this causes spot heating. Light bulbs fail in this manner. The insulation also fails from heat and can short. That wire can get very hot before it burns, and that heat can damage the insulation.

    Using components beyond their maximum ratings leads to failure.

    This, by the way, is why selenium rectifiers fail in vacuum tube equipment. They're reliable until stressed and then it all goes kablooey. I've read the papers and databooks from the 1940s and 1950s where the rules are relaxed for consumer items with a short expected lifespan. Welders and elevators used selenium rectifiers and these are still working decades later because they weren't abused.

    Not exactly. Fuses are governed by time-current curves. So the greater the magnitude of overload, the faster it blows. We're not guaranteed that a 1 A fuse actually blows at 1 A, or that it doesn't take many minutes to blow at that rating.

    See, for example, LittleFuse's guidelines:

    The problem is that a fuse can carry many multiples of its maximum current for some time. Resistors also do this, which is why they make poor fuses.

    This is the big problem with fuses. I learned that the hard way many years ago. Relying on a fuse to save you only works in the case of a high-current short.

    Yes, I already acknowledged the incorrectness of that miscalculation and it's clear earlier in the thread. Again, I typically deal in parallel heaters, not series, and went with the typical without thinking. This is why we have design reviews. It is 150 mA, since it is series.

    BUT the fact remains that on startup the load can be many times the normal load. I've seen numbers of as much as ten times, depending upon the heater construction. Expecting three to five woud not be unreasonable.

    It's not OCD. It actually matters. When engineers stop analyzing reliability and considering points of failure in designs everything falls apart: planes fall out of the sky, buildings and bridges fall down, tunnels start dropping concrete on cars, the electric grid collapses, spaceprobes malfunction, spacecraft are nearly lost or actually lost, and motherboards start rebooting for inexplicable reasons when one has not yet finished saving one's work.

    An excellent idea. This is not perfect, because the inrush limiter it is of shorter duration than the heater startup, but it will prevent the worst of the overload.

    The more I think about it, the more I prefer battradio's suggestion of back-to-back filament transformers. They're designed for this sort of overload. That would totally solve the problem in the most elegant and cost-effective way.
     

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