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Scratch Build, -from the beginning...

Discussion in 'Shock Brother's DIY Amps' started by Jewellworks, Sep 24, 2020.

  1. Nickfl

    Nickfl Friend of Leo's

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    Sounds like a PA or intercom amplifier, 70 volt and 25 volt would imply that they are meant for some sort of distributed audio system like you see in restaurants and institutional settings where need to have a high voltage low current signal to cope with very long wiring runs. These systems use a line matching transformer on each speaker. Usually these kind of systems have a couple of different high voltage distributed audio taps as well as traditional 4 or 8 ohm outputs.
     
  2. Nickfl

    Nickfl Friend of Leo's

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    Before you go down the road of Merlin's power transformer estimating math I'll just say that I have found that it is pretty difficult to get an accurate estimate off of that graph he provides and some educated guess work tends to yield about as good a result for way less effort. What is the unloaded voltage on your power transformer secondary, and what type of rectifier setup are you going to use?
     
  3. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    Wouldn't that be in the territory of a 6ca7 aka EL34?
     
  4. Jewellworks

    Jewellworks Tele-Meister

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    on the bench, no load, no caps, no nuthin
    OHM READINGS:
    primary: 5.9ohms

    2ndary:
    HT: 217.4
    HT-a to C/T: 103
    HT-b to C/T: 114.6

    6.3v Filament: .03
    F-a to C/T: .02
    F-b to C/T: .04

    5v Heater: .03

    VOLTAGES: using a variac, i set the input to the primary at 120VAC

    2ndary: HT: 862 VAC
    HT-a to C/T: 432
    HT-b to C/T: 429

    6.3v Filament: 7.53
    F-a to C/T: 3.77
    F-b to C/T: 3.73

    5v Heater: 5.77

    i was planning on using a 5Z4, since thats what i have on hand. plan B is a 5Y3, cz i got one of those too

    im honestly not sure why im doing all this math. i wanted to build an amp, not get a science degree in algebra and book reading. i can copy a schematic all day long and be happy as a clam. this is very frustrating and much harder than i thought it would be
     
  5. Nickfl

    Nickfl Friend of Leo's

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    What did that transformer come out of and do you have a schematic? Was it from the same low power PA amp as the OT?

    430-0-430 is unusually high unless it was originally in an amp with a field coil speaker or with a choke input filter. That is likely to give you around 500v B+ even with a load on it, which is probably way too high for a non GC 6l6.
     
    Last edited: Sep 26, 2020
  6. Jewellworks

    Jewellworks Tele-Meister

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    heres my 9th grade math:
    Rpri= 5.9 (which i will just call 6) +109 = 115
    Vpri/Vsec = 120/430 = .279
    then i went to google and typed this in: "what is .279^2" and i got .077841
    115 (Rpri) / .077841 (Vpri/Vsec)^2 = 1477.37


    Merlin also says "If the amplifier runs in class A then its current demand does not change much with drive level, so as far as the power supply is concerned it looks like a constant resistance."
    i dont know how much this affects our numbers, but theres that...

    where did you get 80ma? just pick a number out of thin air?

    so... 109 (half the transformer secondary resistance) + 1477.37 (the transformer primary DC resistance (reflected to the secondary) + 188 (The internal anode resistance of the rectifier tube, using the number you picked out of nowhere)
    190+1477.37+188=1744.37 (Rs, for “source resistance.”)

    6SJ7: Plate = 3, Screen = .8 =3.8
    6J5: plate = 9
    6L6G: Plate = 54 (0 signal), Screen = 2.5 = 56.5
    the 6L6 also gives a MAX signal rating of 66/7 (plate and screen respectively) = 73
    (but im guessing were not using the MAX signal numbers?)
    so 3.8+9+56.5 = 69.3 ma

    430*1.414 = 608V

    using an online Ohms Law calculator, I put in 608V, 69.3ma and got 8773.44 ohms
    RI = 8773 ohms

    sooo..... whats all this telling me? whats next?
     
  7. Jewellworks

    Jewellworks Tele-Meister

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    it came from an old Ampro projector. it had 3, 6L6GCs, 6SJ7, 6J5, and some other tube, as well as the 5Z4 rectifier. as far as i can tell it was a Bi-amp'd system w separate highs and lows for that "hi-fi" sound. but i never heard it, and it was hacked out mercilessly. i dont have a schematic. this is NOT the same low power PA amp the OT came from. ive measured and rated the Ampro OT (with help from this forum) and its has more taps than ill need. so i was wanting to use the smaller OT with 1 8ohm tap (ignoring the 70v and 25v) the Ampro also had a splitting transformer used as a phase inverter.
    cant i just knock the voltage down with big resistors? or use a sinker to ground?

    what i "THINK" @andrewRneumann and i are working on is how much can the PT handle. im reasonably sure itll handle my lil circuit without going through all this trouble, but whatever... my plan was to get it all wired up, and then measure the voltage at pin3, and adjust the resistor as needed to keep it around 375 without a load. 22uf filter cap before the transformer and a sinker to ground if necessary
     
  8. Nickfl

    Nickfl Friend of Leo's

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    The calculation Andrew was walking you through is to estimate the rectified DC voltage under load, there is a different calculation and graph for estimating a transformer's current rating, but if this ran 3x6l6GC you are fine. As you have seen, the voltage estimation is a fairly involved calculation and the ***** of it all is that once you arrive at a number and plot it on Merlin's graph you will see that just about any number you are reasonably likely to arrive at is going to end up on a section of the curve that lands between 115 and 120% of RMS. This is why I suggest going with an educated guess and only bothering with Merlin's method if you have a compelling reason to think it might be different or you really like doing the math (as I suspect Andrew does).

    So, my educated guess for a 430v unloaded transformer would be 470-500v under load depending on the rectifier tube. Merlin's graph will probably spit out a similar voltage.

    That sounds about right considering the amp it came from was running 3x 6l6gc which would have pulled down the voltage a bit more and because a 6l6gc is a very different tube than a 6l6G/GB and would generally be run at a significantly higher voltage.

    So, the problem for you is that you need to get that down closer to the 370v you were planning on. Adjusting the B+ with power resistors or zeners is fine up to a point, but you are looking at having to drop about 100v. A single 6l6G at 370v is going to be something like 55ma at idle depending on bias, plus a bit more for the rest of the tubes, so lets say 60ma. That means that whatever is dropping that 100v is going to be dissipating 6w at idle and the rule of thumb is to use a part rated for at least 3x so you need a 20w zener or resistor. Its doable, but that is going to put out a lot of heat.
     
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  9. Nickfl

    Nickfl Friend of Leo's

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    If you go with the brute force method of dropping the power supply voltage to 370 or so, your OT should work nicely with a 16 ohm speaker load. The Merlin method of estimating class A impedance gives a ideal load of 6.5K at that voltage (plate to cathode of 352v).

    Or you could go with full voltage and a 6l6GC, which would work out to about the same class A primary impedance I think.
     
  10. Jewellworks

    Jewellworks Tele-Meister

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    i was wondering how much id need to drop. i didnt even consider the heat dissipation, so thats a big eye opener...
    voltages seemed way higher on the bench than i was expecting, but then again, it WAS pulling 3 6V6's and 2 others...

    i was hoping i could get away with using this project, but it looks like im going shopping.
    375-0-375, 75-100ma?
    http://rover.ebay.com/rover/1/711-5...0001&campid=5338148343&icep_item=174181415951

    rectifier is 5Z4, it was in the Ampro
     
  11. Jewellworks

    Jewellworks Tele-Meister

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    Damn. I gave you some wrong info. @Nickfl . The Ampro transformer was running 3, 6V6GTs, plus those other tubes, and not 6L6's. My bad.

    I think our calculations came out a bit higher than your estimations. We came out nearly 70ma at idle, and if you use the figures on the 6L6 data sheet for the MAX currents, its up over 80.
    There are 350-0-350/6.3/5, tranny's everywhere online. 100ma?
     
  12. Nickfl

    Nickfl Friend of Leo's

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    100ma is probably about right, I think 350v is still too high, that will probably be a loaded B+ of around 410v. Check out the chart on the 5z4 datasheet. It looks like if you want 370v B+ at 70ma idle you'll want around 320-0-320 HT supply.
     
  13. Jewellworks

    Jewellworks Tele-Meister

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    im not sure what i "want" as a B+. i only know that the data sheet for the 6L6G shows a MAX of 350v at the plate. so after rectification, and after the first filter cap, and after the resistance from the transformer, i hope to hit that mark.
    but then again, you can hit more modern 6L6s and 6V6s with much higher voltage than the MAX thats listed on their data sheets, so ... 375? im not sure the older 6L6Gs can take it or not... if its too much, i can drop a resistor like i was saying before. it'd be a much more reasonable value and rating than with a B+ of 470... 500...
    thats my thinking anyway...
    i did some research on the transformer i linked above, looking up the radio console it came from, and its tube set. then i looked up the current draw for plate and screen for all the tubes, then added them up, and its 101.5ma. so its got the current needed. HT is a little high at 380vct (i think thats what that means) but again... hit the 6L6G a little hot, or knock it back with a resistor?
    plus, i like the price... ;)

    and again... dont i want to shoot a little higher than 70ma? the max draw listed on the 6L6G data sheet puts that number up around 80ma?
    i suppose theres nothing wrong with dropping the voltage overall either... im stuck in this mindset that Max is Best. more is more. -but im not really going for that in this Vintage style amp. so maybe i need to re-think my plate voltages
     
  14. Nickfl

    Nickfl Friend of Leo's

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    Keep in mind that the plate voltage number is plate to cathode. Presumably you are going with cathode bias on this design, so you have to subtract the cathode voltage from you B+ for the plate voltage number. From the 6L6g datasheet you would be looking at something like 18v cathode voltage, so 352v plate voltage, which should be fine for a 6l6g. That happens to also work out for you OT since 352*352v/19w = 6521 ohms as a theoretical center bias class A plate load and your output transformer will give you almost exactly that with a 16ohm speaker. So, I think shooting for 370v B+ would be ideal.

    Take a look at that 5z4 datasheet I linked to. That 380v transformer will probably give you 440-450v under load, thats not really much better than the transformer you already have. Find a 320-330v transformer instead.
     
  15. Jewellworks

    Jewellworks Tele-Meister

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    @Nickfl ive got an 8ohm OT and an 8ohm speaker. i cant change that. not without buying everything new, and im REALLY wanting to use what ive got.
    rrrrgh... this is frustrating
     
  16. Nickfl

    Nickfl Friend of Leo's

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    You said your OT impedance ratio was 414:1, so with a 16ohm speaker that would give you 6.6k. Obviously you'd need a different speaker but you could at least still use that transformer.
     
  17. Jewellworks

    Jewellworks Tele-Meister

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    So... whats the problem with the 3.3k load on the power tube? its class A load resistance is 5000, so... its better to have more, and not less?

    i just bought an 8ohm, 6inch speaker for this Vintage amp. it was literally delivered a few hours ago. Andrew seemed to make it sound like i was good with that, with the readings and calculations we did.

    forgive my simple mind, but part of the frustration im having here, regarding that PT, is it came from a tube amp, running those 3, 6V6s and all those other tubes, and it didnt seem to have a problem with the high voltage, much less the current draw. all that "extra" voltage must have gone "somewhere"? 6V6GTs cant handle that high a voltage, can they? if its too hot for the amp i want to build, then fine, ill get something more reasonable. or maybe use a 5Y3, or some other rectifier?
     
  18. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    Again I will mention an EL34. AFAIK it can take the ~500v and the 3k-ish OT primary. Seems like an option.
    If the all the tubes used up the max current available from the PT, the HT voltage would drop to a level that would not be so eye opening. You will not be taxing the PT to that extent so you will have to deal with the high voltage. (The high wall voltages we enjoy does not help with this PT either.)

    If the amp was running a biamp speaker system it should have a second OT. You did mention the amp was in disarray so maybe it was missing?
     
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  19. Jewellworks

    Jewellworks Tele-Meister

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    its an option, if thats all im left with...

    that makes sense. so the ideal power transformer for my Vintage build is 320-0-320 at 70ma? then i can use the 8ohm speaker? im still not sure how the OT is a problem... or is it only a problem with my 430v PT?
    i REALLY want to use all these older tubes, and that 8ohm speaker i just got. whats the ideal PT and OT? ill just buy it... -more money... :( or how do i figure it out? all the math Andrew made me do has melted my brain.

    i assumed it was a biamp system. 2 of the 6V6's were in PP using an interstitial transformer as a PI. i dont know what else the 3rd 6V6 would be for. it was 'complete" when i got it, but the leads going here and there were just cut away, so its only a guess.
     
  20. Jewellworks

    Jewellworks Tele-Meister

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    so, if im doing the math right, im shooting for a rectified voltage of around 300, 310v, (100ma) and that will get me a load resistance of 4.7k / 5k which will match the 6L6G. then i should be able to use all my pieces parts? OT , 8ohm speaker and all?
    im looking for a magic bullet
     
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