Transformers, current, power, etc... group discussion

Discussion in 'Shock Brother's DIY Amps' started by jsnwhite619, Sep 12, 2019 at 11:24 PM.

  1. jsnwhite619

    jsnwhite619 Friend of Leo's

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    Been away from electronic stuff for a little while, been building cabinets lately. But, I'm trying to get my mind back in electrical gear again. So, transformers, and I know I've had one or two posts before, but it's the subject that I keep coming back to....

    I would like confirmation or correction on a few bullet points. I'm not quoting anyone, and I'll try to use italics for my own conjecture points

    • Increasing your PT with a higher current rating PT should allow for more headroom and cleaner output at higher levels where the old PT was stressed or exceeding limitations, but not in general.
    • Increasing your OT size - especially in smaller amps - usually involves changing to a reduced primary load than your original. Example: "upgrading" your 15 watt Tweed Deluxe OT to a 22 watt Deluxe Reverb OT increases the headroom and firms up the bass. Is it because it's a "higher rated" OT or is it because the 6.6k allows more current to pass through the PT and power section and that is why you gain headroom? Is the end result because the OT staying cleaner, or is the PT supplying more current? Personal experience: I've used the Hammond 1750E 15-watt 5E3 OT for it, 5f10, 5F11, and Princeton Reverb, the PR being the last build. Until that one, I knew that the OT saturation must be part of the sound I was getting. The PR had amazing cleans - crisp, punchy, and clean pretty well up the dial. I used a 100mA Hammond 290AX PT -- the smallest PT I've used with that OT... the others being 115mA and 138mA. So, that tells me the circuit may have been a bigger factor than either transformer. Also, I've put a Hammond 1750H Deluxe Reverb 6.6k OT on a Harvard build. It is the absolute cleanest 6v6 Tweed amp I've built, and this is my 3rd Harvard, but first one with the 6.6k OT. Everything else is the same. So, is it cleaner because the OT is rated for 20-22 watts and "sounds" different, or is it cleaner because it's 6.6k vs 8k and it's drawing more current through the power section to keep things cleaner & tighter?
    • I know Fender used undersized OT's back in the day to limit bass response and help the low powered speaker situation at the time. But, am I correct that using higher OT primary impedance would allow the use of lower rated PT's as well? Not looking for optimum performance, but if you had a PT that was borderline for a 4k 6L6 amp - optimum performance - would shifting it to a 6.6k OT reduce the current draw, thereby making it a lower output amp suited for the borderline PT? In that line of thought I'm thinking of the Tweed Super/Bandmaster/Pro that I think used a 6-6.6k OT primary.
    Please don't limit responses to my questions. Once you get away from brand preferences, I feel like transformer specs are the real black magic for a lot of people. Anyone can spend $10-20 on caps and tinker with stuff, but I think the expense and potential for catastrophic & expensive failure keeps most average people from going crazy with transformer games.
     
    Last edited: Sep 13, 2019 at 12:55 AM
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  2. Wyatt

    Wyatt Tele-Afflicted

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    The answers to most of your questions are, "yes, but it depends."

    It's all in the data sheets for each tube. Lets look at the 6V6GT for the Deluxe Reverb and Tweed Reverb.

    [​IMG]
    Maximum output is a 6k6 ohms. If we wanted minimal THD, that is at 6k ohms at a minimum of output loss. By the time we go to 8k ohms, output has started to drop off and THD has started to rise, if we go below 6k, output drops off substantially. There are of course other factors involved,

    The Deluxe Reverb puts out more output than Tweed Deluxe for four main reasons ....
    1. Higher plate voltages...410vdc vs 375vdc...plate voltage is the energy converted for output power, you need more to make more
    2. Optimal plate impedance 6k6 vs 8k
    3. Larger mass, interleaved output transformer that can handle more bandwidth
    4. More efficient long-tail PI using a 12AT7 which can deliver more current without compromising operation
     
    Last edited: Sep 13, 2019 at 3:04 PM
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  3. peteb

    peteb Friend of Leo's

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    Great post Wyatt, and interesting questions jsn.




    What makes a DR louder and more powerful than a tweed deluxe?




    I was unsure of how to answer that question but I would have said,


    Bigger OT, more energy transfer. (I like the larger band width comment)
    Bigger PT
    Higher voltages
    (Forgot about the PI, but yes, the LTPI d finitely plays a role in boosting the signal)
    And then comparing your list to mine, I would say more signal from the pre amp hitting the power stage, along with the bigger OT to handle it, has to be a big part of it.






    Do you mean more current or more signal voltage? What the PI passes to the 6V6 is signal voltage, not signal current.




    I never considered that in the effort to offer a full range of products, that they would purposefully hamstring an amp by mismatching the OT. That brings up interesting possibilities including increased THD.




    That’s an interesting graph. What graph is it?
     
    Last edited: Sep 13, 2019 at 8:27 PM
  4. jsnwhite619

    jsnwhite619 Friend of Leo's

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    This website calculator is fun to mess around with. https://thesubjectmatter.com/calcptcurrent.html

    I know I've brought up some of these points in the past, but it still seems to be an area where I can always learn something new. If you play with the OT primary impedance values, you get to see how things could go badly for your PT in a hurry. Setting values for a 325v-0-325v transformer, a GZ34, 2x 6L6 tubes, and 3x 12ax7, here is how the OT primary changes the PT current draw (it also states that it has a 10% safety factor figured in). I assume these would be the numbers if the amp were running maxed out(?)
    • 16k primary = 71.56mA
    • 10k primary = 100.24mA
    • 8k primary = 119.37mA
    • 6k primary = 151.23mA
    • 4k primary = 214.97mA
    • 2k primary = 406.18mA
    SO, does mean that running your 4k - 8ohm amp ext. jack paralleled at 4ohms, your OT would then see the load as a 2k primary, and you could potentially add 90% to your PT current demand?

    And on the other side, going back to the Tweed 6L6 amps, running a 6k vs a 4k allows for a PT with 30% less demand on it? So, a smaller, lighter, and cheaper PT would work.
     
  5. peteb

    peteb Friend of Leo's

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    I agree with your conclusions, except the power tube isn't the only thing requiring current, so the % change at the PT is probably going to be a little less.



    is the current draw for the PT on the primary or the secondary? they may be right for the secondary but look too low for the primary.
     
  6. schmee

    schmee Poster Extraordinaire

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    Here are my comments just from actual experience, not theory:
    • Increasing your PT with a higher current rating PT should allow for more headroom and cleaner output YES. More headroom. My current example would a Princeton Reverb where the only thing non stock at this point is a huge Super Reverb PT. This damn thing holds up even outdoors. No change in filters, no change in phase invertor.
    • Increasing your OT size - especially in smaller amps - Same primary winding but more robust tranny = cleaner amp at more output.
     
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  7. jsnwhite619

    jsnwhite619 Friend of Leo's

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    So, the stock Princeton PT is a 70mA. The calculator I listed above puts a PR at 120mA (assuming maxed out). I guess that a 180-200mA PT would allow a Princeton to do whatever it needed to do without ever missing a beat. No replacement for displacement! :D
     
  8. FenderLover

    FenderLover Friend of Leo's

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    Bigger motor, bigger exhaust.
     
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  9. FenderLover

    FenderLover Friend of Leo's

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    Notice if your Princeton output went from 70mA to 120mA, you would have a Deluxe output. More current, similar B+, less reflected load: 8K to 6.6K.
     
    Last edited: Sep 14, 2019 at 8:46 PM
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  10. FenderLover

    FenderLover Friend of Leo's

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    Exactly. That an AT7 can pass more current is beside the point, it is not used that way, and it is capacitor coupled. Concertina or LTPI, either will pass enough signal amplitude for the output to overload first anyway.
     
  11. jsnwhite619

    jsnwhite619 Friend of Leo's

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    I like that. It's funny how it seems like so much transformer "upgrade" talk always centers around output transformers instead of power transformers. @schmee 's experiences with it seems to confirm the "bigger motor, bigger exhaust" analogy with his Princeton. I guess it doesn't matter how great your transmission is if the engine is still redlining?

    The PR I built earlier this year used a Hammond 290AX, 100mA and the stock 15 watt Hammond 1750E OT. It sounded incredible, cranked with a Les Paul was one of the best overdrive tones I've ever had. The whole time, I'm wondering how people can complain about a Princeton's headroom/staying together/etc...this is the only one I've ever played. I guess my 40% extra current rating on the PT did more than I realized. And maybe why my 115mA PT Harvard & 138mA PT Tweed Deluxe seem not to have the general tweed issues that I generally read about?:rolleyes:
     
    Last edited: Sep 14, 2019 at 9:23 PM
  12. peteb

    peteb Friend of Leo's

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    A bigger OT and PT are going to make their differences.



    What are they? Can these effects be verified? Measured?



    The larger OT must pass more signal. This would show up as more signal voltage at the speaker.




    What I am reading above must mean that when an undersized PT is stressed out with a heavy load, the output voltage will drop. Does the current rise? I’m not sure, but the voltage will drop.



    Voltage sag or a lack of sag in the power transformer would be easy to verify.




    Jason, how much voltage sag are you getting on the B+ on your three over powered 2x6V6 amps? Is it less than would be expected for stock circuits of the same amps? I would think it must be, but maybe by not that much. In the extreme case of Schmee’s 2x6V6 amp with a PT meant for a 2x6L6 amp, it must have significantly reduced B+ sag.




    I have measured power supply sag on different amps. Champs sag not. A SS rectified Bassman head sags about 30 volts. I assume this is power transformer sag. I have measured a 100 volt sag in the B+ of my 6G2 Princeton. I used to attribute that to the sag of the 5Y3 rectifier, but thinking about it, it must be a considerable amount of PT sag as well.
     
    Last edited: Sep 15, 2019 at 11:28 AM
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  13. Wally

    Wally Telefied Ad Free Member

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    Here is an extreme but opposite example of Wyatt’s ‘bigger motor, bigger exhaust’ observation. The fellow who did all of my tech work for four years before he moved was a great tech...well-versed, lots of experience. He had told me a story about a Rickenbacker M16 combo...2 x 6L6, 4X10...that was on his bench years before. The PT chose that time to fry. The owner was upset and took the amp unrepaired. Maybe ten years after he told me that story, just such an amp came to me. It had very little output, and the owner told me that the PT had burned down years ago while being serviced.....it was the same amp. The owner had found someone to ‘work’ on the amp. I still have the tiny little PT that some rank amateur put in. That PT would have been lucky to have powered an early tweed Deluxe. It could in no way handle the demands of that Ric’s circuit, and the results proved it. Little motor...not much out of the exhaust. I rebuilt that amp with an appropriate PT. Last I heard of it, it lived in Montana. Odd note....the fellow who bought it did not want to pay enough to get the original P10Q Jensen’s!!!!! I got to keep those.
    Indeed, thePT has a lot to do with what comes out of the exhaust.
     
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  14. Snfoilhat

    Snfoilhat Tele-Afflicted

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    I just noticed a difference between these calculations and the plate currents on the 6V6GT data sheet that Wyatt posted. Here's 6L6GC so that it's apples to apples comparison.

    One way of estimating current seems to be assuming the B+ voltage drops across the OT primary impedance and you get these very large increases as primary impedance drops. It's a very steep slope.

    But check out how not-steep the slope of the plate current line on the data sheets is with respect to the same large jumps in primary impedance.
    Screenshot from 2019-09-15 08-47-46.png
    You aren't seeing some worrisome jump in plate current between 4k and 2k, for example.

    When the operating point (not shown, that would be on the plate volts vs plate current data sheet) gets pushed toward wide open the plate voltages drop and the screen current increases, but it doesn't look like the tubes can ever pull those huge currents estimated just by taking the B+ and dividing by the OT primary.

    I'm just pointing out the difference -- I don't actually understand. So that's my contribution to the discussion:eek:
     
    Last edited: Sep 15, 2019 at 12:05 PM
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