My JTM45 may run too hot...

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by hacinador, Apr 28, 2016.

  1. hacinador

    hacinador Member

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    745
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    I built a JTM45 copy. It's a great amp with Mustard caps, Piher resistors and very nice transformers from IGPW Germany.

    There are no problems with the amp, it doesn't hum or hiss, all is perfect and it sounds heavenly!

    My problem is that the wall voltage in my house is little "over the top". The "standard" is 230V (I'm from Europe...), but I have 240-245V.

    The HV from PT's secondary should be 350-360V, but I have 370-380V. There is a plate voltage of 475V with KT66s and 520V with EL34B. One of the big elyts is rated as a 500V cap, but it runs at 520V with those EL34B! My voltage for heaters is nearly 7 volts!

    Is it safe? Is it still "ok" for the "sound"?

    I can use a different PT's primary tap and wire the amp for 240V, but then the amp could run too "cold" at other place, because there is only about 210-215V in some clubs etc.

    And I have other amps too, so they might suffer too (on the other hand I have no problems with my amps and I've been living for 3 years in my house...)

    What would you do?

    Thanks!
     
  2. pdf64

    pdf64 Member

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    The procedure is to measure your mains voltage and set the amp's mains input selector to the next tap up from that.
    As 240V is the highest option available on the PT, you should be running your amp on its 240V tap.

    Your tube heaters are rated at 6.3V; some makes may be +/-5% (ie 6.6V upper limit), others may be +/-10% (ie 6.9V upper limit).
    Prolonged exposure to nearly 7V on tube heaters, and 520V on a 500V cap is likely to reduce their working life / bring on early failure.

    If you are concerned about there being a lower mains voltage at other places you may use the amp, then consider taking a voltmeter and following the above procedure at each place you use it.
    But no damage will result if you leave it on 240V permanently and use it at a lower voltage, eg 220V.
     
  3. hacinador

    hacinador Member

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    745
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    Thank you for your post! I know that these conditions are not healthy at all for the amp (or all my amps) :( The problem is that my amps sound much better with 230V than with 215V :(

    Does anybody have some experiences with some "power conditioner" etc.? Or is it possible to use only some form of variac to tame my higher wall voltage?
     
  4. Steppin' Wolfe

    Steppin' Wolfe Member

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    hacinador, what is the plate dissipation with those two different sets of power tubes? +1 with what pdf64 notes about the heater filaments and the cap at the higher voltages with the EL-34's.
    IT may be that those EL-34 tubes are biased way to the cool side of things. Higher current draw will lower the B+. The KT-66's may be biased way hot. So, one set of tubes biasing cold and one set biasing hot might explain that wide difference in the B+ voltage. That is the first thing I would want to know before I started looking for some voltage control.
    After the biasing is understood, then one knows how concerned one should be with the voltage. There are different ways of dealing with the voltage.....Variac, zener diode, voltage regulator/conditioners. Is this amp tube rectified? IF so and if you are running a gZ34, there are other rectifiers that will drop more voltage than does a GZ34. But....biasing first, imho.....
     
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  5. hacinador

    hacinador Member

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    745
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    Thank you for your post Steppin' Wolfe! The amp is an exact copy of the JTM45. It has a GZ34 rectifier. It has the exactly same layout as the original, PT and OT are wired the same as the original, it even has most of capacitors and resistors from that "old" era.

    I tried those two pairs of tubes, because the amp is new and I'm trying to experiment a bit :) ,but I'm going to use KT66s, so the plate voltage should be around 470-480V (with 245V of my wall voltage).

    I tried the amp in other place and there was about 230V from the wall socket (the "right" value) and the amp had about 450V of plate voltage (these value should be the "standard" for the JTM45), about 350V from PT's secondary (OK) and heaters had 6,3V (OK).

    At home, I measured 245V from the wall socket today! I also tried another MM and got the exactly same value. Then I measured the amp with KT66s: Plate voltage was 476V, 374V from PT's secondary, 6.85V on heaters, 5.5V on GZ34 heaters! All values are about 10% higher than they should be.

    I have bias probes on both tubes and I can set current for the pair via small pot (the original JTM45 was made with fixed bias). I can compensate the higher voltage at home with the "right" biasing, but I can't do to much with the voltage for heaters and this may be the biggest problem. I think that the amp can deal with higher plate voltage, but nearly 6.9V on heaters can be "out of spec" :(

    I may try to find some power conditioner, but I don't want to buy some crazy device for 400 bucks :(

    I also need to measure voltages in my other amps, I'm afraid that it can be even worse, because one of my amp should be run at 220V :-/
     
  6. Steppin' Wolfe

    Steppin' Wolfe Member

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    I agree that the heater voltage is a problem. I got sidetracked by the big difference in the plate voltages with the two different tubes. I can't speak to the other amps, but this build is an example of why the PT choice is so important. For vintage repairs or builds, I prefer voltage adjusted PT's that are built to yield vintage voltages at modern wall voltage output. IN your situation, I would also want a 'Universal' PT so that I could select source voltage....100, 110, 120, 220,230,240VAC.
    With the various voltages you have at different places and with the different amps with different requirements that you have, it appears to me that at the minimum you need a Variac if not a more expensive alternative. Then, you would have to do the experiments to know where you need to run the Variac for different amps at different source voltages.
     
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  7. tresspassor

    tresspassor Member

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    You can always lower the B+ with a Mosfet or Zeners. You can also drop a bit of the heater voltage by inserting a small amount series resistance. 1 ohm or less @ 5W rating should do it. But, having a universal PT with multiple input taps would be best.
     
  8. hacinador

    hacinador Member

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    745
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    Thank you very much for your advices guys! I forgot to mention that the PT has several taps in it's primary and I can wire it for 240 volts. But there is a problem in some places where only some 215 volts are present from the "wall". Due to my calculatins the plate voltage would be around 410V (standard is 450V) and heaters would get around 5.4V (standard is 6.3V). Now I run 7% over the standard wall voltage , but with the change on the PT (wiring it to 240V) I would run the amp 11% under the standard at some places. I think that I need to buy some power conditioner that will give me 230 volts anywhere and it will be the best solution for me, because I can use it with my other amps too :)
     
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  9. gght

    gght Member

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    Sounds like a job for a well placed switch!
     
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  10. big mike

    big mike David Grissom Wannabee Staff Member

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  11. Steppin' Wolfe

    Steppin' Wolfe Member

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    There is every reason in the world to wire that PT to a switch that will access the various AC source voltages. IF a person can do without something like an external votlage control device, then one should do without it, imho. From my perspective, you are wasting time and energy wondering about what to do while the solution to your problem is sitting in the amp.
    Once the PT primary taps have been wired to a switch, then one would want to carry a multimeter and know what voltage is being provided at the wall wherever one plays. Select the appropriate primary tap, and play away.
     
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  12. hacinador

    hacinador Member

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    You're right. I was little unsure about my other amps (I thought that they are also "affected" by my higher wall voltage at home) and I wanted to solve "my problem" with something more "global" (to buy a power conditioner that can be used with all my four amps). Yesterday, I made some measurement and I've found that despite of my higher wall voltage all my amps are running just fine. My Bad Cat has 6.4V on heaters and 350V of plate voltage (about a standard for those amps) and my AC30 has only about 6.1V on heaters and plate voltage is also OK. My other amp Orange Rocker 30 can be set for 220V or 240V, so there is no problem at all. I am going to switch taps on my JTM45's Power Transformer, I am going to set it for 240V and it will be ok. I will buy some switch in future (don't want to pay 15 bucks postage for one stupid switch) and I will use it for switching between 230 and 240 volts :)
     
  13. robrob

    robrob Member

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    I have one of these for US voltage and it works great for reducing wall voltage. There should be one for Euro wall power:


    [​IMG]
     
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  14. donnyjaguar

    donnyjaguar Member

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    Call up your electrical power supplier and tell them to fix the problem. Now I realize in Europe things are different than in the Americas and that many homes will be fed from a single, larger transformer station, but I'm pretty sure if you're not third-world they are obligated to fix it.

    I'm in Canada and our local "hyrdo" is responsible if their supply blows our your gear from overvoltage.
     
  15. hacinador

    hacinador Member

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    745
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    Our energetical power suplier claims that there can be a 10% difference from the standard, it means that everythinf from 207 to 253 volts is OK (the standard is 230V). All other energetical supliers claim the same. So my 245V is quite OK :D
     
  16. hacinador

    hacinador Member

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    745
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    Great! They also make it for "our" voltage and it's quite cheap. I'll look at it. Thanks!
     
  17. GilmourD

    GilmourD Member

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