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JTM45 Sounds beautiful then Main Fuse blows!

Discussion in 'Amp Tech Center' started by koochems, Feb 19, 2018.

  1. peteb

    peteb Friend of Leo's

    Apr 25, 2003
    Cascadia
    That's right, Clint points out that you have measured the screen current directly, even though a little bit indirectly and it does check out, 2 mA is alright.


    Better numbers will help.

    Because right now the cathode current does not equal the screen current added to the plate current.


    Until better numbers are found, it looks like more current or electrons are entering v5 at the cathode than electrons are leaving v5 at the screen and plate combined.


    And that would make problems for the fuse.
     
    Last edited: Mar 11, 2018

  2. Snfoilhat

    Snfoilhat Tele-Holic

    Age:
    37
    621
    Apr 8, 2016
    Oakland, CA
    40-18033.png
    AC power supply stuff

    5VAC winding out of 2A total:
    1.9A GZ34

    6.3VAC winding out of 5A total:
    2.6A 2@KT66
    0.9A 3@12AX7
    ____________
    3.5A

    HT power supply stuff
    B+ estimate 345V x 1.4 = 483V. 483V - 20V (rectifier) = 463V @ 150 mA

    Cathode current estimate = 28 mA + 34 mA + 2 mA + 2 mA + 2 mA= 68mA

    I don't understand how the people at Metroamp came up with target plate voltages of 400VDC if this is the power transformer they recommend. So setting that number aside, I think all of your voltages are too low.

    Your amp is drawing 95% of the 5VAC supply, and only 70% of the 6.3VAC, and only about 50% of the HT. So your measured voltages should be even higher than the nominal 5VAC, 6.3VAC, and 463VDC.

    To my understanding, that implies a problem bigger than just getting it biased correctly.

    This PT has international primaries. Is there any chance you got the USA (120V) and Japan (100V) mixed up? Do you have a safety bleeder resistor installed that should be 220K but you misread the color bands and installed 2200? A shorted tube? Bad OT (I think someone mentioned this earlier)? Some crazy high wattage pilot light?
     
    koochems likes this.

  3. koochems

    koochems TDPRI Member

    14
    Jan 19, 2016
    denver
    Well I've uncovered something a bit distressing... I have 2 16ohm greenbacks wired in what I remember parallel but it was so long ago they are actually wired in series! This increases the load to 32ohms when the amp impedance selector set to 8ohms! Sheesh, not sure if this is the root cause but will rewire the speaker cab in parallel to match the amps ohm setting. Hopefully this hasn't damaged the OT or speakers by now. Also, the magnetic components 40-18039 45 Watt OT is installed.

    Voltage drop readings are as follows:
    V4 = 0.780 VDC
    V5 = 1.124 VDC

    Measured across pins 4 and 6. These values provide lower plate current in turn higher screen current. How to get the screen current under control, hmmm?...

    I have not tried swapping tubes to see if the spark followed. Is it still worth trying knowing my load is wired incorrectly and my screen current is higher than expected for each power tube?
     

  4. clintj

    clintj Friend of Leo's

    Apr 4, 2015
    Idaho
    Get that load fixed first and foremost. Once that's done, see where you sit as far as tube numbers. The tube flashing over may have helped saved the OT windings from damage by limiting voltage, however the tubes may be damaged now.
     

  5. peteb

    peteb Friend of Leo's

    Apr 25, 2003
    Cascadia

    I was wrong, the screen current is fine.

    I should have used your 1 or 2 volt drop from your voltage tables.


    Because the screen resistor is 1k, the voltage is the milliamps.


    1 volt is one mA, two is two, .78 is .78 and 1.124 is 1.124.


    If the current thru the tube is 30 milliamps, one or two mA thru the screen is only 3 or 6 percent which neither are too high.


    I agree get the load right.
     

  6. koochems

    koochems TDPRI Member

    14
    Jan 19, 2016
    denver
    I rewired the speakers in parallel (load = 8 Ohms now) and took more readings. Most everything is the same! Still seeing 2.8AC in heaters... It sounds like this may be an issue? Shall I replace tubes with new and risk damaging in attempt to replicate the issue?

    Only noticeable differences:

    Cathode Current (measuring pin 8 with mV DMM setting:
    V4 = 24.0mA
    V5 = 30.8mA

    Voltage drop across pin 4 and 6 with 8 Ohm load
    V4 = 0.616 VDC
    V5 = 0.992 VDC

    OT resistance is a bit lower @113.1 and 104.0 Ohms. I Really appreciate the help all!!
     

  7. clintj

    clintj Friend of Leo's

    Apr 4, 2015
    Idaho
    Are you running direct from wall power, or are you on a limiter right now? If it's wall power, pull all the tubes and see what unloaded heater voltage is. I'm hoping there's no transformer damage from the tubes arcing like they were.
     

  8. peteb

    peteb Friend of Leo's

    Apr 25, 2003
    Cascadia
    The numbers look fine, for idle, DC conditions.

    Maybe the Ac is the problem??
     

  9. clintj

    clintj Friend of Leo's

    Apr 4, 2015
    Idaho
    That impedance mismatch quadrupled the impedance seen by the power tubes, which does one very nasty thing. It drastically raises the AC voltage at the plates under load. At the low side, it could drop plate voltage to near zero easily. At the high side, plate would skyrocket to insane values and cause that light show the OP was seeing.
     

  10. koochems

    koochems TDPRI Member

    14
    Jan 19, 2016
    denver
    The amp is plugged directly into wall power.

    I removed all tubes and tested VAC with power ON and standby OFF:

    3.0VAC supplied across heaters (should be 3.15VAC)
    4.9VAC across pins 2 and 8 from the rectifier (should be > 5VAC)

    Ugh... does this mean my power transformer sustained some damage since these voltages are lower than expected?
     

  11. clintj

    clintj Friend of Leo's

    Apr 4, 2015
    Idaho
    Never saw an answer to snfoilhat. You do have the PT primary configured for your wall voltage, right? Blue as common, and the appropriate lead chosen for your wall voltage? (And what is your measured wall voltage?)
     

  12. koochems

    koochems TDPRI Member

    14
    Jan 19, 2016
    denver
    I have the GRY, ORN, WHT, and BRN heatshrunk together on top the PT, with the BLK 120V routed to the mains switch, earlier build pic below. I do NOT believe I have a safety bleeder installed unless it was on the original schematic. Is this for amp safety, draining caps? Fairly confident the power tube isn't 100% as there were multiple arc flashes V5.

    I still haven't tested with new tubes in, but to clintj's point I tested the unloaded heater voltages and they are lower than expected so it seems isolated to the circuit / transformers?
    PT pics.JPG
     

  13. peteb

    peteb Friend of Leo's

    Apr 25, 2003
    Cascadia
    I think that you have found that the amp works, even with the 32 ohm load.

    Now that the speakers are rewired, you should have fixed the root of the problems.


    have you tested and played the amp with the rewired speakers?

    If the heater voltage is a little low, the amp should still work.


    didn't the amp sound beautiful with the filament heaters at 5.6V and the wrong load?

    it can only get better.
     

  14. koochems

    koochems TDPRI Member

    14
    Jan 19, 2016
    denver
    LOL. It did! I was so happy with the sound. I'll try and make her sing today after work. I'm sure the headroom, tone has to be drastically different now... I'll find out tonight!
     

  15. koochems

    koochems TDPRI Member

    14
    Jan 19, 2016
    denver
    Replaced the old tubes with the new tubes and let them heat up. Took readings, and most all was the same except B+ is now 475Vdc (+10Vdc difference) and the cathode current is relative similar between the two power tubes - which make sense as the previous set were most likely damaged. I'm really happy with the sound! The tube pulsing is almost completely gone, I played the amp in a dark room to find any variation in the screen grid and there is the slightest bit when the amp is pushed but nothing concerning or out of the norm. Appears the load may been the biggest contributing factor. I'll for sure keep in eye on the amps performance going forward. Really appreciate all the feedback from this thread, I would not have thought to check my cab without your expertise. Learned alot! Thanks again!
     
    peteb and clintj like this.

  16. nasdak

    nasdak Tele-Meister

    177
    Mar 22, 2013
    france
    i don't think the amp would work at all if heater and OT primary where really connected ?????
     

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