Transformer Hum Issue/PT groundI’m thinking it’s a PT ground issue

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by rockometeramp, Feb 23, 2019.

  1. stoddad

    stoddad Member

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    Since the last posted on my Mojo 1484 build when I was pretty sure I was having OT issues, I installed a new OT. I've used Weber speakers and in general have liked Ted Weber's stuff, GRHS. So, I got their version of the 1484 OT. I had to build a bracket to raise it a bit so it fit in under the turret board, and the nuts are non standard, but otherwise it all fit and connected well.

    After I had done other clean up, re-dressing and some rearranging, I was hopeful the amp was going to sound good. It did.

    At least during the start up procedure it did. The light bulb current limiter acted exactly as it should, which it really had not done before. The voltages were still high, but with it plugged into the limiter it was close to dead quiet, and sounded good when played, no hum.

    So, I took it off the limiter to check the voltages. Hum. Not dangerous sounding like with the other OT, but enough to be bad. I checked the filament voltage. 7.2. With the limiter on it had been 6.3, exactly. So, I am thinking if I fix the voltage on the heater leads, I'm going to fix the hum. Also in researching hum issues, I saw that putting the output tube filament wiring out of phase (switching the leads around from the first output tube to the second as you wire in parallel) helps with hum. So, I did that. That did reduce it a bit, but it needs to be reduced more.

    I thought about diodes to lower heater voltage, but decided to go with resistors instead. Since affordable 10w .150 Ohm resistors are hard to come by, I'm going to build a broad with two sets of 5w .33s wired in parallel to fit between the PT heater leads and the tubes. This is bulky and will be a tight fit, but it should do the trick.

    Since the issue is still hum, I thought I should post an update to this thread.
     
  2. stoddad

    stoddad Member

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    So, Merlin B at Valve Wizard says a global approach to voltage issues would be better. It would help fix the high plate dissipation, as well as the high heater voltage that seems to be the major source of my hum. Indeed, my wall voltage is 124.8VAC. I hadn't noticed modern voltage causing issues on my 67 blackface Pro Reverb. So, hadn't thought this way.

    Anyway, he sent a link to an article on building a voltage bucking transformer for vintage amps, but Rob Robinette posted plans and drawings that look promising on the Telecaster discussion Page
    http://www.tdpri.com/threads/bucking-transformer-build.494003/

    Has anyone built one into a new amp based on vintage specs? I just assumed Mojo would have taken this into account when selecting the PT. You guys have any thoughts on stepping down the wall voltage? Thanks
     
  3. stoddad

    stoddad Member

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    I did build the bucking transformer. One side of it brings my wall voltage down to 111VAC. This has brought the heater voltage to 6.27VDC, so pretty much spot on. This improves the hum and brings it to an acceptable level for me. Not quite dead quiet, but pretty good.

    This has brought the plate dissipation down, but still nowhere close to 70% of a 6L6 GC's max. 30W is the rated maximum. So, 70% is 21W. Even with the biasing pot set as cold as it will go I have 29W on one side and 30W on the other. Though I have researched several methods for fixing plate dissipation, I'd like to get ideas for what has worked for you guys.

    The amp sounds good, but will probably eat output tubes.

    Thanks for any feedback.
     
  4. pdf64

    pdf64 Member

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    To reduce power tube dissipation at idle, the magnitude of the bias voltage applied to their control grids should be increased.
    If assistance is required to do that, please provide the voltage variation range of the stock circuit and a link to an accurate schematic.
     
  5. stoddad

    stoddad Member

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  6. pdf64

    pdf64 Member

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    The only 1k I can see is the shared screen grid resistor? But the image is small and I can't zoom in to a gif file easily.

    Bias refers to the negative voltage between the control grid and cathode.
    Are you sure that the bias supply components are correct? It's common for its reservoir or smoothing cap to be fitted with reverse polarity.
    If all good, remove the power tubes and measure the bias voltage range of control (lowest and highest negative voltage magnitudes), at the pot wiper and at terminal 5 of the power tubes.
     
  7. stoddad

    stoddad Member

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    PDF, I definitely checked everything in the my bias circuit -- I had not gotten any of the cap backward etc. I had a couple of resistors that were a little too far out of spec though. This actually indicated a method, as you had suggested, to increase the negative voltage on the grid to fix my biasing issues. Also with respect this thread, the solution did reduced hum, as well.

    Turns out I had somehow damaged the 1K /5W resistor in the bias circuit. This was probably due to the cabinet design I had built being too tight against the chassis. I basically had to pound the chassis (without tubes in, of course) to get it out of the cabinet after practice last week to do further work on it. Anyway, after that, dead amp. I had to test everything again.

    Ultimately, I found the bad resistor, and ordered a new one. That fixed that issue, but in all that, the amp actually got closer to a normal bias with a plate dissipation that was still not great -- like 100% of max 30 watts, but closer to what is has been even with the pot turned all the way cold. So, I probed all the components in the bias circuit again to find the 47K in the power feed to the bias pot was out of spec -- it read around 56K. So, I changed it out for a fresh 47K, and my plate issues were back -- over 30 watts plate dissipation. I had been researching changing Bias Supply Dropping Resistor (which is the 220K before the diode) in this circuit, before this. Seeing the effect of the 47K on the voltages, the voltage drop, hum, and plate dissipation, I thought maybe this resistor would be a place to test for a way to fix the issues.

    So, I installed the only thing I had that was not radically too much or too little of a change -- a 68K resistor in this position. It put everything in a manageable range. I was able to dial the pot within the 70% of max plate dissipation, and all the ranges for the voltages and grid voltages are exactly in the typical range for the 6L6CG's. After playing it for a while I had to increase the bias pot setting to more like 80% of max to get the overdrive I wanted, but it's pretty much the amp I have been wanting without the worry of blowing power tubes.

    I think that Mojo should have done a better job on this build, but much like Rocco's endeavors, if you can figure out the issues, and redesign some, you can build on what Mojo has started. I have certainly learned a lot from all of you, and from the process of building this amp.

    I am grateful for your suggestions and help.
     

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