First Success: New Power Tubes And Rebiasing

Discussion in 'The Workbench' started by drgordonfreeman, Jan 8, 2017.

  1. drgordonfreeman

    drgordonfreeman Well-Known Member

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    I finally picked up my skirt, grabbed my balls, and put in new power tubes in my JVM. I bought some Ruby EL-34 BSTR's from Doug Tubes. After these, I'm thinking of trying some KT-77's, so any specific tube suggestions are welcome.

    Okay, so starting down this little journey, I was pretty nervous, mostly about trying to not die. :lol:

    I bought a little, cheap $10 multimeter from Fry's, some extra long connector cables, and some rubber gloves. Once I had the chassis out and tubes swapped, I first measured the plate voltage from pin #3. Interestingly, first thing I noticed was that the inside of my JVM was slightly different than the other ones I've seen posted online. In mine, there's a wire connection soldered between the pin #3's of each set of power tubes.




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    I went to measure the plate voltage at pin #3, and the first mistake was made. I thought I had the multimeter set on DC, but instead it was set on AC! When I connected the ground to the chassis and touched the lead probe to pin #3, I nearly emptied my bowls. I felt like Art from "The Burbs"




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    Having learned my lesson about safety, I double checked everything and set back in. I measured pin #3 at 475V DC. I then touched the ground and lead probes to the testpoint pins to take measurements for the bias pot adjustments. The left contact point measured at 65.7mV and the right contact pount measured at 75.4mV. I used a precision screw driver to very minimally adjust the bias pots. They aren't kidding when they say these things are sensitive. After a really tedious, iterative back-and-forth process to adjust the pots just right, I finally got them to to where I wanted them. I wanted to set them to 71mV each, and I got really close. I was able to get each pot tuned to 70.9mV. I wasn't about to try adjusting to get to a perfect 71, because one, who gives a shit if it's .1mV off, two, who can actually hear that difference (not I!), and three, my hands aren't stready enough to make that small of a twist on the pot.




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    After getting it set, I unplugged everything and let it sit for a few hours (mostly because I was hungry and needed lunch :lol:). I took one more set of measurements, and I got the same readings. I plugged in a guitar, turned the master up to 11 o'clock, and strummed a g chord. It sounded great! Better than my expectations, honestly.




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    Now that I've learned how to do it, I think I'm going to try KT-77's in it after I've had some fun with these new tubes.

    Also, as a side note, one of the screws holding the chassis was stuck tight, and I stripped it when trying to get it unscrewed. Does anyone know an easy place I can buy these screws? I didn't want to go up to the hardware store and spend all day hunting for this specific type of screw...




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  2. johan.b

    johan.b Well-Known Member

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    God job.
    The bolt is metric M6..Any better nut and bolt store should have them, maybe not in black, but still. ..my nut'n'bolt guy has them, but is a long way from Texas
    J
     
  3. drgordonfreeman

    drgordonfreeman Well-Known Member

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    Thanks, Johan. That helps a lot. That'll help focus my search down at the hardware store. Yea, being 3,000 miles away, I don't think the USD is that strong to justify the trip for a 75¢ screw.:lol:

    On the KT-77's, I've already been doing a little bit of research online, and it looks like you have three options: JJ's, Golden Lion's, or $$$ NOS. I've seen a few posts on the JVM Forum about the Golden Lion's being excellent, so I'll probably try those first.
     
  4. Matt_Krush

    Matt_Krush Active Member

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    The JJs are fine too.
    I have them in my JVM and a home build.
     
  5. Jethro Rocker

    Jethro Rocker Well-Known Member

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    Good job! With my multiple amps, I boight a 100 dollar bias probe from Eurotubes so it measures PV and bias current. The tube fits into socket on probe which fits into amp. Easy and safe, for me!! Incidentally, the little blue wire from pin 2 to the little clip connector on output section on picture #2 is the one you can splice the pot into for neg feedback mod.
     
  6. drgordonfreeman

    drgordonfreeman Well-Known Member

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    You mean for the KT-77's? Do you know what the difference is between the JJ's and the Golden Lion's?


    Yea, I considered getting the bias probe, and I still may eventually, since it adds safety.

    So, I can't seem to find that blue wire you're referring to in pic #2...:scratch:
     
  7. Matt_Krush

    Matt_Krush Active Member

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    Yeah, KT77s.
    Other than cost, no I haven't compared them.

    I just picked the J&Js because they were cheaper and I wanted to hear, if I could tell a difference. It was my first time using KT77's...and JJs for power tubes.

    I like them, and will buy them again.
     
  8. Jethro Rocker

    Jethro Rocker Well-Known Member

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    In pic 2 there is a small white plastic clip on left side connecting to output PCB that runs 2 small blue wires with the bigger ones over to the main board. One comes from pin 2 on main PCB plastic clip. I snipped it and ran the pot off there as per mamy that have done the mod. Sounds hreat IMO it's essential!!
     
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  9. drgordonfreeman

    drgordonfreeman Well-Known Member

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    This gives me confidence then. Thanks, Matt.


    Ah, got it. I see now. Well, that doesn't seem like a big deal to do...
     

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