Nad- Jet City J20h, New Tubes & Bias

Discussion in 'Project Depot' started by Beyer160, Feb 13, 2018.

  1. Beyer160

    Beyer160 Well-Known Member

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    Jet City had a sale recently, and I picked up a JCA20H 20 watt head for about the price of a boutique dirt pedal.

    These amps are awfully bright with the stock Chinese tubes, so I ordered a pair of '80s NOS Russian Reflektor 6n14n (EL84) tubes from KCA, as well as a pair of JJECC803S and a GE JAN 12AT7. There was a slight problem with my order, but Mike handled it quickly with no hassle. Definitely going back to KCA when it's time for new tubes.

    The other thing about these amps is that they're biased pretty cold from the factory, probably to prolong tube life and allow users to roll power tubes without having to open the chassis. Fortunately, it's easy as pie to bias these things- the hardest part was getting all the screws out of the chassis.

    IMG_4617.jpg
    WARNING: THE VOLTAGES IN A TUBE AMP CAN KILL YOU DEAD. DON'T ATTEMPT THIS UNLESS YOU FULLY UNDERSTAND THE DANGERS INVOLVED.

    A pair of alligator clips are your friend- attach a clip to one probe and clip the other end to the point you want to test. You never want to stick more than one hand inside an amp at a time, old school electronics guys recommend working with one hand in your pocket to help resist the temptation to grab something and wind up completing a circuit with your body.

    Also, make sure you have your head plugged into a speaker load and your MV at zero. Standby needs to be off to take measurements.

    The first thing to do is measure the plate voltage between the B+ and ground (you can use the chassis as ground reference, you don't need to clip to the solder point under the B+ joint), mine came to 390vDC once it settled down. EL84s are only rated for 300v, and most designers push the envelope on voltage- this is why EL84s tend to wear out pretty quick. The Reflektors are a Russian military tube, so they should handle more abuse than modern commercial production.

    I plugged this info into the Weber Bias Calculator:
    http://www.tedweber.com/webervst/tubes1/calcbias.htm

    and it told me I was looking at a maximum (70%) 21.5 ma plate dissipation. A 50% "cool" bias would be around 15ma, a 60% "average" bias would be around 18ma.

    I reset my VOM for current (don't forget to do this!), clipped one probe to the B+, then measured current in ma from B+ to each secondary leg of the OT- the blue and brown points circled in the picture.

    Mine came out to 11.7 and 12.5- WAY cold. I fiddled with the bias trimmer until I got it to 19.7 and 18.5.

    After than I swapped V1 for the 12AT7, and V2 and V3 for the JJs. V1 and V2 are the cascaded preamp gain stages, V3 is the PI. I wanted to reduce first stage gain to give it a little more clean headroom. If you're looking in from the back panel, these tubes are in reverse order- V3 is right behind the EL84s, V2 is to the right just behind the OT, V1 is on the end, pretty well hidden behind the OT. I labeled mine, but neglected to take a pic before reassembling the amp.

    If I was trying to be scientific I would have only made these changes one at a time to gauge how much effect each one, had, but I didn't. With the stock tubes and cold bias the amp has a pronounced, shrill high end- I immediately zeroed out the presence knob, rolled the treble to 9 o'clock, and didn't touch them again until after changing tubes. After the bias and new tubes it's still got that EL84 brightness, but I could bring the presence and treble back in without this happening:

    giphy.gif

    It's a 20 watt amp and I'm not trying to use it at bedroom levels, so I usually leave the MV full up like an old skool non-MV brit amp. Before you couldn't get any clean headroom with humbuckers, now it's clean 'till about noon. Full up on the gain with a Strat is crunchy but not full-bore distortion, but hey, that's why Jimi used a Fuzz Face.

    I did also try a JAN GE 6829 in V1- I think they were computer tubes, but it's got the same pinout as the 12XX7 family. It sounded good, but it has a little less gain than a 12AT7. The 6829 didn't give me quite enough gain, the 12AT7 was juuuust right.

    Now the amp has a very Marshall-y vibe (the design was based on the JCM800) with solid '70s rock crunch on tap, as well as some sweet low-gain, edge of breakup sounds. For around $275 brand new including the new glass, I'm happy with my purchase. It's a nicely made amp, too- good thick traces and solid circuit board. If you're looking for a no-frills amp to crank up for some dirty rock fun, definitely take a look at these.
     
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  2. Beyer160

    Beyer160 Well-Known Member

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    This is what it looks like on the outside: Screen Shot 2018-01-26 at 1.59.29 PM.png
     
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  3. mozz

    mozz Well-Known Member Supporting Member

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    Most people don't like a 12At7 in V1. Have you tried 5751 or 12ay7? My bias calculator for fixed bias ( Rob robinette) is showing me 21.5ma at 390v for 70% on 6bq5s. When you up the current , the plate voltage drops, so you have to measure again and recalculate. 390 is very high, anything over 320 makes me worry with that tube.
     
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  4. Mandelorian1138

    Mandelorian1138 Well-Known Member

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    I use a 5751 in my JCA20 combo and love it. Cleans it up just enough.
     
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  5. Beyer160

    Beyer160 Well-Known Member

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    -I don't have a 5751 handy, but a 12ay7 has about the same gain factor as the 6829 I tried, which wasn't quite enough for me (I know there's more to it than that, but based on gain alone I wouldn't want a 12ay7 in that position).

    -Thanks for pointing out the voltage drop, I totally forgot to mention it. What I actually did was use two meters- one for the plate voltage and one to measure current, so I could have them going simultaneously. I probably should have included that part, but it wasn't strictly necessary to explain the procedure and I felt like I was writing a novel as it was.
     
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  6. despiadado

    despiadado Well-Known Member

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    Welcome to the Jet City club! I have a 100HDM, which is like the SLO's little brother in a few senses. They're very good amps for the price :)
     
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  7. mozz

    mozz Well-Known Member Supporting Member

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    Don't get caught up in the internet lore of tube gain. A 12ax7 has a gain of 100, but......in most circuits with stock resistors it is putting out a gain of ~ 50. Then some gain is thrown away in the tone stack if that is next. A 12at7 has lower plate resistance, that sometimes can drive a tone stack better. I myself love 12at7 in the PI (driver) stage. More than once I have measured amps putting out more watts with a 12at7 driving the outputs. Slamming a V1 12at7 with pedals also is great.
    PM me for some 12at7s ( have a lot).
     
  8. Beyer160

    Beyer160 Well-Known Member

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    I hear you about tube gain- I know there's a lot more to how a tube behaves in a circuit than just the gain factor, but I wasn't aware that "lower gain" tubes could push the circuit harder in certain circumstances. I did find the 12at7 gave me more headroom than the stock 12ax7, but not as much as the 6829 (which was too much- this is a Marshall derivative, after all). My holy grail tone is Pete Townshend's P90 SG / Hiwatt combo from the Live At Leeds era, this amp is definitely in that ballpark with a 12at7 in the V1 slot. Nice sparkly clean sound when you lay back, but it snarls like a mad dog when you dig in. Now all I need is a 4x12 loaded with Fanes...
     

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