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JJ 6V6S idling at 15.5W but not redplating, leave it alone?

Discussion in 'Shock Brother's DIY Amps' started by Badside, Jun 22, 2016.

  1. Badside

    Badside Tele-Holic

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    Cathode biased power section, into a Hammond 1760H (6.6K Deluxe Reverb OT).
    Cathode bias resistor is a 360 ohms.

    The amp has a "dual voltage switch", basically choosing between the 115V and the 125V primary taps on the 272FX PT, and it's diode-rectified. When using the 125V tap I get about 375V on the plates, with 28V dropped on the resistor (347V plate to cathode, 77.7mA). Assuming roughly 5% screen current, that gives us roughly 12.8W per tube of anode dissipation.

    However, when switching to the 115V tap, HV jumps to 405V and 31V is now dropped across the resistor (374V plate to cathode, 86.1mA), doing the same math yields 15.3W per tube.

    Officially the 6V6S is a 14W tube, but I've read that it'll do 16W, even 18W without breaking a sweat (bigger plate structure than a classic 6V6GT). It sounds glorious and is not redplating no matter how loud I'm playing it.

    The higher voltage mode has a distinct quality that makes it very desirable (more headroom and tighter, useful for loud clean tones or higher gain stuff). The lower voltage mode is more "brown sounding" if you will. Good for bluesy or tweedy tones.

    Should I leave it alone?

    My main worry is that this amp is for a friend, and years from now when he's on the road and needs new power tubes, maybe he won't be able to find JJs. I could always remind him to never use the high-voltage mode without JJs...
     
  2. 5F6Animal

    5F6Animal Tele-Meister

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    Leave it! 110% is almost normal for non voltage corrected 5E3 builds. Same plate voltage, same current.

    The TAD 6V6 is also a 14W, larger plate tube that handles some abuse.

    Any old 6V6GTA is also a 14W tube. There's a lot of vintage Fender 5E3's running 12W 6V6GT'S that plug right in to the wall. No buck trannie, no variac, no problems!
     
  3. bparnell57

    bparnell57 Poster Extraordinaire

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    As long as you have 450 volt B+ filter caps, I'd say you're fine, but I'd check the filament voltage. Even 10% over-voltage on tube filaments can cause significantly shorter life, according to "Getting The Most Out Of Your Vacuum Tubes", which is a technical piece from the 50's or 60's that is available for free online and that I respect highly.
     
  4. Fred Mertz

    Fred Mertz Tele-Afflicted

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    The only difference between the 6V6GTA and the 6V6GT is that the GTA has a controlled heater warm up feature. They are both 14 watt tubes under the Design Maximum rating standard. The 12 watt rating commonly referenced for GTs was under the old Design Center rating standard. See page 387 of The RCA Receiving Tube Manual RC30.

    Edited to add link.
    http://www.tubebooks.org/tubedata/RC30.pdf
     
  5. Badside

    Badside Tele-Holic

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    I had a few 10W resistors on hand so I figured why not try it.

    Tried a 560 ohm first, that brought it down to 76% dissipation at idle on high-voltage mode, but low-voltage was a lowly 68%. No noticeable crossover distortion, actually sounded very nice, but it lost some of the "chewiness" of cathode bias, pretty much sounded like fixed bias at that point. There was a bigger tone difference between both modes though, but the low-voltage mode was lacking punch.

    So I settled on a 470 ohm... for now. 92% in high-voltage, 76% in low-voltage. Sounds good, rounder than the 560ohm, but a bit cleaner and tighter than the 360ohm.

    I have a band practice tonight, we'll see how it fares in a band setting.

    I also changed the bypass cap from a 220uF to a 33uF, mostly because the old one was rated for 50V and I like to be at least 2X the bias voltage. It didn't really change anything tone wise, 33uF with a 470R still has a -3dB point of 10Hz (that's 3 octaves below the low-E!).
     
  6. Badside

    Badside Tele-Holic

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    Filters are 500V (cap cans cause I like how they look :) ).

    Filament voltage is actually one of the reasons I put the dual voltage option. At least in my house, the 115V tap had the filaments at over 6.5V, while the 125V tap brought it down to about 6.1V. So I figured: leave it on the 125V tap, but you have the 115V tap option when you're recording and you don't care about punishing the tubes a little more. We're talking about a 3% difference both ways though. The 272FX is rated for 5A and I'm not even pulling 2A, so it's under very light load.
    (I chose this PT to have the option of running EL34s or 6L6s)
     
  7. danlad

    danlad Tele-Meister

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    They (whoever they are) say that bias cools when overdriven so that is why you can get away with overdissipating 5E3 type output sections. I did do a quick and dirty test once, cheap digital voltmeter and dummy load resistor and the bias at the cathode and at the grids really did jump up strumming E chords at full pelt, while the anode voltage sagged. Hard to say by how much though with my lo fi equipment.
     
  8. Badside

    Badside Tele-Holic

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    As I understand, that's the theory behind biasing cathode-biased amplifiers much higher, because they can't "maintain" the higher bias point under load. Another reason I lowered the bypass cap (with a high enough bypass cap this point is made moot).

    As for the anode sagging, since mine is SS-rectified and has a higher rated PT (173mA) I can't rely on that too much.

    Plotting the load line on JJ's datasheet made me realize I was over 100% dissipation pretty much the whole time until just before one of the 2 tubes shuts off and shifts into class B. The lack of any redplating perhaps confirms how solid the JJs are (they are said to be more like a 6L6 than a 6V6)
     
  9. danlad

    danlad Tele-Meister

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    Your amp sounds not a million miles off mine in the output section. SS rectifier but a 125ma transformer so presumably my voltages fall way sooner, got about 380v to anodes iirc, 250 ohm resistor and I have stayed with a big cathode cap (100uf I think) and the bias still went walkabout. The coupling caps charging had as much an effect on my voltmeter readings I think, so the two in tandem must have quite an effect. Just a shame I couldn't see how much exactly by showing it on proper test equipment. And at what sort of audible volume and distortion levels it kicks in enough to have a real cooling effect on the bias.
     
  10. jman72

    jman72 Tele-Afflicted

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    The B+ on my 5e3 is about 380 V (measured when I built it). A set of TunSol 6v6s started red plating almost immediately when I fired it up (but they sounded great). I switched to JJ 6v6s and they've been going strong with no problems for almost two years (knock on wood!).
     
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  11. tubeswell

    tubeswell Friend of Leo's

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    If you factor out the screen current (about 2mA per tube), you might find that your JJ6V6S plate dissipation is closer to 14W
     
  12. Badside

    Badside Tele-Holic

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    My math did include 5% of screen current, which works out to roughly 2mA, which is confirmed by measuring the voltage drop across the screen resistor.

    Either way, used it with the band yesterday with the 470 ohm resistor that biases it at 12.75W (91%) per tube and it sounded just as good, perhaps a little less "round" but only I would notice it. Don't think it's worth making the amp "JJ-only" for such a small difference. Especially that I've had a few JJs go bad in recent weeks (mostly preamp tubes though, I'm switching to Tung-Sol)
     
  13. tubeswell

    tubeswell Friend of Leo's

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    Yeah I'm not to crazy about their pre-amp tubes. But I have to say that their 6V6S is about the best modern production 14W output tube available AFAICT (which is Irish for 'a faict')
     
  14. hackworth1

    hackworth1 Friend of Leo's Vendor Member

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    I concur. For new production tubes, the JJ 6V6S represents the best value.

    tubeswell is a tube amp master who surely knows his stuff. We are fortunate to have him share his expertise on this forum.
     
  15. tubeswell

    tubeswell Friend of Leo's

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    Gee shucks :rolleyes:
     
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