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6v6 -> 6L6 confusion

Discussion in 'Shock Brother's DIY Amps' started by ejb222, May 4, 2016.

  1. ejb222

    ejb222 Tele-Holic

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    here's the skinny. I brought my homebrew amp to a tech today to help trouble shoot a couple issues. One thing he decided to do was change the cathode bias resistor on my parallel 6V6s (single ended) to a 500R. This helped bring my plate dissipation down to 75ish%. Then he threw in a pair of 6L6WGS in place and the amp worked...and sounded great. But I'm confused as I thought my OT would not like the different impedence from the change in tubes.
    some numbers to help:
    plate to cathode v is 344V
    voltage across 500R cathode resistor 27.6v
    OT is edcor 10w 8ohm 5k

    I didn't think these tubes were interchangeable
     
    Last edited: May 4, 2016
  2. JD0x0

    JD0x0 Poster Extraordinaire

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    The OT's usually aren't that fidgety. There's definitely better spec'd iron for the application, but that doesn't necessarily mean you will run into issues. I'm curious as to why the bias was brought down so much, and then 6L6's were added. Class A cathode bias likes a bit higher dissipation, and running a 6L6 will definitely be running it really cold. Right now, I believe you're at ~30% dissipation with the 6L6's. That's really low for class A, IMO.

    I swapped a 6V6 for a 5881 in a single ended amp and have had no issues. Right now, I believe I'm running at about 92% dissipation and the amp sounds really healthy and big for such a low wattage.
     
  3. ejb222

    ejb222 Tele-Holic

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    so we kept the 6v6s in. He biased them a bit cold as they were running at about 115% previously. I might bring it down to 400R in order to get the amp back to about 93%

    do you think the 6L6 were not true to their sound running at only 30% dissipation...i really liked the sound
     
  4. 5F6Animal

    5F6Animal Tele-Meister

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    It looks like you should run two 6V6's at 4 ohms and a SINGLE 6L6 at 8 ohms.

    And yes, the bias is way too cold. 240-300 ohm cathode bias resistor should work a lot better for either tube/tubes.

    Check out Rob's bias calculator.

    https://robrobinette.com/Tube_Bias_Calculator.htm
     
  5. ejb222

    ejb222 Tele-Holic

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    thanks. I've been using that sight to help understand what I've been doing.
    thank God for the kindheartedness of people to put a site like that together.
    so currently with my OT made for a 5k impedance and 8ohm load, 6L6GC in parallel would be under that 5k impedance as direct replacements. 4ohm load would be more appropriate with the same OT no???
     
  6. 5F6Animal

    5F6Animal Tele-Meister

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    I'm a bonehead!

    6V6's like to see 8k, 4K parallel....close enough to [email protected]

    6L6's like to see 4K, 2k parallel. A single 6L6 @ 8 ohms would be fine.

    You shouldn't run two 6L6's unless your PT can handle the extra current on the heaters and high voltage.
     
  7. Wyatt

    Wyatt Tele-Afflicted

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    The OT does NOT "see" impedance from the tubes...it's the other way around...the tubes see the impedance provided by the OT.

    The OT doesn't care...show it a 8-ohm speaker load and it'll reflect 5K back to the tube plates regardless of what they are.

    How much impedance the tube plates see determines how efficient they operate. BTW, tube amp designs have used all sorts of mismatches to control the tube operation.

    BTW, If you look at the tube data, in cathode-biased, 6L6's what to see higher impedance (9K) than fixed biased designs (5K), at least in push-pull operation.
     
  8. Commodore 64

    Commodore 64 Friend of Leo's

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    [​IMG]

    Regarding the impedances, the above shows you that power output and distortion are affected by the load resistance (primary impedance). The values up there aren't all that useful because your tube isn't using those values, but I think the graph can help you understand. Different impedances affect different tubes differently.

    In the case above, if you were only at 1k, you'd be getting significantly less audio power out of the tube than you would at 3 or 4k. Make sense?

    How is this useful in practice? Well in my case, I wanted to find the best cathode resistor value to run a couple of my amps with. Here's what I'd do.

    1. Using a breadboard, I'd jumper in a resistance in parallel to the existing cathode resistor.
    2. Calculate the resistance value.
    3. Play a clip through the amp. Used a looper, so I could use the same clip over and over.
    4. Crank the volume until it sounded like distorted mush. Dial it back a tad.
    5. Observe the voltage across the speaker terminals with a multimeter.
    6. Record the max voltage I saw.
    7. Use Ohm's law to calculate audio power.

    Here's a vid I recorded of the process.


    Using that process, I graphed the response of cathode resistors vs. power output and selected the best. IN the case below, my initial value was 270, and I got significantly better performance with ~200 Ohm.

    [​IMG]
    Here's other data I collected in the process:
    [​IMG]
     
    Last edited: May 5, 2016
    asdTerror, danlad and tubeswell like this.
  9. tubeswell

    tubeswell Friend of Leo's

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    Tubes pass tube current. The hotter they are biased, the more tube current they pass. Biasing controls DC current, but the OT impedance controls the current under signal conditions. (We call the OT load under signal conditions a 'reactive load' because the load changes all over the place depending on what's happening with the speaker and the signal). The most efficient load is the one that gets the optimum power transfer through the OT for a given amount of current. The OT transforms high V(ac) (and lowish current) on the primary, to low V(ac) and higher current on the secondary (to drive the speaker coil better). The higher the voltage swing at the primary (for a given V.A), the better the transformation to high current at the secondary. But, if there isn't enough V.A at the primary, then there won't be 'enough' V.A on the secondary either.

    By lowering the OT impedance, you lower the reactivity of the load (because you lower the V.A transformation). With a lower reactive load, the tubes will pass more overall current under signal conditions, but there is less voltage swing on the tube plates going to the OT primary (all other things being equal). Hotter biasing tends to shorten the life of the tubes and reduce the efficiency of the Pri:Sec V.A transformation.

    There's a few other* aspects at play besides the signal and the theoretical obtainable V.A transformation, but these are the most significant variables. Besides those, the V.A transformation 'equation' works within a certain band of tolerance, but it is dynamic and it changes depending on how where you are on the spectrum of infinite voltage or infinite current. As you approach either extreme, the V.A transformation equation tends to fall apart. (That's my take on it anyhow).

    *Amongst the other variables is the tolerance of the output tube electrodes to V.A abuse, the L/C reactance, magnetising current saturation point, and inductance leakage of the OT, and the reactivity of the speaker(s).
     
    Last edited: May 5, 2016
  10. Commodore 64

    Commodore 64 Friend of Leo's

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    Incidentally, that graph above, was a 5881 Self-SPlit topology. Below is a LTP topology with 6V6. Used the same measurement method, the 6V6 LTP put out more audio wattage than the 5881 Self-Split. And it's also less sensitive to the cathode resistor value/voltage.
    [​IMG]

    (I prefer the tone of the self-split, current fave is EL34 self-split.)
    [​IMG]
     
    Last edited: May 5, 2016
  11. ejb222

    ejb222 Tele-Holic

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    thanks Commodore 64. And everyone else. This is super informative. I'll have to dig into this more. I also see that the plate current on these 6L6s are way to high for my OT that says 80ma max according to the specs. And my PT is only rated for 175ma max. So I guess a well biased 6L6 amp will have to wait. But thanks for.all your input. I'll dig into your posts more and ask questions as I have them.
    OH...and even order harmonics are more desirable then odd correct? I'm going to bust out my data sheets and take a closer look at the 6V6s biasing. I feel the distortion my amp produces isn't very "pretty". Guess I'm not sure what it "should" sound like as a single ended 6v6s in parallel with 6sl7 preamp.
    again, thanks a bunch
     
  12. elpico

    elpico Tele-Afflicted

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    It's good you're watching the limits of your transformer set. If you have the heater current to light them though there's no reason to avoid the 6l6 tubes. It's not like 6l6 *need* to produce a certain amount of power. They have a maximum power rating to keep them from being destroyed, but there is no minimum rating. You can run a 6l6 tube as a 1w output stage if you want. You can run any big tube at the same voltage and current as a 6v6. You won't get more power out, just a different sound. If you like the sound that's all that matters.

    Tubes don't have "likes" or "wants". They don't desire any particular bias current, power output, or OT impedance ratio. It's your desires that dictate those values in your designs. If you use a tougher power tube than necessary in an amp like this generally three things happen: lifespan is extended, the clean sound gets cleaner because the tube is distorting less (not talking about clipping here, but the amount of distortion that's always added), and you may get relatively more/tighter bass. Many people would like cleaner cleans and tighter lows, but if that's not your sound a simple change to the negative feedback resistors can add that color/distortion back in without changing the amount of power being used.

    On your second point about even vs. odd order distortion, no that's just internet folk "wisdom" that gets repeated without really questioning or understanding it. When you play a note on your guitar you're actually producing the root note plus every harmonic, both even and odd. They're all there in your signal already. If odd order distortion sounded "bad" then teles and acoustic guitars would sound especially horrible because they have louder harmonics relative to the root note than most guitars. (what we call treble or brightness in guitars is actually those harmonics). IMO when we add distortion with an amp what we want is to add LOTS of new frequencies, all of the harmonics, but we want to do so evenly. If one range has a lot of distortion components being added and another has very little it will sound bad, and the range with a lot (usually the upper mids) will easily become so densely crowded it turns into irritating noise before you can get the other ranges to the thickness you want.
     
    Last edited: May 6, 2016
  13. Commodore 64

    Commodore 64 Friend of Leo's

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    175 mA is probably OK for your PT. I have an Edcor 120mA running the 5881 self-split above, and it barely even gets warm. A quick (and relatively reliable) way to see if your PT is hacking the load is to put your hand on it after playing. If it's to hot to leave your hand on it, you are probably at the limits (it should be warm, even slightly hot).

    And every Edcor I've ever used (and they are my preferred iron) is pretty stout.

    Really, it's probably as simple as which one you like better tonewise.
     
    Last edited: May 5, 2016
  14. ejb222

    ejb222 Tele-Holic

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    but what about the OT? It's a 10w 5k 8ohm rated at 80ma max. I assume the max current is the sum of the current running through the plates of the output tubes? The tech said I was pushing 15w out of the transformer as it sits now with 6v6s at about 75% dissipation. But can I really push the current above the 80ma max? I suppose not all iron is created equal so maybe some companies aren't as pushable as others?
     
  15. mRtINY

    mRtINY Friend of Leo's

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    It's likely that the 80mA is "per leg". There are some spec's out there on mis-match, but 80mA is way to high for that....


    -tINY
     
  16. Commodore 64

    Commodore 64 Friend of Leo's

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    You have a schematic for your amp? What phase inverter? If you are on cathodyne, and there isn't big grid stopper, consider adding one. 500k to 1 meg. Merlin Blencowe says this is the key to good distortion on cathodyne, and having implemented this (with the folks over at ppwatt), I agree.

    good info here: http://www.ax84.com/bbs/dm.php?thread=389061

    http://ax84.com/bbs/dm.php?thread=488611


    EDIT WHOOPS: Forgot it was parallel SE. That's why your distortion is loose and unpretty (IMHO).

    Tonally I much prefer PP amps. And the transformers are cheaper as well. So I'll leave the above in case you wanna explore a new topology.
     
    Last edited: May 10, 2016
  17. ejb222

    ejb222 Tele-Holic

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    thanks. Now you have me looking at PP OTs.. Any advantage to class AB vs class A pushpull?
     
  18. mRtINY

    mRtINY Friend of Leo's

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    A vs A/B is only a matter of setting the bias.....

    A/B will give you more output.
     
  19. tubeswell

    tubeswell Friend of Leo's

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    In a push-pull amp, the output stage is biased in such a way that it runs in Class A at low volumes, and then as the driving signal (from the PI/driver) gets bigger, it changes to Class 'A/B'. This is where Class B operation kicks in for part of the signal cycle, but Class A predominates for the majority of the rest of the signal cycle. The Class B mode kicks in when the PI/driver signal is big enough. With the right amount of (slightly cooler) bias, a sufficiently big grid voltage swing will force the output tube that is on the side that experiences the negative signal-grid voltage swing, to go into cutoff. In this condition, there is no current through the tube, so that side of the OT primary is removed from the reflected load. When this occurs, the OT's Pr:Sec impedance ratio is effectively halved. This results in a halved load resistance - causing more signal current to flow in the opposite side of the output stage (the 'conducting' tube) and resulting in the steeper "B' load line - during this 'B' part of the signal cycle for the conducting tube. This condition pulls the plate voltage (in the 'on' tube) down further than it would normally go on the 'bottom peak' part of the plate signal swing. This results in a bigger peak voltage swing, and more power-transfer through the OT to the secondary - which means more current to drive the speaker harder. The output tubes can tolerate this temporary exceedance of operating conditions during the Class B part of the cycle, because they are in cutoff for the same amount of time on each alternate part cycle, so the average dissipation remains as normal.
     
    Last edited: May 12, 2016
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