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Dual SE vs Push Pull

Discussion in 'Shock Brother's DIY Amps' started by Jewellworks, Oct 4, 2020.

  1. Jewellworks

    Jewellworks Tele-Meister

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    I'm considering building a dual SE amp, with Parallel OT tubes. I was wondering how the output compares to a Push Pull? If the tube data sheets say the Wattage is 10W, am I correct is assuming having 2 in PP is 20W? (Probably less, but for arguments sake, they add together). So I would need an OT to handle 20, 25 watts. How does parallel tubes add up? What's the calculations for PP vs parallel SE?

    Thanks
     
  2. elpico

    elpico Tele-Afflicted

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    Short answers (all assuming 10w tubes):

    single ended = 5W OT
    parallel single ended = 10W OT
    push pull = 20W OT


    Longer answer:

    Single ended amps can only operate in class A. The maximum output of a class A amp is only half the power rating of the tube. With the losses of real world parts it will always be less than that, but keeping things simple, if your tubes are rated 10w then a 5w output transformer is enough for a single tube and for two in a parallel SE use a 10w OT.

    Push pull amps can operate class A, class B, or anywhere in-between. Class A is still class A, it doesn't matter whether it's push pull or parallel single ended, so for a class A push pull amp you need a 10w OT. A perfect class B amp can make roughly 3x as much power as the class A version, so a 30w transformer would be the upper limit. Most push pull amps operate in between between those extremes, in class AB. There a 20w transformer would be good.

    Note that the power is only one of the OT's ratings. You need to select a transformer with an appropriate impedance for each of these cases too.
     
  3. Jewellworks

    Jewellworks Tele-Meister

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    @elpico -thank you for that clear and concise answer
     
  4. Jewellworks

    Jewellworks Tele-Meister

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    yyyyeeeeeahhhh, im struggling with that as well at the moment.
    without going into a ton of backstory and depth and detail, i have a previously calculated draw of around 70ma, and a peak voltage will be 452V. ohms law says the load resistance will be 6457ohms. due to the winding ratio i measured and calculated on my OT, i get a load resistance of 3.3k w/an 8ohm speaker, and 6.6k w/an 16ohm speaker. however, all i can use in this particular amp is an 8ohm speaker. so 3.3k, when the tube wants to see closer to 6k. i was told to take a look at the data sheet for the load resistance, but i REALLY dont know how to read that chart.
    Image1.jpg
    is it as simple as this? im still not sure what the intersection is telling me.
    but, my read on this is ... reading where the harmonic distortion lines cross is right where my Rl line is, so id have an almost equal amount of 2nd and 3rd order harmonics, near the 5% mark, which could sound "bad"? is that what is read into this? (the harmonic distortion around 5% of the fundamental, so its pretty low, comparatively) it seems like 5K would be best, (and what the data sheet says the Rl is) -to bring out more of the harmonics? or at least more separation. 6.6k isnt even on the chart, so ... the harmonics "Trend" seems to be going up, so, even more harmonics? and perhaps another criss-cross, but higher in the chart?
    thats how im reading this anyway. am i right?
    and if i were to do this as a dual SE, my cross point would be higher in the chart, but also depending on the Rl, correct? its strange it only goes to 7.5 on the left, but also says the Po (peak output?) is 15 on the right.

    and what else -if anything- CHANGES if i only present a 3.3k load when it wants to see 6.4k-ish? any circuit considerations i need to adjust? or is it just a change in the harmonic distortion levels? that seems too simple...

    thanx in advance
     
  5. elpico

    elpico Tele-Afflicted

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    That chart is telling you what happened with various resistive loads in one particular amp. In this case a single tube, single ended amp where the power supply is 250V at both the plate and the screen. Different supply voltages or different power amp types would produce different curves and different numbers. So would loading the amp with a speaker instead of a resistor.

    Given all that, the chart doesn't tell you anything specific about your amp. So why does it exist? What I'd encourage you to take away from this chart is this: given that hearing is logarithmic, these curves are actually quite flat. The loudness of the amp for example would be the same using a very wide range of loads. Where does it show that? Look at the output power curve, Po. In this amp anything from 2k to 6k would be within 1dB of each other, which is around the threshold of being an unnoticeable difference / smallest detectable one. That's a 3:1 range, quite forgiving. That's what this chart is really trying to tell you. It's necessary for audio tubes to be tolerant of a very wide variety of loads and this is the designers showing "how good we did" at that goal.

    Why is it necessary for the tube to be tolerant of a wide variety of loads? The OT and speaker have only one load printed on them after all? Because those numbers are made up. :D Okay okay, they're not made up, but they don't tell you much about the reality of playing guitar through a speaker. If you played a single frequency sine wave at around 250Hz through the amp *and nothing else* the speaker would look kind of like an 8 ohm resistor. (a 6.5ohm resistor plus 1.5ohms of capacitive reactance) This is the lowest load the speaker will ever present to the amp, which is why it's printed on the label. Play other frequencies through the speaker though, and of course that's exactly what we do all the time, and the speaker isn't 8 ohms at all.

    Here's a chart showing the real impedance of a 12" jensen speaker at different frequencies:

    [​IMG]

    Yes you're reading that right. That "8ohm" speaker is already 12ohms by 1kHz and rises to 32ohms by 10kHz, but the lower notes are even worse. Play the open A string and the speaker acts like a 40ohm to 97ohm load, depending on whether the cab is open backed or sealed. And ALL of this added impedance, anything above 6.7ohms, is reactance not resistance, so it behaves differently than the purely resistive loads used in tubes data sheet. The OT dutifully transforms these loads, which are up to 10 times higher than the label suggests, back to the tubes. So it's a good thing they're tolerant of a wide variety of loads.
     
    Last edited: Oct 5, 2020
  6. Jewellworks

    Jewellworks Tele-Meister

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    @elpico -thats the best explanation ive ever gotten for how all this ties together. this has been a big hang-up for me, and i think i can finally move on getting started with this amp build. i cant thank you enough for taking the time to lay this out so clearly.
    One last question... So what does it matter if i use a 5k or 3k or 8k Rl OT ? If it changes that much over the frequency range if the guitar, and they're all within 1db of each other?
     
    Last edited: Oct 5, 2020
  7. elpico

    elpico Tele-Afflicted

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    You'd find that those load impedances do sound and feel a bit different, even if the volume is largely unchanged. Unfortunately I'm not aware of any line you can draw on a chart that plots a sound's "goodness". I see it as something you can only know by trying different loads, playing the amp, and listening.

    I know it's not as easy as we'd wish to try a variety of different loads with off the shelf transformers. It would be ideal if we had a transformer with a dozen different taps you could use to vary the reflected load over a nice range then listen for your favourite one. We don't have that, but you can't let that stop you from picking an OT and continuing the project. Sometimes the perfect is the enemy of the good, as they say, and you have to just jump in someplace and go from there. The most logical place to jump in is to use a value used in past well liked amps. You can hear what half or double that load sounds like by connecting your 8 ohm speaker to the 4 or 16 ohm taps.

    Are you building a single ended 6L6 amp? If so I suggest you take a look at a recent build by fellow tdpri member @jsnwhite619 . The output transformer he used has two primary taps that are intended to reflect either 5k or 8k. By utilizing the 4,8,16 speaker taps as well you could use that transformer to hear 2.5k, 4k, 5k, 8k, 10k, and 16k loads. The upper two are unlikely to be what you want for a 6L6, but that's still a few good options.
     
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  8. jsnwhite619

    jsnwhite619 Friend of Leo's

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    I've never done a parallel SE, but I'll say this -- depending on your desired power & output, a 20-25W SE amp would probably be a monster. My 5881 Tweed Princeton @elpico referenced above was a SE 5881 that essentially ended up as a Blackface Champ circuit with a single tone knob would blow your hair back with a good efficient speaker. I loaded it with a Cannabis Rex and the guy who has it now said he rarely plays it above 3 on the Volume. And that's with a 12ay7.

    Concerning the OT, I tried 8k & 5k with the 5881 and ended up picking the 5k for extra headroom & punch; the 8k was a very mushy Black Keys type sound. I liked it but the cleans were really nice. My OT was a ClassicTone 15W SE option for Tweed Champ/Princeton - same physical size as a 15W Tweed Deluxe OT.
     
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