Saturation

Discussion in 'Amp Tech Center' started by peteb, Sep 2, 2021.

  1. peteb

    peteb Friend of Leo's

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    Saturation is known as overloading the OT to get some unique sounds.

    what amps saturate easily?


    I nominate the hot biased bf/sf champs as most easy to saturate.



    Does the bf/sf champ saturate the OT? How do we know?


    What (fender) amps do you think will easily saturate?
     
  2. str8razor59

    str8razor59 TDPRI Member

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    I had a tweed chassis Champ and it saturated early. I don't know about the SF/BF, but the tweed circuit was awfully sweet.
     
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  3. peteb

    peteb Friend of Leo's

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    I think the tweed amps are definite candidates with the smaller output transformers.

    however, the voltages were lower.
     
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  4. 2L man

    2L man Tele-Holic

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    When output transformer is small/light vs output power the output signal saturate more. Saturation first happen on low frenquency, obviously when the current flow longer time before changing to other direction on push pull OT. So saturation exists also on higher frequency signal when OT is small. On push pull amp tube imbalance increase DC current flow and it saturate OT.

    On SE amp OT bias current is average current and signal change this current higher and lower. On high power, low freaguency signal the current saturate small OT more.

    Tweed Fenders seem to have small/light OTs and Vox AC have big/heavy OTs when their power is compared. SE amps have much bigger/heavier OTs vs amp output power. Bass guitar amp OTs are bigger than guitar amps. HiFi OTs are biggest/heaviest because they need to output very low frequency.
     
    Last edited: Sep 3, 2021
  5. andrewRneumann

    andrewRneumann Tele-Holic

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    What’s the general take on the sonic desirability of this? Good, bad, ugly, depends on taste?
     
  6. uriah1

    uriah1 Telefied Gold Supporter

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    I always thought it was squishy in a good compressed way
     
  7. Gunny

    Gunny Tele-Holic

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    Peavey would have us believe they have this. Some models have a saturation control.
     
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  8. peteb

    peteb Friend of Leo's

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    It sounds like if you know what to listen for it is easy to hear, but otherwise it is difficult to quantify or measure.

    I like the comment about compression.


    I like this technical description I found:

    You can increase the applied field, but you won't increase the induced field (well only slightly)


    At

    https://music-electronics-forum.com/forum/amplification/guitar-amps/theory-design/17230-


    I just looked up causes of compression and the power supply is the main quoted source.


    This actually may explain one of the more unique sounds champs produce.

    This may explain the spongy feel as uriah1 mentions. First I thought it was power supply compression, but it’s not as single ended doesn’t sag.


    Then I thought it was cathode squish from the cathode bias, but this also does not happen in single ended.



    Is the OT compression what causes the spongy, squishy feel of champs that everyone talks about?
     
  9. andrewRneumann

    andrewRneumann Tele-Holic

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    I don’t quite get the analogy. Does the OT take a noticeable amount of time to recover after a loud passage that saturated it?
     
  10. andrewRneumann

    andrewRneumann Tele-Holic

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    That's the common wisdom, but I've begun to have my doubts about it. I think it is generally true that SE doesn't sag as much as PP, but have begun to believe that a certain amount of sag is possible in SE amp circuits and it's enough to be audible.

    Although anode dissipation power is usually maximum at idle, this doesn't mean average current can't increase under signal. If average current increases to the plate and/or the screen, this would cause sag due to any resistances between the power tube and the supply (screen grid stoppers, screen supply droppers, OT resistance, rectifier, PT secondary, reflected PT primary). The amount of sag will be a function of how much resistance is between tube screen and plate and the power supply, and importantly--the amount of capacitance in the supply. The supply capacitors will attempt to source the required current to maintain voltage, but if they are relatively small and the resistance in the power supply feeding the capacitor is high, it will sag over time. If the resistance feeding the capacitor is high, it will also take more time to top up the capacitor and recover from the sag at the end of the loud passage.

    That's my theory at least. I have experimented with different screen supply caps on an SE amp and found they made a very noticeable difference in the compressive sag (and recovery time) during loud passages, so it's not all speculative.
     
  11. northernguitar

    northernguitar Friend of Leo's

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    VOX amps have saturated chime.
     
  12. peteb

    peteb Friend of Leo's

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    I am not sure if that is the source of what I hear or not.
    Just saying.



    It’s not sagging down like the power supply (which is very hard to hear) it is a limit, the power out can’t exceed the capabilities of the O T.


    don’t doubt, don’t question, measure.

    measure the current flow in a single ended amp while you play, while you overdrive the amp.

    the current draw in pp amps changes with signal, not with single ended, not at all.

    understanding the different behavior between single ended and PUshpull amps helps in understanding tube operation.
     
  13. andrewRneumann

    andrewRneumann Tele-Holic

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    Would love to see some OT “compression” measurements if they are out there.
     
  14. andrewRneumann

    andrewRneumann Tele-Holic

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    Good news... in Guitar Amplifier Overdrive, A Visual Tour, Neumann and Irving devote an entire chapter to the topic of OT saturation. They tested a small OT see the effects of saturation and it's basically just clipping the top and the bottom of the waveform. They also tested a 6V6 and a Champ size OT and found that the signal level was not enough to cause saturation in that particular circuit.

    Interestingly, they also found that clipped signals from the the 6V6 were subject to extra OT distortion. The clipped tops of the wave are essentially short segments of DC. The OT attempts to maintain current by releasing some of its magnetic energy during the clipped portion of the wave. This causes the clipped signal to retain some of it's curvature for a while while the magnetic field starts to collapse. I'd post a picture but won't for copyright reasons. It is analogous to the sloping tops of clipped signals elsewhere in the amp caused by capacitance, but it's caused by the release of magnetic energy instead of the storage of electrical energy as in a capacitor.

    I'll also take the time to clarify what I mean by "sag" in SE amps. What I am talking about is local drops in voltage at individual B+ nodes, especially the screen node. This is indeed a far cry from the global sag in a true AB amp where the average plate current increases so much under signal that it drags down the whole circuit. All I am saying is that local parts of the SE circuit could sag under signal, and these voltage sags could be audible in my estimation. I am fully aware that a SE amp biased for current in the middle of its load line will draw the same average plate current under signal.
     
  15. peteb

    peteb Friend of Leo's

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    Do you have a link? I would be interested in seeing that.

    I have scoped my champs and PP amps. I am familiar with the differences.

    what causes those weird excursions in BF/SF champs if the is no sag or squish?


    If you understand that, why do you think single ended amps still sag, even if only a little.

    put a meter on the plate and jam on a champ. The DC voltage on the plate and the cathode are fixed.

    single ended sag none.

    my 6G2 sags about 100 volts.

    my Bassman head with SS rectifier sags about 30 volts.
     
  16. Bendyha

    Bendyha Friend of Leo's Silver Supporter

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    Last edited: Sep 7, 2021
  17. peteb

    peteb Friend of Leo's

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    thanks Bendyha,
     
  18. jsnwhite619

    jsnwhite619 Friend of Leo's

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    I used Hammond's 1750E OT for everything 2x 6v6 I've built except one amp. It's a Princeton Reverb OT clone, their version is listed as 8.5k, but it's small and self-limiting compared to a 6.6k Deluxe Reverb style, and that's why I like it. My take on saturation is as described above - the extreme ends on both sides start clipping and cutting off, and you are left with fat mids. Think about Tube Screamers and so many overdrive pedals that boost the mids - they are simulating a saturated OT sound. High gain and overdrive - to me - have more grit, grind, crunch to it, layered dirt. A cranked amp with saturated OT for me is kinda like the sound a humbucker neck pickup above the 12th fret when you get that violin sustain, fat mids, train whistle sounds. You lose definition of the EQ and balanced tone, it's more of a howling overdrive. Like how Clapton's "Woman tone" had the treble rolled off - the saturation level has the same woman tone and the tops & bottoms of the waves are being clipped and you get the middle of it.

    The last SE amp I built was this year and a 5881 version of a 5f2a, all components sufficiently over-spec to handle it. Hooked an iPod into the input, clipped my meter lead on the plate, then on the B+ after that. I absolutely watched the voltage drop as I turned up the volume and/or bass on the amp with music playing through it. I can't remember how much, but it reliable dropped as the volume increased, and would pop back up when I paused it or turned the knob down.
     
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  19. peteb

    peteb Friend of Leo's

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    thanks Jason,



    It’s not class A then.


    Likewise

    if your push pull amp’s power supply is not sagging, probably because the volume is half way up or less and you are only hitting the amp with a moderate signal, that is not operating in class AB, even though it technically is because you can only determine class of operation at full output.
     
  20. jsnwhite619

    jsnwhite619 Friend of Leo's

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    The original quote was "single ended sags none". I assume the entire machine must run at 100% efficiency because it delivers constant steady voltage & current at all moments and the power transformer, rectifier, nor filter capacitors can ever be taxed beyond their capabilities? Nothing ever struggles, nothing ever needs to "catch up", and the entire system cannot be pushed beyond it rated capabilities to cause a drop in voltage?

    I know the back & forth about the subject, I always say I'm never getting into it again, but I never listen to myself. But, single-ended amps are always talked about like "nothing you do to the power supply matters, it doesn't matter what rectifier you use, everything operates the same all the time because singled-ended/class A doesn't sag". It took 15 seconds with a meter and sound source to watch the voltage drop in operation. Turn up the volume & add bass, watch the voltage drop. This was a single 5881 power tube 5f2a amp.

    This guy wrote a research paper on a similar build with nothing mentioned about "sag" except the voltage drop he witnessed in testing it, 15v or more loss at full volume vs idle.


    upload_2021-9-15_13-30-4.png
    All that being said and on the subject of compression & saturation, I think that people often hear and like compression and "bloom" from the OT slipping in & out of saturation, or maybe the filter cap recovery time when you dig in hard. But, on my Harvard-type amp I keep messing with, I got a "good" clip of a 5y3 sagging and trying to keep up - sounds terrible. But, I've never gotten a clip quite like this before. Les Paul into a parallel input, amp on 12, 5y3 and in the 380v B+ range. It runs through the chord progression 3 times, that's the amp at 10, 11, then 12 at the end. That is one starved power supply. It sounds like the mic clipping, but I could hear it and feel it in the room. That was coming out of the speaker.








     
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