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Thread: What is Transformer Saturation??

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    What is Transformer Saturation??

    I've been away for a long time.. I forgot that this is the place to get real answers! Sorry...


    When you play a big amp at home at a hairs volume, the tone seems to be stale. But it seems to sound and feel way better loud even though I'm nowhere near clipping the Output Section of an amp that wasn't meant to clip the Power Tubes... What do you think is going on here? Can you have Core Saturation without clipping the Power Tubes?

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    Supporting Member TomCarlos's Avatar
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    Here is an easy read... found it on the Sweetwater Web site - Core Saturation.

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    Well. That explains why a Bassman 100 OT is 2x the size of the PT...

    Definitely no reduction in bass & will rip your headoff on 10. Yeah, I tried it.

    Justin

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    "Wow it's red! That doesn't look like the standard Marshall red. It's more like hooker lipstick/clown nose/poodle pecker red." - Chuck H. -
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    Old Timer tedmich's Avatar
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    me after the second movie?
    Click image for larger version. 

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    My guess that what is happening is because the ear is not as sensitive to low and high frequencies at low volume. So, when you turn it up you start to hear the less mids and it's more balanced.

    https://en.wikipedia.org/wiki/Equal-loudness_contour

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    That, plus maybe filtering effects of the volume pot at low volume (even with a treble bleed cap) as well as the speaker cone inertia and mechanical damping that's not being overcome by the signal. Also reduced back EMF from the speaker. Perhaps also the filtering of the speaker fret cloth at low volume attenuating highs. All kinds of ratios and relationships could change. I haven't looked, but how does the response curve of a speaker in an enclosure look at low volume? My thought is that it wouldn't be linear for all frequencies.

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    Can you have Core Saturation without clipping the Power Tubes?
    In principle yes - if the OT is underdesigned.

    Saturation doesn't require power tube clipping. It is caused by too high primary voltage amplitude at low frequencies (excessive voltage-time area). It always starts at low frequencies.
    When the core fully saturates, the OT acts as a short to the tubes and is no longer able to transfer power to the load. Beginning saturation would primarily show in weak bass response.
    I don't think that OT saturation is desirable at all.

    I think most of us will agree that even beginning core saturation at medium output power is very unlikely as that would render the amp unusable at full power.

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    Last edited by Helmholtz; 03-30-2020 at 09:03 PM.
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    Transformer saturation is very unlikely to happen in a real amp. It doesn't look good on the scope and doesn't sound good either.

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    When this topic cropped up previously, I think it was Malcom Irving that put forward the hypothesis that transformer saturation was impossible when driven by pentodes (or beam tetrodes).
    As the core starts to saturate, primary inductance starts to collapse, the power that the tube can develop across the load reduces; hence the verge of saturation is a self limiting condition.

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    Quote Originally Posted by pdf64 View Post
    When this topic cropped up previously, I think it was Malcom Irving that put forward the hypothesis that transformer saturation was impossible when driven by pentodes (or beam tetrodes).
    As the core starts to saturate, primary inductance starts to collapse, the power that the tube can develop across the load reduces; hence the verge of saturation is a self limiting condition.
    Makes sense as pentodes are high impedance current sources. Nevertheless bass response will suffer and power tube distortion will increase by heavy current loading.

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    Thanks for all replies! I'm trying to better understand my turned up tones so that I can try to reproduce them quietly at home.
    In other words, I'm OCD about having the right settings on the amp that represent what I would play really loud, so I can leave them set that way when I record at home with the master turned down.

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    Sorry, but that's not going to happen because of a number of things. Google and read about the "Fletcher Munson Curve" for one. Also, depending on exactly "how loud" you're talking, some of your sound may be power tube clipping, which you won't get at low volumes. It's simply not possible for an amp to sound the same at extreme volume changes. That's not to say you can't arrive at a good low volume sound with some tweaking. Use your ears instead of knob pointers. A power soak might get you closer, but still.........

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    Also the speaker cone has increasing distortion levels with volume, and depending on the mix of notes played. You may also have heard about cone breakup and resonances, and feedback from speaker to guitar strings. You may well have pointed to one of the more sublime technical aspects of playing a guitar at different volumes.

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    ...... and then there's the room itself. Different rooms have different acoustics- different reflections, different EQ hot spots, etc., etc. Even if all other things were the same (which they're not), the settings you used for the gymnasium wouldn't likely sound good in the bedroom.

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    I aware that our ears perceive tone differently at different volumes. The only thing I won't get on a recording at home is the speaker vibe. That's okay.. I can still get the frequencies proportional at the quieter volumes of a recording. The amps I'm using are the Fryette Deliverance D120 and a Sig:X both with KT88's. THose amps are all preamp gain.

    If my living situation was such that I could turn things up really loud I wouldn't have the need to start this topic!

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    Quote Originally Posted by leadfootdriver View Post
    The only thing I won't get on a recording at home is the speaker vibe.
    Or acoustic feedback to the strings, or any circuit/transformer distortion after the MV, or voltage rail sag related compression.

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    There is yet another OT core related effect. An OT's lower corner frequency depends on its primary inductance, which strongly varies with applied signal voltage. Inductance is low at low signal voltage levels and increases to a maximum at medium to high output, from where it drops caused by beginning saturation.

    Consequently OT bass response improves with increasing output until partial saturation sets in.

    If this effect actually has audible consequences would depend on a number of factors, especially on the low level inductance of a given OT and the bass responses of amp and speaker. Also global NFB will tend to mask the effect.

    BTW, a bigger OT core doesn't mean higher inductance and/or better bass response.

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    Last edited by Helmholtz; 04-01-2020 at 04:18 PM.
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    But at least a big core stops people worrying that it would sound better if it had one (rather than a more modestly sized core)

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