Tube Amp Saturation VS PreAmp Saturation

Discussion in 'Glowing Bottle Tube Amp Forum' started by Mike_LA, Sep 15, 2021.

  1. Mike_LA

    Mike_LA Tele-Afflicted

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    Greetings,

    Well I was struck by PeteB's thread https://www.tdpri.com/threads/saturation.1080711/

    But my question is substantially different, so another thread about saturation.

    I have Marshalls, all with master volumes.

    I quite enjoy the preamp distortion brought on by overdriving some (or all) of the (6) 12AX7s in the preamp.

    I have never taken one out into the countryside and dimed it so I don't know how much (different, better) it is than what I know.

    I guess I'm going to listen to the little 1 watt on the top and listen to what it does.

    Who wants to take a stab at describing the tonal differences, perhaps with examples?

    Thnks .. . . .

    6101_JVM_Stack_P1020083.jpg
     
  2. Thomthom098

    Thomthom098 TDPRI Member

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    Greetings ,

    Apologies in advance for being off topic but can't help asking about middle amp : did you split a 6101 Combo into head + cab ? What are all those custom knobs / switches then on the cab ?

    Thanks
     
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  3. kennl

    kennl Tele-Afflicted

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    power amp distortion involves the synergy between the output transformer and speaker(s)
    all sorts of things happen there in a fashion that's different from the relative linearity of preamp distortion
    you need to hear and FEEL it to totally grasp the experience
     
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  4. doghouseman

    doghouseman Tele-Holic

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    Agreed. My experience is that it sounds more spongy and gets more like a fuzz pedal as the volume increases. There is definitely a sweet spot that is sometimes hard to find at a decent volume. That is the problem with it. Preamp distortion is more controllable, if you dont have an attenuator, and pre amp distortion can be smoother sounding, if that is a sound you want.
     
    Last edited: Sep 16, 2021
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  5. dan40

    dan40 Friend of Leo's

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    Raw and thick are two adjectives that come to mind when pushing the power tubes into overdrive. Preamp OD can be a bit buzzier sounding (but still sounds great with the right circuit) when compared to power tube distortion. Much of this comes from the interaction between the output transformer being driven hard and the speakers receiving all of that extra power. The type of power tube makes a difference also. Driving EL84's hard sounds much different than a larger tube like the EL34.
     
  6. Frodebro

    Frodebro Doctor of Teleocity Ad Free Member

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    Often times it's the phase inverter tube that is distorting, not the power tubes themselves. I have heard Marshalls with post phase inverter master volumes that had that characteristic "power tube crunch" at volumes so low that the power tubes were basically idling.
     
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  7. Tim S

    Tim S Tele-Afflicted Gold Supporter

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    If power tube distortion is all about the interaction between the transformer and the speaker(s), then do attenuators disrupt that interaction? Can good sounding power tube distortion occur at less than 100 decibels? I’m curious.
     
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  8. Mike_LA

    Mike_LA Tele-Afflicted

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    Yes indeed, I did split a 6101 combo into a head and cab, then used the space in the combo cab to house a 6G15 reverb that I built from a weber kit. So the stack has 2 heads in case of failure. . . .

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  9. Mike_LA

    Mike_LA Tele-Afflicted

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    I don't know about the PI in the circuit but I get glorious Marshall tone with everyone home . . . . with the master volume turned way down
     
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  10. 2L man

    2L man Tele-Holic

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    Reactive attenuators which impedance graps follows typical loudspeaker graph does not change power amp distortion as much as pure resistive attenuators.
     
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  11. doghouseman

    doghouseman Tele-Holic

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    Just ran my Vox AC15 wide open, about a week ago, and it was a little too raw for my tastes. I thought it would sound great, but it wasnt really what I expected. I guess that is the EL84 sound?

    Are 34s smoother than 84s?
     
  12. Timbresmith1

    Timbresmith1 Tele-Afflicted

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    The Vox is Class A vs AB. A different animal, regardless of output glass.
    The Vox do get a bit woolly all the way up. Knock it back a notch and it will roar.
     
  13. dan40

    dan40 Friend of Leo's

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    EL84's do have a different sound than EL34's. The larger EL34's are the sound that you heard from countless British and American rock bands throughout the 60's and 70's. Many of these bands were using Marshall Plexi's and this was the most common tube up until the late 70's when Marshall starting fitting amps with 6550's here in America. 6L6's show up in many of the larger Fender amps that bands were using back in the day. They have a slightly different overdriven sound when compared to the EL34's.


    With an attenuator, you are still hearing the interaction between the power tubes and the OT. The attenuator will reduce the signal level reaching the speaker though, so you do lose out on some of the spice that the speaker provides when it's being pushed to it's limits.
     
  14. printer2

    printer2 Poster Extraordinaire

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    The Vox 15 is Class AB. In the same way that a 5E3 can have the idle adjusted hot or cold (using different cathode resistors). The Vox is biased hot, but it still does enter Class B.
     
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  15. adjason

    adjason Poster Extraordinaire Silver Supporter

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    Maybe not a good example but music man amps generally only have power tubes- they do not sound good overdriven to me- gotta have preamp tubes in my opinion
     
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  16. MilwMark

    MilwMark Doctor of Teleocity Ad Free Member

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    The difference of navel gazing. And suggestion.

    Ok snarky. But seriously. I just worry about whether things sound and feel right in musical context and if so I don’t wonder about “better”.

    I have a JCM600 with Gain and Volume on the OD channel plus Master:
    - Master low and channel volume up. Sounds great. Feels great. Less bass. Less punch. Less loud/soft dynamic range.
    - Master up channel volume low. Sounds great. Feels great. More bass. More punch. More loud/soft dynamic range.

    So live I use Master Up.
     
    Last edited: Sep 17, 2021
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  17. printer2

    printer2 Poster Extraordinaire

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    Might be more true than some would want to admit. Mind you there are amps where the output tube distorting is the sound. The 18 Watt, when the output tube is overdriven the bias shifts and crossover distortion shows up. It gives the amp a little more 'edge'. Depending on the speaker it may sound buzzy. that is when the bias is turned up (and hopefully the tube take it) or zener's or the like are used to keep the bias from shifting. I have a little 5E3 styled amp and I adjusted the bias, slammed the output, did not sound quite right. Increased the bias, went through the same routine until I like the sound when it was fully cranked. I then scoped the output and the bias was set, wait for it.... ...the output just about developed crossover distortion.

    An 18 Watt into a resistive load.

    [​IMG]

    Do you want bias shift with an inductive load with your crossover distortion?

    [​IMG]

    May not be bad with a 'warm' speaker but playing clean it might be a different story.
     
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  18. Lynxtrap

    Lynxtrap Tele-Afflicted

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    One thing to factor in with Vox amps (AC15 and 30) is the lack of negative feedback. That is an important aspect of their sound and behaviour IMO.
     
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  19. printer2

    printer2 Poster Extraordinaire

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    "This speaker sucks in this amp but works in that amp."

    NFB lowers the effective impedance of the output stage. Because of that the speaker puts out sound time a transistor amp, generally flatter. Without NFB the amp acts as a current source more, where the impedance of the speaker rises (resonant frequency and the rise in the high end) delivering more power at the higher impedances than an amp with NFB. Without NFB the clipping should seem to distort earlier than a linear amp output, although I am not sure of that. The NFB amp should be cleaner and have a wider frequency response clean up to clipping but once it clips it will be more sudden. Acting more like a transistor amp (which clips hard for the same reason). ?Forget the thought that there is just a little compression as compared to a SS amp, the tube amp with NFB will clip and make square waves just like a SS amp.

    SS Marshalls can sound reasonably close to tub Marshalls when dimed. A little different with edge of breakup. Still more going on with the amp in overload. designers have come a long way once they realized the low dampening did a lot of the tube sound so they introduced the effect by adding current feedback to the SS amp. When SS amplifiers came available for hifi use they threw more gain into the amp than needed in order to take a lot of NFB which then cleaned up the amp. They missed the fact that the loop has to be fast enough so that the NFB fixes the highs, see slew rate if you want to dive in. Same occurred in instrument amps. But rather than mild clipping in hifi use guitar amps can spend a lot of their time in clipping, not always pretty. But that is a different tangent.
     
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  20. Larry F

    Larry F Doctor of Teleocity Vendor Member

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    I don't navel gaze. I have specific things that I try to bring out or suppress whenever I plug in. If I don't have the opportunity to get to know an amp well, I like to shape the sound with my hands, in real time.

    Ugh, how I dislike the term "navel gazing." But I admit that "naval grazing" can be quite a lot of fun, depending on context.
     
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