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  • Engl Savage question: asymmetrical distortion

    I have acquired some time ago an Engl Savage 120 (built in 2011) which I guess never sounded "right", just kinda average. I have retubbed it, re-biased, tried all kinds of tubes in, but it never sounded as brutal as I would like it to be. And I am mostly talking about the high gain distortion on channels 3 and 4 (CrunchII and Lead). It always felt as missing something to sound full/right. The Clean and CrunchI channels sound alright, but I rarely use them. I also own an Engl Fireball 60 which is just great.
    Now I have come to think that there might be something wrong with the actual distortion or something else is faulty. I have a general understanding of electronics and tube amps and I am able to repair faulty components, but it is a pain to remove the main PCB to check all the components one at a time. I could also add that the PCB is of very poor quality, thin traces and the like, easy to destroy. Therefore I am hoping someone might be able to chime in on this.

    Engl Savage E610 Schematic: engl-savage-120-type-e610-schematic.pdf

    I have done some tests with the oscilloscope, applying a 1KHz, 150 mV input signal to see what kind of output do I get across the speaker output. I have tested channels 2 and 3 (Crunch I and Crunch II) on roughly everything set to noon and this is where I can see some interesting results.

    - On CrunchI I get a good distorted sound which has the fundamental frequency as well as 3rd harmonic quite high and so on. I assume this is good, as the signal looks symmetrical.
    Output waveform as well as frequency components in purple (1KHz, 3KHz peaks):
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    - On CrunchII I get a distorted sound which has the 2nd harmonic very high and as far as my knowledge goes, it appears the preamp tube is cold biased somehow (not properly biased by the cathode resistor + capacitor) as it is kinda clipping/distorting on the low level input.
    Output waveform as well as frequency components in purple (1KHz, 2KHz peaks):
    Click image for larger version

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    Looking over the schematic, I can see that V2b preamp tube is responsible for Crunch II and there is a switch S11b which adds the capacitor in parallel to the cathode resistor. I do not know where that switch is located and maybe there is something about it that changes the waveform or there is something faulty in there. Following the schematic I can see that switching to lead channel adds the V3b gain stage. I have tested the output on the lead channel and it is actually symmetrical with just fundamental + third harmonic, not much content in the second harmonic. If there is something faulty at the V2b stage, I assume is roughly affecting the subsequent gain stage as well, which might explain the sound lacking full body, but could it be that V3b might correct any misbalance further on? I am a little puzzled.

    Basically I have a few questions. Is anyone able to judge if this asymmetrical waveform is the way it is supposed to look on Crunch II compared to the symmetrical Crunch I?
    If this is not how it is supposed to look like, should I then look at replacing the V2b cathode resistor and bypass capacitor/switch? Or could there be other issues to look for?

    Also, maybe it is worth mentioning, but when the amp is turned on with Master Volume set to MIN, I get a 400 kHz sine output (range over a few Volts) that goes away when I turn up the volume. Is this normal?
    Appreciate any suggestions/advices.

  • #2
    Official Engl schematics are inaccurate. They are purposely made to serve only as an approximate guide in their architecture and avoid copies. Missing filters, components with erroneous values and even containing certain elements that can not make sense.
    Savage 120 is closer to Ritchie Blackmore model in terms of overdrive levels but with greater dynamics. Lower than in a Powerball, for example (in case you have these references).

    Comment


    • #3
      If I interpret the schematic correctly, gain stage around V2b is NOT responsible for Crunch II. In fact it is bypassed in Crunch II mode and amployed only in Lead mode. Crunch I is basically on that separate "channel" so when you're comparing Crunch I vs. Crunch II you're actually comparing operation of the signal passing through two entirely different signal paths.

      Also, gain stage around V3b is always in the circuit, whenever the amp is on Crunch II or Lead modes.

      S11b is somekind of "boost" switch. It bypasses cathode (even at quite low frequencies) so it basically just provides overall gain boost at full audio spectrum. No, unless something's faulty there it shouldn't affect DC bias conditions at all.

      And as said, the V2b gain stage is only active in Lead mode.

      Comment


      • #4
        Thanks a lot for this, I have taken another look and you are indeed right, V2a seems responsible for the Crunch Gain and not V3a. It also appears that the switch entitled L.B. is actually the Lead Boost push button on the front panel so it has no influence on the asymmetry shown in the waveform.

        Comment


        • #5
          400khz on the output is not a good sign - something is causing oscillation

          When scoping the output I find it helps to ignore the control positions and first get the best sine-wave output at the highest level at the point where the peaks are just about to level off. This may not coincide with a 12-noon approach. Then increase the volume and/or gain and see how the clean waveform transitions into distortion. If something isn't right I work back from the output; grids of the output tubes, input to the PI and back through the gain stages to look at what's happening on the grid and the output (as seen after the coupling cap) of each section.

          I would resist changing any components at this stage.

          Comment


          • #6
            I want to thank you all for valuable advices. Some follow up on the discussion after some more testing and scope tracing.

            First of all, I have tested my other amp, the Engl Fireball, and it does not have any asymmetric distortion, nor 400 kHz at the output when master volume is set to 0.
            The Engl Savage has this kind of waveform at the speaker output when volume is set to 0:
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            Secondly, I have investigated where exactly does the distortion get in, and what kind of artifacts do I get when the sine wave starts plateauing.
            It seems a clear sine wave can be obtained on the Clean Channel easily, no matter the gain. On the CrunchII, I get a perfect sine wave at the output only when gain is smaller than 7:45 o clock. Then it starts distorting and shows asymmetry. I am showing here some output captures as the gain goes through 8 o clock and 9 o clock:
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            As suggested, I have done some testing to trace the asymmetry and it seems that the asymmetry happens somewhere in between the output of the first stage and the input of the second stage by increasing the gain. I have captured the following scope screens where:
            - output of the first stage (red): Plate of V1B (pin 1), voltage displayed centered around 150 V
            - input of the second stage (blue): Grid of V2A (pin 7), 0 V offset
            - output across the speakers (yellow), 0V offset

            Crunch gain: 7 o clock
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            Crunch gain: 9 o clock
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            Crunch gain: 12 o clock

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            Crunch gain: 15 o clock

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            It kinda clamps the upper waveform as the gain is increased, while the lower waveform is amplified as needed. In the schematic, it looks like a high pass filter, but I wonder if the capacitor is actually causing the problems and if there is a DC leak into the signal or similar. I am now relying on the schematic provided as I have not yet taken the PCB to have access to the capacitor itself or any other components.
            I will eventually remove the main PCB to get access to the components, but before that, any advice would be appreciated.

            Comment


            • #7
              As suggested, I have done some testing to trace the asymmetry and it seems that the asymmetry happens somewhere in between the output of the first stage and the input of the second stage by increasing the gain. I have captured the following scope screens where:
              - output of the first stage (red): Plate of V1B (pin 1), voltage displayed centered around 150 V
              - input of the second stage (blue): Grid of V2A (pin 7), 0 V offset
              - output across the speakers (yellow), 0V offset

              Crunch gain: 7 o clock
              scope_29.png

              Crunch gain: 9 o clock
              scope_30.png

              Crunch gain: 12 o clock

              scope_31.png

              Crunch gain: 15 o clock

              scope_32.png

              It kinda clamps the upper waveform as the gain is increased, while the lower waveform is amplified as needed.
              You have a diode there, anode is grid, cathode is cathode, so it is "pointing towards ground" so it will clamp the positive peaks, above its forward conduction voltage, think about 2 or 3 V peak.
              In the schematic, it looks like a high pass filter, but I wonder if the capacitor is actually causing the problems and if there is a DC leak into the signal or similar.
              It IS a highpass filter, but it also is a positive peaks clipper, thanks to the built-in diode.
              Advice is do not mess with it, what you see is normal and expected.

              Unless itīs a cathode follower, where grid to cathode voltage is not referred to Ground, standard Tube gain stages canīt handle more than, say, 2V RMS at the grid without clipping positive peaks.

              I can see that V2b preamp tube is responsible for Crunch II and there is a switch S11b which adds the capacitor in parallel to the cathode resistor. I do not know where that switch is located and maybe there is something about it that changes the waveform or there is something faulty in there.
              Nothing faulty.
              Tube stages will clip above a certain level, basically when the waveform peaks reach either positive rail or tube gets saturated (so they actually never reach real Ground, but 30 to 60V above it).
              Also both types of clipping bare different: top one is more square/harsher; bottom saturation is more rounded/smoother.
              Designer can choose to have more of one or the other , which will lead to unsymmetrical clipping, or both about the same, *just* by choosing idle plate voltage.
              Depending on his goals he may have almost symmetrical, what he did on V3b ; harsher top clipping or somewhat smoother bottom rounding, as he did in V2b.
              None is "broken" but designer choice.
              Imagine this: 3 "flavours" of distortion with just a tiny bias change.
              Thatīs why many amp schematics "look almost the same" yet they sound different.

              Is anyone able to judge if this asymmetrical waveform is the way it is supposed to look on Crunch II compared to the symmetrical Crunch I?
              If this is not how it is supposed to look like, should I then look at replacing the V2b cathode resistor and bypass capacitor/switch? Or could there be other issues to look for?
              No issues.
              You may mod the circuit, but then it will be a "Bungimeala" and not an "Engl"
              Juan Manuel Fahey

              Comment


              • #8
                Originally posted by J M Fahey View Post
                Thatīs why many amp schematics "look almost the same" yet they sound different.
                Exactly. In that ENGL amp pretty much all preamp gain stages are generic common cathode circuits, therefore they all "look almost the same" because they follow a standard circuit topology.

                The catch is in component values the circuits use. Almost all gain stages have 100K plate loads but look at their cathode (DC) resistances: One stage may have 1K, another may have 1.5K. This affects gain somewhat (very little in regard of AC because all cathodes are bypassed), but even more it affects the bias point (idle plate voltage) oif the gain stage and therefore stage's overall clipping symmetry (or asymmetry). So, that tiny little difference makes the circuits perform differently even though they technically "look almost the same". Well, in this regard they don't and that's the big deal.

                You probably see even greater difference when you compare those stages to the operation of the gain stage that is employed in "Lead" mode. Plate load is 330K, cathode resistance is 3K3. Circuit architecture is no different (it's the same generic common cathode amp) and schematically it LOOKS the same as all others but circuit is again different because the component values are different. It will also operate differently.

                Comment


                • #9
                  Painting with a VERY broad brush, Fender (and many others) started to use basic circuit blocks pulled straight from tube Manuals or Datasheets, but then continued developing them year after year, often with input from Guitar players, until result was unique, recognizable, and very successful.
                  Once they established the ground base, others basically tweaked that design to death.

                  It can be said that many other amps are just "tweaked Fender": original Mesa Boogie were "Fender Blackface with an extra tube", original Marshall were "tweaked Fender Tweed", high gain amps (JCM800 on) "tweaked Marshall with an extra tube" and so on.

                  Not my words, Fender themselves stated it :

                  Juan Manuel Fahey

                  Comment


                  • #10
                    Unless itīs a cathode follower, where grid to cathode voltage is not referred to Ground, standard Tube gain stages canīt handle more than, say, 2V RMS at the grid without clipping positive peaks.
                    This is another vital detail in understanding tube overdrive circuitry: As a rough rule of thumb, the grid begins to conduct (and its impedance drastically decreases) whenever grid voltage turns more positive than the bias voltage. The result is signal clipping at the grid. The bias voltage in a "self-biased" common cathode circuit is practically cathode's DC voltage, generated by voltage drop across the cathode resistor.

                    Thus overall DC bias point will also establish threshold to grid conduction and grid clipping. For example, 1K develops less voltage drop and lower cathode bias voltage, and will therefore result to earlier grid clipping than, say, 1.5K, which in turn generates higher cathode bias voltage. The "2 VRMS" figure quoted by J.M. Fahey is in the ballpark of cathode voltage using "standard" biasing with 1K - 1K5 cathode resistance and typical high voltage B+ supply to plates. The cathode bias voltage will be roughly about 1 - 1.5 VDC with such resistance values.

                    Note that with grid clipping as topic we're specifically discussing about signal at the grid. The signal amplitude at the plate of a previous gain stage (feeding the grid) can be very high, and in typical designs this high amplitude signal may then subsequenrly be either attenuated ...or not attenuated. The signal attenuating circuit (or lack of it in some cases) will naturally contribute to how high amplitude signal reaches the grid, and therefore how much grid clipping will actually take place.

                    Grid clipping clips positive waveform peaks at the grid but an inverting amplifier (like the generic common cathode amp) shows this as clipping of negative waveform peaks at the plate.

                    In addition, lower DC bias voltage also results to lower idle plate voltage, which means there will be more "headroom" for positive signal swings at the plate before clipping to B+ voltage limits. Higher DC bias voltage in turn results to higher idle plate voltage (plate voltage is closer to B+ voltage) and therefore decreases headroom of positive signal swings.

                    So: Higher bias increases headroom to grid clipping but decreases headroom to plate clipping. Lower bias decreases headroom to grid clipping but increases headroom to plate clipping. Plate and grid clipping will have somewhat different characteristics: Grid clipping is basically "softer" than plate clipping.

                    What comes to overdrive, bias point technically allows to choose whether asymmetric clipping characteristics are softer or harder. With proper bias point clipping characteristics can also be turned quite "symmetric", as in both positive and negative peaks of the waveform will clip about the same magnitude, although with different characteristics of "softness" or "hardness" (so some asymmetry will always prevail). This is usually referred to as "center bias", meaning headroom of positive and negative signal swings is about equal, and idle plate voltage roughly half of the B+ voltage. (Note that these are just rough rule of thumb guidelines).

                    Typical common cathode preamp gain stages of tube guitar amps deliberately employ something else than center bias to introduce asymmetry and resulting greater amount of even order harmonic distortion. ENGL's 330K plate, and 3K3 cathode -resistance combination of "Lead" mode's additional overdrive circuit, on the other hand, is a great example of choosing bias point that instead result to greater degree of symmetric clipping characteristics, and therefore to lesser degree of even order harmonic distortion.

                    If you take a look at classic "Soldano circuit" you will quickly notice quite atypical common cathode gain stage with 39K cathode resistance. Going by what you just learned you realize its there to introduce certain overdrive characteristics: "Softer" grid clipping is minimized due to high headroom for grid signal swings before bias voltage is exceeded, and "hard" plate clipping is enhanced due to limited headroom of positive signal swings at the plate. Overall result are highly asymmetric and "hard" clipping characteristics". I believe that classic Soldano circuit was derived from certain Marshall design, where the cathode resistance had somewhat less "drastic" figure of "only" 10K.

                    There is also one special characteristic of overdrive to consider, that is intermodulation characteristics. Intermodulation distortion means additional harmonics created by process of intermodulation whenever a complex signal (complex signal is a signal with more than single frequency) is distorted.

                    Magnitude of intermodulation distortion can be decreased by distorting the signal with limited bandwidth, because limited bandwidth decreases amount or magnitude of those several frequencies a complex signal contains and therefore reduces their effect to intermodulate. There is a BIG difference whether the signal distorts while signal's bandwidth covers the entire effective range of guitar's frequencies, or whether the signal distorts while the bandwidth is limited to, say, mid-range frequencies.

                    As a rough rule of thumb, the more there are lower frequencies in the distorting signal the "fuzzier" or "fartier" the resulting tone of overdrive is. This is why you see many traditional overdrive circuits drastically reducing low end before signal clipping. Balance is chosen between "fartiness" (and overall incomprehensibility and weak note separation) and having the tone turn too "thin".
                    Similarly the more there are high frequencies in the distorting signal the more "fizzier" the resulting overdrive tone is. If high frequencies are reduced too much the overall sound may sound too "muddy" or "dark". So it's always a compromise and pretty much reflects taste or tonal goals of the designer(s).

                    Fender amps traditionally employ high cathode bypass and coupling capacitances. Result is distortion at quite large bandwidth (and associated intermodulation effects). Marshall amps traditionally employ low cathode bypass capacitances to limit low end and resulting intermodulation while coupling capacitances might have modest values. On the other hand, they often employ "treble bleeder" circuitry in interstage coupling to enhance higher frequencies (which is the same thing as decreasing low end). Then there are amp designs like classic Mesa, Rivera and ENGL that employ low coupling capacitance (to limit the low end) but still highish cathode bypass capacitance. All such design choices will affect overdrive's tone probably much more than the actual characteristics to clip (soft vs. hard, symmetric vs. asymmetric) as intermodulation has very great dominance in creating overall harmonic content of distortion.
                    Last edited by teemuk; 05-19-2018, 01:49 PM.

                    Comment


                    • #11
                      Thanks a lot for all these amazing tips in how the overdrive symmetry/asymmetry is employed in amp designs. I can now indeed confirm that after testing my other amps, I get a blend of symmetry/assymmetry. For example, testing my Engl Fireball, the signal is distorted symmetrical, while a Mesa Boogie Dual Rectifier (Multi Watt) has an asymmetrical distortion for the yellow/red channel emplying high gain (at least in the modern mode). So I now understand that this is not something that can be traced to be "problematic" or not, it is part of the sound signature.

                      Just to clarify, I am not interested in modifying anything on the Engl Savage, I was just curious how to trace down the issue with my amp, as the distortion that I get out of it is still problematic and not fully enjoyable, so to describe.

                      I have taken the Savage motherboard apart and measured all the components in circuit, visually inspection for anything faulty as well as poking all the components/wires while the amp was on, trying to find if there is an issue, could not really get to anything. At first, I have thought that maybe the optocouplers could be the culprit so I have even taken them out (in the schematics they are valued as LT1011, but they are in fact LT3011, Excelitas DC opto-isolator, basically an LDR, unfortunately NOS and not so much available lately as replacements). Investigating the circuit, I was able to deduce that they are only used when Rough/Smooth mode is engaged and they only act as simple (ON/OFF) for adding/removing some resistors to the circuit affecting the smoothness in the Crunch/Lead mode. Using the schematic and datasheets it looks like they basically get engaged when the Smooth button is engaged and they add some 0.3 kOhm resistance in a few selected branches of the circuit affecting the tone. I have also checked the power supply part, changed the power supply filter cap between the 540 V - 380 V lines due to 100 Hz hum induced in the power amp part. I have managed to get less hum now, but there is still some amount left. As the amp is fairly new (2011), I am not yet sure if I need to look at all the caps from the circuit for DC leakage.

                      I don't know if the "lifeless" sound is caused by any of the mentioned circuitry. For example, the CrunchI channel with full gain sounds like a Fender Bassman with max gain so to say (maybe not a fair comparison, but just in gain amount) and it sounds far from the brutal metal tone that is supposed to be, at least according to some youtube videos that I can compare with. Also, the CrunchII and Lead channels, while they have considerable more gain, they are not articulate, not punchy, no life or character in the sound, get very mid-rangey somehow and lacking tone separation. The clean channel works great and is clean and clear, and the CrunchI sounds ok, but the gain is really low and the CrunchII and Lead are lifeless, dull, muddy. I am quite sure that the issues are in the preamp stage, as if I slave out the pre-amp to another power amp, I get similar results.

                      So now I am out of ideas what to look for further. I would be grateful for any advices in what to look for/what or how to test the non-articulate or what appears to be faulty distortion. The tips I got so far with starting off from a clean sine are a very good start, but I do not know how to actually check if the distortion is as it should be.

                      Comment


                      • #12
                        Originally posted by bungimeala View Post
                        So I now understand that this is not something that can be traced to be "problematic" or not, it is part of the sound signature.

                        Just to clarify, I am not interested in modifying anything on the Engl Savage, I was just curious how to trace down the issue with my amp, as the distortion that I get out of it is still problematic and not fully enjoyable, so to describe.
                        Thereīs nothing "broken" in your amp, itīs just not equalized the way you like.
                        Meters - scopes - etc. can help find broken or improperly working components, canīt say a word about "taste".
                        I see a contradiction here.

                        I have taken the Savage motherboard apart and measured all the components in circuit, visually inspection for anything faulty as well as poking all the components/wires while the amp was on, trying to find if there is an issue, could not really get to anything.
                        Because thereīs no Technical issue.
                        At first, I have thought that maybe the optocouplers could be the culprit
                        WHY???
                        so I have even taken them out (in the schematics they are valued as LT1011, but they are in fact LT3011, Excelitas DC opto-isolator, basically an LDR, unfortunately NOS and not so much available lately as replacements). Investigating the circuit, I was able to deduce that they are only used when Rough/Smooth mode is engaged and they only act as simple (ON/OFF) for adding/removing some resistors to the circuit affecting the smoothness in the Crunch/Lead mode.
                        Ok, so they are just switches,they do not affect sound or even less taste by themselves. So leave them alone.
                        Using the schematic and datasheets it looks like they basically get engaged when the Smooth button is engaged and they add some 0.3 kOhm resistance in a few selected branches of the circuit affecting the tone.
                        So they do not affect Tone.
                        I have also checked the power supply part, changed the power supply filter cap between the 540 V - 380 V lines due to 100 Hz hum induced in the power amp part. I have managed to get less hum now, but there is still some amount left.
                        Cool, but thatīs not what brought you here.

                        As the amp is fairly new (2011), I am not yet sure if I need to look at all the caps from the circuit for DC leakage.
                        Same thing.
                        I don't know if the "lifeless" sound is caused by any of the mentioned circuitry.
                        Sound is the result of *all* of the amplifier, and definitely switches and power supply (which is working properly) do not affect that.

                        For example, the CrunchI channel with full gain sounds like a Fender Bassman with max gain so to say (maybe not a fair comparison, but just in gain amount) and it sounds far from the brutal metal tone that is supposed to be, at least according to some youtube videos that I can compare with.
                        Youtube videos are nice and entertaining but are FAR from showing accurately some kinds of sound.
                        You donīt know what was used to record them, any kind of postprocessing, whether some special guitar or pickups were used, if the player used some clean booster on the floor .... evem moving the microphone **1 inch** can completely change results.
                        I suggest you find some friend (or rehearsal/recording Studio or a shop with some padded back room) who has another Engl Savage and let you test both side by side.
                        I fear there is a high possibility they both sound the same if set the same way.

                        Also, the CrunchII and Lead channels, while they have considerable more gain, they are not articulate, not punchy, no life or character in the sound, get very mid-rangey somehow and lacking tone separation.
                        See above. Compare it to another Engl Savage, same settings.
                        The clean channel works great and is clean and clear, and the CrunchI sounds ok, but the gain is really low and the CrunchII and Lead are lifeless, dull, muddy. I am quite sure that the issues are in the preamp stage, as if I slave out the pre-amp to another power amp, I get similar results.
                        So maybe thatīs the nature of the beast.
                        So now I am out of ideas what to look for further. I would be grateful for any advices in what to look for/what or how to test the non-articulate or what appears to be faulty distortion.
                        Want ideas? OK, you may try some of these if you wish, or any other somebody suggests or you can imagine.
                        1) try a clean booster ahead of the amp.
                        This alone can work miracles.
                        It may add lots of harmonics simply by clipping what before did not, bite, aggressiveness, you name it.
                        2) switch speakers.
                        Often ignored, they can turn day into night and viceversa.
                        Just watch this example: exact same amp, settings, player, riff, guitar, through 15 different speakers:

                        So much for people who agonize over cap colour or resistor composition , speakers have, what? 100 times more influence on sound?
                        No double blind tests needed here

                        Plan B: donīt waste time, SELL the amp you donīt like, buy one you do.
                        Juan Manuel Fahey

                        Comment


                        • #13
                          Distortion is a form of signal degradation. It is pretty easy to define what a good "clean" signal is, but trying to achieve just the right kind of "broken" can be pretty maddening.

                          Comment


                          • #14
                            J M Fahey
                            I believe I have read your posts on another audio website. I think bunglmeala is trying to discover what makes the magic in an amp combo by toying with the circuits but as you succinctly stated with your advice to try another amp, that effort is often wasted. In my experience with combos, the speaker is usually the weakness when the electronics are in the ballpark. I don't want to revive an old thread or threadjack this one so I'll just link my question here and hope to get a reply there.
                            Last edited by yldouright; 05-22-2018, 10:55 PM.

                            Comment


                            • #15
                              Originally posted by yldouright View Post
                              J M Fahey
                              I believe I have read your posts on another audio website. I think bunglmeala is trying to discover what makes the magic in an amp combo by toying with the circuits but as you succinctly stated with your advice to try another amp, that effort is often wasted. In my experience with combos, the speaker is usually the weakness when the electronics are in the ballpark. I don't want to revive an old thread or threadjack this one so I'll just link my question here and hope to get a reply there.
                              Please check your link. It needs to be modified. It takes me to a blank "start a thread" dialog.
                              If it still won't get loud enough, it's probably broken. - Steve Conner
                              If the thing works, stop fixing it. - Enzo
                              We need more chaos in music, in art... I'm here to make it. - Justin Thomas
                              MANY things in human experience can be easily differentiated, yet *impossible* to express as a measurement. - Juan Fahey

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