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Learned lesson about overdoing a Good Thing !

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  • HaroldBrooks
    replied
    Originally posted by Helmholtz View Post
    I was surprized to see from Zollner's measurements that open back cabinets actually reproduce lower frequencies than closed ones. Reason is that the bass response of speakers sharply drops below their bass resonance. So the 2x12 speakers in an open back cabinet might have a bass resonance at 80Hz, while the same type of speaker in a 4x12 closed cabinet will have a resonance at 120Hz (the enclosed air volume increases cone stiffness and thus resonant frequency). But the lower damping of the closed cabinet will produce a pronounced peak at 120Hz, which gives the impression of tighter and stronger bass.
    The SPL at the bass resonance also depends on the output impedance of the amp. A higher impedance (e.g. Vox AC30, Tweed Deluxe, Marshall 18W) emphasizes the resonance.
    I've noticed this too, that some open backs generate more lows below the resonant frequency of the speaker, even though there is a sizable potential for cancellation of front to back sound waves, but it depends on the size and depth of the open back cabinet, and it's position in any given room (back wave reflections cancelling or enhancing).

    Same thing for the closed back cab with regards to size, but the lack of a back wave is a bit more predictable regarding bass production. A big 4 x 12 original Marshall cabinet with a large amount of insulation does a good job with lows indeed, because of it's sheer size and if you included absorption material that makes things "Bigger" acoustically.

    But humans can hear higher frequencies better and louder with less energy, so a big cabinet will get the nod often when it generates bass in a more reasonable range for guitar, perhaps above the resonant frequency of some common 12" speakers.

    Then to confound the issue further, there is mutual coupling at a certain distance out in front of a cabinet. You can never win getting this to be absolute, IMHO.

    So this means I should still use a larger cabinet ! And stop trying to be a bass player and guitar player all at once !
    Last edited by HaroldBrooks; 10-09-2019, 11:38 PM.

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  • Helmholtz
    replied
    hopefully in closed back cabs
    I was surprized to see from Zollner's measurements that open back cabinets actually reproduce lower frequencies than closed ones. Reason is that the bass response of speakers sharply drops below their bass resonance. So the 2x12 speakers in an open back cabinet might have a bass resonance at 80Hz, while the same type of speaker in a 4x12 closed cabinet will have a resonance at 120Hz (the enclosed air volume increases cone stiffness and thus resonant frequency). But the lower damping of the closed cabinet will produce a pronounced peak at 120Hz, which gives the impression of tighter and stronger bass.
    The SPL at the bass resonance also depends on the output impedance of the amp. A higher impedance (e.g. Vox AC30, Tweed Deluxe, Marshall 18W) emphasizes the resonance.
    Last edited by Helmholtz; 10-09-2019, 08:55 PM.

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  • Helmholtz
    replied
    But then I'd like to be involved in the design and qualification process.

    (Regarding small amps I am actually quite happy with my little Super Champ from the 80s. I don't really like much bass in my guitar sound and don't think it helps in the mix, but if needed I would connect to one of my bigger speaker cabinets - or use a Vibroverb or Super Reverb.)
    Last edited by Helmholtz; 10-09-2019, 07:57 PM.

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  • Leo_Gnardo
    replied
    Originally posted by Helmholtz View Post
    Nice! That's where I grew up and studied physics.
    It was just a wild guess. Plus, Germany should not be underrepresented. No shortage of good engineers & scientists of all sorts from there.

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  • Helmholtz
    replied
    "Heidelberg"
    Nice! That's where I grew up and studied physics.

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  • Leo_Gnardo
    replied
    Originally posted by Helmholtz View Post
    I see. The fact that sine shape distortion starts at lower power with lower frequencies confirms beginning saturation. Saturation depends on magnetizing current which increases at lower frequencies by nature of the inductance.

    But my main point was that bass response will get worse at higher output, especially with clipping.
    We are definitely in agreement!

    To the owners of amps with small OT's, I say what can you expect with a fingertip size core? Not much.

    Leo F after all intended the Champ to be a "student amp." Something cheap and practical, just to get started. Once the basic techniques are learned, time to graduate into a Princeton or Harvard. High hopes!

    There should be a CalTech, an MIT, an Oxford, how about a Heidelberg.

    About 20 years ago the guitar-wiz head of the math department at our local Ivy League was thinking about having me build him a custom 2x10 amp, sort of a double Harvard. We were going to call it the Vassar, have it tolexed in the school colors grey & pink. Never did come about, oh well. Good to imagine though.

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  • Helmholtz
    replied
    Depending on several factors, I measure 4 to 6 watts "clean" output from 400-500 Hz on up. And half to one watt at 100 Hz.
    I see. The fact that sine shape distortion starts at lower power with lower frequencies confirms beginning saturation. Saturation depends on magnetizing current which increases at lower frequencies by nature of the inductance.

    But my main point was that bass response will get worse at higher output, especially with clipping.

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  • Leo_Gnardo
    replied
    Originally posted by Justin Thomas View Post
    I loved playing my 79 Vibro-Blaster through my 68 2x15" CTS-loaded Bassman cab...

    Justin
    You ... are ... radical! And more guitarists should do similarly. There's a whole lotta good tone to be found.

    Helmholtz, fwiw I make power measurements just below the clipping point. On Champs not so easy, the waveform starts bending way out of shape, so I select the point where the sine wave still is looking like a sine should. Depending on several factors, I measure 4 to 6 watts "clean" output from 400-500 Hz on up. And half to one watt at 100 Hz. One occasion I was able to experiment with a much better single ended OT, and got good looking sine output all the way down to 100. That was just a quick experiment though. The customer really didn't want to spend for an upgrade OT and was happy recording his amp as-is. Besides, too much clean "bass" might beat up the stock speaker.

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  • Helmholtz
    replied
    In Champs especially, and other similar amps at the low power end of the spectrum, the output transformer's low frequency response rolls off at a steep rate below 400 Hz.
    As a constant primary (magnetizing) inductance can only produce a moderate -6dB/octave roll-off, a steeper roll-off means that the core is driven into partial saturation. This would happen at high output. In the power amp's linear range NFB will at least partly compensate. But when the power stage clips, NFB loses its power. All this means that bass response will be better at moderate output.
    Last edited by Helmholtz; 10-09-2019, 04:52 PM.

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  • Justin Thomas
    replied
    I loved playing my 79 Vibro-Blaster through my 68 2x15" CTS-loaded Bassman cab...

    Justin

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  • Leo_Gnardo
    replied
    Originally posted by Chuck H View Post
    I've had some success actually LIMITING extreme low end from really small amps. Like Champ size and wattage. My reasoning being that with such a low end deficient cabinet and speaker anything below 100Hz is wasted energy through the amp. Why not reserve that power for frequencies the amp and cabinet CAN reproduce.
    In Champs especially, and other similar amps at the low power end of the spectrum, the output transformer's low frequency response rolls off at a steep rate below 400 Hz. Without a better - and more expensive - transformer (which amp owners are loath to pay for!) it truly is folly to try & boost low frequencies. Better to use a larger speaker, certainly. I've changed the output jacks in a hundred or more Champs so their owners could plug into 10, 12, even 15 inch speakers, hopefully in closed back cabs. Do they ever use them? Rarely if ever.

    Another factor which I expect misleads guitarists into thinking "there must be more bass available from my amp" is listening to recordings made with microphones jam up to the grill cloth, and maybe even closer, accentuating the "proximity effect" which boosts bass quite a bit. To me, that's another reason to mic up your amp & run it thru the pa/monitor system. In some cases I skip the stand (another excess burden!), roll up a mic in a towel with the head sticking out a bit & stick it in the back of the amp. Picks up plenty of signal, mellower tone, and out of sight = secret weapon. Just don't forget to remove it before hauling the amp offstage after the gig.

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  • Chuck H
    replied
    About the best you can do with any small or small-ish open back combo is to stiffen the power supply with more main filtering and use a speaker that has good low end. Even with this most players looking for more bottom end won't be satisfied because what their really looking for is a tone that's beyond such a simple rig. Something I've seen many players do with BFish type designs in combo cabinets is just cranking up the bass and then complaining about farty low end.?. I have to explain to them that such amps have an overabundance of LF through the preamp. Far more than the power amp and power supply is capable of reproducing well. The cabinet and speaker system are the limiting factors. So turn up the bass until the amp starts to fart, then back it down until it just stops farting. That's all you get. The amp is now making all the useful bass it can. (<period)

    An increase in filtering won't really give MORE bass, but it will tighten it up a little. Then it's up to the cabinet and speakers.

    I've had some success actually LIMITING extreme low end from really small amps. Like Champ size and wattage. My reasoning being that with such a low end deficient cabinet and speaker anything below 100Hz is wasted energy through the amp. Why not reserve that power for frequencies the amp and cabinet CAN reproduce. This is done in the preamp by reducing coupling cap values, reducing cathode bypass cap values or reducing the tone stack LF pass frequency. The end result would be the ability to advance the bass knob one or two more numbers with just a little more and tighter AUDIBLE low end in the final EQ.

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  • Leo_Gnardo
    replied
    Originally posted by HaroldBrooks View Post
    470uf 50v cap. I must admit that I was driving the amp input with a Seymour Duncan "Flat" boost (25db gain) and a with about another 6db of gain from a stomp box EQ, and at the front of all of that, a compressor. LOL, a bit over the top. At such high levels I've found with almost ANY amp there will be some type of raucous noise along for the ride, making it hard to use in a performance. The metal type amps like Soldano, Mesa, etc... are more carefully thought out I believe.

    I only own and modify the old stuff, as a matter of fact, my newest amp is a 1986 Marshall JCM 800, and the next is a 1965 Gretsch 6162. Most of my amps are from the 1950's and 1940's, so you can see I am trying to fit a square peg into a round hole, but still have the square peg at the end of it all. Still, I have had some good success so far, so I am encouraged that I can have an amp that sounds good at a modest crunch level, but sound at least 'OK' when you drive the crap out of it !
    Thanks for the reply. Late 80's when amp modding started to take off, some customers were begging for "more lows" from their funky old combo amps. Sometimes I'd boost an output tube cathode bypass cap to 100 uF from its original 20, but that's as far as I went. How many times I lectured disinterested customers "hey you have an open back cab here, no matter what I do to boost lows, they cancel out anyway." I convinced a very few to cover up, at least partially, the open backs on their combos. Or do like the Rolling Stones, plug your gem's output into a closed-back cab. Always got a "wow!" demonstrating this last technique, running Champs, GA-5's, tweed Deluxes, all sorts of dinky amps into my Altec-loaded 4x12. But nobody wants to carry an extra cab to the gig, not even a single 12. So... mic it up & put it in the PA if nothing else. Gosh, that would take a mic. And a cable. And even a stand. Too much baggage! Fooey. Might as well deliver sermons to the fish.

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  • Helmholtz
    replied
    Originally posted by Mick Bailey View Post
    I wonder if the sound is a beat frequency caused by PSU ripple (due to increased current draw) and imperfect cancellation in the OT.
    As for very low frequency beating the two frequencies involved can only differ by a few Hertz, this effect would only occur at special notes. (BTW, at power stage clipping the OT can't cancel ripple any more.)

    I still tend to the motorboating at high output theory.

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  • Mick Bailey
    replied
    I wonder if the sound is a beat frequency caused by PSU ripple (due to increased current draw) and imperfect cancellation in the OT.

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