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Thread: Learned lesson about overdoing a Good Thing !

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    Member HaroldBrooks's Avatar
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    Overdoing a Good Thing !

    Recently I've been trying to extract as much lower Bass as I could from the power section of my smaller amps, figuring if some bass is good, then more would be better !

    Well I think I caught myself "Over doing" it once again, this time it has to do with adding a huge Cathode Bypass cap to the power tubes in a push pull 6V6 amp setup (amp has a single 12" Jensen speaker).

    I learned that you can get away with a huge caps on single preamp tubes (as in 470uf 50v) to reduce hum, by creating a kind of short to dump the heater noise to ground. I figured a large cap might be good for the push pull power section as well, to get that last bit of low end... But it seems I caused an obnoxious swirly noise on the decay of low, and very high gain power chords. I just noticed this effect tonight, and it was driving me nuts as to why that was happening, as all seemed well with the amp for the last month or so, playing at lower gain settings.

    The clue came when I backed off the bass settings severely using a stomp box EQ, and the nasty sound lessened quite a bit, so on a hunch I removed the huge cathode bypass cap I had added from the power tubes all together, and that seems to have fixed it, the nasty sound is gone !

    So I am going to give myself the same advice I routinely give others, don't try to achieve unnatural levels of bass from your smallish amp. If you need a "Bigger" sound with more bass, hook up your amp to a bigger multi speaker cabinet, with perhaps bigger speakers with more natural bass.

    A small amp and speaker has a limit as to the bass it can generate.

    Any comments regarding this are welcome, as I am still learning a lot about amps and mods, and only know the basics, but enough to cause trouble for myself it appears !

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    Last edited by HaroldBrooks; 10-08-2019 at 07:50 AM. Reason: Shorter, same content

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    The clue came when I backed off the bass settings severely using a stomp box EQ, and the nasty sound lessened quite a bit, so on a hunch I removed the huge cathode bypass cap I had added from the power tubes all together, and that seems to have fixed it, the nasty sound is gone !
    Even a "huge" cathode bypass cannot produce more bass than no bypass cap. So there must be a different reason. It might be associated with the bias shift caused by the cap which lets the tubes run cooler at large output compared to no cap.

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    Cheaply made amps tend to have OTs that are barely adequate for the application, and may get pushed close to saturation when overdriven. Increasing the cathode bypass cap value (along with the increase of mains voltage in North America) may act to push the OT fully into saturation, which may be characterised by a ratty thin tone.
    May describe what happened here?

    How does it sound if the stock value bypass cap is fitted, and the mains is adjusted so as to achieve a heater voltage of 6.3Vac?

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    Bent Member Chuck H's Avatar
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    I interpret that Harold ADDED a cap to the existing circuit. We don't know if there is an existing bypass cap at this time.

    Another possibility is that higher levels of LF in the output generate more current in power amp leads. Leads that may be interacting with more sensitive low level signal leads in the amp. What you heard could have been an oscillation. But I like pdf64's angle too.

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    It's possible that you were on the edge of motorboating. This is the low frequency equivalent of high frequency oscillation. You sometimes run into it if you keep increasing bass response in the circuit until the power string bypasses are no longer completely effective in removing those low frequencies from the power supply to the tube. It's a little odd to get right on the edge of it, though

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    Amazing!! Who would ever have guessed that someone who villified the evil rich people would begin happily accepting their millions in speaking fees!

    Oh, wait! That sounds familiar, somehow.

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    Quote Originally Posted by Chuck H View Post
    I interpret that Harold ADDED a cap to the existing circuit. We don't know if there is an existing bypass cap at this time.

    Another possibility is that higher levels of LF in the output generate more current in power amp leads. Leads that may be interacting with more sensitive low level signal leads in the amp. What you heard could have been an oscillation. But I like pdf64's angle too.
    I think you may have hit upon it Chuck, and here is why. Yes I added a cathode bypass cap to the shared cathode resistor of the power tubes where there was none, and I also noticed that when I leaned over the amp with my guitar boosted to the moon, and created an audio feedback because of the close proximity. The feedback tone oscillated like a tremolo, and when I removed the cap again, that oscillation of the feedback was gone, so I have the strong suspicion you guessed correctly. The amp's layout is a hodge podge of wires and never intended for the big guitar signal I am giving it. Good show Chuck.

    Actually there are other things perhaps afoot, as both pdf64 and Helmholz indicated as well. The amp 6V6 power tubes are currently biased from the factory using a common 220ohm resistor, and this puts both tubes very, very close to the edge of 14watts each for plate dissipation, although there is no red plating, so that may be a factor that contributed. Again, I noticed from the start that the output transformer is also a bit small for this type of amp, and the amp is from 1957 so yes, the wall voltage in my house is around 126vac these days, and that is a factor.

    I ignore nothing you gents see fit to comment on, as you all have a tremendous wealth of experience individually, and collectively.

    Thanks for the excellent observations so far, but please keep them coming if you have any more !

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    Last edited by HaroldBrooks; 10-08-2019 at 07:15 PM.

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    Yes I added a cathode bypass cap to the shared cathode resistor of the power tubes where there was none
    So this means that you compared a "huge" cathode cap to no cathode cap as I assumed?

    No cathode bypass cap in the original circuit would actually be surprizing. Acc. to Langford-Smith (R.D.H.) a class A PP stage doesn't need a bypass cap as cathode signal currents of both tubes compensate and there is no signal voltage across the cathode resistor (similar to the tail of a LTPI). But a class AB stage should have a cap for best performance.

    Please post schematic!


    The feedback tone oscillated like a tremolo
    This strongly supports R.G. Keen's hypothesis of latent motorboating. It also means that your power supply filter caps have critically low capacitance wrt. the bass response of the amp.


    It is probably a combination of several effects.

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    Last edited by Helmholtz; 10-09-2019 at 08:14 AM.
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    Old Timer Leo_Gnardo's Avatar
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    Wondering what was the capacitance of this "huge" bypass cap that caused the problem?

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    Quote Originally Posted by Leo_Gnardo View Post
    Wondering what was the capacitance of this "huge" bypass cap that caused the problem?
    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 !

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    Last edited by HaroldBrooks; 10-09-2019 at 08:08 AM.

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    Quote Originally Posted by Helmholtz View Post
    So this means that you compared a "huge" cathode cap to no cathode cap as I assumed?

    No cathode bypass cap in the original circuit would actually be surprizing. Acc. to Langford-Smith (R.D.H.) a class A PP stage doesn't need a bypass cap as cathode signal currents of both tubes compensate and there is no signal voltage across the cathode resistor (similar to the tail of a LTPI). But a class AB stage should have a cap for best performance.

    Please post schematic!




    This strongly supports R.G. Keen's hypothesis of latent motorboating. It also means that your power supply filter caps have critically low capacitance.


    It is probably a combination of several effects.
    Interesting comment on the power supply caps Helmholz, I just checked those early today in the only way I know how, that is to parallel a good big 450v cap of at least the same value, and see if the wide open volume hum changes at all. So far they all passed that test, but I realize this test is superficial and a bit flawed, as it misses another test and something I only know a little about, "ESR" entirely.

    I agree that it's probably of few things, one being that I was pushing a 1958 Zenith Hi-Fi amp I am making into a guitar amp beyond what was intended gain wise, the other is my lack of any coherent respect for the prior stage resonances. I know nothing about this, but I read it in a search for similar problems with too large Cathode bypass caps.

    By the way, the oscillation is still there, but much, much lower. Now I have to put my guitar pickup practically on the amp to get it to oscillate. But since I removed the 470uf cathode bypass cap I installed, the funky swirly sound on power chord decay is gone, as in 100% gone, and that is good. The oscillation I can invoke doesn't seem to effect the tone or response one bit. But then still, it was much worse with the big cathode bypass cap in place, so there is that.

    Here's the schematic, and I think you've seen it from my other post. Sorry, wasn't trying to keep it a secret, I just sometimes limit details until someone asks, but from now on I will try to post all my schamtics up front to avoid the mystery. You guys can read deeply into these things, me, not at all at this point.

    Thank you for your help, and I have already changed the Volume control negative feedback as per your suggestion, and the distorted amp tone sounds even better than before.
    Click image for larger version. 

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    Oh, this schem looks familiar (from your other thread). Thought this was a different amp.

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    Just my opinion...

    I think smaller amps often limit bottom end because it makes more demand of the power supply and speaker. A 3" speaker in a table radio is never going to make a big bottom end, so why task the amp with it? Same really with an 8" Champ speaker. It can still be PLENTY loud without the big bottom. We try to drag more bottom out of the amp, and we bang up against the power supply limits.

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    that is to parallel a good big 450v cap of at least the same value, and see if the wide open volume hum changes at all.
    Paralleling a good cap is a good test, but open volume hum at idle is no good indicator for sufficient capacitance. While higher capacitance should reduce ripple (120Hz), the hum you hear is most probably not (only) ripple.

    Rather I would test if the paralleling stops the oscillation at high output.

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    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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    Quote 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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    Quote 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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    Bent Member Chuck H's Avatar
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    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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    Quote 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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    I loved playing my 79 Vibro-Blaster through my 68 2x15" CTS-loaded Bassman cab...

    Justin

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    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.

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    Old Timer Leo_Gnardo's Avatar
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    Quote 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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    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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    Old Timer Leo_Gnardo's Avatar
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    Quote 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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    "Heidelberg"
    Nice! That's where I grew up and studied physics.

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    Old Timer Leo_Gnardo's Avatar
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    Quote 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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    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.)

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    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.

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    Last edited by Helmholtz; 10-09-2019 at 09:55 PM.
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    Quote 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 !

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    Last edited by HaroldBrooks; 10-10-2019 at 12:38 AM.

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