Loose low end

[daz]

I now have a lot of what i want happening in my amp, but the one thing that has never changed no matter what i do is the loose low end. It's gotten better, but never goes away. I have used duncan's calculator and tried smaller bas pots and caps, larger slopes, nothing there works. i have had every value imaginable as plate and cathode resistors and bypass caps. Had voltage dividers in the preamp, then removed them and replaced with grid leaks. tried every coupling cap in the amp reducing them at times to very small values. You name it, i've done it. It's no problem reducing lows, that does nothing to help which is why tone stack and coupler tweaks do nothing. even using tone stack values that the duncal calc shows to roll off the very lows doesn't work. I'v tried voltages from high to very low in the preamp. the output section has gone thru a million changed and has been fixed bias in the past. It's diode rectified but has been tube before.

I know this leaves very little for anyone to suggest, and if thats not bad enough i can't even list all the things i've tried because i'd be typing all day and never remember them all. But maybe someone else has been thru this b4 and found something that i have yet to think of. If i can whip this issue the amp will then be ripe for final tweaking because that part is easy IMO. It's fixing sonic flaws that is hard. so any ideas?

[octal]

I've forgotten- does this amp use NFB?

General tips:
Smaller coupling caps (you've tried this)
More NFB
More PS filtering
Stiffer PS (diode VS tube recto)

Especially if the amp is non-NFB, using a speaker with a lower Qms spec will tighten the bass. (Lower Qms= speaker has more self damping of its bass resonance.) An open backed cab is also going to help.

Good luck,

Nathan

[daz]

It has NFB, and i have tried a lot of things there too along with all else you mentioned. The thing is, this issue i assure you is not a matter of the typical tweaks to tighten up NORMAL bass response that just happens to be a bit loose due to things like too big of couplers or lack of NFB. It's a flaw in the circuit or parts, i'm sure of it. I'm hoping someone who's been thru this has an idea from experience.

heres the newly edited schematic

http://i24.photobucket.com/albums/c3.../myEL34new.jpg

[Chuck H]

The thing about low end is that once the amp is making all the bottom end it can any extra becomes flab. This goes for the lower mids too. That's one reason why you see small coupling caps in cascade style preamps. With a high gain amp the amount of bottom end you can implement that the amp will accurately reproduce is really quite small. Adding more in an attempt to get more bottom end is futile because the amp simply can't create any more so instead you get mush. You can design with smaller coupling, bypass and filter caps in the early stages and open up the later stages to get bigger, tighter more clear bass response under OD conditions. The rub is that this makes for a pretty lame clean tone.

I had the same problem with my first Marshall mods. Cascaded like an 800 only not. I used: V1a) .0022 decoupling cap, 2.2uf cathode bypass cap with a 1.5k cathode resistor. 500k "gain" pot. V1b) .022 decoupling cap, 15uf cathode bypass cap with a 1.5k cathode resistor. the V1b decoupling cap was followed by a 1M resistor with another .0022 cap across it to further trim and tune the bass. 1M "volume" pot. V2a used a 1.5k cathode resistor with a 1uf bypass cap. V2b was the cathode follower. The filter cap for both stages of V1 was only 10uf. The filter for V2 was 22uf. A master volume followed the tonestack. The rest of the amp was pretty common values. The OD was grand. The clean tone was lacking.

That was one of my first mods. Arriving at those values took forever. I have skipped some circuit details in the interest of clarity. My point is just that it's a tradoff between OD and Clean with respect to bass response in Marshall type amps. Believe it or not you can actually get closer to a "one channel fits all" type amp using the Fender topography. But even Fender amps, when stock, have the common complaint of flabby bottom end when overdriven. People just don't know to turn down the bass knob. Turning down the bass knob in a Marshall type amp will not help because the tonestack is later in the circuit. So you have to take out the extra bass in the hard wiring. That makes it impossible to get a dynamic bottom end response for cleaner and slightly distorted tones.

IMHE

Chuck

[daz]

well then, is there a way to roll off bass w/o having to insert and entire tone stack say in between the 1st and second stage? If i could put a one knob tone control there maybe that would do the trick, and i could tweak it till i'm happy then use a set resistor in place of the trimmer. No?

[loudthud]

Check the following schematic:

http://www.el34world.com/charts/Sche..._BASS_1992.pdf

Right before the phase inverter is the Slope control. It's a network that rolls off the low end. In some Gibson and Vox amps there is a similar network (without the pot) to remove the Tremolo oscillator signal from the guitar signal.

What different speakers have you tried?

[daz]

Actually, i wasn't taking my own advice into account when i mentioned rolling off lows. As i said, it's not a matter of removing lows, it's a flaw. i know this because every way i've tried including those that remove lows can leave me hardly any bass yet whatever lows are left have the same fuzzy or loose sorta sound. And having replaced all the filters and the signal caps recently i know it's not a faulty cap. I just went back to a 100k on V1A's plate. It never seems to sound as good, but this time it may actually sound better, tho i have to keep evaluating it and go back to 220k in a while to be sure. But it helps a bit. Tho i know it probably won't be 100% fixed until i find whatever circuit flaw or bad component is causing it. The other amp like this one my friend has is the same tho, so it's gotta be a design issue.

speakers....i use a EV12L and a celestion 100 at times and have tried a all-tone, a jensen, and a couple others that were in some fender tube amps and i'm not sure what they were. It's not the speakers, trust me.

[pdf64]

The 0.022uF coupler at the input to the phase splitter is letting sub lows through, because that 1M grid feed resistor is bootstrapped by the nfb of the long tail pair. Makes the equivilant resistance that the cap sees much higher, maybe 10M. So try reducing it's value, try 4n7 in the first instance.
The way you've configured that first volume/gain control is a really neat way of rolling off the lows as the control is increased, due to the 220k grid feed resistor - a built in slope. But maybe change the coupling cap before it to 0.01uF to take more lows out when it's up high, there will be little difference at lower clean settings.
Orange amps had an 'FAC' control, which was a multi-position rotary switch for different coupling cap values, so you could progressively roll off the lows, see
http://www.schematicheaven.com/newam...ge_otr_120.pdf
Peter.

[loudthud]

You've tried a wide range of speakers. Have you tried anything different in the output tran$former department?

[Hardtailed]

Maybe I'm beyond my knowledge here but isn't cathode-biased a bit looser in the low-end department by default?

What you want to do is cut as much as you can get away with right at the first stage (smaller bypass cap on V1a will help) and get it back later in the chain.

On my Plexi/800 hybrid, I used 0.022 coupling caps all around and the low-end is still really tight. I have the usual 0.68uF cathode bypass cap on the first stage (which is actually V1b since I went with the 2204 layout), but the 2nd one is a huge 220uF.

It's not "huge" sounding by any means though, but it's the most perfectly balanced amp in a band context I've ever used.

Also, hopefully you're using a sealed cab, not open-back.

What cathode bypass caps are you using on the power tubes?

[daz]

The 0.022uF coupler at the input to the phase splitter is letting sub lows through, because that 1M grid feed resistor is bootstrapped by the nfb of the long tail pair. Makes the equivilant resistance that the cap sees much higher, maybe 10M. So try reducing it's value, try 4n7 in the first instance.
The way you've configured that first volume/gain control is a really neat way of rolling off the lows as the control is increased, due to the 220k grid feed resistor - a built in slope. But maybe change the coupling cap before it to 0.01uF to take more lows out when it's up high, there will be little difference at lower clean settings.
Orange amps had an 'FAC' control, which was a multi-position rotary switch for different coupling cap values, so you could progressively roll off the lows, see
http://www.schematicheaven.com/newam...ge_otr_120.pdf
Peter.

Not sure what you mean by "220k grid feed resistor. the only 220k in the amp is the V1A plate R. I tried the PI input cap just recently, but i'll give it another try, tho i've tried every coupler in the amp several times and they alway make the tone skinnier and the bass remains loose.

You've tried a wide range of speakers. Have you tried anything different in the output tran$former department?

yeah, a 4k heyboer. But the lows are the same with that. Maybe a tad better, but the tone is also skinnier.

Maybe I'm beyond my knowledge here but isn't cathode-biased a bit looser in the low-end department by default?

If it is, it isn't in this amp because i had it fixed bias for months, and in fact going back to CB was one of the best things i did for it. The lows were the same with fixed.


EDIT:just tried a .0047uf at the PI input and same result as last time....skinnier tone, bass still loose/fuzzy, and like last time, motorboating for some reason.

[Chuck H]

Would that be motor boating with the standby in the "play" position?

I have to say that even though you have tried a number of speakers that doesn't mean you have tried and appropiate speaker/cabinet. A closed back or detuned/ported cabinet is paramount. Preferably with at least two 12's in it. An open back cabinet wll never give the classic Marshall bottom end...Never. Even closed or ported, less than two twelves will come up short too. You've got to move some air. A common malady with combo amps, open back cabs and one 12 cabs is not enough bottom end. As a result players and designers drive too much bass through the amp to imply bottom end and you lose definition. The exact cabinets and speakers that most Marshalls are played through is a big part of the tone. Even the impedance curve of a speaker design can change the response of your amp to make it more/less bass and tighter/looser. Impedance swings on different speaker models can vary greatly.

I am currently trying to pick speakers for an amp model. It's a real bitch because this amp will be sold as both a combo and a head. With some speakers the combo sounds thin AND the head into a closed 2x12 cab is too boomy. The resonance of the circuit, speaker and cabinet all come into play. This is a pretty crunchy amp by nature and even still a pair of the wrong speakers in the 2x12 cab made the bottom end loose and mushy. With several other test speakers the bottom is nice and tight and not lacking in balance or volume. So it's definitely the speakers in this particular case causing the mushy bottom. This could also be happening to you if you have the wrong speaker/cabinet system. Most amp styles have a prefered type speaker/cabinet system that produces a characteristic tone. Veer too far from that and you get something very different.

I am still auditioning speakers. I've heard of guys collecting 5, 10 even 20 speakers in quests like this.

It can be a tough one. But a good start for you would be a closed back cab with at least 2x12's in it. Then you may have to re tune the amp for a more appropriate amount of bass through the circuit and the speaker system would have your back.

Chuck

[Steve Conner]

That is a good point. An open-backed cabinet is always going to suck in the bass department, and there's nothing you can do in the amp to compensate it. You can EQ it out at low volumes, but then when you increase the volume, the cone will just flap like crazy and give you nasty farting distortion.

Also, new speakers need several hours of loud playing to break them in, and their bass response can change radically over that time as the suspension loosens up.

[daz]

I know you guys are the techs and i certainly don't even resemble one. But in this case i honestly know what i'm talking about....it ain't speakers. I played bars for 25 years and owned more amps than any person should be allowed to. Tried every speaker known to man in nearly every amp known to man, and this is one thing i know for a fact....this fuzzy bass has nothing to do with speakers. In that 2.5 decades of playing out i mentioned, 95% of it was spent using 1-12" open back combos. I have never in my life encountered and amp with this kind of bass that was fixable thru speakers unless the particular speaker that was in it when i first encountered the issue was bad, and then any other average speaker would have cured it. If i can try an amp with about 5 different speakers like i did with this and the problem persists, then even if a speaker DOES "fix" the issue, the amp is still the problem and the speaker would just be a fluke and a a bandaid.

And Chuck....no, with that smaller cap there the motorboating happens the entire time the amp is on and while i'm playing. With the .022 there it only happens as it's powering down. And oddly, it only seems to happen at home and not at work ! so i guess it's the wall voltage difference that sets the perfect balance to start the boating in motion.

[Steve Conner]

Well, in the light of stuff I posted earlier in your other threads, if the amp is just on the verge of motorboating, then there's probably enough phase shift in the feedback loop to do weird things to the bass as well.

I'm not prepared to try and talk you through fixing that, though, except just to suggest that you go back to the standard Marshall power amp schematic for the model your amp is based on (yes, fixed bias and all )

In particular, though, you once mentioned that your PI supply node had no filter cap. This could be a source of unwanted feedback that might explain the above problems. Did you try adding a filter cap and did it make any difference?

Also, low end depends on the room you're playing in. Every amp I've ever played at home, guitar amp, bass amp, stereo set, whatever, has sh*t low end in my living room, because the room itself has a nasty upper bass resonance.

PS, if you've tried every speaker and every amp known to man, and you're still not happy, maybe you need counselling