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Tightening up a loose sounding low end

2/6/2004 11:18 PM
Tightening up a loose sounding low end
Most people say the Marshall 18 watt build has a tight low end, but whether it's because it's subjective or something else, mine is not tight at all. It's the build with the 2nd channel that has some preamp drive to it, and this is where it's got the problem. What are the areas within a typical valve circuit that defines how loose or tight the low end is?  
The problem i find is that the thing seems to have a huge mushy low-mid to it, and the treble is ratty sounding so i have to keep it rather low. This makes for a boomy loose sound thats just horrible. As for the notion it's a mistake in the circuit, i've gone over it a 100 times and i'm certain it's wired right with the correct component values. Where should i start experimenting to remedy this problem? I've already tried cathode cap changes.
2/7/2004 7:59 AM
Well it can have alot to do with the guitar your using. I don't mean to pull a total "DUH" on you here but it surprises me every time I swap guitars with a given amp. My humbucker guitar is very fat and my single coil guitar has very strong upper mids and treble. With both extremes there is no one good amp for both guitars. My point is that if your expecting a certain sound that you "heard" through one of these amps, but yours doesn't sound like that, it may not be the amp at all. Your amp might sound just right with another guitar.  
O.K I mentioned that to mention this... The goal now would be to tweak the amp you built to your guitar and playing style. It sounds like you need to make some changes in the basic design to get it to behave like you want.  
The very first thing I would do is raise the preamp voltages. This will make a huge difference in the bottom end tightness and the stability of the top end. The amp will litterally "come to life". Just lower the B+ resistors to pick up the preamp voltages. O.K. Also useful for a tighter bottom end is a choke to feed the screens. Lose the resistor and install a $25.00 choke suitable for almost any amp that uses a choke to feed the screens. Anything from 4-15 henries should make the improvement you want. More? Make sure your bypass caps are reasonable. There is very little advantage to any bypass cap bigger than 25uf. Bigger caps won't give you more bottom end for guitar. They will just change the reactive overtones and loosen the bottom end. This is a very desireable effect with some designs, like the 1959, but it's not working for you. So experiment with lower value bypass caps to see what sounds best.  
If your really cranking the amp it would help to convert it to a fixed bias arrangement. I love cathode bias and use it on most of my builds, but it will get more mushy than fixed bias when pushing the amp.  
Decoupling caps... Try replacing any .1 caps with .022 caps. Especially in the PI stage and for the grid feeds on the power tubes. You won't hear much difference in the low end in these areas between .1 and .022 decoupling caps but it can help stabalize fartiness (is that a word?).  
SPEAKERS??? Are you using good tight speakers? How does your amp sound through a 4X12" cab" It would suck to put alot of work into the amp only to find out your speakers were at fault.  
2/7/2004 4:46 PM

The guitars definatly aren't the problem, and i've tried cathode cap changes, plus they are only 1uf anyway. Speakers also aren't the issue. It's the preamp drive thats muddy in my 18 watt build. Cleans are tight as is the drive channel if it's set clean. The one thing i haven't tried is lowering plate resistors to get more voltage, so i will try that. Thanks for the ideas.
2/7/2004 5:08 PM
Bruce /Mission Amps

"The one thing i haven't tried is lowering plate resistors to get more voltage, so i will try that."
You might have a harder time tweaking this amp then you first thought. ;)  
If you think that's what it takes to raise or lower tube's plate voltages, then you need an amp building refresher course ha ha!!  
Never use the plate load resistors to change the delivered B+ to a tube.  
That's what the B+ rail's dropping resistors are for.  
2/7/2004 11:54 PM

Ok, well i tried it and it did change it by about 25 v and tightened it up a bit. Nothing like i wanted tho. So why shouldn't i do it this way? I'm not questioning you, i just really don't know and would like to. But anyway, i'll but the stock ones back and drop it as you said. Thanks.
2/8/2004 4:26 AM
Wild Bill

Dazco, the plate resistors are not there to drop voltage. They are there to provide the load for the plate. The amplified signal voltage swings across the plate resistor. A smaller resistor means a smaller swing. There's more to it of course but suffice to say that the plate resistor value influences the gain.  
That's why some higher gain circuits use 220k instead of 100k. And with a different plate load resistance you need a different value cathode resistor. You are adjusting the cathode bias to the preamp stage to be correct for the different plate load. Otherwise you get one side of the waveform clipped.  
Much better to change the dropping resistors in the filter string to get a higher voltage fed TO the plate resistor!  
It would help you a lot to go to Randall Aiken's site and look for his FAQ on voltage amplifiers. I refer to it often with these problems.  
---Wild Bill
2/8/2004 4:16 PM
Bruce /Mission Amps

I haven't built the 18w amp you are messing with, but I've worked on real 18 and 20 watt Marshall amps and they never struck as needing to be tightened up much.  
Bill's explaination of the why's of plate voltage is correct.  
You can increase the preamp headroom a little by raising the plate voltage as long as the tube is idling in it's correct linear range.  
As Bill alluded to, that means altering the cathode resistor if you change the B+. Most 12AX7 triodes live quite well in the 100K to 220K range.  
Remember, these single triode stages are actually little baby single ended class A amplifiers so they need to biased in such a way as to keep them linear no matter what signal you hit them with if you want them to stay clean.  
Woofiness or muddiness is usually a too much gain at low frequency kind of thing.  
If your preamp stages can pass huge amounts of low freqs, it's easy to muddy up the tone by having too much low mids and bass.  
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