Suggestions for specific Low-End tweak I'm looking for

[lgehrig4]

I have a 50W JMP'ish that I built a number of years ago and hav been tweaking ever since. Actually, started at an JTM-45 with RS transformers, then Drake, then JTM 50 and now its like a mix of JTM/JMP/JCM.

I really like what I am hearing when dialed up but one final adjustment I'd like to make is specifically to the low end when overdriven. When I have the amp cranked to around VH territory, the bottom end doesn't quite compress and crunch the same way a the other frequencies do. Sounds a tad cleaner than the rest of the frequencies if that makes sense.

I don't necessarily want to decrease the bass, but rather get it to compress/crunch more in line with the mids and highs. When playing guitar I'm specifically speaking to the low E string tones.

Any suggestions? I have no idea to begin.

[Roe]

Just add more bass by increasing caps

[lgehrig4]

I'm not following you. I'm not looking to increase the bass. I'm looking for the bass to response to be more in line with the higher frequencies in regards to compression and gain when I turn it up. My amp is already on the verge of having too much bass and if anything I'd like to reduce it, but that aside, I just want the bass to saturate more.

I'm not looking to turn the amp into a tight metal machine, but right now the low end could use a little more crunch to go with the higher frequencies.

[Travis_HY]

First, I would try reducing the two 0.1uF coupling caps in the output of the phase inverter to something smaller like .022uF if they aren't already The difference is a reduction in frequencies below 10hz which are entirely unneeded in a guitar amp as far as I am concerned. If you like the bass compression the 0.1uF gives you, 0.047uF might be a consideration, but it will be almost exactly the same as the .1uF's sonically IME. The 0.047uF's might be just a touch less bass.

After reducing the value of those coupling caps, I'd try a pre-set Resonance circuit network in the negative feedback loop.

Wire a 220k resistor and a .0047 in parallel. Then solder this little network in series between the feedback resistor wire to whatever speaker tap you are using for the negative feedback. Or make it a variable 1M pot to tweak the amp to work with different cabinets. I like the pot with combos because those cabinets generally need some extra low end poop.

That makes the bass response a little more prominent because it compresses the low mids. It also it balances out the EQ so the Middle and Treble controls giving a little more distinction to each of them. Without the Resonance, the EQ seems more subtle and it really only sounds good when you get it cooking to my ears.

It's an easy thing to try out. If you don't like it, just take it out and report back about what you thought about it and maybe I can help you from there.

All the best! Happy soldering!

[Roe]

I'm not following you. I'm not looking to increase the bass. I'm looking for the bass to response to be more in line with the higher frequencies in regards to compression and gain when I turn it up. My amp is already on the verge of having too much bass and if anything I'd like to reduce it, but that aside, I just want the bass to saturate more.

I'm not looking to turn the amp into a tight metal machine, but right now the low end could use a little more crunch to go with the higher frequencies.

you need to introduce more bass early in the circuit to get more saturated bass, otherwise the bass will be cleaner and thinner than the mids and top-end

[Stevem]

I don't follow you either, as what discribe is the opposite of what should take place and what does take place during the 50 years I have seen in playing and working on amps!

That being said the first thing I would do is to increase the resistance of the feedback resistor to lower the global feedback voltage, or even start off with a big step by moving the feedback tap off from where it is to the next lower OT impeadance tap and see if that's too extream.

Also you said that the amp started out as a JTM45 which would have had a rectifier tube, is the amp SS rectified now I would guess, as that's a great big change in and of itself in terms of low end crunch!

Is your Bass pot open all the way when your talking about this issue?

[pdf64]

Without a current schematic, it seems kinda pointless responding?
Need to know where you are, and what you've already tried and rejected.

Also, are you sure the issue isn’t the speaker cab, have you tried the amp with any others at all, preferably a variety?

[Phil_S]

...are you sure the issue isn’t the speaker cab, have you tried the amp with any others at all, preferably a variety?

What do I know? Not much. This was my first reaction, change the speaker to something that has better bass response. This might be the driver or it could be the box it's in. Addressing this could be considerably more challenging because changing the cab isn't like changing a cap, and changing the driver can be expensive. On that note (punny, huh?), I can understand someone's resistance to looking a this end of the signal chain. If you have an open back cab, one thing that is relatively easy is to cut blanks to make it partially closed or fully closed.

[lgehrig4]

First, I would try reducing the two 0.1uF coupling caps in the output of the phase inverter to something smaller like .022uF if they aren't already The difference is a reduction in frequencies below 10hz which are entirely unneeded in a guitar amp as far as I am concerned. If you like the bass compression the 0.1uF gives you, 0.047uF might be a consideration, but it will be almost exactly the same as the .1uF's sonically IME. The 0.047uF's might be just a touch less bass.

After reducing the value of those coupling caps, I'd try a pre-set Resonance circuit network in the negative feedback loop.

Wire a 220k resistor and a .0047 in parallel. Then solder this little network in series between the feedback resistor wire to whatever speaker tap you are using for the negative feedback. Or make it a variable 1M pot to tweak the amp to work with different cabinets. I like the pot with combos because those cabinets generally need some extra low end poop.

That makes the bass response a little more prominent because it compresses the low mids. It also it balances out the EQ so the Middle and Treble controls giving a little more distinction to each of them. Without the Resonance, the EQ seems more subtle and it really only sounds good when you get it cooking to my ears.

It's an easy thing to try out. If you don't like it, just take it out and report back about what you thought about it and maybe I can help you from there.

All the best! Happy soldering!

Thank you. This is a good start. Already did something similar to the negative feedback recently, but I can't remember the values at the moment, only that the overdrive sounded better.

[lgehrig4]

I don't follow you either, as what discribe is the opposite of what should take place and what does take place during the 50 years I have seen in playing and working on amps!

That being said the first thing I would do is to increase the resistance of the feedback resistor to lower the global feedback voltage, or even start off with a big step by moving the feedback tap off from where it is to the next lower OT impeadance tap and see if that's too extream.

Also you said that the amp started out as a JTM45 which would have had a rectifier tube, is the amp SS rectified now I would guess, as that's a great big change in and of itself in terms of low end crunch!

Is your Bass pot open all the way when your talking about this issue?

Good catch. I've actually been going back and forth between SS and tube rectifier and right now I settled on tube. I prefer the slight squish I get from it. My bass pot it almost off. The amp has more than enough bass. With the SS rectifier the amp is punchier, little louder and tighter, but what I'm describing is still present. The bass stays too clean I want the bass to compress and crunch up a little more. Right now when the amp is cranked up the bass sounds slightly cleaner than the other frequencies.

[lgehrig4]

Without a current schematic, it seems kinda pointless responding?
Need to know where you are, and what you've already tried and rejected.

Also, are you sure the issue isn’t the speaker cab, have you tried the amp with any others at all, preferably a variety?

It's not an issue. I'm trying to voice the amp slightly different. The speaker cab is a Wizard 4x12 with G12M Creambacks. Sounds amazing with every Marshall type amp I've had or currently have, including this one. I'm simply being picky and trying to see if I can achieve this change without pedals.

As far as a schematic, it's a 50W marshall with with a few circuit value changes. I figured what I asked was specific enough and that there are areas in the circuit to address that? You don't need the schematic to know that changing the value on presence cap wouldn't help.

[pdf64]

Detail matters, especially so with such a nebulous query.
It’s hardly that big of an ask for you to sketch out the schematic, note the values, and take / upload a photo of it, surely?
A Marshall 50 is not a specific thing, there are variants.
What you’ve got in front of you may be obvious to you, but that doesn’t work over the internet.

[Stevem]

Well if that's the case then I would replace your 50 watt OT with like a Hammond 30 watt OT which when driven hard in itself will roll off low end , add some crunch and make you open up that bass pot more and drive either a PI, the outout tubes or both harder and give you more of what your missing.

The Hammonds are over built and should not burn up on you too soon!

[JD0x0]

When I have the amp cranked to around VH territory, the bottom end doesn't quite compress and crunch the same way a the other frequencies do. Sounds a tad cleaner than the rest of the frequencies if that makes sense.

This to me sounds like you need to increase the cathode bypass cap(s) a bit so that low frequencies have a bit more gain. I would also calculate your high pass at the coupling caps. I tend to use the smallest possible value that wont shave too much lows, to prevent blocking, when using larger cathode bypass caps.

IIRC when reading about EVH's mostly unmodded marshalls, they mention a 'mod that increases bass' on the bright channel. My assumption is they increased the cathode bypass cap a bit on the bright channel to give it a bit more 'umph' in the lows.

From what I understand, when you cut low frequencies with smaller cathode bypass caps, that effect is a sort of negative feedback on the tube at lower frequencies, this would decrease gain, as well as clipping in those frequencies, just as removing the bypass cap would do this to all frequencies.

[lgehrig4]

Well if that's the case then I would replace your 50 watt OT with like a Hammond 30 watt OT which when driven hard in itself will roll off low end , add some crunch and make you open up that bass pot more and drive either a PI, the outout tubes or both harder and give you more of what your missing.

The Hammonds are over built and should not burn up on you too soon!

Change the transformer just to make a slight adjustment to the way way the low end compresses when pushed? I’m sure that what you’re suggesting would accomplish that, in addition to changing many other things that I don’t want changed.