low end and how to reduce it?

[daz]

We all know there are ways to remove or accentuate various frequencies of treble, but are there any ways to do the same for low end? I don't mean reducing low end, but reducing certain areas of it. In my hi gain 18 watt the lows extend too low. Seems almost all the way down to subsonic levels. So consequently it gets a bit flabby and howls when the volume is turned up even when i turn down the pre gain to compensate. i have to turn the pre gain down too much because of this. So are there ways to deal with this? I have already tried changing the bass cap value in the stack.

[chipaudette]

I think that the easiest way to cut bass is to change the value of the coupling cap after the first (or second) tube stage. Cutting the value in half will raise the cut-off frequency by a factor of two. Keep cutting it in half until you've eliminated the bass that you dislike. Fine tune as necessary.

Note that coupling caps must be rated for high voltages. Use a similar rating as the coupling caps in your amp right now.

Chip

[daz]

I tried that, but by the time i got rid of the sub lows, most of the rest of the low end was gone too.

[Shea]

The vibrato channel of the Marshall / Watkins 18-watt circuit has two coupling caps in series, with a 470k resistor connected between the junctoin of the capacitors and ground. Then the 2nd coupling cap connects to the volume pot.

The 1st cap and resistor form a high-pass filter, so they shave off some bass. The 2nd cap and the volume pot form a second high-pass filter. So, you have two high-pass filters in series -- a two-pole high pass filter. This works more effectively than making the coupling cap in a single-pole filter smaller. With a two-pole filter, the bass response drops off twice as quickly below the knee frequency. IIRC, it drops 6 dB per octave compared to 3 dB per octave for a single-pole filter. With that steeper dropoff, you can get enough bass response down to the lowest frequency that you want to hear, and then have it drop off sharply below that point to prevent the flubs.

In a circuit that has more stages to it, like a 50 or 100 watt Marshall, there are a lot of different points in the circuit where some bass is shaved off. The net effect of having a little bit of bass shaved off in several different parts of the circuit sounds better, IMO, than trying to cut out a lot of bass at one point. It's that steep dropoff thing. But the 18-watt circuit only has one gain stage and then the phase inverter, so there are fewer places to cut bass. So inserting that extra high-pass filter was a good idea, IMO.

When I used to make single-channel variants of the 18-watt amp, I based it on the tremolo channel and just left out the tremolo circuit, instead of basing it on the normal channel. One reason why I did that is because I thought the extra high-pass filter in the tremolo channel (which isn't in the normal channel) results in tighter bass response.

Shea

[daz]

Shea......mine is a single channel 18. It WAS as TMB, but i cascaded it then tweaked it ad nausium. But the one thing i cann't seem to cure is that low end that extends WAY too low. the pre has been thru endless configurations even cloning a JCM800 exactly. yet i have this ultra low bass that the amp just cannot reproduce w/o huge iron, and filters i guess. I've tried reducing it in several places but it always loses too much and never gets tight enough. I'm starting to think it absolutly much be the power section, and i'd gladly covert it to fixed bias if i thought that would do it. I just don't know if it really would, but i have gone to cathode bias on other fixed bias amps and the opposite never happened. (bass getting loose) I tried much bigger cathode caps to no avail. The PI is also suspect, tho i'm not sure what to do about that. I have the PI values set to Marshall standard sizes.

[Shea]

You want to try smaller cathode bypass caps to tighten up the bass, not bigger ones.

Using the PI values from the 18 watt amp might help a little bit. IIRC, it uses a bigger tail resistor than the regular Marshall amps. IME, that makes the tone a little bit tighter, for lack of a better word. It doesn't cut bass, but the effect is sort of the opposite of mushiness.

Shea

[daz]

I didn't mean a bigger cathode on the AX7's, i meant on the power tubes. Some say a bigger cathode cap tightens it up, but it didn't do that at all for me. As to the preamp, i've gone down as low as .1uf on those but all it does is thin out the tone and the bass stays just as flabby and big. I tried a bigger tail R on the PI today in fact. Funny you mentioned that because i had no reason to believe it would help, and unfortunatly it didn't. or if it did i couldn't really tell.

[pdf64]

Have you tried some known good tubes in there? It might well be that your el84s are weak or tired, or you just dont like their tone. Peter

[daz]

yes, in fact two sets of known good jj's and a known good set of eh. several preamp tubes too. it's not tubes, speakers, guitars. its the amp.

[Guitarist]

A few successful things that have worked here that you've likely tried:

.0047-.01 1st stage cap
1-2uF cathode caps (5uF max)
remove master volume (esp. PPI)
quality speaker

you could insert an eq. in the loop (passive off CF/MV would work)

G.

[chipaudette]

Daz,

If you post a schematic of your amp (pre+power), we might be able to help a little more. If you feel that you have sub-bass going on, perhaps you deviated from a "typical" setup in some unintended way...a way that permits the sub-bass. If you post the schematic, one of us might be able to spot the problem.

One thing that I'm going to look for is your feedback circuit (if you have one). It's possible that you have an extra cap in there by accident that prevents bass in the feedback line. That would result in a huge boost in bass in your output. Furthermore, the deeper the bass, the more boost. Kinda sounds like your initial complaint.

Chip

[daz]

I'll try to post one later. I'll have to edit a schematic because at this point it's deviated from the last one i posted in another thread.

[Sweetfinger]

I will generally try to tame flubbiness with the 1st stage cathode bypass cap. try as small as a .47

[daz]

thats none of the first tings i tried, but i want even smaller to .1 uf on both side of V1. Like i said, no amount of high pass filters ot cathode caps etc etc etc work because all they do is add treble, not remove bass. So i end up worse because now theres a ton of flab and the rest is treble. I think you get the pic. The problem is the bass must be removed, not add highs. With the bass on 0 the bass is so low in frequency and theres so much the power supply can't handle it. I may have figured it out tho, but i won't be able to tell till monday when i can crank it up to hear it. (neighbors ya know)

[Liam]

Like i said, no amount of high pass filters ot cathode caps etc etc etc work because all they do is add treble, not remove bass.

That's not how it works. A high pass filter can't add anything, it either lets things through or it doesn't. A schematic with the current resistor and cap values will enable someone to figure out where the real lows are getting through.

The only thing a high pass filter does is to remove low frequencies, which is exactly what you want.