Marshall Super Bass Filtering?

[shocki]

I just finished my Marshall build with using this layout.
http://www.ceriatone.com/images/layo...sCeriatone.jpg

It sounds reallly great and does not hum at all which is something I was not expected from my first build
One thing I do not like though is that the amp has a very "stiff" feeling. Probably great for tight rock riffs and typical for this amp. But I would like to have it a bit more sag and more of a looser feeling.

This layout uses all 50+50 caps. I was wondering if changing to 32+32 in the preamp section and maybe also in the power section would make a big difference?
Will changing the caps to a lower value actually decrease the bass or just will not make it as tight ?

[loudthud]

An easy way to experiment with sag is to simply disconnect one of the 50uF cap sections at point "C" on the layout. That will give you a feel for what more sag will do.

I think you would get more benefit from a Resonance control or simply reducing the amount of feedback in the output amplifier. The feedback is controlled by the 47K resistor between the impedance switch and the Presence control. Try just disconnecting one end or increase the value to perhaps 100K to 220K reduce the feedback. This will lessen the effect of the Presence control and give you more gain.

[shocki]

Cool idea. So this will reduce filtering after PT to 50uf instead of two times 50uF. I am just wondering what effect changing filtering in different stages has. So changing in the preamp section as opposed to changing the cap at the PI or power tube screen or after the PT.

[loudthud]

In general, filter changes in the preamp don't make much difference until you get down to the neighborhood of 10uF and then there is a dependence on what is happening in prior filter stages upstream. The voltage sag is just too slow. The screen grid node is the important one and it is influenced by the first filter node.

When the power amp demands current, ripple increases on the first filter node (the reservoir cap). The choke removes the ripple between the first and second (screen) node, but drops the voltage in proportion to the amount of ripple. More ripple means more drop. The cap at the screen node controls the speed at which sag occurs and how fast it can recover.

Besides ripple on the first node, each half cycle of low bass notes also takes a divit out of the B+ voltage. This sounds like a weakness in bass response and a slight blurryness to the tone. So there is a compromise that has to be struck between strong bass and sag induced compression.

The suggestions for changes to the power amp feedback affect tone from a different angle. A lower damping factor will loosen up that stiff feeling and let the speaker's personality come through. I like a multi-resonant tone from two or more different speaker sizes.

[olddawg]

How loud are you playing the amp. If you are not cranking it it will sound sterile IMHO. You also might consider a stomp box compressor.

[shocki]

Besides ripple on the first node, each half cycle of low bass notes also takes a divit out of the B+ voltage. This sounds like a weakness in bass response and a slight blurryness to the tone. So there is a compromise that has to be struck between strong bass and sag induced compression.

But I would only notice this at higher volumes when the PT cannot deliver enough current and so the caps make up for this? I really like the bass response like it is. Very balanced. I also hear this stiffness at lower volumes.
Also you mentioned the resonace mod. Is this something like a depth mod? I think at lower volume I really would benefit from this. My Soldano has this and I really like it.
I only want to do mods or try out things that can be totally bypassed. A pot as the feedback resistor will do no harm to the sound. I don't know about the depth mod with a pot. Can you point me to a schematic?

The suggestions for changes to the power amp feedback affect tone from a different angle. A lower damping factor will loosen up that stiff feeling and let the speaker's personality come through. I like a multi-resonant tone from two or more different speaker sizes.

Ok. I will try this. When I look at a 5F6 Bassmann for example. One big difference is the filtering (which is really low) and also the feedback value is half of it. I think both of it has to do with it.

How loud are you playing the amp. If you are not cranking it it will sound sterile IMHO. You also might consider a stomp box compressor.

I have to test it at lower volumes. It gets better when I turn it up.

[shocki]

OK. Found something.
Depth/resonance mod? - Marshall Amp Forum

I just tried the negative feedback with a 250K resistor. At lower volumes it really does alot for loosening the sound. Sounds much closer to Bassmann kind of sound which I like alot. Also it sounds already a bit like a depth switch. Probably enough for me. But I boost the Marshall with pedal and turn it up I like it better with the stock resistor value. It is much tighter that way. But I think I leave it. It is really nice to have both options.

I also tried just using one part of the first cap. So it will give me 50uF instead of 2x50uF. Which ist very similar to 32+32. I had the feeling I got more bass but the bass was not as tight. Stil I am asking myself if this is something you would only notice at higher volumes.