Cathode bypass caps

[greiswig]

On my low plate D'Lite build, I'm tinkering with taming the bass. And I notice something kind of interesting that I wanted to run past this group.

#124 uses 4.7uF cathode bypass caps on V2. If you've got too much bass, I agree that it's better tonewise to lower the coupling capacitance than to lower the cathode bypass caps. But increasing the cathode bypass caps has an effect that might be useful.

It clearly lowers the bottom end a fair amount. But (and this is the part I'm wanting to get feedback on) it also seems like it gets rid of some low-mid mud. I'm just guessing, but often when you have a passive filter, there is a knee at the cutoff frequency that actually is up 2-3db. I'm wondering if that is the low mud that some of us are hearing, and if it might help to actually lower where that knee is (i.e. get it out of the way) and deal with the bass via the coupling capacitance?

Anybody else tinker with this?

[David Root]

Well, so they are! I think I missed that on my build and made them 10uF like V1.

[greiswig]

Well, so they are! I think I missed that on my build and made them 10uF like V1.

Happy to have helped for a change. Note, too, that #183 also used smaller bypass caps on V2.

But seriously, not much traffic on this topic. David, I'd be curious as to whether you feel it necessary to revisit or over-large cathode bypass caps or not, and what coupling capacitance you have coming into V2a and b.

I'm still wondering about what is causing what I seem to be hearing. It seems to me like having larger bypass caps makes the bass go deeper (extends it down), but simultaneously pulls some of the low mid mud out (maybe around 140Hz?), which in my amp is a good thing. I hypothesized that there might be a knee above the cutoff frequency that causes the low mid hump. Randall Aiken, in his excellent tech section on his web site, has the following:

"What this means is that the partially bypassed cathode results not in a first order highpass filter with a -6dB per octave slope extending down to DC, but rather a shelving equalizer, in this case with a response that starts at 30dB, is 33dB at 1.2KHz, and then rolls off to a maximum of 36dB above that. It looks like this:"
[img:626:435]http://www.aikenamps.com/triodegain1.gif[/img]

The image certainly doesn't show any kind of bump where the frequency starts to roll off, but I swear I recall that passive filters like this (in crossover networks and elsewhere) have that kind of a bump right above the cutoff. Maybe they work differently in this location.

Hard to deny what my ears are hearing, though. Just seeking an explanation.

[vibratoking]

The image certainly doesn't show any kind of bump where the frequency starts to roll off, but I swear I recall that passive filters like this (in crossover networks and elsewhere) have that kind of a bump right above the cutoff. Maybe they work differently in this location.

Active and passive filters can have the 'bump' that you are referring to. It depends on the filter order, which is how many reactive components it has, and the value of those components. For the bump to exist, the filter must be second order or higher. If the system is underdamped, then the 'bump' will exist to some degree. It depends on the location of the poles and zeros of the system.

Here is a link that briefly discusses 2nd order systems and has plots that show the 'bump'. It has some pretty pictures that will allow you to easily see the phenomenon.

http://lorien.ncl.ac.uk/ming/robust/freqplots2.pdf

Here is another link if you really want a deeper understanding. Warning, it is very mathematical and technical.

http://web.mit.edu/2.14/www/Handouts/PoleZero.pdf

[Structo]

George if you are using 20uf or above isn't that a fully bypassed cathode?

I implemented what I told you about and I like it a lot.

I have gravitated to humbuckers for these amps and with the .68uF bypass cap on CL1 it is much better than the 4.7uF I had there originally.

Then I have the 4.7uF on a switch, when I throw that the result is 5.38uF.

I can definitely tell that in that 5.38uF setting the tone is much fuller and looser on the bottom.
But with a single coil guitar is sounds pretty good.
I am also in the midst of trying something with the mid cap in correlation with the bypass cap switch so we'll see how that goes.

Then we have the FET input which apparently a lot of guys plug their SSS strat into.
I'll have to mess around with that.

With my 2x12 with the EVM 12L's even with the .68uF bypass cap, there is plenty of bottom but it is more focused and not as muddy.
I have my bass pot at 9 o'clock.

Some Marshall amps have a 330uF! on V1a and a .68uF on V1b! (1969 Super Lead)

While other Marshalls have a .68uF on V1a and a 10uF or bigger on V1b.

So many different ways to skin a cat.

Ultimately, we want tight bass that we can dial in as much or as little as we want but it seems there is always a compromise.

The endless tweaking continues.

[martin manning]

The Rk-Ck does indeed produce a shelving response, but with the 1k5 and 5uF, the high-frequency knee is at ~45 Hz. It's 3 dB down there and it's effect will taper off over the next two octaves. The D-string on a guitar is 147Hz, and even with the 5uF you'd have essentially no roll-off at that point. A 10uF would lower the knee to ~22Hz.

[David Root]

I haven't played my 124 in a while but I don't recall any problems with muddy bass.

The one I had too mucho bass on was my Bluesmaster, which I did fix with lower value pots. But even there I don't recall it being muddy, diffuse.

[talbany]

George I know yours is a low plate but thought I would throw this out there..So!!
Not exactly sure if I can reveal the exact ODS number here so let's just put it in the 12x range built in 84 in it's original state built and voiced by the man for a vintage Strat and sounds absolutely wonderful with single coils..

Amp is..

High Plate Classic 100w

150k Slope
300 pf Treble cap
5uf bypass V1-V2
Treble bleed
3K3 NFB resistor
1K5 presence pot w/1uf cap
220k feeding 500K OD trigger
100k Ratio 100k drive
No bright cap on master

Hope this Helps..

Tony

[jelle]

what's the treble bleed?

[talbany]

what's the treble bleed?

HF taper..

[David Root]

Tony I think Jelle means what values on the treble bleed?

[Structo]

Tony is that a 6L6 amp?

Interesting about the OD entrance trimmer.

I take it that this amp must have a Deep switch with the 300pf treble cap?

Presence is different.

How about V2 snubbers?

[talbany]

Tony is that a 6L6 amp?

Interesting about the OD entrance trimmer.

I take it that this amp must have a Deep switch with the 300pf treble cap?

Presence is different.

How about V2 snubbers?

6L
Deep
230 pf snub
500k/22nf taper

T

[greiswig]

Very, very interesting. Thanks in particular for the technical links, Vibratoking. I'm going to have to digest those for a bit!

I'd wondered about phase, too, since I've dealt with that in a recording studio and had issues with phase on a bass track that, when fixed, cleared up a bunch of low mid mud.

For now, I'm going to trust my ears...so far as the room that I've been testing the amp in will let me. I know that room ain't exactly bass friendly to begin with. So I'll wait and see how the amp performs at a gig coming up. Tonight (Snohomish, WA "Taste of the Blues" Fest!) I'll be using my TK Metro instead of the big amp. Easier to swap out.

So anyway, for now I'm going with 8uF bypass on both the V2 sections, and keeping the .0047uF//4.7M. We'll see if it sounds at a venue the way it does right now. But to me it sounds very good...like the bass was extended a bit lower, but a bump around 100-140Hz got smoothed way out. I'll report back on what I find...and see if I can wrap my head around the docs Vibratoking sent!

Tony and Tom, thanks for your suggestions, too. I may look into those.