Taming the Bass

[Structo]

Today I disconnected the 10K tail on the Bass pot and inserted my decade resistance substitution box.

Then I put the .01uf mid cap back in and experimented some more.

The lower the resistance there the later the bass pot comes on in it's rotation.
It "turns on" later.

This is what I want so I ended up with a 1K resistor there.

I'll give that a try for a while.

I wish we had a decent tone stack modeler for this Skyline stack.
I had emailed Duncan a long time ago asking him if he would be willing to make a tweak to his tone stack calculator or a program change to include this stack and told him how many guys would be pleased with that....
Never heard a word back.

[gary sanders]

i got rid of the 1n cap across lug 1 and 3.Made my mids better

[Joost]

So has anyone tried placing a fuzz filter across the plate resistors? What value might work? Or were the Other places mentioned better?

No, I am talking about low end bass.

The noise you describe is probably the often described fizzies in the overdrive.

Sort of a high pitched buzziness that accompanies high frequencies.

There are a few ways to address that.

Are you familiar with smoothing caps?

This is where you take a ceramic cap with a value of say, 50-100pf and put it as a shunt to siphon away the high fizz.

This is why the "snubber" caps on V2 are there (The 250-330pf caps).

You can try putting a cap in parallel with a plate resistor, or between the plate resistors, or from the plate to ground or plate to cathode.
You just have to experiment with placement and value.

I also feel that perhaps the GNFB or Presence circuit may be part of the issue with the fizzies as well. But I'm still experimenting with that.

It can be aggravating.
I thought I had those all but eliminated but I fear they are back in my amp now......
Right now I am wondering if power tube rattle may be partly to blame.
I have some tube dampers on the way...

[greiswig]

Gil used a .0047 usually which cuts a lot of bass and a 4.7meg to ground at the input of the OD stage. I've used the .05 before and is about all I needed.

Resurrecting an old thread here. I'm still not completely content with the balance of bass between the clean and overdrive channels. I like the bass at noon on the clean channel, but things get woolly when I switch to overdrive, particularly on anything other than a bridge pickup.

I have one of Brandon's Twin-style OPT's, and doubled the first filter cap stage when I started using larger tubes (KT-88s) and running 520V on the plates. So it's already pretty heavily modified, but it is a D'Lite chassis.

Looking through this group, I came across this post from tubedogsmith. I had remembered .02uF as a commonly-used pre overdrive cap addition, so .0047uF looked small. My build currently uses a .01uF.

So my question is, how low have people gone on this value? And what role does that 4.7M resistor play? Is it critical to the tone, or is it merely to discharge the cap and prevent popping noises?

[tubedogsmith]

g, that cap is inserted right before the 220k in the OD entrance with the 4.7m resistor going from where the OD relay connects with that cap and ground. The V1b .05 cap and V2a .01 cap are still there. does that answer the question.

[markusw]

So my question is, how low have people gone on this value? And what role does that 4.7M resistor play? Is it critical to the tone, or is it merely to discharge the cap and prevent popping noises?

I used 4n7 and 6n-something on two builds and I like the sound, FWIW. On one of the two builds I had something like 14n before which I thought was too much. All a matter of taste I guess.
The 4M7 is added to prevent popping. You could also wire it from before pre-od-cap to gnd. Doesn't matter. The cutoff frequency is determined by the value of the cap and the 220k-plus-100k-OD trim.

Regards,

Markus

[Structo]

I remember Gil commenting on this mod and he said that the resistor should be in parallel with the cap, not to ground.
I know the mod was further talked about and a lot of people did use the resistor to ground as a pop filter.
But, the original mod is the cap and resistor in parallel.

[greiswig]

I remember Gil commenting on this mod and he said that the resistor should be in parallel with the cap, not to ground.
I know the mod was further talked about and a lot of people did use the resistor to ground as a pop filter.
But, the original mod is the cap and resistor in parallel.

Hmmm...not that I doubt you, Tom, but I've been searching for some of Gil's postings and can't find any that directly refer to whether the resistor is parallel to the cap or not. Can you point me to something? I must not be using the right keywords.

...meanwhile, I'll give it a go and see if it helps.

[Structo]

It's mentioned in a few different places but this is a good one.
I know later on he came on to post that while what some of us were doing was fine, it was not what he had suggested.
Notice in his post that he uses the "//" to symbolize parallel.

https://tubeamparchive.com/viewtopic.php?p=56875#56875

[greiswig]

It's mentioned in a few different places but this is a good one.
I know later on he came on to post that while what some of us were doing was fine, it was not what he had suggested.
Notice in his post that he uses the "//" to symbolize parallel.

https://tubeamparchive.com/viewtopic.php?p=56875#56875

Thanks, Tom. Still not clear to me that he meant the // to signify parallel, but if you're sure of it, that's good enough for me to try it at least.

I'd be interested in someone who knew more about the impedances and R/C values involved modelling what the differences would be with and without the resistor, in either parallel mode or with the resistor to ground.

[markusw]

It's mentioned in a few different places but this is a good one.
I know later on he came on to post that while what some of us were doing was fine, it was not what he had suggested.
Notice in his post that he uses the "//" to symbolize parallel.

https://tubeamparchive.com/viewtopic.php?p=56875#56875

Thanks, Tom. Still not clear to me that he meant the // to signify parallel, but if you're sure of it, that's good enough for me to try it at least.

I'd be interested in someone who knew more about the impedances and R/C values involved modelling what the differences would be with and without the resistor, in either parallel mode or with the resistor to ground.

A while ago I simulated both variants in Spice to find out it doesn't make a difference how you wire the 4M7.
https://tubeamparchive.com/viewtopic.ph ... late#94834

Regards,

Markus

[markusw]

Hi,

this is how I understand it. Please let me know if I miss something:

connected in parallel with the pre OD cap (e.g 10n) the 4M7 together with the 220k/100k-OD-trim forms a voltage divider, like a 5M volume pot set to about 6% (the signal is attenuated by 24,4 dB).
Thus, 6% of the "unfiltered" signal passes. The "unfiltered" signal has an fc (a -3dB point) of 0.64 Hz (a high-pass filter consisting of the 50n V1b output cap and 4M7+220k+100k resistance to gnd). So it's flat well down below the audible range..
The 10n pre OD cap plus the 50n V1b output cap in series give 8.3n. The 8.3n net capacity together with the 220k/100k-OD-trim forms a high-pass with an fc of 59.9 Hz. (edit: with a 4n7 pre OD cap you would shift the fc to 115 Hz, btw).
So if you connect the 4M7 in parallel with the pre OD cap you have a 59.9 Hz high-pass plus 6% "unfiltered" signal.

If you connect the 4M7 resistor from the "input" of the pre OD cap to gnd you basically remove the 6% "unfiltered" signal.
The fc of the high-pass filter remains the same 59.9 Hz.

I doubt that the 6% "unfiltered" will make a difference.

What do you think?

Peace,

Markus

[ElectronAvalanche]

Hi Tom,

https://tubeamparchive.com/viewtopic.ph ... ht=#165912

ayan wrote:


For what it's worth, that it not actually the case. What I did -- and never in an amp with 100K plate load resistors -- is put a .0047uF (so, 4.7nF, NOT 47) in PARALLEL with a 4.7 Meg resistor before the 220K pre OD trimmer (100K).

Cheers,

Gil

Most of the signal is cut back by the 4M7 in series with the 220k, only the higher frequencies that can pass the 4n7 are not attenuated. Thus there is a bass cut. Cutoff-freq can be calculated.

Cheers,

Electron