Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: phsyconoodler on September 30, 2011, 11:30:43 am

Title: tone thickening mods
Post by: phsyconoodler on September 30, 2011, 11:30:43 am

I recently came across a couple of mods that are touted to 'thicken' or fatten the tone of an amp.
One mods was to add a 330pf cap across the input grid resistor and the other was to add a .0033uf cap in series with the negative feedback wire to the speaker or impedance selector.
  Anyone use these mods?I'm going to try them of course,but I was wondering if they are viable mods and don't muddy up stuff too much?

Title: Re: tone thickening mods
Post by: TIMBO on September 30, 2011, 03:20:17 pm

Is this one of the mods you are describing, I have no idea if it does make any difference as this amp was designed by a guy called Chris Cornell from the UK and i built it as per his schem, can't say i seen it added to other amps.Great little amp. Also the Marshall jubilee has a cap in series with the input resistor, i'm thinking that this would colour the input signal  :dontknow: Thanks

Title: Re: tone thickening mods
Post by: Tone Junkie on September 30, 2011, 03:46:53 pm

Phsyconoodler I do know that the .0033 with the -feedback is a built in resonance mod adds alot if you have shelved bass in the preamp so as not to be muddy they will usually run it in perallel with a 1meg resister. you can search and there is a formula to use to dial in what you want, i just use my cap switching box to find what sounds best, because I seem to be mathmatically challenged  :l2: . they have some good discussions at sloclone forum.
Bill

Title: Re: tone thickening mods
Post by: phsyconoodler on September 30, 2011, 04:32:31 pm

Timbo,
  I don't see either mods on your stinger amp and the Jubilee has the cap in series and not bypassing the 68k grid resistor.

I'm going to try the resonance this weekend and the bypassed input grid thing and I'll report the results.

Title: Re: tone thickening mods
Post by: jjasilli on September 30, 2011, 06:45:20 pm

A cap bypassing the input grid resistor:  for DC operation this seems to resemble series grid biasing, though this omits a cathode resistor:  http://www.tpub.com/content/neets/14178/css/14178_48.htm (http://www.tpub.com/content/neets/14178/css/14178_48.htm)  This would seem to alter the bias point of the stage, if already cathode biased.  For AC signal operation I'm thinking it's a notch filter, though I'm not sure that's the exact terminology.  Here's some dizzying math:  http://www.allaboutcircuits.com/vol_2/chpt_4/4.html (http://www.allaboutcircuits.com/vol_2/chpt_4/4.html)

Cap in series with a global NFB loop -- sounds pretty much like a simplified, fixed Presence control.

Title: Re: tone thickening mods
Post by: phsyconoodler on September 30, 2011, 06:56:15 pm

Hmmm....presence control? The cap goes to ground in a presence circuit but this one gets put in series with no ground reference.

Title: Re: tone thickening mods
Post by: jjasilli on September 30, 2011, 07:53:43 pm

Probably depends on the values in the circuit, and where the series cap is placed in relation to the series/shunt resistors in the NFB loop.

Title: Re: tone thickening mods
Post by: plexi50 on September 30, 2011, 09:54:48 pm

Quote from: phsyconoodler on September 30, 2011, 06:56:15 pm

Hmmm....presence control? The cap goes to ground in a presence circuit but this one gets put in series with no ground reference.

I have built a few amps using a resonance pot. And man it does work nice. Tightens the bass up nice to
Earth is resonance / Sky is presence. I dont think you can get the whole effect just using a cap alone
It really come alive and is very noticeable using a pot. Gets the full sweep range of more frequency change

Title: Re: tone thickening mods
Post by: Jack_Hester on October 01, 2011, 04:44:24 am

Quote from: plexi50 on September 30, 2011, 09:54:48 pm

I have built a few amps using a resonance pot. And man it does work nice. Tightens the bass up nice to
Earth is resonance / Sky is presence. I dont think you can get the whole effect just using a cap alone
It really come alive and is very noticeable using a pot. Gets the full sweep range of more frequency change

Do you have a pdf of the schematic for this amp, that I can look at?  I can follow your layout, if I hand-draw a schematic while I'm doing it.  I'm interested in what you've done and it's effects.  Thanks for posting the layout.

Jack

Title: Re: tone thickening mods
Post by: Boots Deville on October 01, 2011, 08:38:40 pm

The AX84 "October" schematic shows an example of a Resonance control.

At AX84.com it can be found under "Associate Projects".

Here's a direct link to the PDF.  The resonance option is on page 3.
http://ax84.com/october/October_Club_101010.pdf (http://ax84.com/october/October_Club_101010.pdf)

-John

Title: Re: tone thickening mods
Post by: Tone Junkie on October 02, 2011, 12:48:40 am

Here is another one showing the resonance control this is SDM,s site it has some great info like Sluckeys site does just differant stuff.
Thanks Bill
http://home.comcast.net/~jbjdav26/Resonance1.jpg (http://home.comcast.net/~jbjdav26/Resonance1.jpg)

Title: Re: tone thickening mods
Post by: darkbluemurder on October 05, 2011, 09:08:43 am

Quote from: phsyconoodler on September 30, 2011, 11:30:43 am

I recently came across a couple of mods that are touted to 'thicken' or fatten the tone of an amp.
One mods was to add a 330pf cap across the input grid resistor

I read about that as well but I think the guy that posted it made a reading error. The cap value should probably be 330nf. With a 330pf over a 68k you get a corner frequency for the boost which is so high that not even bears, cats or dogs can hear a discernible difference. Well, bats maybe :)

Quote from: phsyconoodler on September 30, 2011, 11:30:43 am

and the other was to add a .0033uf cap in series with the negative feedback wire to the speaker or impedance selector.
  Anyone use these mods?I'm going to try them of course,but I was wondering if they are viable mods and don't muddy up stuff too much?

I used a .01 cap in two 100W Marshalls. Really pumps up the bass.

Cheers Stephan

Title: Re: tone thickening mods
Post by: phsyconoodler on October 05, 2011, 10:38:25 am

Does it do it in a good way without muddying up the tone?

Title: Re: tone thickening mods
Post by: jjasilli on October 05, 2011, 10:50:33 am

I read about that as well but I think the guy that posted it made a reading error. The cap value should probably be 330nf. With a 330pf over a 68k you get a corner frequency for the boost which is so high that not even bears, cats or dogs can hear a discernible difference. Well, bats maybe :)

Seems right, also a cap in that position will defeat the grid stop function of the input resistor, and pass RF noise into the first stage.  So if a cap is used, it might be good to follow it with a grid stopper.

Title: Re: tone thickening mods
Post by: plexi50 on October 05, 2011, 12:45:28 pm

Quote from: Jack_Hester on October 01, 2011, 04:44:24 am

Quote from: plexi50 on September 30, 2011, 09:54:48 pm

I have built a few amps using a resonance pot. And man it does work nice. Tightens the bass up nice to
Earth is resonance / Sky is presence. I dont think you can get the whole effect just using a cap alone
It really come alive and is very noticeable using a pot. Gets the full sweep range of more frequency change

Do you have a pdf of the schematic for this amp, that I can look at?  I can follow your layout, if I hand-draw a schematic while I'm doing it.  I'm interested in what you've done and it's effects.  Thanks for posting the layout.

Jack

Here you go Jack. Sorry for the delay in answering your question.

Title: Re: tone thickening mods
Post by: Jack_Hester on October 05, 2011, 01:10:44 pm

Quote from: plexi50 on October 05, 2011, 12:45:28 pm

Here you go Jack. Sorry for the delay in answering your question.

Thanks a lot!  I'll give it a good study now.  Have a good one.

Jack

Title: Re: tone thickening mods
Post by: Jack_Hester on October 06, 2011, 04:57:39 am

Plexi50 -

By the way.  What software did you use to make your schematic?  Very clean look.  Thanks.

Jack

Title: Re: tone thickening mods
Post by: plexi50 on October 07, 2011, 06:01:15 pm

I did not make the schem or layout. I got it off the web 5 or so years ago when there was a lot of new stuff poppin up online

Title: Re: tone thickening mods
Post by: xm52 on October 09, 2011, 08:32:19 am

A cap parallel to the input grid resistor in this configuration is, effectively, a high pass filter. Some call it a bright cap.  The lows are removed so the input sounds brighter.

(http://s2.postimage.org/4ljf1y1c7/input_grid_cap.jpg) (http://www.postimage.org/)
 (http://www.postimage.org/)

Title: Re: tone thickening mods
Post by: HotBluePlates on October 09, 2011, 08:37:32 am

Don't look at it as removing lows.

There is a voltage divider on that input circuit, made of the 100k resistor and 5.6M resistor. The cap across the 100k allows highs to bypass the 100k resistor, unattenuated. The rest of the signal is attenuated by the divider. By this action, the highs appear "boosted" when they are simply not reduced along with the rest of the signal.

So the cap by itself doesn't remove lows, so much as it allows highs to not be reduced.

Title: Re: tone thickening mods
Post by: tonewood on October 11, 2011, 03:35:41 pm

Quote from: phsyconoodler on September 30, 2011, 11:30:43 am

I'm going to try them of course,but I was wondering if they are viable mods and don't muddy up stuff too much?

Phsyconoodler- did either of the mods pan out?

Thanks

Title: Re: tone thickening mods
Post by: moonbird on October 11, 2011, 04:20:40 pm

Quote from: HotBluePlates on October 09, 2011, 08:37:32 am

So the cap by itself doesn't remove lows, so much as it allows highs to not be reduced.

So then it is like a "treble bleed" installed on the vol pot on a guitar - is that right? thx.

Title: Re: tone thickening mods
Post by: jojokeo on October 11, 2011, 04:42:24 pm

Quote from: moonbird on October 11, 2011, 04:20:40 pm

Quote from: HotBluePlates on October 09, 2011, 08:37:32 am

So the cap by itself doesn't remove lows, so much as it allows highs to not be reduced.

So then it is like a "treble bleed" installed on the vol pot on a guitar - is that right? thx.

yep, that's all it is.