OD entrance treble bleed

[brewdude]

I just recently added the recommended treble bleed circuit before the OD entrance on my D'Lite. I used a .05uF cap with the a 10M to ground.

I am trying to understand how it works. Is it a low pass filter with a high cut off frequency? It looks different than the example I saw drawn out in some internet calculator I found, in that the example of a low pass filter I found had the resistor going to ground at the other side of the cap. I'm guessing that there is something more to this that I have missed because the calculated frequency using this calculator did not seem to be a reasonable value.

Anyway, what I really want to know is how does changing the value of this cap affect the frequency response? Will a smaller cap bring the cut off frequency down, bleeding off more highs? Or, vice versa?

I also noticed a note on the ODS101 schematic implying that a slightly smaller resistor (150k-200k instead of 220k) before the OD entrance trim pot would clear up some of the low midrange mud. Can someone comment on this resistor?

[bluesfendermanblues]

If you want the OD less bright, you need the treble bleed after(!) the OD, not in front of it or you're at risk of killing tone.

The treble bleed is a low pass filter and works pretty much like the 'tone' control on a typical less paul or stratocater. In order to find the sweet spot for your particular setup, taking your guitar and speaker of choice into the equation, I suggest that you try the treble bleed with a (1nF=0.001uF) capacitor with a 500k trimmer.

Dont be too scientific about which capacitor to use for treble bleed - any capacitor type will do - ceramic, poly or polyprop.

Don't try to caclulate cut off frequency and the like......use your ears to trim it. And do so at different volume levels and be prepared to reduce the amount of high cut, as the amp matures and gets played in, which will mellow the amp tone and reduce the need for treble bleed, alltogether. The 150k-220k (about halfway on a 500k trimmer) sounds about right, but lets your ears decide.

[brewdude]

I have seen what you are describing. A sort of tone control after the OD. That would probably be my next strategy if what I was trying to describe doesn't get me where I want to be. In fact it may be a more flexible and intuitive means to managing the over bright character of the OD relative to the clean channel. However, it seems likely to be a little more involved to implement in my existing amp.

Perhaps I am confused and have misunderstood what the mod I described in the original post is intended for. I have seen this OD entrance mod coming from the channel switch to a 10M resistor to ground and a .05uF cap in series to the overdrive entrance's 220k to the 100k trim pot to the 68k grid stopper to OD1.

If I have got it wrong, what is the purpose of inserting the 10M and the .05uF combination?

What is the effective difference between using a .047uF and .0047uF cap?

[bluesfendermanblues]

I have seen what you are describing. A sort of tone control after the OD. That would probably be my next strategy if what I was trying to describe doesn't get me where I want to be. In fact it may be a more flexible and intuitive means to managing the over bright character of the OD relative to the clean channel. However, it seems likely to be a little more involved to implement in my existing amp.

Perhaps I am confused and have misunderstood what the mod I described in the original post is intended for. I have seen this OD entrance mod coming from the channel switch to a 10M resistor to ground and a .05uF cap in series to the overdrive entrance's 220k to the 100k trim pot to the 68k grid stopper to OD1.

If I have got it wrong, what is the purpose of inserting the 10M and the .05uF combination?

What is the effective difference between using a .047uF and .0047uF cap?

Got ya!

What you are describing is the Gil Ayan OD entrance bass recuction, which is a high pass filter that dampens low frequencies and is usefull, if you have a muddy OD channel that tends to fuzz out on the low strings.

What is the effective difference between using a .047uF and .0047uF cap?

The .0047 takes more low end than the .047 uF.

Start with the high value and reduce to taste and untill the mud goes away.

[Dr d]

+1 on Fenderbluesman. The 220k resistor you are refering to controls the bandwidth of the frequencies entering the OD. Raising this value shaves off some of both the high and low frequencies and vice versa. Hope this clarifies.

[Structo]

Also with the treble bleed (OD tone control), be sure to have the resistor or trimmer to ground, not the cap.

I tried it both ways and with the resistor to ground it worked much better.

[brewdude]

Thanks guys.

It seems I was a bit confused. I think I will leave the OD entrance mod as is. It seems pretty good on the low end to me at the moment.

I will have to consider the treble bleed after the OD2.

[Structo]

What the OD entrance mod does is place the CL2 .02 cap in series with the .05 when in overdrive.
This changes the OD input effective capacitance to .014uF.

I had that mod for a while but removed it in the end.
In reality the ODS 101 non HRM schematic is what I have changed to and find it gives me the best D tone I have gotten with my amp.

The treble bleed circuit can be used to tame the high frequency fizz or shrill overtones on OD.

It goes on the CW lug of the Level or OD volume pot with the cap to the pot lug and the trimmer to the ground buss.

[talbany]

The Treble bleed works great if the OD is flat out too bright in all settings..Much of the brightness here comes from where you have the drive pot set (preceding gain stages) ...If your one who likes allot of gain in the tone (using 100k pot) and you set it pretty high your decreasing the amount of signal applied on the carbon trace of the pot.. Less signal passing through the carbon = brighter tone.. Same with a volume control.. This is why say a 250k pot there will be darker because your getting the same amount of gain but the pot is set lower so the signal is passing through more of the carbon attenuating the highs keeping the amp from being to bright another reason why I prefer a 250k there.. ala 124

If the OD becomes a bit to bright at your drive settings and you want to shave off a bit of the sting, you can do a reverse bright cap (wiper to bottom) say 250pf (find one to taste) put it on a switch or better yet a push pull pot..This way you can pop it in when you crank the drive
Just a suggestion..

Tony

[Structo]

OK, this is getting confusing, because different amps have those two controls labeled differently.

#124 has the 250k pot on the Ratio control, which is the volume for the OD channel.

HAD labeled the OD gain, Level and the OD volume, Ratio.

On my amp the OD gain is labeled as Drive and the OD volume is Level.
See how this is confusing?

I remember discussing something with Gil Ayan and he was using the #124 terms and it was before I had seen the #124 layout.
I got very confused because it is the opposite of my amp.

For example, a Bludotone I have pictures of has the pots labeled as Drive and Volume which seems more logical for what they do.

Lately I have had my OD drive and Level at 12:00 because I use the Dumbleator Output pot as a master so those OD settings seem to give me a good balance with the clean.

[Dr d]

Tom, Tony is definately refering to the first stage control of the OD, be it the Level(#124) or Drive (BLudo & D'lite) and it does have a positive effect on taming highs/introducing more low end content. Try it!!!!

[brewdude]

What the OD entrance mod does is place the CL2 .02 cap in series with the .05 when in overdrive.
This changes the OD input effective capacitance to .014uF.

What is the purpose of the 10M going to ground? Is it to eliminate popping when switching to OD?

[Structo]

Yes

[Guitarman18]

Tom wrote:

#124 has the 250k pot on the Ratio control, which is the volume for the OD channel.

Tom, the 250K is on the Drive/Level(#124 label) pot, not the ratio (1988 version). The ratio (100K) remained unchanged throughout it's evolution AFAIK.

Cheers,

Paul.

[Structo]

Hi Paul,
OK, I looked at the photos again and I hate to say it but, Tony the #124 layout has the 250k on the Ratio (lead volume) not the Level.
That was confusing me.
It should be on the Level.

See why it was confusing?

Everything said it should be the OD Drive/Level that got the 250k but things were contradicting and confusing because different amps call it different names.

Thankfully I installed the 250k pot in the right location yesterday.

The hand drawn layout has it right, but calls it Drive.