Tuning bass on the OD channel

[TheHandsomeOrk]

Something that helped me is: not looking at the bass content but rather look at the lack of high freq.
Raising the GNFB resistor to 10K was my solution and measuring the entire range of caps in the amp. I replaced 4 caps that were off spec.

Yep, an accurate cap tester goes along way. I've measured plenty of the ps series and got readings like .01=.016, .047=.053 ect. so sometimes it's like hitting a moving target even if you've got the right infomation and this forum.

[greiswig]

the 100k 150k level and drive feeder resistors.. Going up in value will cut some lows however will also effect gain and mid content and tone so these are out..

Hi, Tony. This is interesting...I wasn't aware of this. What sorts of effects does it have on the mid content?

One other thing to think about may be to lower the OD trim pot from 100k to 70k or 50k and set it about half way will also help with the low end..

Not sure I understand this one. So in a "Tweeked" D'Lite build, you set the trimmer with, for example, 25k to ground. This means there is a 220k resistor, followed by 75k of the trimmer, followed by a 68k resistor into pin 2 of V2. That means that there is a total of 363k resistance after the coupler, with 25k to ground.

If I put a 60k trimmer in instead, the total would drop to 318k if I had 30k to ground. Is that what you mean?

In your experience, what's the effect on the bass when doing that, and why does it work?

Again, I really appreciate this...it's what I love about this group, because I am learning principles, not just making fixes.

[greiswig]

By having series caps to reduce bass does the bass get totally shaved off and dumped or highs go through first, arrive first at the listeners ear giving the impression of less bass?

Then bass frequencies are filtered and not passed to the next stage. The idea of some frequencies arriving before or after others is called group delay. Group delay is normally applied to circuits as far as I know. It can be calculated for a given circuit topology. A delay of frequencies can also occur after the amplifier output due to speaker cable, speakers, etc...

Wow! This is new to me. Can you give me a reference? If this is an actual issue for simple circuits like this, I could see how phase issues might start to add up.

[brewdude]

I can't really offer much in the discussion of how all the different little nuances of the circuit effect each others effect on the amps bass. However, I recently swapped the .01uF V2a coupling cap with the .005uF V2b coupling cap and I like what it did for me.

[talbany]

Not sure I understand this one. So in a "Tweeked" D'Lite build, you set the trimmer with, for example, 25k to ground. This means there is a 220k resistor, followed by 75k of the trimmer, followed by a 68k resistor into pin 2 of V2. That means that there is a total of 363k resistance after the coupler, with 25k to ground.

If I put a 60k trimmer in instead, the total would drop to 318k if I had 30k to ground. Is that what you mean?

In your experience, what's the effect on the bass when doing that, and why does it work?

Again, I really appreciate this...it's what I love about this group, because I am learning principles, not just making fixes.

_________________
Here is some info on the roll off of the different pot values..100k/250k Drive and level

https://tubeamparchive.com/viewtopic.ph ... 250k+drive

It was directed at the Drive and level pot's however you get the idea..
BTW..Some here recommend changing the 220k resistor to a 270k if you use the 50k trimmer..To me a matter of taste..

Hi, Tony. This is interesting...I wasn't aware of this. What sorts of effects does it have on the mid content?

In my expierments with different values here Up-ing the value of these resistors can help reduce lows but also chokes the mids and reduces gain.. I always come back to Dumbles values..Perfect!!

Tony

[vibratoking]

Wow! This is new to me. Can you give me a reference? If this is an actual issue for simple circuits like this, I could see how phase issues might start to add up.

If you Google it, you will find plenty of good explanations. It is normally discussed/taught in regard to filters, but it applies to almost any circuit with an amplifier or reactive components. It mainly comes into play for nonlinear phase response filters because the group delay is the derivative of the phase response vs frequency. By the way, there are plenty of nonlinear circuits in a guitar amp. It takes a lot of attention to detail to control the phase response/group delay in a circuit. I have never done it in a guitar amp, but I have built relatively few of them.

It also comes into play in hi end speakers. You can find many examples of speaker systems where the woofer, midrange, and tweeter are located different distances from the listener in an attempt to phase align the different sources.

[dogears]

Tony, changing the pot to a larger pot increases the bass. Comparing the gain figures is pointless unless you set the divider the same. If you do, you will find more bass.

Do the math on the RC network. Changing from a 100K to a 250K is similar to doubling the .01uf on the board to a 02uf.

[talbany]

Tony, changing the pot to a larger pot increases the bass. Comparing the gain figures is pointless unless you set the divider the same. If you do, you will find more bass.

My ears tell me the same thing..I like the 250k Drive in the 100w and the 100k in the 50's..matter of taste!!

Do the math on the RC network. Changing from a 100K to a 250K is similar to doubling the .01uf on the board to a 02uf.

Didn't know that THX..

Tony

[Structo]

I thought larger resistance values attenuated the highs?

As in the two interstage resistors on V2 (100K and 150K)

I hooked up a resistance decade box to where the 150K goes and I was able to place pretty high values that dulled the high end before cutting the signal.

[dogears]

You are correct. The increased resistance makes the Miller Effect much more pronounced. It also adds bottom.

I thought larger resistance values attenuated the highs?

As in the two interstage resistors on V2 (100K and 150K)

I hooked up a resistance decade box to where the 150K goes and I was able to place pretty high values that dulled the high end before cutting the signal.

[talbany]

It also adds bottom.

My ears tells me raising the impedance there cut's both mids and sure top end and bottom/everything.. Our ears being more sensitive to higher frequency's notice this change more.. The perceived result is less highs and mids giving the end result as the perception of more bottom..But not really adding bottom.. Is this a correct assumption?.. AFAIK..The 100k resistor/Drive pot only relates to the Miller effect

Tony

[dogears]

Tony, the added resistance is part of the RC computation. More bottom. Check the formula.

It also adds bottom.

My ears tells me raising the impedance there cut's both mids and sure top end and bottom/everything.. Our ears being more sensitive to higher frequency's notice this change more.. The perceived result is less highs and mids giving the end result as the perception of more bottom..But not really adding bottom.. Is this a correct assumption?.. AFAIK..The 100k resistor/Drive pot only relates to the Miller effect

Tony

[talbany]

Here I am on Vacation in sunny Puerto Rico doing RC Computations
I know seek help!!

.047 cap followed by 220k into 100k and 50k pot both set @24k
Here you go!!


RC Computations using 100k trimmer set at 24k
Gain @10Hz 0.05151 (-25.76 dB)
Gain@20Hz 0.06629 (-23.57 dB)
Gain@ 40Hz 0.07251 (-22.79 dB)
Gain@ 82Hz 0.07438 (-22.57 dB)
Gain @ 100Hz 0.07458 (-22.55 dB)
Gain @ 200Hz 0.07490 (-22.51 dB)
Gain @ 400 Hz 0.07497 (-22.50 dB)
Gain @ 800 Hz0.07499 (-22.50 dB)
RC Computations using 50k trimmer set at 24k
Gain @10Hz 0.02616 (-31.65 dB)
Gain@20Hz 0.03568 (-28.95 dB)
Gain@ 40Hz 0.04027 (-27.90 dB)
Gain@ 82Hz 0.04175 (-27.59 dB)
Gain @ 100Hz 0.04191 (-27.55 dB)
Gain @ 200Hz 0.04216 (-27.50 dB)
Gain @ 400 Hz 0.04223 (-27.49 dB)
Gain @ 800 Hz 0.04224 (-27.49 dB)

Tony

[dogears]

Tony,

I think the sun and margaritas are clouding your usually superb intellect!

Just get the -3db point. No need for all this gain mumbo jumbo. Apples to apples, the bigger the resistance to ground after the cap, the lower the breakpoint.

[talbany]

Tony,

I think the sun and margaritas are clouding your usually superb intellect!

Just get the -3db point. No need for all this gain mumbo jumbo. Apples to apples, the bigger the resistance to ground after the cap, the lower the breakpoint.

.. They are rather heavy on the pour here.. Probably not a good time to be looking for break points anyway.. ..Off to the Beach..AGAIN

Tony