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Blues Jr - Tone Assement -8

Posted by Jim Black on 4/24/2015 to Mods

This week we take some measurements to see how far off we are from our goal. We need to ask questions like "Did I Accomplish what I set out to do?" "Is it just me?" "Am I asking too much?"

Stock Blues Jr.

I fired up the Blues Jr and I must say this amp is dark! Way too dark for my taste. There it is, I just said it. My Taste I relied on Bill's comments to guide my decisions and found that we have different views about how the highs are defined.

" and the moderate highs keep the ice pick tones out of my ears."

Ice pick tones don't sound appealing.

The screen shot shows the Blues Jr. before the mods. All the controls are set at 6. And, the frequency response includes the speaker and the room. Notice the bass region is low compared to the mids and highs. This is exactly what I heard and didn't like. Also, checkout the scoop around 200Hz.


 

The next screen shot is after the mods. The bass really came out. The mods did their job but the midrange got pulled back and buried. Notice that the notch at 200Hz is still there. 200Hz might seem like a low frequency but it is right around an open G string.

Modified Blues Jr.

Nothing against Bill here. We just have a different idea about high frequencies. I have to admit that when I was tested for hearing a few years back my audiologist said I have "Led Zeppelin ears." That's code for a distinct roll-off of high frequencies above 10Khz. I guess it comes from standing next to the drummers crash cymbal for so many years. Oh yeah and the hundreds of really loud concerts.

I equate the mismatch to a movie critique that you find to be opposite to your taste. Any time Roger Ebert didn't like a movie, I knew I would love it. What's wrong with Mars Attacks? Ak-Ak, Ak-Ak-Ak. Love that flick.

What do I like in tone? My favorite amp of all time is a Fender Super Reverb. I don't expect the Blues Jr. to sound like a Super Reverb but I think we can touch some of the beauty in that tone.

I have to figure out what needs to be changed. First, the twin mod doesn't seem useful since I don't care for a massive midrange scoop. Cool experiment, but it has to go. I decided to focus on the tone stack. The super and twin reverb tone stack have the same topology as the Blues Jr. So, a comparison is easy to do.

Default Stack
  Stock Mod SuperTwin
Treble250p250p250p250p
Mid22nF15nF22nF47nF
Bass22nF100nF100nF100nF
After having a look at the table I can see that just changing the mid range capacitor may give me what I am after. It is cheap and quick. I'm going to change it to the 47nF cap that the Twin Reverb uses. This goes past the Super 22nF with hopes it compensates for the extra bass we added. The billm mods dropped it to 15nF and I could hear the mid range missing. The mods also changed the coupling caps so we have more of all the frequencies. I did look at the specs for the Swamp Thang speaker and that speaker should be able to produce what ever we can put in it. I don't think it is an overly dark speaker. Next time we remove the twin stack mod, change the cap, take more measurements and listen again. Keep your fingers crossed...

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