AA763 champ bypass cap question

[catnine]

On the SF champs there is a 22 uf bypass cap on the preamp /input of the 12AX7 on the second gain of the other troid of V-1 12AX7 there is a 2 uf bypass cap .

My goal is just to try to cut some highs , I can do this backing off on the treble control but was wondering if I raised the 2 uf to a 22 uf if this would even make much difference?

Do you think they used the 2 uf cap because of the light built 8 inch speaker that always seemed to flab out on bass notes at higher levels?

[Chuck H]

That 2uf cap has a -3db knee at bout 150hz. This is a bit of a generalization because the apparent effect of the smaller cap is audible around 850hz. This is probably because a bypass cap on a typical 1.5k cathode provides about 6db gain. So the -3db point (that is typically quoted) represents a 50% decrease in the offered gain. A 22uf cap will have a -3db at about 30hz. But is more apparent audibly apparent around 100hz. So the real world shift would be about a 4db boost at 100hz. Since 850hz (where it is now) is really a midrange frequency I don't think you would percieve a high vs low shift by changing to a 22uf cap. More like a bass boost. So instead of just turning down the treble knob, you would also have to turn down the bass knob.

There are some good mods for tonestack bypass that can get the amp closer to the tone of the earlier (5f4?) model which has a more pronounced midrange character. If that sounds like what you need.?.

Chuck

[catnine]

I tried the 15k mid tone stack lift but personally hated the outcome of that . I put it back , I had it on a switch but never found it useable to me .

It was just a thought I had using a 25 uf in place of the 2 uf but seems like this won't be the best idea . maybe a alnico speaker would warm the amp up . I did put a JJ EC83 in place of the ruby 12ax7 and that brought the highs down quite a bit since the JJ is darker sounding .

I guess I expect far to much out of a simple champ . I had a 73 SF and it sounded much better with a weber alnico sig8s than this build does with a weber ceramic sig 10s . so perhaps a different speaker is in order .

[Bruce / Mission Amps]

On the SF champs there is a 22 uf bypass cap on the preamp /input of the 12AX7 on the second gain of the other troid of V-1 12AX7 there is a 2 uf bypass cap .

My goal is just to try to cut some highs , I can do this backing off on the treble control but was wondering if I raised the 2 uf to a 22 uf if this would even make much difference?

Do you think they used the 2 uf cap because of the light built 8 inch speaker that always seemed to flab out on bass notes at higher levels?

I think what Chuck is trying to say is that using a 22uF cap instead of the 2uF does not really do anything more to the highs, it just adds more low freq gain to the stage.
Any value cap will add gain at higher frequencies.
So a 2uf will have the same gain at around 160Hz on up as a 22uF cap does, but not as much low freq gain.....(by the way, it has been my experience that with respect to audio and coupling or bypass caps, the power of five to ten is the minimum you can use to actually hear a difference).... note: 2uF is about 1/10 of 22uF.
Now, if you are looking to drop some treble response, simply solder any 500v ceramic, .001uF to .0022uF across the first or second preamp stage's 100K plate load resistor.

[Chuck H]

Or you could try a conjunctive filter. That will leave the highs in the preamp and sound a little more like a darker speaker. A 10k 5W resistor in series with a 1500pf 1000V cap across the OT primary leads will take out the "bacon frying" highs and smooth out the top end overall. If you want more effect you can try larger cap values.

Chuck

[catnine]

It seems that using a 22uf bypass will basically boose the gain on the lower frequencies . Not what I want . I just want to cut some highs and backing off the treble does seem to cut the clarity at especially low levels . I can back off on the guitar volume and up the amp vol and get there .

I don;t know why but the guitar controls always at least to me seemed to be a tone killer so I always leave the guitar vol and tone dimed . I just have strats with stock USA PU's and controls.

I can get breakup but only past 6 on the amp or if I back the guitar to 2 and the amp on 9 then it's ok . I have a OD pedal which is a rocktron austin gold since it seemed to be the least OD gain pedal I could fine , don't like buzz saw sounds. At 6 this AA763 champ is quite loud not a bedroon amp by any means.

I will try the suggestions unless they kill the what I call clarity or openess.

[Bruce / Mission Amps]

...
I don;t know why but the guitar controls always at least to me seemed to be a tone killer so I always leave the guitar vol and tone dimed . I just have strats with stock USA PU's and controls.
...
I will try the suggestions unless they kill the what I call clarity or openess.

Kind of goofy but I think the best, raw sounding guitar I've used is one with the volume and tone controls completely GONE.
Pickups switched right to the jack...

[Chuck H]

I once wired up an Ibanez Iceman with a DiMarzio Super Distortion in the bridge and a pull pot that bypassed all the controls and hardwired the bridge pickup straight to the jack. This was in the early 90's you understand But it WAS a monkey tone machine. Loved it.

I have disabled the tone control on two different professional players guitars on their request. They felt the guitars sounded better without them and they both complained that they were always checking to be sure they were on 10. I told both that I could make the control do something else, but niether wanted to.

Chuck