Why my amp doesn't like some OD pedals...?

Happyorange27 · January 31, 2011, 11:11:39 AM

I've got some screamers and also a tube preamp by Seymour Duncan. They all sound fine through a solid state PA or a Fender Twin even. But it seems that if the screamers are on full drive or the worst culprit, the Duncan OD on the OD channel, I get a harsh clipping type of distortion through my Blues Junior. The best thing I can do is to somehow buffer the pedals down by sticking some other pedals in between the OD and the amp as to try to reduce the levels, but the nasty signal still occurs to some degree. Like I said, it's not really a problem with other amp situations. By the way, I even turn the preamp volume on the BJ down to like 1, but it's still there. What's going on???

P.S. I know there are mods to be done to the BJ which i plan on doing but I want to make sure the amp (brand new) is not faulty otherwise. When I'm not using the heaviest drive, the amp is freaking sweeeeeet.

Happyorange27 · February 10, 2011, 12:11:41 PM

I'm going to attempt to put a 12AY7 in place of the 12AX7 to give me more clean in the preamp. We'll see.

cactuskeeb · March 22, 2011, 04:40:17 AM

How'd this work out for you?

Happyorange27 · March 22, 2011, 06:35:48 AM

I'll be ordering that tube next week. Thanks for the bump on this Keeb. Some say it only changes the gain slope and hence lowers overall loudness. You would get same results if 12ax7 was on volume 3 as a 12ay7 on volume 7 for example. Some say this really did help pedal inputs because the gain breakup is stretched over more decibels. I will try to run same pedal setting with both tubes to realize the difference. I should use a decibel meter as well for speaker output.

cactuskeeb · March 22, 2011, 11:07:51 PM

I'm not sure what seymour duncan preamp tube thing you have but I'll just assume you're talking about swapping tubes in the blues jr. ...

So, yeah, three 12ax7s in a row in the blues jr. might be the culprit...

I would try substituting one 12at7 in the middle, and then work from there. I don't know if a 12ay7 is what you want because the gain rating is about half that of a 12ax7. Best to take a quarter of a slice off the top, before you try cutting it in half.

Why would you want to mod a blues jr, anyway? You know well enough, it's never going to have the headroom of a Fender Twin, let alone a solid state PA

And if this is any indication of what you can expect with a relatively new blues jr, you're probably going to be fighting a losing battle the whole way (by the way, I like how the guy who does the webpage where I found this picture talks about Illinois capacitor putting out a "bad batch" of caps...while I tend to think every capacitor (ever) made by Illinois is essentially just one more part of a never-ending, bad batch of capacitors:

Poster · March 23, 2011, 01:54:58 AM

yeah most new PCB fender amps are shrill when pushed. crap caps :0( cheap trannies. cheaper chokes.

Happyorange27 · March 23, 2011, 07:42:01 AM

Cactus, I definitely changed out those caps along with several others on the BJ. And yes I'm getting the distortion from the amp, not the pedal unit.

As far as trying the second tube first, it sounds like it has almost no effect according to Billm.

QuotePutting a lower-gain tube into V2 is pretty much the same thing as turning down your master volume. So keep a 12AX7 in there and don't worry about it. - Bill m

Please read the rest of this page to see where I'm getting my original advice. http://billmaudio.com/wp/?page_id=131

I will however look into going with a 1/4 step drop before a 1/2 step drop. Good advice.

Well modifying a BJ is just my entry level way to get my feet wet in this hobby. I think it has plenty of headroom (I'm talking about dynamics/loudness not clean headroom), at least for my lower volume use. If anything I'd like to have some compression in there

I will be upgrading the transformer eventually too. Again I don't have much experience with any other bad ass amps so I'm digging this amp over my solid state Fender immensely.

Keep bearing with me guys, as I slowly grasp all of these concepts and try to catch up to you

cactuskeeb · March 23, 2011, 10:10:57 AM

Quote from: Happyorange27 on March 23, 2011, 07:42:01 AM

Keep bearing with me guys, as I slowly grasp all of these concepts and try to catch up to you

I hope for your sake you never "catch up" to Poster and I. I'm not sure you realize yet what everyone else knows, which is that neither of us are to be looked up to for our so-called knowledge of things. I'll be the first to admit, while it has calmed down in the last year, I always used to supplement my knowledge of gear with made-up (but rhetorically powerful) nonsense just to appear somewhat authoritative on certain subjects. Take, for example, everything I said one year about Uni-vibes. It's too painful for me to reread anything I wrote on this forum about vibes. It's embarrassing to see (read) both *what* I wrote and *how* I wrote it.

Happyorange27 · March 23, 2011, 10:23:22 AM

Those are humble words which makes me feel a little better. I try to be pretty humble and not assert myself as an authority of anything. If I have some experience I will share it but I think I always speak with disclaimers. Maybe that makes me wishy washy but oh well.

It's cool if you guys do in fact get all passionate and definitive about stuff, it's your right. So don't take it too hard. The result of some of your rants is that it's allowed dudes like me to look that stuff up and do more research and on occasion, blow a hole in your theory!!!

Rock on good people

cactuskeeb · March 23, 2011, 10:24:51 AM

Maybe Bill M is correct but I would need some further insight as to the exact layout of the preamp stage in the blues jr. I'll read some schematics (I hope...depends on whether they are available). Then, I'll get back to you. I think Bill M reminds me of fifty other mod-merchants who sell their services online...for the most part, they never listen to any reasoning except their own.

Well, I know very little about master volume amps, so I won't attempt to argue with Bill here...

But, for your own sake, I would try the swap anyway. I've never listened to anyone who advised me to "not worry about" something without following up with an (in my mind) adequately detailed explanation as to why I shouldn't bother.

Happyorange27 · March 23, 2011, 10:30:29 AM

http://support.fender.com/schematics/guitar_amplifiers/Blues_Junior_schematic.pdf

Make sure you look at the pages dated 2001 because they have the old and new model on the same pdf.

cactuskeeb · March 23, 2011, 11:18:22 AM

Quote from: Happyorange27 on March 23, 2011, 10:30:29 AM
http://support.fender.com/schematics/guitar_amplifiers/Blues_Junior_schematic.pdf

Make sure you look at the pages dated 2001 because they have the old and new model on the same pdf.

With three 12ax7s in the preamp of a single channel design, that's more "high-gain" than any vintage Fender design (i.e., the design that would work well with your pedals) even if said vintage design was itself modified for more gain (either through *passive* circuitry -- as in hijacking the vibrato tube to add an additional gain stage to either channel of a classic, two channel blackface -- OR -- *active*, as in Billm's "Clean Boost" module). In either case, you're just adding loudness that will drive the output tubes harder.

Perhaps what you want is to modify the Blues Jr. to work on 6v6 rather than el-34 power tubes.

Swapping out one or two of the 12ax7s will affect gain structure of the amp in a different way. But it just might be what you're looking for to stop this "clipping" that's going on with your OD pedals and the blues jr. But just understand, you're running pedals through an ultra-modern, high-gain beast of a preamp stage that has been effectively disguised in the form of the not-very-loud and rather unassuming Blues Jr., as a result of the master volume control being there to quiet everything down.

Happyorange27 · March 23, 2011, 11:30:00 AM

I appreciate your leg work here. Yes the clean boost module will be important to help me out so I can get louder without adding preamp gain. I'll just buy some of these lower gain tubes and play with swapping them out. Others have testified that this really helps with the pedals in front of the amp. Thanks man.

cactuskeeb · March 23, 2011, 07:50:53 PM

Quote from: Happyorange27 on March 23, 2011, 11:30:00 AM
I appreciate your leg work here. Yes the clean boost module will be important to help me out so I can get louder without adding preamp gain. I'll just buy some of these lower gain tubes and play with swapping them out. Others have testified that this really helps with the pedals in front of the amp. Thanks man.

No problem. I think I'm on a bit of an amp circuit high right now. I figured out that a lot of the ceramic disc caps I was using in my amp were not temperature stable. I have a couple original sprague cases that say whether or not the caps inside are temp stable. I then realized, through a process of deduction, that the ones with the codes X5E and X5F (along with NPO) are def. temp stable, while other codes are either a definite no or as yet undetermined. I swapped out the treble caps and phase inverter input cap for ceramic discs that had the X5E code printed on them. The frequency response must have stabilized a lot as a result of doing this; in particular, the bass frequencies are now no longer all over the place and I get really full, articulate bass regardless of where I'm playing on the neck.

Also, I decided to try something I read about last year but can't for the life of me remember where i read it from. Anyway, if you look at a blackface deluxe or deluxe reverb layout (ab763), you'll see a 22 or 27K resistor in the phase inverter section. To cause the amp to break up sooner, swap that resistor out for a 10K (or, play around with it, like I did, trying a 6.8K first, which ended up sounding too harsh tho. I love what this did for my amp.

Happyorange27 · March 30, 2011, 07:47:07 PM

Cactus I got a 12ay7 (cheap). I ran an experiment below.

Just go to my photobucket albums and see the 12ax7 and 12ay7 albums.
http://i1239.photobucket.com/albums/ff504/happyorange27/12ax7/7b3d2541.jpg
The pictures may have uploaded out of order but you figure it out. The experiment was crude.

Summary: the harshness is reduced to the ear when high gain is applied on my preamp tube pedal. It's not 100% gone but extremely better and tolerable. I just won't set the pedal at full gain. As for overall volume loss, it's negligible. I've still got gobs of volume to dispense. Mind you I can always upgrade to the clean boost mod, higher output tubes or stronger output transformer.

A $9 improvement way worth the money.