Amp mods and feedback control

The lowdown on the Mississippi Sax. Just for Google, this section is about harmonicas.

Amp mods and feedback control

Postby blues devil » Mon Dec 15, 2008 6:27 pm

Hey, recently bought a Fender Blues Deville reissue 4X10 as the gigs we are getting are proving too much for my existing rig. I have been advised to swap out one of the stock 12AX7 preamp tubes for a 12AU7 lower gain tube to minise feedback when dring the amp hard. I have playted through a similar amp with this sewt up and it sounds gooooooooooooood with little or no feedback issues! Not surprisingly Fender say this may damage the amp - anyone had experience of a preamp tube swap that has resulted in any damage to the amp? Also has anyone found an affordable alternatie to the Kinder anti-feed back unit?
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Re: Amp mods and feedback control

Postby ricochet » Mon Dec 15, 2008 8:07 pm

Won't hurt the amp to do that tube swap, but it's a big step down in gain from a 12AX7 to a 12AU7.

Here's a typical table of the "amplification factors" of various 12*x7 types:

12AX7 = 100
12AY7 = 45
12AT7 = 60
12AU7 = 19
5751 = 70

The actual gains of these tube types in a particular circuit (that was optimized for a 12AX7) won't rank exactly in those proportions, but it gives you an idea.
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Re: Amp mods and feedback control

Postby barbequebob » Mon Dec 15, 2008 8:37 pm

When those amps first came out at the same time the Bassman reissues did in the early 90's, that amp actually sounded more like a real Bassman than the Bassman RI actually did. First thing to do is always plug into the #1 input on the Normal channel, and the master volume turn all the way up so it totally negates this circuit. Then the next thing to do is to turn the treble completely off, and then find a bass and middle to your liking and never set the presence control beyond 8. Why so low on treble?? This is where the feedback problem comes from, and in music stores, there's nobody in those stores who know anything about anything remotely related to harmonicas, let alone play them, and those people there are the WORST people to listen to, and high treble is fine for guitar, but for harp, that's the worst thing possible. Also remember this, all harp mics are at least 10 times more powerful than any signal coming out of a guitar or bass and so unless you have something like the Kinder AFB box, you can't turn the volume up beyond around 3 or even 5 ever (unless you don't mind feedback headaches)!!!

If you need a smaller rig that's not as loud, get yourself something simpler like a Pro Junior. It's the most powerful 15 watts around.
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Re: Amp mods and feedback control

Postby jeffl » Mon Dec 15, 2008 9:17 pm

ricochet wrote:Won't hurt the amp to do that tube swap, but it's a big step down in gain from a 12AX7 to a 12AU7.

Here's a typical table of the "amplification factors" of various 12*x7 types:

12AX7 = 100
12AY7 = 45
12AT7 = 60
12AU7 = 19
5751 = 70

The actual gains of these tube types in a particular circuit (that was optimized for a 12AX7) won't rank exactly in those proportions, but it gives you an idea.
I agree with Rico. I've never had to swap down below a T or a Y to get what I needed, but I keep spares of all of 'em in my bag.
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Re: Amp mods and feedback control

Postby blues devil » Mon Dec 15, 2008 9:25 pm

ricochet wrote:Won't hurt the amp to do that tube swap, but it's a big step down in gain from a 12AX7 to a 12AU7.

Here's a typical table of the "amplification factors" of various 12*x7 types:

12AX7 = 100
12AY7 = 45
12AT7 = 60
12AU7 = 19
5751 = 70

The actual gains of these tube types in a particular circuit (that was optimized for a 12AX7) won't rank exactly in those proportions, but it gives you an idea.


Thanks man - that is a big assist!
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Re: Amp mods and feedback control

Postby NEONMOONY » Tue Dec 16, 2008 12:03 pm

Thanks man - that is a big assist!


Would that be a guy who likes rear ends? Or a proctologist or colorectal surgeons perhaps?
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Re: Amp mods and feedback control

Postby jbone1 » Tue Dec 16, 2008 12:04 pm

a couple weeks ago i had the bassman out at a jam, with a crystal mic plugged into it. a fairly new harp guy asked if he could play through it, and since it was on stage already, i told him to go ahead. i had it set where i like it, very low treble and presence, mids about half, bass way up. this bassman has no master volume. i had the mic in channel one.

as the guy started to play with the house band i noticed the guitarist next to him tweaking knobs, and the sound just got worse and worse! when i went up after a few songs i noticed the amp was set pretty much how a guitarist would set it for good guitar tone. bbq is dead on about this. YOU need to set your own amp and follow some rules of thumb for good harp sound and tone. i promptly reset everything and played a set with some guys. when i got down the new harp guy asked me why my set sounded better, and i told him, NEVER let a guitarist set his sound- learn to set it himself! the road to hell is paved with good intentions.

and on a related subject, i got to blow through an old masce p.a. head recently with a couple of vintage 12's. tone nearly off and volume at about 3, and tone to burn. there's not much that can touch the vintage stuff sometimes, and the less knobs the better.
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Re: Amp mods and feedback control

Postby ricochet » Tue Dec 16, 2008 2:27 pm

NEONMOONY wrote:
Thanks man - that is a big assist!


Would that be a guy who likes rear ends?

Big ones, I'd think.
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Re: Amp mods and feedback control

Postby barbequebob » Tue Dec 16, 2008 4:36 pm

The big reason why those vintage tube amps sound better is because the circuitry is MUCH simpler and there's no BS to deal with like pre and post gain, master volume, fat switches, etc., and it's as simple as plug in and play, and that's what I prefer.
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Re: Amp mods and feedback control

Postby ricochet » Tue Dec 16, 2008 7:34 pm

I like the one or two knob thing myself.

(Bob wouldn't approve of the "bright cap" I just put in my Valve Junior to brighten it up for lap steel, though!)
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Re: Amp mods and feedback control

Postby blues devil » Tue Dec 16, 2008 9:44 pm

NEONMOONY wrote:
Thanks man - that is a big assist!


Would that be a guy who likes rear ends? Or a proctologist or colorectal surgeons perhaps?


Yes I would be that guy (likes rear ends). :D
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Re: Amp mods and feedback control

Postby blues devil » Tue Dec 16, 2008 9:48 pm

ricochet wrote:I like the one or two knob thing myself.

(Bob wouldn't approve of the "bright cap" I just put in my Valve Junior to brighten it up for lap steel, though!)


Guys this quickly degenerating into a bad Carry on movie (do you guys get thoose in the states? - If not they were a very unfunny series of films where almost all of the jokes involved some form of sexual double entendres)
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Re: Amp mods and feedback control

Postby ricochet » Tue Dec 16, 2008 9:55 pm

Surely not here! :shock:

I'm going to log off and go play with my Johnson.
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Re: Amp mods and feedback control

Postby bottleneck » Thu Dec 18, 2008 2:06 am

i fortunately have hundreds of NOS tubes,including peanut tubes.i try to pick out my pre-amp tubes in a blindfold test at as close to full gig volume as i can get.
a 12au7 is one thing at the house,but maybe too much kill at a gig.

nobody has been able to tell me whether all three pre-amp tubes go to both channels on a bassman RI so i have to compromise between harp and slide.

as far as a kinder AFB,i'd like to know definitively if it is truly anything more than a noise gate.it is a pretty expensive noise gate if not.

however,if there is more to it than that,and it works better than a noise gate ,i'd buy one regardless of cost.

as a cheaper alternative i have been using a nf-1 noise gate since the late 70's or early 80's to cut feed back.this was shown to me by (then)philadelphia harmonica great billy burns,an amazing player who now lives in florida i think.he was doing a bassman and bullet combo back then,years before mainstream!

anyway a noise gate can be set so that it cuts your signal when you aren't playing,hence less feed.

in extreme volume settings you could also use a graphic eqalizer to tune out feedback,and also a volume pedal to come up for your solos,although i try to avoid bands THAT loud.

here's a funny thing about feedback.if you've been playing harp proffessionally a while you learn to avoid feedback.however,when a band gets too loud,you can't really avoid it.what i find funny is that it then becomes the bands fault that the harp feeds back,but they sure get mad at the harp player,don't they,haha.
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Re: Amp mods and feedback control

Postby bottleneck » Thu Dec 18, 2008 2:14 am

oh,another thing i think helps with feedback is to have a little mic variety.you might find that different mics feedback more so at different nights and different places.say a green bullet at smoky joe's and an astatic at smoky joan's.
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