Amp mods and feedback control

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Re: Amp mods and feedback control

Postby ricochet » Thu Dec 18, 2008 12:19 pm

bottleneck wrote: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.

Look around on http://schematicheaven.com/ and you'll likely find it.
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Re: Amp mods and feedback control

Postby jbone1 » Thu Dec 18, 2008 12:30 pm

i find a volume pot on a mic to be indispensable. different rooms, bands, etc do make for different needs on stage. with my small amp i usually turn the amp up to where it does feed back at "rest" and then cut my mic volume. then at solo time, with a too-loud band, i can turn the mic up just when i'm doing a solo, and cut it back down so it doesn't feed back at rest. not a perfect solution but works pretty well.
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Re: Amp mods and feedback control

Postby barbequebob » Thu Dec 18, 2008 6:11 pm

Bottleneck, all 3 preamp tubes on a Bassman RI affects both channels, just like on the real deal (which I've owned since 1983). At the input jack end, that's where the voicings differ. I don't go for using anything other than the stock tubes (on my real Bassman, from input to rectifier, it's: 12AY7-12ax7-12ax7-5881-5881-GZ34/5ar4) because, much as you say, it's fine in the house, but on a gig, they sound like crap because everything gets so overcompressed and you lose important things like dynamic touch, and to me, this is just a really poor excuse for not taking the time out necessary to develop acoustic tone and chops first because there's absolutely no rig that's ever been made or will be made that can hide that, but too many harp players are just flat out too hard headed to understand that.

The Kinder AFB box is more than just a noise gate because what it also does is that it properly matches the impedance of the mic and the amplifier to get the most out of it and get whatever distortion you want, but it works totally different than a master volume control does and those controls just destroys dynamic touch altogether. A crystal/ceramic mic is ultra-high-z, and most GB types, bet CM/CR are upper medium-Z impedance and dynamics are really lower mid-z in impedance. Generally speaking, anything over 600 ohms is considered high-z, but there's a huge difference and impedance matching can be amazing. Even in amps, you'd be surprised at the different impedances at the input jack end too.
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Re: Amp mods and feedback control

Postby jbone1 » Fri Dec 19, 2008 12:07 pm

well i'd like to get my mind wrapped around this right. if i have an amp that's biased for a crystal, does that then affect how the amp will sound with a dynamic or cm, and vise versa? or is this only if one uses a kinder afb or noise gate?

so far with the bassman i have i have used both crystal and cm at different times. using the bass, mid and high knobs to tailor my sound to the room. seems to work well, and this is with no extra devices between mic and amp. so....?
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Re: Amp mods and feedback control

Postby ricochet » Fri Dec 19, 2008 2:41 pm

It's not really an issue of biasing, but of the input impedance of the amp. A crystal mic is extremely high impedance, meaning it'll put out a good voltage but very, very little current. A low impedance dynamic mic can put out way more current at low voltage.

A tube amp with a high input impedance, like one with a 1M resistor between the first tube grid and ground, will work well with any of these. A tube amp with a small resistance on the first tube grid, or a transistor amp, won't respond as well to the very high impedance mic. It'll lose higher frequencies, especially. It's like trying to fill a bathtub with an open drain with a small, high pressure hose nozzle vs. a great big hose with lots of water gently pouring out.
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Re: Amp mods and feedback control

Postby barbequebob » Fri Dec 19, 2008 2:44 pm

No, amps come already setup at the factory in terms of impedance. The 50's tweed amps generally are about 1meg impedance, which mates up perfectly with crystals/ceramics, but then when you get to the black face era amps, the impedance is lower and dynamic mics and CM/CR types, which have a lower impedance matches up perfectly. The Kinder AFB box allows you to mix and match perfectly. On the Kinder site, there's also just this circuit alone as a seperate device and I think it's called a maximizer. By having the impedances properly match, everything is optimized. If you play a number of different amps, you'll always find that some mics work better with certain amps than others, well this is one of the reasons why.
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Re: Amp mods and feedback control

Postby ricochet » Fri Dec 19, 2008 2:56 pm

barbequebob wrote:No, amps come already setup at the factory in terms of impedance.

Exactly.
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Re: Amp mods and feedback control

Postby bottleneck » Fri Dec 19, 2008 10:37 pm

hmmmmm,after thirty five or so years of harp playing,i think i'm getting somewhere.haha.this is really helping!even though i've discussed similar setups a zillion times.bear with me.

SO,the way to get a RI bassman or similar bassman to sound good for both harp and guitar is to have ,say,the normal channel input(s)impedance match a ceramic/ crystal and have the bright channel input impedance match the guitar ,which wouldn't be TOO bad for a CM/CR ?

am i close here?
for my show tonight i'm going to leave the power tubes in,so i don't have to worry about bias and put in a new rectifier and pre amp tubes.i'll try that first,i notice my rectifier tube is twice the size of the 5Ar4's i have in my tube box.fortunately i have LOTS of tubes!now i just have to decide between amperex,GE,RCA,westinghouse,mullard etc.

i'm starting with the tube set up BBQ bob recommends .

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Re: Amp mods and feedback control

Postby bottleneck » Fri Dec 19, 2008 10:45 pm

and who's got a kinder link?

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Re: Amp mods and feedback control

Postby ricochet » Sat Dec 20, 2008 12:03 am

Actually, the 1M input resistor works great with guitar pickups or low impedance mics as well. I can't imagine what the benefit of the common 68K resistors in later model amps might be. I wouldn't build one that way.
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Re: Amp mods and feedback control

Postby barbequebob » Sat Dec 20, 2008 5:30 pm

What I do on my Bassman (the real deal) can be done on the RI's as well, and that's plug into the #1 input on the normal channel, set up the volume where you want it with the just nearly to the point of feedback. Then set the volume control on the bright channel no higher than 6. What this does is because the two volume controls on the amp interact with each other, turning this control to 6 (and NO higher unless you want volume loss) puts it on maximum gain and really does boost the bottom end more. I learned this trick from the guy who puts out kendrick amps.

The website for Kinder is http://www.kinder-instruments.com.
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Re: Amp mods and feedback control

Postby bottleneck » Sat Dec 20, 2008 9:22 pm

i have to admit my RI bassman sounded better last night than it has in years.

BBQ bob,are you talking about the second channel volume up with the channels linked by a chord?or just turning the bright channel volume up without them being linked since they are not discreet?in any event i'll try it at tonight's show.

now,the original poster is talking about a blues deville,which i had before the bassman RI.you can get those things to sound like four trombones,haha.what an amp that was!

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Re: Amp mods and feedback control

Postby barbequebob » Sat Dec 20, 2008 9:58 pm

No I am not, and I don't believe in linking them at all because, for my ears, it makes it sound mushy. Just turn the other channel'svolume control absolutely NO higher than 6 max. This is explained in full in a chapter of Gerald Weber of Kendrick Amps' book which explains all of the differences between a real 4-10 Bassman and the reissues.
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Re: Amp mods and feedback control

Postby bottleneck » Sun Dec 21, 2008 6:19 am

yep,my bassman RI was good at tonight's show too.i like the new set up and tried what BBQ bob said about the two channels which made a difference in sound.i started out on a victoria 5/12 went to a silver panel deluxe by the end of the first set then finished the night on the RI bassman,

now let's get down to brass tacks:is the kinder AFB+ worth getting?

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Re: Amp mods and feedback control

Postby barbequebob » Sun Dec 21, 2008 7:37 pm

I don't have one, as most of the time, I deal with musicians that actually do listen (real pros, and not the usual BS from musician wannabees at open jams), and I've never needed to have one. However, for those do who have it, they love it. In the Harp King amps, this circuit is already there. It does allow one to play in a wide variety of venues in terms of volume, and you can actually turn the amp volume all the way up, stand as close as 2 feet away and not feedback at all. It's an excellent feedback suppressor, "grit" adjuster, as well as for properly matching mic and amp impedances, but it does not function as a noise gate, a circuit which never suppresses feedback at all, and it's a totally different animal. I think they are worth it, tho. Noisegates are for cleaning up the signal when there are things that will interfere with the signal, such as far too many effects pedals on the chain, and every one added to them introduces noise, and also if you use the wrong power adapter for any effects.
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