Joe Strouzer
14 posts
Apr 20, 2013
7:29 PM
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Hey folks,
I've just got my hands on an old Jt30 with a cm 151 old stock element.
It's sounding amazing in the studio but I'm having some real issues with feedback at live shows... sound tech tonight could not handle it and I had to switch back to my old Mic (sm58)
I'm playing through a lone wolf delay and a fender twin.
Any one got any advice on how to reduce feedback?
I love the sound of the Mic and would like to use it as much as possible.
All the best
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www.josephstrouzer.com
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Joe Strouzer
15 posts
Apr 20, 2013
7:34 PM
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I also realised I have another problem with spelling astatic...
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www.josephstrouzer.com
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jbone
1234 posts
Apr 20, 2013
8:58 PM
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Is it a cm or a crystal element? The 151 is an original crystal element if I'm not way off here.
What ir may be is that you have a particularly hot controlled magnetic element like a 99 series Shure. My best thing I can do with one like that is drop the gain and highs and tone and bump the low end and or bass, then set volume to just before feedback hits. Does the mic have a volume pot on it? That can allow you to set the mic hot and keep it down until solo time, then crank it and drop back off afterwards. A mic feeds back at rest, not when it's being used. Keeping volume low or off between harp parts may be an answer.
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Greg Heumann
2106 posts
Apr 20, 2013
10:34 PM
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It is a microphone's job to pick up sound waves including those made by your amp. Feedback in general is not the mic's fault - although certainly some mics are more sensitive than others. You CAN tame an overly sensitive mic with an inline volume control. However I bet it isn't the problem.
The most COMMON causes:
Problem: amp not loud enough to hear so you turn it up until it feeds back. (You didn't say what Twin you had. If it is 15W it probably isn't enough. If it is 85W it probably is, read second problem)
Solution: Get a bigger amp
Problem - amp SHOULD be loud enough but any time you come even close to what it can do the feedback leaps out of the amp so you have to steer way clear of the feedback zone, reducing max attainable volume
Solution: It is still set up for guitar. Too much gain. Calm it down via tube substitution
Problem: Can't hear the amp so sound guy has added harp to the monitors. Harp in monitors ALSO creates feedback in a hurry
Solution: Get the harp out of the monitors. Put your amp where you can hear it better.
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/Greg
BlowsMeAway Productions
See my Customer Mics album on Facebook
BlueState - my band
Bluestate on iTunes
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Bigtone
208 posts
Apr 20, 2013
10:40 PM
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In my experience a Fender amp and a crystal are a bad match for feedback. Fender amps period when I play gigs just doesn't work, they feedback a lot. Even with treble on 0 bass on 0-10 there is still that brightness and feedback. Also I find crystals much more feedback prone than a CR. Try a different speaker. I also find that the way you cup a make makes all the difference. My friends sit in at my gigs and the minute they play the mic feedsback like crazy but when I use it it doesn't feedback. I dont know it is a hard problem a lot of us have
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Kingley
2542 posts
Apr 20, 2013
11:22 PM
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Some very good advice so far. I'll try and condense it into a few simple bullet points for you.
First things first though. What element is in the mic? Is it an Astatic MC151 or a Shure Controlled Magnetic? If it's a crystal and is going bad (as they all do eventually) then I'd swap it out for a Shure CM.
The amp is a Fender Twin. What model of Twin is it? Have you tried changing the preamp valves for lower gain ones. This will often improve the amp for harp.
Do you have a good cup on the mic? Often feedback can be an issue for players who haven't learnt to get a good seal on their cupping.
How do you have the tone controls set on the amp? For example if you have the treble turned up you will get feedback much sooner than if you don't.
Last Edited by Kingley on Apr 20, 2013 11:23 PM
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5F6H
1613 posts
Apr 21, 2013
6:14 AM
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As Greg rightly says, the feedback is not the mic’s fault.
Prior to the introduction of the RI Bassman and discontinuation of the Astatic crystals, Twin Reverbs with JT30s (with crystal elements) were fairly popular harp rigs, though that’s not to say they are particularly easy to get on with, feedback-wise...or back-strain-wise! ;-)
The preamp circuit is basically similar to contemporary Super Reverbs/Bandmasters etc, & even the Blackface Concert. All of which have been well employed as harp amps. A couple of ways in which your amp differs is: 4x power tubes and more subsequent gain in the power section, 2x12” speakers that make the amp more prone to feedback than 1x12/mix of 10”/12”/multiple 10”s.
Many Twins have an externally accessible bias pot (sometimes bias balance, sometimes bias adjust for both pairs of tubes equally), first I’d get a tech/suitably experienced friend/bandmate with some bias probes (no need to open the chassis with these, super safe) to check the bias, aim for 10-15mA per tube, use just enough current to get the tone to “hold together” & not choke on long held notes/chords (won’t be a problem at 15mA, but maybe at significantly less current). If the amp is just fitted with a bias balance pot, unbalance the bias…keep the hotter tube at 35mA or under, lower tube at 10-15mA and see what happens when you get a 15-20mA mismatch?
You could even replace 2x6L6 (total tubes fitted) with 2x EH KT90, both tubes biased cold, as above.
The next bit of advice is going to sound counter-intuative, but remember, your biggest problem at the moment is curbing feedback – remove either the 2 outer, or the 2 inner power tubes & rewire the speakers in series. This will still give good volume and raise the feedback threshold, wiring the speakers in series corrects the impedances that the tubes see.
Perhaps obtain a 12” to 10” speaker converter ring and lose one of the 12”s (maybe even both if you have some spare 10”s, but 2x10” will require regular, hotter bias). If you stick with 2x12” try some Eminence Alphas, cheap, dark, less feedback prone than many others (but I’d really aim to lose one 12” at least).
Try a 12DW7, or 12AY7 in the 12AT7 slot, next to the power tubes. This is the phase-inverter tube, this is also the tube that I would focus on changing to reduce feedback in your amp. The 12DW7 may stress a plate resistor in the amp, but it may last years & is quick & cheap for a tech to fix, if it fails in the worst case scenario.
By all means try lower gain preamp tubes in V1 & V2 (preamps farthest from the power tubes), works for some people, not so much for others (I personally prefer 12AX7 here…but that doesn’t mean it’s the only way to go), 12AY7 & 5751 are both feasible subs…if you drop too much gain you might lose tightness in the notes & the amp may go loose/mushy?
As Paul (Kingley) says, tone control setting is a big factor, don’t be afraid of turning up the middle & keeping the treble way down. I don’t recommend turning the bass full up on this amp…keep it around 5-6 tops (bass control on tolex Fenders has a bigger effect than on tweeds, too much can make the amp muddy and leave you dialling in too much treble to compensate, making your life harder).
All these suggestions are non-invasive & easily returned to stock…nevertheless they are the “tip of the iceberg”. If you, or someone you know is technically minded, I have other suggestions but these involve working on the circuitry.
I have owned a Twin for nearly 15yrs, anything you can think of I have probably tried, I have a pyramid of reject 12” speaker that I have accumulated in that time, I haven’t run it with 4x power tubes for at least 10years, it’s still loud (comparable with 4x10” Concerts, Bassmans, Victoria 45410 etc) with just 2 (28mA per Sovtek5881) power tubes, I run mine with 2x10” or a 3x10” cab.
If the amp chassis is in good order, you probably have a good basis for a harp amp…bearing in mind what I have written above.
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Last Edited by 5F6H on Apr 21, 2013 6:21 AM
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Kingley
2549 posts
Apr 21, 2013
6:39 AM
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5F6H - Mark does that running just 2x6L6 and wiring in series trick, work on any amp with 4x6L6's?
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5F6H
1614 posts
Apr 21, 2013
7:05 AM
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Hi Kingley, yes, certainly works on fixed bias Fenders with 4x power tubes (Twin/Dual Showman). Cathode biased amps must always have all power tubes installed as the number of tubes fitted actually biases the amp (fixed bias has an additional circuit that biases the amp, irrespective of number of power tubes fitted).
The output transformer works a bit like a gearbox, it might expect to see ~2000ohms for 4x6L6 on the tube side & 4ohms (for instance) on the speaker side. When you remove the two inner/outer tubes you raise the impedance on the tube side to 4000ohms, so you ideally need to do at least the same on the speaker side. Rewiring the Twin's 2x8ohm speakers in series gives 8000ohms tube side (about the same as a Blues Deluxe) & 16ohms speaker side (safe for the tubes).
If you had a 100W amp with a 16ohm cab & alternative speaker taps, you could remove 2 of the 4 power tubes and plug the 16ohm speaker cab into an 8ohm chassis socket. Same effect.
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Last Edited by 5F6H on Apr 21, 2013 8:38 AM
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Kingley
2551 posts
Apr 21, 2013
8:21 AM
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Thanks Mark. All very interesting stuff.
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barbequebob
2249 posts
Apr 22, 2013
8:14 AM
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One thing to remember with any mic is to turn the treble down, or with some amps, turn it totally off because that makes the problem much worse. High treble settings are fine for guitar but NOT for harp. The other thing to remember is if you are using an amp like the Fender Blues Junior or Vibro King, and I'm gonna put this in all caps because of its major importance is MAKE SURE THE FAT SWITCH IS ALWAYS TURNED OFF!!!! Why?? Remember, the vast majority of amps have NEVER been designed for the instrument, just guitar, bass or keyboards and the fat switch makes the gain way too high for harp mikes and the signal from harp and harp mikes is easily 10 times more powerful than that of a guitar, bass or keyboard.
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Sincerely,
Barbeque Bob Maglinte
Boston, MA
http://www.barbequebob.com
CD available at http://www.cdbaby.com/cd/bbmaglinte
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Tuckster
1274 posts
Apr 22, 2013
10:20 AM
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After reading all of this,I started wondering: Why do we always brag about how hot such and such mic is? Shouldn't we want just the opposite? As long as there is enough signal to drive the amp into the "sweet spot".
It would make feedback problems much simpler.
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barbequebob
2252 posts
Apr 22, 2013
10:23 AM
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The main reason for nting a hot mic is more for the midrange, as that's where the harp range actually is, but if you tend to play too hard all the time, the chances of you feeding back often increases dramatically.
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Sincerely,
Barbeque Bob Maglinte
Boston, MA
http://www.barbequebob.com
CD available at http://www.cdbaby.com/cd/bbmaglinte
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Joe Strouzer
16 posts
Apr 23, 2013
6:22 AM
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Hey folks, thanks so much for the response.
unforunately the amp doesnt belong to me so changing tubes is kind of out the question right now.
Thanks for all the info though its definately going to be useful in future.
Right now I'm going to stick with my 58 for live shows and keep the JT30 for recording until I can get all the gear to sort it out.
All the best
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www.josephstrouzer.com
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Greg Heumann
2109 posts
Apr 23, 2013
8:18 AM
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Q: "Why do we always brag about how hot such and such mic is? Shouldn't we want just the opposite?"
A: PROVIDED you have the feedback monster under control, a hotter mic drives your preamp tube harder resulting in more attainable distortion.
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/Greg
BlowsMeAway Productions
See my Customer Mics album on Facebook
BlueState - my band
Bluestate on iTunes
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barbequebob
2253 posts
Apr 23, 2013
8:49 AM
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Greg's got it right. I've played some 1950's JT30's back in the late 80's that had crystals so damned hot that I couldn't even turn the amp on, even set the way I normally do on my real '59 Bassman, with the treble turned completely off (mind you, this is years before the Kinder AFB box got invented). I found out later that with the really hot crystals or ceramics, the bass response was as strong if not stronger than the midrange presence rise on those cartridges and if you ran the bass on most Fender amps past 6 or 7, you'd feedback like freaking hell.
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Sincerely,
Barbeque Bob Maglinte
Boston, MA
http://www.barbequebob.com
CD available at http://www.cdbaby.com/cd/bbmaglinte
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HawkeyeKane
1605 posts
Apr 23, 2013
8:58 AM
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"I found out later that with the really hot crystals or ceramics, the bass response was as strong if not stronger than the midrange presence rise on those cartridges and if you ran the bass on most Fender amps past 6 or 7, you'd feedback like freaking hell."
No lie there. My JT-30 has an MC-564-1 ceramic in it. Bright and hot as an arc welding rod. When I first got her running, I ran her through my Alamo Fury (which is pretty close in circuitry to a Fender Musicmaster Bass amp), and had the Bass & Treb zeroed out. As I brought the bass up, that sucker started squealing like a banshee.
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Hawkeye Kane
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Frank
2267 posts
Apr 23, 2013
9:15 AM
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Hot mic...To me, this means the mic brings the amp alive as if awakening it from the dead and projects the harmonica in a way that I can hear the electricity being infused into the notes and I can actually feel the power of the amp course through my body via the mic :)
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chromaticblues
1383 posts
Apr 23, 2013
10:25 AM
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Joe I would try using a Shure CM element with that amp and see if you like it. It won't have the high midrange bite of the astatic, but it should sound good enough and I find they don't feedback with bright Fender amps as much.
Something to try!
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