Re: [Harp-L] Feedback

[MARK BURNESS <markwjburness@xxxxxxxxxxxxxx>]

Greg wrote: "But when all is said and done, the single most valuable change you 
can make to make an amp more harp friendly is to reduce the gain of the amp 
through tube substitution." In many cases tube subs are very useful, but some 
amps are much less tolerant to lower mu tubes (most guys I know that run brown 
tolex Fenders like Concerts & Bandmasters use 12AX7, John Kinders amps used 
12AX7). In many high powered amps, running the phase inverter at high voltages 
(the preamp tube that is next to & drives the power tubes), subbing a 12AX7 for 
a 12AU7 can burn out the plate resistors, I wouldn't do it in a Blues 
Deluxe/Deville, as these occasionally fail even with a 12AX7 installed (if this 
resistor does fail it can usually be replaced for the tech's minimum 
charge/bench fee, so if you feel it gives a substantial benefit you may well 
decide the risk worth taking). In all cases it's probably best to not try and 
put all your eggs in one basket regarding feedback, output tubes have gain too 
and this can be reduced by rebiasing cooler, in a sympathetic amp/design (as 
long as this doesn't push up operating voltages beyond what tubes & caps will 
stand). Feedback can be affected by all stages in the chain - mic, preamp tubes, 
any guitar oriented bright caps & tone filters that increase high end, power 
tubes & bias, rectifier & speaker choice. Sometimes one or two aspects can 
provide a useful advantage, other times you have to utilise other avenues, 
either seperately, or in combination..





________________________________
From: Greg Heumann <greg@xxxxxxxxxxx>
To: harp-l@xxxxxxxxxx
Sent: Sunday, 1 May, 2011 18:03:59
Subject: Re: [Harp-L] Feedback

Hi, Vern

I think Rick's answer is pretty accurate but I will take exception to a few 
items. First of all, gain is not the same as volume. You cannot change a mic's 
gain - you can only change its volume. Think of gain this way: Imagine you have 
a 400hp sports car - with a throttle pedal that only moves 1/2 inch. It is very 
hard to drive smoothly. Now the same car, with a throttle pedal that moves 5 
inches, would be easier to control. A typical Fender circuit 3 12AX7 amp, for 
harp, is like that first example. The single most effective thing you can do to 
help with feedback is to reduce the gain by substituting some lower gain tubes 
into the preamp section. For the above circuit which you'll find in MANY amps, 
replacing V2 and V3 with 12AU7 tubes is usually the right choice. Foot to the 
floor, you will still have full throttle, because there is STILL enough gain to 
drive the power tubes to their saturation point. The original setup has WAAAY 
more gain than is necessary to fully drive the power tubes. That is done because 
guitar strings and pickups do not couple nearly as efficiently to sound waves as 
does a microphone. Guitar players like very gainy amps so they can get 
Hendrix-style feedback - but for harp it just makes the amp edgy and twitchy and 
hard to drive. 


So - to your first question - the gain of the mic is fixed and is a function of 
the design of its element. Lowering its volume doesn't change the gain (much) - 
it just lowers the input signal to the amp, including the mximum input the amp 
will see. For that reason, volume controls are indeed very handy because they 
allow you to kill feedback instantly if it does occur. But they don't change the 
fundamental sensitivity of the amp.

Phase is not so important - if you picture a sound wave from the speaker in 
relation to your mic, you only need to move 1/2 a wavelength to be in or out of 
phase with that signal. Once you have sound at multiple frequencies, you just 
can't win. 


As for the directional properties of mics, all bets are off when you hand hold 
them. Directionality of vocal mics like an SM57 or 58 is accomplished by 
allowing some stage sound in behind the mic's diaphragm - which does provide 
some phase cancellation. But this assumes the mic is in free air, mounted on a 
stand. Once you put your hands around the mic, you're creating a resonant 
chamber AND a "funnel" - sort of like a satellite dish - that totally changes 
the mic's directional properties. You CAN help to control feedback by leveraging 
this, but it has nothing to do with the directional properties designed into the 
mic. 


And yes "If the responses of two mics are different at different frequencies" 
you could correct that with an equalizer. 


But when all is said and done, the single most valuable change you can make to 
make an amp more harp friendly is to reduce the gain of the amp through tube 
substitution. 


/Greg

http://www.blowsmeaway.com
http://www.bluestateband.net
http://itunes.apple.com/us/album/duracool/id343874731


> On Fri, Apr 29, 2011 at 9:28 PM, Vern <jevern@xxxxxxx> wrote:
> 
>> Every time I see a report on the feedback tendencies of a mic, I have the
>> following questions.
>> 
>> Q. Why is the gain of the mic more important than the gain of the entire
>> system including the mic and the amp?
>> 
>> It seems to me that the following things are important concerning feedback.
>> - The amount of sound (at each frequency) from the speaker that enters the
>> mic.
>> - The phase of that sound with respect to the source and the speaker.
>> 
>> Q. How can that involve the mic unless it is allowed to affect the overall
>> system gain which would affect speaker loudness?
>> 
>> Q. If the loudness at the speaker and the positions of the speaker and mic
>> don't change, how can a different mic affect feedback?
>> 
>> Q. If the responses of two mics are different at different frequencies,
>> could not this be corrected by equalizer controls?
>> 
>> Vern
>>