Re: Fender amps - bad sounds when the voltage drops (question for the technicians)
- From: "Phil S." <psymonds_no_spam@xxxxxxxxxxx>
- Date: Sat, 20 Oct 2007 09:40:52 -0400
<GrovesAustin@xxxxxxxxx> wrote in message
news:1192852165.733558.261440@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
I just got in from an outdoor gig, and I'm hoping somebody here can
explain a technical problem we had. During loud sections, my guitar
amp(s) experienced some really unpleasant, flabby, low frequency
distortion. Since it happened primarily when I hit a power chord, or
when I kicked in my boost pedal, and more so after it got dark and the
lights were on, I surmise that it was related to momentarily sagging
line voltage. Oh, and the power was coming through 100ft extension
cords.
I was using my primo rig that is reserved for good (and close-to-home)
gigs - a vintage blackface deluxe reverb and a vintage blackface
vibrolux reverb in parallel - volume at 4, miked through a large PA.
The vibrolux was the worst, and I replaced it with my early eighties
fender Concert amp for the second set. I eventually took the Deluxe
out of the mix too, and just used the Concert. Even then, I
occasionally heard the flabby distortion, but not as bad.
This is the second time this has happened, and ironically the first
was at the same venue a year ago...but that time we were running from
a large generator, and after dark when the generator was also powering
big lights, we had this problem. Today, were running on power company
power, meaning the power situation was completely different.
So...the PA equipment did not seem to be affected (Crown and QSC amps,
Allen & Heath mixer, etc). I may have heard it in the bass amp, but
I'm not certain (70s Acoustic solid state head). But the guitar amps
were sounding terrible when the problem occurred (all three of them,
but to varying degrees).
My tech questions are:
* does it make sense that the problem was caused by sagging line
voltage, or is there another explanation to consider?
* does it make sense that my fender tube amps would be the weakest
link on the stage in terms of handling sagging line voltage?
* Is there a way to make these amps handle sagging line voltage
better, or alternatively, are there (toneful) guitar amps that would
tolerate this situation better?
* Is there a practical way to regulate the line voltage in a gig
situation?
By way of clarification: the distortion that I experienced was sort-of
a combination of low-frequency breakup with possibly a 60hz or other
low frequency buzz. It was clearly triggered by loud passages, but
when it wasn't happening everything was very sweet and toneful.
Thanks for any help or ideas,
John
First, do you have any idea of the parameters of the power source? (Amp
rating -- see the circuit breaker, and take a meter with you to check the
voltage at the source.)
You really should consult an electrician on this. A casual conversation
might illuminate a great deal. But really, you can do the math yourself.
Add up all the power requirements of all the pieces of equipment you are
running on the circuit. Then add 10% just for a margin of error. Once you
know your power requirement, you might be able to use this calculator to
understand what's going on:
http://www.powerstream.com/Wire_Size.htm
For example, if you run a 100' 14AWG wire carrying 120VAC from the source
and load it with 15A (which is really too much load), it will put out about
112VAC at the other end. The loss of 8V may not seem like a great deal, but
consider, if you are starting with 117V, it will drop you below 110v, which
is clearly too low for your equipment.
One thing you can do to help is get the "fattest" cord possible. In this
case you might need to build your own. I know I'll get flamed by the
electricians here (Hi, Dave) because I'm out of my depth, but I think I'm on
the right track. I suspect the problem here is that your typical household
15A circuit is fed by 14AWG wire and a 20A circuit uses 12AWG. What's the
sense in using a higher capacity cord (i.e., 10AWG) than the source feed?
This doesn't make sense to me.
Anyway, it's basic physics and a few calcuations. Either get closer to the
source or you need a fatter pipe to assure less line loss. Your typical 15A
household circuit is not adequate for your gig and with the 100' cord, even
a 20A feed isn't really enough. IIRC, a 40A feed requires 8AWG wire. A
100' of 8/3 wire will cost you a bundle -- maybe $100 or so.
As for the sweet sound you describe, today's line voltage can be as high as
125v or even close to 130v, which is considerably above the 115v or 117v
your BF amp was built for back in the day. That line loss is working to
your advantage here until you start to push the amp. Since the power feed
is already probably marginally adequate, the excess demand associated with
the loud parts makes the sag very obvious. It probably got a little better
when you dropped out on piece of equipment simply because you lowered demand
on the circuit.
I suspect you will find that you are overloading the circuit but not by
enough to trip the breaker. I wouldn't be surprised if you touched the 100'
cord and found it is quite hot to the touch.
.
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