wrote:
Meat Plow wrote:
On Tue, 26 Dec 2006 07:19:09 -0800, meow2222 Has Frothed:
Meat Plow wrote:
On Tue, 26 Dec 2006 00:13:43 -0800, meow2222 Has Frothed:
mm wrote:
Is it dangerous to the output transistors or something to run a radio
without speakers?
with tr amps its harmless, do what you like with the vol ctrl. With
most valve sets its harmless, but perhaps not all. Sometimes valve amps
were fitted with spark gaps on the output transformer HT side.
Some vintage valve guitar amps had a shunt prong across the 1/4" spkr
output jack when the plug wasn't in it. IIRC Marshall amps had a warning
tag that to operate the amp without a load or with a resistive attenuator
would void the warranty. I know that operating most any class AB1 valve
amp is risky to the OT.
I had an old amp that would spark across the spark gap with no load
connected. But whats the mechanism for that? No load cant take any Vs
higher than normal run voltage, unless theres some kind of LC thing
going on in the op tf, and no load means no damping.
Not sure, I just trusted the word of my elders that damage could occur.
Maybe some of the deeper thinkers in alt.guitar.amps could explain exactly
what mechanism(s) is/are involved in damaging a AB amp with no load. Now
that you've (sparked)laughs my curiosity, I'm eager to know.
I see thread has already jumped the gap to the guitar group, lets hope
it sparks someone's curiosity there too. Perhaps someone's resistance
to contributing will break down, and they'll tell us something that
resonates with us all.
NT
http://www.geofex.com/tubeampfaq/taffram.htm
also:
"This varies from amp to amp. Some amps have no problem at
all with being unloaded, as long as no signal is being passed.
Others (certain Boogies, for instance) will break into oscillation
when the impedance of the feedback loop changes when the
speakers are disconnected. If your amp is wired so that a
switching jack is used to throw a short across the amp's
output when the plug is removed, then everything will be
fine. If you have a standby switch, it's always a good idea
to put the amp on standby when removing or connecting
speaker cords."
The speaker presents a “load” on an amplifier, just as a “flywheel”
presents a load on an internal combustion engine.
http://guitargeek.com/chat/showthrea...threadid=46544
"SPEAKERS & OHMS!!!! Everything you need to know about SPEAKERS & OHMS!!!
Ohms:
A value of electrical resistance. The lower the number, the less
resistance. The higher the number, the greater the resistance.
Speakers are rated at certain ohms (or impedance), most commonly, 4, 8,
and 16.
Your amplifier is also rated at a certain impedance, or has multiple
taps for speaker configurations. It is important that you match
these numbers up.
EXAMPLE --- 8 ohm cabinet---8 ohm jack on amp
EXAMPLE --- 4 ohm cabinet---4 ohm jack on amp
EXAMPLE --- 16 ohm cabinet---16 ohm jack on amp
It is typically safe to mismatch one step higher
EXAMPLE --- 8 ohm cabinet---4 ohm jack on amp.
EXAMPLE --- 16 ohm cabinet---8 ohm jack on amp.
Now for the do-not do's.
NEVER EVER hook up a cabinet with a lower ohm rating than the head.
EXAMPLE ---4 ohm cabinet---8 ohm jack on amp...THIS IS A NO-NO!!!
You will cause your output transformer to overheat, and eventually, it
will blow.
NEVER - Turn on a tube amp without a speaker load attached to it. This
can cause an output transformer to blow in a short time.
The only instance where it would be safe to turn on a tube amp without a
load would be on amps that have a standby switch (Class AB typically)
This would only be for visual diagnostics (tube heater check, bulb
check, etc) BUT, DO NOT take it off of standby. While it's in standby,
only the tube heaters are working, and will not cause damage to the
output transformer.
Also, NEVER use instrument cable to hook up a speaker cabinet. The
speaker jack can put out a high enough voltage to burn the small wire in
instrument cable, thus leaving the amp without a load, and frying the
output transformer. ALWAYS use a good heavy speaker cable.
General Speaker Info
Sensitivity - This number indicates how efficient the speaker works.
Generally, this number will be between 89 and 105. If you replace a
speaker with one of a higher sensitivity rating, you will also increase
the apparent volume.
Generally speaking, an increase of +3dB will double the overall loudness
of the amp.
Wattage - How much the speaker can handle. Smaller, combo amps usually
have a speaker rating close to the rating of the amp. If when cranked,
you don't like the sound of the speaker breaking up, then you might look
into changing it out with one of a higher wattage. Some like the way a
speaker sounds while being pushed to it's limits, and some don't. It's
all in your presonal taste.
Wiring up multiple speakers.
When wiring speakers, there are three ways to do it
SERIES: one wire from the input jack to the "+" terminal of one speaker,
from the "-" terminal of the same speaker to the "+" of the next
speaker, last wire from the "-" terminal to the input jack. When wiring
speakers in series, multiply the ohms by the number of speakers used.
Also, when one speaker blows, it will cut power to the other speaker as
well, leaving both speakers not working.
EXAMPLE: jack------"+ '8ohm' -" ----"+ '8ohm' -" ---jack=16ohms
8x2 = 16
PARALLEL: One wire from the input jack to the "+" terminals of both
speakers. Other wire from the input jack to the "-" terminals of both
speakers. When wiring speakers in parallel, divide the ohms by the
number of speakers used.
In this case, when a speaker blows, the remaining speaker will still work.
EXAMPLE:
jack----"+ '8ohm'"------------ "+'8ohm'"
jack----" - '8ohm'"-------------"- '8ohm'" = 4 ohms
8/2 = 4
This is also the way you will want to hook up multiple cabinets to an amp.
EXAMPLE: (2) 8 ohm cabinets ----4 ohm jack on amp.
EXAMPLE: (2) 16 ohm cabinets---8 ohm jack on amp.
SERIES-PARALLEL:
This is the most confusing. The total speaker load will equal the ohms
of one speaker. Most commonly used on speaker cabinets that contain 4
speakers. This theory only works with 4 speakers of the same (ohm) rating.
In this case, if a speaker blows, it will take out it's series
counterpart, leaving two speakers working, and two not working, hence
doubling the impedance.
Starting at the upper left speaker, you will run one wire from the input
jack to the "+" of that speaker, then from the same terminal to the "+"
of the speaker below it.
Then run a wire from the input jack to the "-" of the upper right
speaker, then from the same terminal to the "-" of the speaker below it.
Finally, connect the "-" of the upper left speaker to the "+" of the
upper right speaker, and the "-" of the lower left speaker to the "+" of
the lower right speaker.
Wiring Multiple Cabinets
Sometimes, it's inevitable that we need to run multiple cabs of
different impedances. Well, here's how you figure out how to solve that
problem.
EXAMPLE:
The amp has 4, 8, and 16 ohm taps.
Cabinet A = 8 ohms
Cabinet B = 16 ohms
In this case,
1/8 +1/16 = 3/16
Divide the back number by the front number to get the impedance.
16/3 = 5.33
You would want to run both cabinets from the 4 ohm taps, or daisy chain
them if (2) 4 ohms taps aren't available. Remember, it's safe to run a
slightly higher mismatch.
....And just as a reference
(2) 8 ohm cabs = 4 ohms
(2) 4 ohms cabs = 2 ohms - *typically unsafe*
(2) 16 ohms cabs = 8 ohms
(1) 8 ohm cab + (1) 4 ohm cab = 2.66 ohms - *typically unsafe*
(1) 4 ohm cab + (1) 16 ohm cab = 3.2 ohms - *somewhat unsafe*
The two above examples would typically be unsafe to run at, although,
the very bottom example would be better to run, since it's closer to 4
ohms. This would get by in a pinch on some occasions.
[disclaimer]
The information provided above is a general reference. In some
circumstances, it may not be appropriate to mismatch impedances. Use
common sense, and consult the manufacturer if possible. If you are in
doubt, the safest bet is to find a cab that has a nominal rating of that
of your amp. I am not responsible for damages caused from mismatching
impedances.
[/disclaimer]