Why is it bad to run an amp with no load?
Reuben Wilson
Could anyone enlighten me as to why it is dangerous to operate a tube
amp with no load? How can this damage the output transformer? It's
obvious to me why a shorted output is bad (the output stage has to drive
way too much current) but when the amp is unloaded, nothing should be
happening, because the amp has no load to drive, correct? All technical
answers would be greatly appreciated (Lord Valve? Ned? Danny? Anyone?).
I have submitted a false e-mail address, in an effort to shield myself
from spam.
Thanks,
- "Reuben"
Danny Russell
It has to do with the output transformer and the way a two-sided
bilateral conversation takes place through it. Driving a tube output
section into the primary side of an output transformer who's secondary
is properly loaded by a speaker cabinet would be like slamming your full
body-weight into a cushioned wall. Driving a tube output section into
the primary side of an output transformer who's secondary is open or
unloaded would be like slamming your full body-weight into a brick
wall. That ain't the most technically accurate answer, but it's
colorful enough to remember well, and it intuitively gets the point
across. More correctly, the primary gets rattled heavily by back-EMF or
flyback much like an uncoiling spring. (The AC voltage thrown into the
O.T. primary has no where to go, and consequently gets thrown back
albeit at a much higher voltage than it went in.) The harder you slam
it, the harder it kicks back. These abnormally high voltage spikes look
for a weak spot in the tubes, sockets, or O.T., then arc across with
much smoke and fanfare. Try it , you'll see. -Danny
Matthew L Reed
RF amps especially require a balanced load to operate correctly. Remove the load from your RF amp and your output tubes will melt down in seconds, output transistors will pop in about a half second.Ugly. And expensive.
Chris Mohrbacher
Reuben Wilson wrote in message <361671...@earthlink.net>...
>Hi everyone,
>
>Could anyone enlighten me as to why it is dangerous to operate a tube
>amp with no load? How can this damage the output transformer? It's
>obvious to me why a shorted output is bad (the output stage has to drive
>way too much current) but when the amp is unloaded, nothing should be
>happening, because the amp has no load to drive, correct? All technical
>answers would be greatly appreciated (Lord Valve? Ned? Danny? Anyone?).
>I have submitted a false e-mail address, in an effort to shield myself
>from spam.
>
>Thanks,
>
>- "Reuben"
I ought to make a FAQ of this...
A tube amp with no speakers DOES have a load, but unfortunately it's a
highly inductive load. Specifically the output transformer, with no speaker
load, becomes an inductor. Inductors have the property of storing current as
magnetic energy and then releasing it in the form of large voltage
transients which can be many times larger than the original voltage. When a
signal is run into said transformer-now-inductor and the signal changes
quickly enough (pretty much describes a guitar amp cranked up) then the
generated voltage spikes will arc over inside the transformer, the tubes
and/or the sockets, soon destroying them.
So it's not the tubes causing the destruction, it's the output transformer,
which you're pretty much stuck with in all guitar tube amps.
Chris Mohrbacher
Ned Carlson
<rwi...@earthlink.net> wrote:
>Hi everyone,
>
>Could anyone enlighten me as to why it is dangerous to operate a tube
>amp with no load? How can this damage the output transformer?
Mr. Russell gave a good account. But part of it is that tubes
tend to act as current sources.They want to keep stuffing
the same current into the load no matter what it is (albeit, not
sucessfully). If you disconnect the speaker load, the reflected
impedance of infinity (open load), is infinity. Any AC current across
infinity impedance is infinity voltage....IOW, enough voltage
to blow holy shit out of output transformers, sockets, tubes,
and so forth. OK, maybe in practice, it's not infinity, but
enough that running tube amps into an open load is
a bad, bad idea.
Ned Carlson Triode Electronics "where da tubes are!"
2225 W Roscoe Chicago, IL, 60618 USA
ph 773-871-7459 fax 773-871-7938
12:30 to 8 PM CT, (1830-0200 UTC) 12:30-5 Sat, Closed Wed & Sun
http://www.triodeel.com
Your Start Page for Tube and Tube Amp info on the net...
http://www.triodeel.com/tlinks.htm
Fahid Yohaid
>
>Could anyone enlighten me as to why it is dangerous to operate a tube
>obvious to me why a shorted output is bad (the output stage has to drive
>way too much current) but when the amp is unloaded, nothing should be
>happening, because the amp has no load to drive, correct?
No,, not correct,,,,,,, what is happening is that the tubes
are conducting on the primary side of the output transformer.
The current through the primary winding creates a magnet field
around the output transformer.
When the tubes stop conducting, the energy in this magnetic
field has to go somewhere. Normally it would be dissipated by
the load,,,,,but with no load the only path it has to discharge is
through the output tubes or by arcing across the windings of the
transformer. Since the output tubes are turning off,,
their resistance become quite high, so in order for this magnetic
flux energy to discharge out of the output transformer, the voltage
has to become extremely high to overcome all of the high resistance
paths available for discharge. With all of this high flyback voltage
generated,,,,,it's simply a matter of what breaks down first,,,,the
output tubes, or the insulation on the windings of the transformer.
This is why tube amps usually have grounding jacks at the output
to close the output winding and offer this flyback voltage somewhere
to go if there is no speaker connected.
If you want to know more about this phenomena, study the operation
of automobile ignition coils,,,,,,it's basically the same phenomena.
JuArKa97
>snip
> No,, not correct,,,,,,, what is happening is that the tubes
>are conducting on the primary side of the output transformer.
>The current through the primary winding creates a magnet field
>around the output transformer.
> When the tubes stop conducting, the energy in this magnetic
>field has to go somewhere. Normally it would be dissipated by
>the load,,,,,but with no load the only path it has to discharge is
>through the output tubes or by arcing across the windings of the
>transformer. Since the output tubes are turning off,,
>their resistance become quite high, so in order for this magnetic
>flux energy to discharge out of the output transformer, the voltage
>has to become extremely high to overcome all of the high resistance
>paths available for discharge. With all of this high flyback voltage
>generated,,,,,it's simply a matter of what breaks down first,,,,the
>output tubes, or the insulation on the windings of the transformer.
> This is why tube amps usually have grounding jacks at the output
>to close the output winding and offer this flyback voltage somewhere
>to go if there is no speaker connected.
Right on Fahid. I'm the Mesa warranty repair guy here and I recently got a
Mesa 400+ from the local music store (brand new). This amp has 12 6L6s in
Push-Pull Parallel. Cranks out 400 watts or so. Apparently someone in the
store plugged the amp in and tried to play thru it without a speaker connected.
When I pulled it apart, one of the ceramic tube sockets on the 6L6s was burnt
to a crisp. It would probably take an acetylene torch to deliberately do the
kind of damage to the tube socket that had occured. You too can have your very
own plasma torch!
Juel E. Rowden
Rowden Sound
JuAr...@aol.com
Spokane, WA
Richard Hopper
recording without connecting to a speaker cab?
There must be a way around this to simulate a loaded speaker cabinet.
Could it be as simple as just placing an 8 or 16 ohm resistor in the
signal path?
I am asking this as I actually want to try recording in this way, but am
restricted to playing through headphones!
Any advice would be appreciated.
--
Richard Hopper
Jeppard3
them cuz you're gonna juice em, i can tell. Your amp will not know it isnt a
speaker. remember: It MUST be a wirewound!!!
Lord Valve
Lord Valve Speaketh:
Yeah, your amp *will* know it isn't driving a speaker. Any
tube amplifier will respond differently when driving a purely
resistive load, rather than the reactive load a speaker presents.
This *will* change the tone. (This is why "power brake" type
gizmos never sound *exactly* right.) Using a load resistor
on an AC-30 is a particularly bad idea, since those amps work
the crap outta the tubes to begin with. If you *must* do this,
use a resistor rated at 100 watts, and (forgive me, I don't recall
if the AC-30 reissue has this, but the old ones did) use the 16-ohm
tap if at all possible...this will minimize the stress on the OP
tranny's secondary.
Lord Valve
Visit my website: http://www.freeyellow.com/members2/lord-valve/
Good tube FAQ for newbies. Click the e-mail link and request a
tube catalog. I specialize in top quality HAND-SELECTED NOS and
current-production vacuum tubes. Good prices, fast service.
TONS of gear and parts in stock...let's DEAL!
"I'm not an asshole, but I *play* one on the Internet." - Lord Valve
Chris Mohrbacher
Lord Valve wrote in message <701iuo$b...@dfw-ixnews5.ix.netcom.com>...
>In <19981014020714...@ng109.aol.com> jepp...@aol.com
>(Jeppard3) writes:
>>
>>A 50 watt wirewound resistor is what you need. get some with heat
>sinks on
>>them cuz you're gonna juice em, i can tell. Your amp will not know it
>isnt a
>>speaker. remember: It MUST be a wirewound!!!
>
>Lord Valve Speaketh:
>Yeah, your amp *will* know it isn't driving a speaker. Any
>tube amplifier will respond differently when driving a purely
>resistive load, rather than the reactive load a speaker presents.
>This *will* change the tone. (This is why "power brake" type
>gizmos never sound *exactly* right.) Using a load resistor
>on an AC-30 is a particularly bad idea, since those amps work
>the crap outta the tubes to begin with. If you *must* do this,
>use a resistor rated at 100 watts, and (forgive me, I don't recall
>if the AC-30 reissue has this, but the old ones did) use the 16-ohm
>tap if at all possible...this will minimize the stress on the OP
>tranny's secondary.
...repeat...
>Using a load resistor on an AC-30 is a particularly bad idea, since those
amps work
>the crap outta the tubes to begin with.
If that's the case, would it be worth considering using a resistor value a
little greater than 16 ohms, like 20 or 25 ohms? I'm thinking that would
reduce the stress on the tubes and output transformer, without significantly
increasing the risk of frying the OT from voltage spikes. Just an idea.
FWIW both of my Marshall Power Brakes incorporate a resonant circuit (10 or
15 mH inductor and 150 uF capacitor) that causes it's impedance to be
approximately double at around 120 Hz. Coincidentally (?) this impedance
peak closely mimics the behavior of my Celestion Greenback mounted in a
closed-back cabinet. Apparently dealing with an increase in impedance, at
least down low in the spectrum, is business as usual for amps.
Chris Mohrbacher