cellardweller
New member
Since supposedly only the heaters are on when an amp is switched to standby, is it true then that it is safe to switch cab's when the amp is on standby?
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goldtopchas
New member
I wouldn't just to be safe. It would depend on the amp. With most standby switches, no voltage is present at the rectifier tube /diodes until it is switched to "ON. But I think there are designs that don't follow that set up. Not sure about after it's already been on. Since the caps store a charge that would have to be discharged, I would say no.
Just flip the power off first. Is there a reason why you wouldn't want to do it this way? Just wondering...
Just flip the power off first. Is there a reason why you wouldn't want to do it this way? Just wondering...
Track Rat
Just Your Average Sized Member
I agree with Goldtop. I've seen a few different standby schemes.
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ggunn
Crystal Flavolian
You can switch cabs any time as long as you are not actually playing; you don't have to power down or even go into standby. It's only when a tube amp tries to make sound and there is no load connected that there is a problem.
cellardweller
New member
It MIGHT facilitate recording being able to swap out cabs on the fly, without letting the tubes get cold at all....
Actually, I'm just curious on a theoretical level....
I'd read that the aforementioned premise was true on the Boogie board
http://forum.grailtone.com/viewtopic.php?t=23012
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dementedchord
Psychotic State alumni ch
you should be ok if it's on stand-by... the HT or B+ voltage is removed then... worked on lotsa amps and dont remember seeing where the primary cap is still in the circuit... some secondary caps are in most cases but they are there to help decouple the stages from each other and dont hold much juice and flow back to the output stage is not a big concern... as to some of the other comments i wouldnt assume that the tranny cant burn if the speaker is gone and no signal is present...
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ggunn
Crystal Flavolian
The only time that you need worry about a no load condition on a tube amplifier is when you have signal running through the output stage. What can hurt the power tubes and output transformer is large voltage swings across the primary of the OT with no load on the secondary. There is no DC component to the voltage on the secondary (a good thing, otherwise the speakers would fry), so there is no risk to the amp by opening the speaker circuit when you are not producing signal. To be safe, I'd turn down or unplug any instruments plugged into the amp so that you won't send any signal through the amp by accident, but powering down or putting the amp on standby is simply not necessary.
Also, any damage that you can cause typically isn't instantaneous even at worst case (playing on an amp with no load); before I knew about the damage it can cause, I used a little Supro tube amp that had a preamp output installed as a distortion unit and ran it BTTW with no speaker for months before it finally took out the OT.
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boingoman
New member
The problem is that the amp doesn't need any signal to drive itself to full output with no load. It will do that just fine on it's own, no matter what the volume setting is, because it's the output transformer basically amplifying it's own internal noise. It goes to full power and fries itself.
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Whale Bone
New member
Last week an engineer plugged in and turned on my '65 Ampeg fliptop for 2 hours without plugging in the speaker. It was in standby atleast, and I can find no harm ...atleast not this year. I recorded with it for four days with no trouble. Was scared at first because I'd just had the input transformer replaced.
I've played into my amp, on, speaker disconnected, for a minute or two more than once while I figure out the obvious reason for no sound.
And if you're worried, turn off the amp, swap cabinets, and turn it back on. The tubes will still be hot. We're not talking about hi-fi gear that one wants to leave hot 24/7. It's an instrument amp that goes hot and cold and gets bounced around in between. I don't think you need to worry either way.
I would think it would take a very long time with the amp on (not in standby), without a speaker load, without signal to cause a problem. There's probably always a little current in the primary of the output transformer due to noise/hum or imbalances between the power tubes.
A signal, though, would certainly cause the transformer pain if unloaded. But depending on the strength of your signal, I would expect not to fry to the transformer instantly.
I've played into my amp, on, speaker disconnected, for a minute or two more than once while I figure out the obvious reason for no sound.
And if you're worried, turn off the amp, swap cabinets, and turn it back on. The tubes will still be hot. We're not talking about hi-fi gear that one wants to leave hot 24/7. It's an instrument amp that goes hot and cold and gets bounced around in between. I don't think you need to worry either way.
I would think it would take a very long time with the amp on (not in standby), without a speaker load, without signal to cause a problem. There's probably always a little current in the primary of the output transformer due to noise/hum or imbalances between the power tubes.
A signal, though, would certainly cause the transformer pain if unloaded. But depending on the strength of your signal, I would expect not to fry to the transformer instantly.
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boingoman
New member
Then in many cases you would think wrong. Like I already said, you don't need an input signal for the trannie to fry itself. Depending on the situation, a trannie will fry itself very quickly indeed. Like in minutes. Or, it could sit all day. Probably not, though.
Actually the hum is partially from the imbalance, not the other way round, but basically, yeah. And that's one of the things that will drive an amp to full power with no load attached.
The signal strength doesn't matter. The fact that the amp will swing itself up to full output power and fry does matter.
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tom18222
yes
http://www.musiciansfriend.com/product/Radial-CabBone-Speaker-Cabinet-Switcher?sku=153910
if you got one of those, you have no worries, and you can do it on the fly.
if you got one of those, you have no worries, and you can do it on the fly.
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boingoman
New member
I don't have time to go into it, but for one thing, of course it's OK if the standby switch is in play, because there's no B+, and so the tubes won't amplify anything.
And an amp can easily drive itself into oscillation with no speaker attached, I'll lay it out later.
Pieces yo.
And an amp can easily drive itself into oscillation with no speaker attached, I'll lay it out later.
Pieces yo.
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ggunn
Crystal Flavolian
Sorry, I don't buy that. I have never known this to happen and I doubt that it ever could. For one thing, the output transformer is a passive device and cannot amplify anything.
Lt. Bob
Spread the Daf!
ditto ...... while you could have something microphonic that could conceivably cause the amp to oscillate up to full power, you'd surely know that there was some problem while the amp was playing.
Otherwise .... if you're saying that the tranny hum causes an amp to amplify up to full power, then you're essentially saying that an amp is always at full power because that same noise would be there if there was a load and amps clearly don't put out full power all the time.
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dementedchord
Psychotic State alumni ch
re: amps burning up with no load and no signal present...
the way it was explained to me is that the instability is caused by the "long tailed pair" which is a feedback from the secondary of the output tranny...
the way it was explained to me is that the instability is caused by the "long tailed pair" which is a feedback from the secondary of the output tranny...
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Whale Bone
New member
On the Ampegs, and pretty much anything else I've looked at, the power amps do sit at 'full power' all the time - it's just a matter of whether or not you are asking them to do anything. What's varied is the amount of signal fed into them. With no signal to modulate the DC voltage on the power tubes there's no AC voltage being created to go through the transformer. With no effective input on the transformer, there's no need for a load to keep the secondary windings happy, i.e. unfried.
By this logic (or just my understanding if I'm wrong about this) a little hum, whether from the preamp or the power amp, isn't going to tax the output transformer too much. On the other hand, driving the amp full-on with no load is going to tax the output transformer plenty.
I am prepared to be wrong about all of this, but I'm pretty certain I've got the basics right here.
By this logic (or just my understanding if I'm wrong about this) a little hum, whether from the preamp or the power amp, isn't going to tax the output transformer too much. On the other hand, driving the amp full-on with no load is going to tax the output transformer plenty.
I am prepared to be wrong about all of this, but I'm pretty certain I've got the basics right here.
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dementedchord
Psychotic State alumni ch
and is what's used in the vast majority of amps... exceptions are the cathode"self biasing" and class A designs...
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ggunn
Crystal Flavolian
That's the way I see it. too.