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Newbie here (no electrical knowledge).

Question: For a 15 watts tube amplifier without a headphones output, is it safe to use the external speaker out jack to plug it to headphones, mixer or USB interface, while short-circuiting the FX loop (that is, leaving a dummy jack plugged into the FX lopp In (Send), without it being connected to anything)?

I wanted to buy the Monoprice 15 Watts tube amp and I thought of using it with headphones when at home. But it has no headphones jack.

It does have: A) an external speakers out (so you can plug it to a cabinet), impedance 8-16 ohms; B) an FX loop.

I have both a mixer and an USB interface, so if I couldn't plug the "external speakers" output directly to the headphones, I imagine I could plug it into one of these. The problem is that sound still comes out of the regular cabinet when something is plugged to the external speakers jack. To mute the combo cabinet, one can insert a 1/4'' plug in the FX loop In (Send), without it being connected to anything (that is, nothing is plugged into the FX loop Out (Return)). My fear is that this might damage the amplifier.

I have read contradictory information on the internet regarding this. This post, for example, says it's OK to mute the cabinet by plugging something in the FX loop In (Send), and then use the external speakers output to connect it to headphones. But I also read people commenting this will blow the amp in due time.

Does anyone know what is the correct electrical information? Will I blow up the amp if I do the above?


EDIT: I apologize, I think I might have gotten some things wrong. This guy says he's doing the opposite of what I suggested: he's connecting his headphones to the "FX send" jack and plugging a dummy plug into the "external speaker" jack (to mute the amp). But the commentary right after his suggests it might blow up the amp

Relevant links:
https://www.tdpri.com/threads/monoprice-15-watt-tube-amp.609524/page-22#post-7561910
https://www.tdpri.com/threads/monoprice-15-watt-tube-amp.609524/page-43#post-8874099

http://talk.philmusic.com/index.php?topic=276298.0 (apparently, the Monoprice amp is a Laney Cub 12R clone)

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To be sure I understand, you want to plug headphones into your speaker out on a tube amp head?

I would say absolutely not. It doesn't matter what you have plugged into your FX loop. If you want signal from your cabinet out, that comes from the power amp.

A speaker out sends a "hot" load from your power amp to the speaker. If that load is not resolved somehow (a load box or a speaker), you are going to blow up your amp. And there is no way your headphones are going to safely accept that amount of power.

There are amplifiers that have ways around this. I own four heads, two of them can be safely used without a cab/load box, one is "allegedly" safe to use but most don't recommend it, one absolutely can not.

Here's the breakdown of those heads:

Orange Microdark. When you plug headphones into the headphone jack, you get cabinet emulation and do not require a load on the cab out. There is no standby on this little wonder of an amp (frankly for the money I don't think you can get a better amp of this class), you just plug in the headphones and you're good for silent practice/recording. This amp is an EXCELLENT (!!!) choice for this purpose (and it sounds great).

Hugh and Kettner Tubemeister: Very smart amp, has onboard attenuator. You put it on standby and set the attenuator to "silent" and use the emulated out (which is not a headphone jack, it's meant for running to a PA or interface etc) and you do not require a cab/load. There is no headphone jack. Excellent amp for the silent recording/practicing purpose, but pricey.

Marshall JVM410h: Allegedly, if the presence and master volumes are at 0 and you are in standby, the power amp is not engaged (or at least, is engaged so minimally no damage is done), and you are fine to use the cabinet-emulated out or preamp send for silent recording. There is no headphone jack. This monster is a great amp, but very expensive and way, way more horse than you'd need unless you're playing a mid/large sized venue.

Randall Diavolo 45: Under no circumstances can you run this amp without a cab or load box. Standby, presence/master at 0, doesn't matter. Run it without a load and you're going to blow the transformer.

So how do I get headphone capability out of these amps (with the exception of the MicroDark, which has a headphone/cab emulation jack)?

Every single one of them has an speaker-emulated out. I run that into an interface (like a Scarlett or some such) and use the headphone jack on the interface. This lets me dial up the amp hard, but cut the gain going into the headphones by using the gain control on the interface. You can get any tone you want.

On the Marshall, for silent recording, I run my guitar into a Helix, run a send out of the Helix into the guitar input of the amp, run the parallel send of the amp back into the Helix receive, and on the Helix put a cabinet model in the path. I can then use the headphone jack on the Helix. Alternatively, I run the emulated out into an interface and again just use the headphone jack on the interface.

On the Randall, the same as the marshall; parallel send, or emulated out, into interface of some kind, use the headphone jack on the interface.

On any of the above, it is ALWAYS recommended to run a cab or load box on the cab out, even if the manufacturer says it's ok not to. This isn't so much because you don't believe them, it's because it only takes an unnoticed bump on a master volume, or an accidental flip of the standby, or (in the microdark) you unplug your headphones and forget to turn the amp off, to blow a tube amp that doesn't have anywhere to send the load. And again, some amps simply can't be run without a cab/load box under any circumstances.

To that end, getting a load box for people that want to use tube amps for silent recording/practice is a VERY GOOD IDEA. I have a Rivera load box that also acts as a cabinet emulator. You plug your cab out into the load box, pick the cabinet emulation you want, and either use the headphone jack on the Rivera, or run the Rivera out to an interface that has a headphone jack. The load box can eat the whole load of the power amp, so you can run your amp REALLY hot, dial back the output on the load box, and get a full-on tube signal at low output volume. This is a standard in studios.

But would I ever say "sure just plug a headphone into a speaker out?"

Total disclosure, I am not a pro amp tech. But I've been around them a long time, know some guys that are really knowledgeable, I'm really careful with my research but still made some of the rookie mistakes, and this really sounds like one of them. I would be willing to bet some money that if you asked the amp manufacturer "headphones into power amp out", there'd be a pause on the phone, followed by, "...you want to do what?"

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  • Thanks a lot for your answer! There's a lot I didn't know, thank you very much for taking the time write it. I do have some further questions though: this post reddit.com/r/Guitar/comments/3fji3h/… says it's safe to plug the "FX send" to an audio interface and a shorted cable to "FX return" to prevent any signal from going to the power amplifier. Do you think this would still harm the amp? – flen Mar 28 at 2:39
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    It sounds like a hack, and Iam fairly certain would not apply to all amps, my Randall would still need something, a cab or load box. An FX loop feeds signal to the power stage but does not drive it, you could still have a load I wouldn’t do it. Get a load box with cabinet emulation, and if it has a headphone jack use it, or run the load box out to an interface and use the headphone jack on that. It is absolutely the safest way to run a tube amp silently. – Tim Consolazio Mar 29 at 7:16
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If you have 1/4" Headphones with 8-16 Ohm resistance, just plug them into the external speaker jack. You will need a 1/4 Mono to Stereo converter plug (Mono Male, Stereo female), but that's all.

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  • Thank you! The only problem is that if the external speaker jack is used, it doesn't mute the regular speaker (sound will come out of it as usual, but now it's being duplicated to the external speakers, if I understood it correctly). To mute it, I need to to also plug something into the FX loop In, but I don't know if it damages the amp to have a plug in the FX loop that doesn't get plugged back into the FX loop Out – flen Mar 24 at 2:29
  • Ahh, I see. MonoPrice doesn't release wiring diagrams for that amp (or pretty much for anything), but you'd probably find that by plugging into the FX loop, you're losing the power amp side. If the power amp is transistor on that amp, and only the preamp has tubes, you're good. Otherwise, you need an electronics person to wire a new socket to the external speaker jack - one that breaks the thru connection when something is plugged into it. A simple enough job, and a common socket type. – PeteCon Mar 24 at 2:43
  • Thank you again!:) People who have been dissecting the Monoprice said it's a copy of the Laney Cub (or something close to a Fender Blues Junior IV). I'm sure the power amp is all tube too. I'll try to see if someone in my university could wire the new socket – flen Mar 24 at 3:26
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    Headphones are used to having mW put through them. Are they going to cope with all that extra power?Turn the volume right down, and it may sound awful. – Tim Mar 24 at 7:37
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As far as I know, the external cabinet output must be after the power amp stage, otherwise your signal wouldn't have enough power to drive the speakers. So if you interrupt the signal path at the effects loop, you would mute both the internal and external speaker at the same time.

You might be able to plug the effects send from the amp into your USB interface or mixer and use headphones that way.

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  • Great idea! I think you're right. Though I think there's the problem Tim mentioned in his answer, the amp will probably still create a load. I'm not sure of what would happen here if nothing is connected to the "FX return" and to the "external speakers" jack – flen Mar 28 at 2:33
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    I don't have enough reputation to comment on TIm's post, but as far as I know, shorting the cable is the trick used by cables like these thomann.de/gb/cordial_csi3rpsilent.htm to mute the amp when the cable is disconnected. Unfortunately I don't have one available to try. – Vegard Mar 30 at 6:07
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    Further, my understanding is that since it is a combo amp, it always has a load for the power stage - the built-in speaker. Of course, at this point you're not using the amp as the manufacturer indended so everything is at your own risk. If you could get the manual frmo the manufacturer, maybe that will say something? I was only able to find the manual for the 5-watt version, and that one doesn't seem to have an effects loop or external cabinet connection – Vegard Mar 30 at 6:12

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