Help modifying output jacks please?

[CaptainCrunch]

Two questions in modifying the outputs on the back of a small combo unit -

1) The phones/line out jack disables the speaker when used, how can I change it so the speaker is not disabled (only will use this as a line out and want the speaker functioning)?

2) I want to add an output for an extension speaker, which will need to disable the combo speaker. How do you accomplish the disabling of the combo speaker when this output is in use?

In both cases, could a toggle be added to allow a choice as to whether the speaker was disabled or not?

Thanks!

 

[Kent S.]

Re: Help modifying output jacks please?

Conceptually speaking it's not hard, however it depends on how the amp is laid out, some amp's incorporate a dummy load for the speaker (running an amp with an infinite load can burn some up also) ... some tend to mute the speaker by killing the signal to the power amp section (I'm including PI here as well), feed it nothing but a grounded jack, and it sits there happily doing nothing (idle). Some low powered SS amps do something else altogether.
If it incorporates a switching jack (duh ... it does as it kills the sound from the speaker), then the jack could be changed to not mute the sound, or a switch added to do either.I have some old SS Fenders that have a headphone jack and a switch to mute the speaker ...
Case in point also, those Fenders have a preamp out, and plugging headphones into that is very different from plugging them into the headphone output; However, the preamp is a line output ...Not sure what they are doing with a jack that's a combo of both.

Adding a switching speaker jack, that kills the internal speaker when it's plugged into, is easy ... but being able to connect the internal speaker as well, that's going to mismatch the impedance ... plus I've always been leary of putting my hands on switches that are carrying lethal amounts of current/voltage on the other side (I don't particular trust most switches). Anyway, that aside, if you have PCB then you'll probably have to cut some traces et al, PTP is easier, but you should have a tech do it (the reason that you are asking how to do it, means that you aren't sure how to ... and that is a big safety issue), plus it will save you the time in tracing thru your amp and schematics to find what goes where.

 

[CaptainCrunch]

Re: Help modifying output jacks please?

Conceptually speaking it's not hard, however it depends on how the amp is laid out, some amp's incorporate a dummy load for the speaker (running an amp with an infinite load can burn some up also) ... some tend to mute the speaker by killing the signal to the power amp section (I'm including PI here as well), feed it nothing but a grounded jack, and it sits there happily doing nothing (idle). Some low powered SS amps do something else altogether.
If it incorporates a switching jack (duh ... it does as it kills the sound from the speaker), then the jack could be changed to not mute the sound, or a switch added to do either.I have some old SS Fenders that have a headphone jack and a switch to mute the speaker ...
Case in point also, those Fenders have a preamp out, and plugging headphones into that is very different from plugging them into the headphone output; However, the preamp is a line output ...Not sure what they are doing with a jack that's a combo of both.

Adding a switching speaker jack, that kills the internal speaker when it's plugged into, is easy ... but being able to connect the internal speaker as well, that's going to mismatch the impedance ... plus I've always been leary of putting my hands on switches that are carry lethal amounts of current on the other side (I don't particular trust most switches). Anyway, that aside, if you have PCB then you'll probably have to cut some traces et al, PTP is easier, but you should have a tech do it (the reason that you are asking how to do it, means that you aren't sure how to ... and that is a big safety issue), plus it will save you the time in tracing thru your amp and schematics to find what goes where.

Thanks Kent, I gotta try to pick your brain a little more here, I should add this is just one of the small Valvetronix amps I'm talking about here, no power tubes, just a pre-amp tube and obviously PCB. I would like to know these mods are doable inexpensively before I get one - the 50W version does have the speaker output, so I guess I am assuming they could have easily incorporated this feature into the smaller versions if they wanted to...

On Q #1 - I follow in as much as there could be several different ways this is set up - I was hoping that it would be a similar situation to what you mentioned in another post (I realize different situation) "put a break jack (a phone jack that will break the connection from the preamp to the power amp section when a plug is inserted)". Is it possible they just used a break jack here, if so, how would I tell? and any potential problems created by replacing if this is the case?

Q #2 - So just using a "break jack" for a speaker output, even though disabling the internal speaker, would still create an impedance mismatch, i.e. the amp would still see the internal speaker load?

 

[Kent S.]

Re: Help modifying output jacks please?

...........the 50W version.........

Well, yes or no, a switching jack that kills the internal speaker when another cabinet i plugged into ... Yes. Keeping the impedance so that the internal speaker can run along with the cabinet ... cost wise probably no.

.......... problems created by replacing if this is the case?

On the power amp input, yes that jack is a switching jack ... otherwise it wouldn't cut off the main preamp signal ... I'd have to say yes.
The only way you can tell really is to look at the schematic... and potential problems would also be answered there as well ..
If it has a tube preamp, then it's probably still running around +120VDC to +190VDC on the plates of that tube(s) ... Don't think that SS amps can't kill you, they can ... even a pure SS amp with +15VDC supply for opamps is still got lethal current at the speaker's side, not to mention the power supply primary side.
If you interupt the input to the power amp input section, and feed it no signal (a grounded input) it's like a master volume sitting on *0* ... No sound, the power section still has it's speaker to load the system appropriately.
Again, until you go thru the schematic, you don't know how the particular amp is laid out ... If you aren't skilled in this area, I'll say it again ... Take it to a tech, ... pay the money, and get the job done safely and correctly.
There are many books available about service to your amps, and mods, if you are developing an interest in this stuff that's a place to start, along with a good and through understanding of basic electronics (including the bloody math!)
If not, and it's just a one time, *I just want to get this done to mine...* type of thing ... see above note about having it done professionally.

Q #2 - So just using a "break jack" for.......

No ... If you have say, an 8 ohm load, and the jack disconnects that speaker, when another 8 ohm cabinet is plugged in ... well you still have 8 ohms ...Nothings changed.
***But if you plug in that 8 ohm speaker and the jack doesn't disconnect it, then you have 4 ohms (two 8 ohm drivers running in parallel) **
BTW, only switch when the amp is at idle, or better yet turned off.

Back to 4 ohms, this may be okay. Many amps do this, normally something like 80Watts @ 8 ohms, 100Watts @ 4ohms, that sort of thing. Many of those are actually 4 ohm amps, with an 8ohm speaker in them, so when you plug in another 8 ohm speaker ... you get what the amp wants to see ... but generally isn't necessary.
You need to know what the minimum load is your amp can handle and don't exceed it.
Fortunately most SS power sections treat higher impedances more favourably, in other words, if your amp is rated for 8 ohms, you put in a 16 ohm speaker in it, and have another cabinet rated at 16 ohms. Run either and you just don't get quite as much power, run them both and you are back to spec. Having an extension speaker jack installed just parallels off of the normal speaker connections (or speaker jack), a switch to kill the internal speaker would just disconnect the speaker's leads and hold them open. A switch jack would preform the same option automatically.
Are the mods inexpensive, relatively speaking yes, I got ready to talk about unless you wanted to change the output tranny ... Then remembered we were talking SS output stages here. The preamp output/power amp input is probably cheaper than an actual FX loop, as that normally requires extra buffers and circuitry ... but if you want an actual pedal level loop, then get one of those installed.