No Speaker Load

[MikeStew]

Mesa Mark5 25 Head. Manual says "Never run the amp without a load" - sure, of course. At risk of overstating the obvious, this is a concern for ANY amp head because by definition there is no speaker naturally attached like you would have in a combo. On the M5:25, to use the CabClone out without a speaker there is a very tiny Speaker On / Speaker Off switch in the back which in the Off position presumably routes to an internal load to prevent damage to the head. Failure to have this tiny, easily forgotten switch on the back of the amp in the correct position would be the death of your amp.
SO, to dummy-proof my head (which I use mostly in the CabClone/Speaker Off mode) I am thinking of adding a 20W 8ohm resistor and just plugging it into the 8ohm speaker out jack. Any thoughts on this approach?
Sidenote... why isn't this already included in the design? (Speaker On <and> Nothing Plugged In = Default Internal Load Engaged). It would be easy to implement. It's almost like they are begging for catastrophic user-error. Good way to sell more amps, I suppose.

 

[user 41014]

I like the idea.
If implemented on a switchable jack, you could have it switched out when a speaker is plugged in.
Unless your amp's components start playing dominoes, like power tubes, it's the output transformer that you will briefly be able to fry an egg on. Mark V OT is prob replaceable. Mark II, nope.
How about a 8ohm, 20W sealed in a extension long enough to have a clip to clip on to the back of the amp to prevent strain on the amp jack connection, and with switchable jack out for speaker load?
Patent it and of course the main thing is a cool name and slogan; maybe, "take a load off and save your amp from taking a dump"?
Ok, it needs some work but you get the gist

 

[GJgo]

Sidenote answer- that's what they did in the MkVII.

 

[MikeStew]

I like the idea.
If implemented on a switchable jack, you could have it switched out when a speaker is plugged in.
Unless your amp's components start playing dominoes, like power tubes, it's the output transformer that you will briefly be able to fry an egg on. Mark V OT is prob replaceable. Mark II, nope.
How about a 8ohm, 20W sealed in a extension long enough to have a clip to clip on to the back of the amp to prevent strain on the amp jack connection, and with switchable jack out for speaker load?
Patent it and of course the main thing is a cool name and slogan; maybe, "take a load off and save your amp from taking a dump"?
Ok, it needs some work but you get the gist

Yep, "Put The Load Right On Me" - Planning to mount the resister inside the head with a short bit of speaker cable to a 1/4" right angle plug. Using a speaker? Just temporarily unplug the resistor load. A slightly clunky solution but seems like it would be effective safety measure.

 

[MikeStew]

Sidenote answer- that's what they did in the MkVII.

Good to know - thanks. No excuse for not including it on all Mesa heads.

 

[tbonesullivan]

A lot of techs will use a dummy load like that when they first are powering on and testing an amp, as the alternative could be a speaker making a VERY LOUD noise. As long as it has the power rating and cooling to dissipate any power coming at it, it should be fine as long as it's wired correctly.

 

[user 41014]

Yep, "Put The Load Right On Me"

I see you jumped right to a pretty heavy jingle!

 

[user 41014]

Wrapping my head around some master g-train and your new invention I've got the 1st mod:

double the resistors in series for 16ohms, or better yet just a 16ohm R. The safe mismatch drops the primary side of the OT amps in half limiting its max power to half its original watts.

20W of 16ohm resistance give you "40W protection" on a 8ohm tap and while one might think that is paltry protection for a 100W amp, a 100W is likely Class AB and if it is accidentally on at idle (no signal to amplify) its protection needs to me don't seem to be 100W.

Of course a 16ohm 50W load would be fool proof for 95% o amps excluding bass.

 

[MikeStew]

Wrapping my head around some master g-train and your new invention I've got the 1st mod:

double the resistors in series for 16ohms, or better yet just a 16ohm R. The safe mismatch drops the primary side of the OT amps in half limiting its max power to half its original watts.

20W of 16ohm resistance give you "40W protection" on a 8ohm tap and while one might think that is paltry protection for a 100W amp, a 100W is likely Class AB and if it is accidentally on at idle (no signal to amplify) its protection needs to me don't seem to be 100W.

Of course a 16ohm 50W load would be fool proof for 95% o amps excluding bass.

Yep, more resistance maybe warranted in some cases. For the Mark5 25 head I'm thinking 8ohms is plenty as it's just an emergency override measure - and that's all the amp would expect to see anyway.

 

[MikeStew]

Well here's the idea installed, fwiw. Thanks everyone for the input.
As a bonus, the Speaker On/Off toggle is now blocked by the right angle connector so it would be quite difficult to accidentally bump it on anyway.