Adding an external speaker jack to a combo amp

funstuie

Hi, 

I have a Marshall Lead 12 Combo amp and I want to add a speaker out so I can connect it to an external cab. Just so I dont blow it up, does anyone have a quick guide to doing such a mod? 

I have read that I just need to cut the cable from the amp into the speaker and add a jack on the amp side and a plug on the speaker side. Does that sound right? 

ecc83

ICBM will surely have the full SP but personally I would leave well alone.

The internal speaker is 8Ohms and a Celestion G10, not a bad model? The danger is an external feed could get shorted and the schematic shows no protection AFAICS.

The circuit also uses a current feedback trick to simulate the higher OP Z of a valve PA abd this might be a problem if the speaker lead is long enough. The circuit also uses the horrid jack switching technique to get a headphone output. This is a known source of trouble, if not in this, in other transistor amps.

Dave.

ICBM

I've fitted these with speaker jacks in the past. The amp section is identical to the head version they made for the original micro-stack. There may not be any proper protection, but actually these are the most robust 12W solid-state amps ever built - at least that I've ever seen or heard of - and I doubt there's any risk.

The way I've done them is simply to ream out the cable hole in the chassis floor and fit a jack - make sure it's the plastic insulated type like the ones on the front panel, so it doesn't couple the ground to the chassis - with a plug on the cable.

This is what the chassis looks like inside - I've seen less beefy 100W amps!

http://www.freestompboxes.org/viewtopic.php?f=2&t=4592

And yes, the combined headphone/line-out jack is a nasty solution and sounds crap for headphones anyway, so I would probably bypass the switch in it and make it line-out only.

ecc83

Said Eee'd know!  I only had the schematic.

Yes, that looks pretty chunky, should blow the fuse first. Would that ALL transistor amps were such a BSH grade!

Dave

ICBM

ecc83 said:

Yes, that looks pretty chunky, should blow the fuse first. Would that ALL transistor amps were such a BSH grade!

Exactly. This whole series are possibly the very best solid-state amps ever in terms of build quality - basically identical to the JCM800 valve amps, but with transistors. Full welded heavy-gauge steel chassis, ply cabinets, decent speakers, overspec'ed internal components… they also sound great, are loud for their rated power and almost bombproof.

Sadly, I expect they would be thought of as uneconomical to make by modern standards, given some of the cheap crap Marshall have turned out over the last few years.

funstuie

ICBM said:

I've fitted these with speaker jacks in the past. The amp section is identical to the head version they made for the original micro-stack. There may not be any proper protection, but actually these are the most robust 12W solid-state amps ever built - at least that I've ever seen or heard of - and I doubt there's any risk.

The way I've done them is simply to ream out the cable hole in the chassis floor and fit a jack - make sure it's the plastic insulated type like the ones on the front panel, so it doesn't couple the ground to the chassis - with a plug on the cable.

This is what the chassis looks like inside - I've seen less beefy 100W amps!

http://www.freestompboxes.org/viewtopic.php?f=2&t=4592

And yes, the combined headphone/line-out jack is a nasty solution and sounds crap for headphones anyway, so I would probably bypass the switch in it and make it line-out only.

what sort of jack should I look for? When I search on ebay I get thousands of options. 

funstuie

oh and thanks for the responses. 

ThePrettyDamned

ICBM said:

ecc83 said:

Yes, that looks pretty chunky, should blow the fuse first. Would that ALL transistor amps were such a BSH grade!

Exactly. This whole series are possibly the very best solid-state amps ever in terms of build quality - basically identical to the JCM800 valve amps, but with transistors. Full welded heavy-gauge steel chassis, ply cabinets, decent speakers, overspec'ed internal components… they also sound great, are loud for their rated power and almost bombproof.

Sadly, I expect they would be thought of as uneconomical to make by modern standards, given some of the cheap crap Marshall have turned out over the last few years.

I want the Lead 100 MOSFET head.  SO BAD.  

funstuie

any of these the right jack to get:

http://www.hotroxuk.com/amp-parts-service-equipment-294-0/input-sockets-7949-0.html

ICBM

This one:

http://www.hotroxuk.com/mono-input-jack-4-solder-terminals-switched.html

Connect the wires to the terminals on the side you can't see in this pic, black closest to the nut.

funstuie

I found this on another forum. Is this correct:

ICBM

No, because that's for a jack which switches to the external speaker while still leaving the internal one hardwired.

It's much simpler just to use a plain jack with the internal speaker plugged into it.

All you need to do is connect the wires inside the amp to the two terminals on the right in the lower pic, then put a plug on the wires to the speaker with the red going to the tip (centre) connection. If you cut the existing wires at the cable hole they will be the right length.

funstuie

ICBM said:

No, because that's for a jack which switches to the external speaker while still leaving the internal one hardwired.

It's much simpler just to use a plain jack with the internal speaker plugged into it.

All you need to do is connect the wires inside the amp to the two terminals on the right in the lower pic, then put a plug on the wires to the speaker with the red going to the tip (centre) connection. If you cut the existing wires at the cable hole they will be the right length.

OK Thanks for the clarification. 

All ordered. 

AlanP

ecc83 said:

The circuit also uses a current feedback trick to simulate the higher OP Z of a valve PA abd this might be a problem if the speaker lead is long enough. 

Dave.

Hmmm... If that's the schematic labelled 3005.gif, I can't see any unusual "current feedback trick" - it looks bog-standard to me (albeit using darlingtons)...  but no protection though...