Turning combo to head - do I still need a speaker load connected if I Line out to PA?

sev112

Question for 

so I am old and creaky and cannot any more carry my old Marshall AVT150 combo downstairs and into the car , let alone out again and into pubs etc to gig with, or to rehearsal rooms.
NO-one will buy it if I advertise it (as eBay demonstrates and general hatred of it on numerous forums)
so I might as well see how else I can use it, as it doesn't have any resell value.
i like the sounds, so I am wanting to take the amp physically off the combo, and use it as a much lighter portable head with a senate smaller portable cab that I have. 
Pit is top supported /hangs from the top, and the cab of the combo below is self supporting so won't fall apart from a simple surgical separation with a Stanley knife (for the Tolex) and a hand saw.

question: 
whenever we gig, we always go into PA, wherever we are. So then I use a borrowed) portable CRate PowerBlock head which has a line out and doesn't need to have a speaker load on it.
The Marshall also has an emulated line out socket.
So, do I need to keep a speaker load (I've the portable cabinet) on the amp if I am going line out into the PA with the Marshall "head" ?

(my portable cab is the same impedance as the combo speaker)

ecc83

Don't know the amp and the general idea is that you don't need a load on a solid state OP stage. I disagree, I have had amps burn out, presumably because they went instable without a load.

The crunch question is what resistance and power rating to make such a load? How hard do you have to drive the amp to get a decent emulated output? If only modestly something around 8-10 Ohms and 50W rating should serve. You could even bolt it to the chassis and plug it in as needed.

No doubt ICBM will be properly informed/.

Dave.

Danny1969

On that particular amp you could easily remove the power amp section if you wanted to. Or just give it a dummy load if you thought it would become unstable .... or put a jack in the headphone socket which mutes the pwr amp anyway 

ICBM

No, you don't need a speaker load because the amp has a solid-state output section. It's probably less likely to break down when running like that…



In fact, if you're *never* going to use it as anything other than a preamp, you could disconnect the power modules to be really safe - it's a single PCB connector for each one - and then the fans, which would stop the annoying noise they make. Just remember to not reconnect the power modules without the fans as well.

sev112

Thanks guys Appreciate the steers and the potentials alternatives as well

ecc83

Hi IC, My experience was with a medium quality 80W PA. The Zobel network had cooked up. I can only presume the sequence of events was..Big HF pulse, Zobel R cooked and went low, melted cap which shorted, amp went into DC chain reaction ape shit.

A better designed amp with a heftier Zobel network would no doubt have survived  but they tend not to be that for guitars!

Dave.

ICBM

Yes, I've seen a couple like that too. Pretty sure it won't apply to anything like this, because the amp is designed to run with no load when the headphone jack is in use.

DJH83004

IC am I right in thinking the AVT150 has the two separate PCBs with the TDA 72.... chips mounted on them? if so they do have separate power supply plugs to each board so could be disconnected, but I wouldn't worry, as the chips have so much internal protection, I can't see a failure happening due to no speaker load.     

ICBM

DJH83004 said:

IC am I right in thinking the AVT150 has the two separate PCBs with the TDA 72.... chips mounted on them?

Yes.

DJH83004 said:

I wouldn't worry, as the chips have so much internal protection

lol

They're supposedly internally protected against shorts and thermal overload too… for all the good that does!

DJH83004

Ha, Ha I was laughing when I wrote it - just imagining your response 

ecc83

78XX and 709XX regulators are short and thermally protected as well....Ho, ho, ho!

Dave.