speaker degradation

It's worth consideration...many people find that changing the speaker can have a big impact on the tone of the amp. Do you have access to a different speaker?

 

It could be the speaker. It also could be a crappy output transformer.
Many people including myself have gotten rid of dead sounding Marshall amps. I wish I knew then how much a crappy output transformer could ruin the sound and punch and clarity of an amp. Drake transformers were very inconsistent. Some sounded great while others sounded really bad.
I've played through Korg era Vox amps that had the same problem - they also used Drake transformers during that time.

 

This is a solid state amp...does it have an output transformer?

 

I'd try a speaker before giving up on it. I have a Lead 12 from that same series and while it sounds okay with the stock speaker, it kicks ass with a great speaker.

 

While arguably not the worse speaker, 40 years is a long time to hold up, for any electronic component really.

Maybe the bias(resistor) has drifted?

 

speakers are idd a big part of the amps sound, tone and feel.

 

I would try... try to check the speaker... then try to check the amp

 

Oh I didn't know that but no transistor amps do not have output transformers.
Not knowing what those old Marshall transistor amps from the 80s sounded like I have no way of comparing. The best thing would be to try some different speakers and see if it brings the amp to life.

 

Welp, some SS circuits had an OT, but they were typically crap. I don't think Marshall even went that route... Usually the output stage in a SS amp is a pair of big capacitors. I agree with @Ricochet, there are a lot of components that could easily have drifted off spec over the decades.

 

It’s funny, I had the power transformer changed in a cheap ss practice amp. While not as dramatic as an OT swap in a tube amp there was a subtle difference. Every EE says there shouldn’t be(220V being 220V) but here we are.

 

If I can be bothered enough, I’ll swap out the speaker in mine. Don’t hold your breath.
Any particular Celestion you want tested?

 

One dose of an EVM12L is the cure.

 

I don't work on transistor amps - not much tweaking you can do to them so they don't interest me.... The ones I do like such as the Roland JC amps usually never blow up....

 

Roland makes great stuff. There was also a 60W, 12" combo made by Acoustic Control Corporation in the late 70s. Fantastic SS amp. Would love to find one again. Quilter makes good solid state these days.

 

Sorry, no Jensens! I’m not even sure I have a 4Ohm speaker.

 

I used an Acoustic Control 135 2-12 combo back in the 70s (73-78) for about 5 years. It had a nice clean tone but I was unable to get any kind of good distortion out of it without the use of distortion pedals.

 

The 60W 1-12 was similar.

 

Well, If the speaker hasn't been mistreated (schocks, deformations, poor storage with pression on the cone, dust, humidity), I don't think he his the culprit. If not already done, I would check the amp and tubes beforehand.

That said, you may want another speaker and that's fine. But IMHO suspect the amp circuit and tubes first...

A+!