1972 super lead 100W 'burning-plastic-smell'

[SQUAREHEAD]

Well, that about says it all…
So, even after just a few minutes of playing through the amp
(Vol On 8 or 9 into a hot plate) it starts to have a strong 'burning plastic' kind of smell in the room…
I don't know any other way to explain the smell other then it smells extremely poisonous…
It does not smell like tubes,
Any ideas what this could be?

[neikeel]

First things first.The smell of course could be the Hotplate!

I hope you have an amp impedence to Hotplate to cab match?

Take the head out of the cab, and flip it over, well supported. Don't touch anything yet, just inspect for signs of charring, in particular around the PT wires, OT wires and valve bases.

Check your fuses are of the correct values.

Personally, after this if there are no signs of burning and you have the correct fuses in, I suggest you plug your amp straight into a cab, without the Hotplate, and fire up at low volumes, see what gives. Then build up volume to where you were to see if it does it again.

Does the PT and/or OT get hot (or for that matter the choke)?

Most likely issue is the OT which may have a partial short but I have had a PT do this on me (it crackled and sizzled too).

Good luck and remember to drain the filter caps safely before you actually prod around

[SQUAREHEAD]

Thanks Neil, I've got a bunch of old Marshall heads and non of them smell but this one, it is definitely not hot plate, but I will prod and see what looks to be the problem…
Keith

[herbvis]

I would open that amp up and find out what is burning and take care of it asap. Or have a tech do it if you are not confident.

[RagnarMarGunn]

This smells exactly the same as when I toasted the PT in my Metro 100W MV