Combo amp shells

CHrisP86

My DSL401 (just the amp chassis) has been off with Martin at MJW amps having a few mods and a bit of TLC.

So I’ve had a chance to look at the combo shell.

It is completely made of mdf.  Even the speaker baffle.

Given that the convention is to use plywood, what are general thoughts as to whether I’d notice much difference if I made a new shell out of plywood?  Or even if I just made a new plywood baffle?

ICBM

There is a definite difference in the sound of the two materials, but you would notice a bigger difference from replacing the cabinet than the baffle in my opinion. The baffle can actually benefit from being more inert.

fftc

If you are on Facebook you could check out Mark Bartels page and maybe pick up a few hints on what he thinks makes a good baffle.

https://www.facebook.com/pg/bartelamps/posts/?ref=page_internal

CHrisP86

Wow, that is guitar cab wizardry!

Think I might give it a go and see if I notice a change. Hoping the plywood will actually be a bit lighter as well!

ICBM

You could also build it with a conventional back panel arrangement instead of those daft metal grilles...

I think the purpose of them is to increase convection cooling by setting up a cold-in, hot-out path, but the main problem with these amps and heat is that they’re biased far too hot at the factory - the recommended bias setting is 35mA (1.4V measured at the test point), which is nuts - that’s 14W per valve. Set it to 20mA (0.8V or 8W) and it won’t try to cook itself as much! I also increase the cooling gap above the rear of the chassis by adding a couple of rubber grommets around the two top rear bolts, and a heatsink on the filament rectifier.

I expect Martin will do this anyway, it becomes standard practice when you’ve worked on enough of them, as most of us have .

CHrisP86

Thanks, IC.

I’m not sure what bias Martin has set it at but I’m sure it's more sensible than the factory setting!  He has also increased the spec of the bridge rec to avoid the overheating issue.

I agree on the metal grilles.  I've already been using the amp without them and re-positioned the wooden part to close off the bottom half of the cab.  Would do similar if I build a new shell for it.

Modulus_Amps

Birch pywood, which you really should use, is still pretty heavy. But you can cheat if weight saving is what you want to do.

I just finished a 1 x 12 birch ply speaker cab with weight reduction, Steve who does my covering was shocked at how light it was compared to the normal version of the cab.

martinw

CHrisP86 said:

Thanks, IC.

I’m not sure what bias Martin has set it at but I’m sure it's more sensible than the factory setting! 

1.0V, so not as cool as ICBM suggested, but we're on the same page.

Certainly way better than 1.4V.