50w MK3 Initial thoughts...

[Adrian R]

Okay, about 24hrs after getting the MK3 2500 I got it working quite well. it was a MESS. It hadn't been used in probably two decades....I had never seen such a dusty mess. It took hours to clean the dust away... Had to use a razor blade to scrape it away. Everything was completely cleaned. Took out the enclosure and scrubbed with soap and water, blasted it with a water hose. Let it dry completely...some of the blackness had had faded from the cloth... colored that in with a sharpy.

Looks almost new now.

But, like what Marshall did with many 900s, the bias network is out of whack. When I first fired it up it was only idling at 2mA...and that was with the bias pot nearly cranked!

This thing for it's entire life was running horribly over biased.... I put in some GL KT-77s and got it to around 17mA...that's it.

So, it it is going to need a bias network change with the appropriate resistors to get it within normal bias range...which should be around 35mA or so.

Sounds decent enough though...but I know it can be better. Will install a choke too. The DR 900 in the pic has one and it sounds much tighter... Might change the filter caps too, but I like keep it stock...and they seem okay.

 

[mrjones2004x]

Love my 2501 with its mods. Never looked for another amp since. We’ll except my spark go but that don’t count.

Congrats on getting it running well

 

[Neptical]

So, you're saying it smokes your DSL.

 

[Adrian R]

So, you're saying it smokes your DSL.

Nope...

 

[Adrian R]

I finished the sort of pseudo restoration of some sorts. I have attached some pics. It started with removing the chassis from the head shell and removing the piled up decades of dust from it. Needed to use a razor blade to scrape it off, almost like ice from a windshield. I even had to resort to using some sandpaper...then acetone to finally clean it. The enclosure I took out side with some soap and a small bucket of water, with a scrub brush and went to town, complete with hosing off the years of crud with a garden hose. I towel dried it and let sit out in the hot Sun for a hour or so.. Some of the black dye from the basket weave washed away with the piled up dust...so I colored it back in with a black sharpie...


So when I checked the bias, it was way off... With bias pot cranked the most I could get out of a steady idle bias current was 2mA! Normally a 50w Marshall amp idles about 35mA.. I able to get it to idle at 17mA with some hotter running KT-77s (EL-34 variant) but even at that it is still woefully over biased, That amp for it's entire life was running or sounding no where near as good as it was intended to sound.

Going to have to replace resistors in the bias network in order for it to bias correctly. Plus, gonna order a choke for it.

I wonder how many other early 2500s are running in this manner. Anyway, I attached the pics.

 

[Sg-ocaster]

Nothing to do with bias. But you mentioned faded grill cloth. I have a Laney cab that is esentially a 1960A with garbage speakers Im redoing. I took a rag wetted in WD-40 and wiped the whole grill,looks great afterward.

 

[Central Scrutinizer]

Man, you could have grown potatoes in that buildup of topsoil!
Looks good after your cleaning.
Glad you got it running well now.

 

[Dogs of Doom]

hate to tell you but, tone...

is in the dust!

you took away it's mojo!

...

I wonder why the bias was so out of whack. Did you measure the resistors?

 

[Tatzmann]

I have a Laney cab that is esentially a 1960A with garbage speakers Im redoing.

The usual modern Laney cab is nothing like a 1960, besides dimensions maybe.

They use wooddust material and have no fingerjoints. Very few i've seen were made out of plywood, but no fingerjoints. (Sorry for derailing, op)