weak preamp overdrive on 2555 silver jubilee

[Satamax]

There's also C21 which is replaced by a diode!!!!!!! WTF.??.?

[mocklah]

here's the 2550 version. preamp section should be the same, right?

[mocklah]

There's also C21 which is replaced by a diode!!!!!!! WTF.??.?

man, you guys are startin' to scare me!

can any of you guys post pics of your jubilee boards so i can just compare and copy?

[mocklah]

i may have found a schem that's more like matt's & mine.
it's pretty hard to read, but at least the bais resistor says "R22" as opposed to "R23" on all the other schems.

click pic for larger version

[Ray Ivers]

Mike,

I've never owned a Jubilee amp, and the ones I worked on were all a long time ago. I couldn't find any 2555 pix on acplink.com, so that wasn't much help, either. Maybe someone here can pony up some PCB shots for you.

I haven't compared the 2550 and 2555 preamp sections so I don't really know if they're absolutely identical. And as Matt pointed out, I guess it wouldn't really matter anyway...

I sure wish I had an easy way out for ya, but I don't - it sounds like this amp's been tweaked at least a little, and it may not be all that easy to sort out. You may luck out, though, if the stuff that's been pointed out so far is all that's been changed - and if not, it's amazing what a really good visual inspection can turn up, usually in a very short period of time.

Ray

[mocklah]

thanks ray.

there's a few guys on the vintageamps.com forum that have jubilees...i'll ask them to post pics too.

[Enzo]

The 2555 and 2550 share the same pc board, and the preamps appear to be identical when I compare drawings. I am looking at the "STD" versions.

Is 430 all the more the B+ is? I would have expected maybe 50 more volts.

[Ray Ivers]

Mike,

Hey, thanks for that Rev 3 (June '88) print! I would definitely use that one to go thru your amp, as it's apparently the correct one for the one you have - does yours have a date tag?

Ray

[Satamax]

And on this schemo the R29 is a link. And the diode which is in the cap holes might be D8.

[Matt T.]

Sorry I get didn't get back to this thread, I don't have a digicam and couldn't post pix anyway. That's cool you found that different schematic. Regarding the B+, my black tolex 2555 runs at about 455vdc. Just to save time/posts, here's the measured voltages in my 2555 based on a wall voltage of ~122vac.

The 5 nodes are at:
454vdc
450vdc
385vdc
345vdc
327vdc

The preamp plates/cathodes are (starting with V1a):
250v/2.1v
220v/1.6v
235v/1.7v
135v/0.5v

PI plates/cathode are:
263v/253v/41v


HTH.

[R ski]

low power

Lots of good responses

The question, is where is the lost signal?

The quick method which is brutal but will reduce the guess.

The power amp should easily dump 38 volts AC across a 25 to 50
watt power resistor for a short time when lifting off the capcitor
from the master volume and touching the capacitor.

A speaker can be used and the cone will dance will work for a short
period. A speaker outside a cabinet is less painful. I use a old 6 X 9
when determining the power of an amp. If the voltage is low the power
amp is probmatic.

[AintNoEddie]

Hi there! I read your thread in the Plexi Palace and thought I'd sign up here to help a fellow Silver Jubilee user

Here are a couple of good shots from a fellow at the Marshall Amp Forum - it's the Slash model, but it should be the same as yours.

http://www.cryoflove.co.uk/my2555SL/2555sl.htm

I could also post pictures of my 2550 Jube.

FYI, have a look at the two red LEDs inside of the amp. They should noticably glow once the Lead channel is activated. It's probably a bit difficult to see it in daylight, but clearly visible in darker rooms.

Does your Lead Master work properly?

In my 2550 the small switching relais (RL1) was malfunctioning. One pin didn't work, it was strange as the channel LED did go on/off, but the preamp distortion was pretty f*cked up. Once I replaced it everything was working flawlessly again.

Have a look at the amp pictures, and if you want further info on the relais don't hesitate to ask!

Cheers,
Mike

[Matt T.]

One pin didn't work, it was strange as the channel LED did go on/off, but the preamp distortion was pretty f*cked up.

That would make sense...the switch itself both energizes the coil of the relay and turns on the LED but the contacts inside the relay are what actually switch the channels. The Lead Master wouldn't work if it didn't actually switch channels.

[mocklah]

hey, thanks for the ongoing help guys!

Mike, thanks for that link. those are some good pics to study.

i played the amp at rehearsal last night & it sounds petty good cranked. but still not the preamp saturation/compression that i had in my old 2553.

i'm startin' to think my new amp is such a low serial# (00628), that it's a pre or early revision. and maybe some of the future revisions increased the gain. like how satamax noticed in my pic, there is a diode where a cap goes, but revised schems show a D8 there.

i wish could remember the serial of my 2553. might be on the guitar center receipt if i can find it.

...but i'm still gonna look into all the suggestions you guys posted...i actually enjoy the troubleshootng process.

-mike

[AintNoEddie]

Hi again!

In case you haven't already got these, you can different 255x schematics at

http://www.marshallschematics.com/

In the case of my amp, the 2550, there are two versions, one for the earlier models and one for ser.no 3100+. I have the later version. Maybe you can find some board revision details here?

Mike