Jubilee 2555 theory of operation

[Mxzx]

I own an original 2555 and would just like a laymans explanation of what is going on with the circuit in regard to gain structure, and the pull knobs.

From looking inside, the "pull lead master" is utilizing the LED clipping.

Also, why does pulling the "rhythm clip" lower the volume so much? Is that normal?

Thanks!

 

[thegaindeli]

If you do a search, there are plenty of web resources that explain the SJ circuit. You will hear a slight drop in volume when switching diodes. The "clean" setting = diodes lifted.

Tip: For the best SJ tones; toss the G12T-75's (if that's what you have), and load the cabinet with a quad of 444 coned (55Hz) Marshall "Vintage" speakers. These are what were used in the original Silver Jubilee AV/BV cabinets. I don't think Marshall sells these separately, so you may need to find a donor cabinet.

 

[Jethro Rocker]

My understanding is that Lead channel always uses the diode clipping. Go from Ckean to Lead channel, it kicks in and possibly another circuit with V2. Pull Rhythm Clip and diode clip activated for Clean channel which lowers overall volume some.

 

[Mxzx]

I did not see the LED's light unless the Lead was pulled?

 

[thegaindeli]

I did not see the LED's light unless the Lead was pulled?

I'm not 100% certain, but I believe that's because the diodes are lifted out of the signal path. I'm going to take a gandy at the SJ drawings...

 

[Mxzx]

Thanks

 

[dreyn77]

don't worry about the amp makers craft. or the pedal makers craft.

it's just what happens and makes up their electronic bag of tricks.

if you've noticed it, then it's happening but it must be there inorder to get the sound effect they were after at the time of manufacture.

if the volume sound seems to drop, then it drops and don't worry about it.
if it adds noise, then it adds noise and forget about it.

BUT maybe there's another product you could have bought which does the same sound but without the 'problems'.

for example, don't buy the hendrix pedal method if you can't stand some noise. buy the kramer guitar method and you won't have a problem.

 

[mickeydg5]

The LED on the front is an indicator only.
The LEDs in the LEAD channel are only for that channel along with several diodes.
The RYTHYM channel has its own diodes for clipping.

There will be a bit of volume loss when engaging RYTHYM Clip. The diodes "clip" the signal. Amplitude equates to volume so if you remove the tops from AC signal (clipping) you are cutting the volume down. That is why Marshall did not make this function foot switchable being it is offered as a simple choice at the control panel. Incorporating foot switchability would have required more parts including another volume type control.

 

[plexilespaul]

There will be a bit of volume loss when engaging RYTHYM Clip..

hardly noticeable on mine.

 

[Mxzx]

Yes, I'm talking about the 2 LED's on the board. I know the panel is just an indicator.

So when neither of the knobs are pulled, there is no diode clipping going on?

Thanks