Welcome to Solid State Guitar Amp Forum | DIY Guitar Amplifiers. Please login or sign up.

March 29, 2024, 07:46:49 AM

Login with username, password and session length

Recent Posts

 

Prototyping a Different Preamp in an Existing Amp

Started by Dino Boreanaz, January 15, 2018, 02:10:59 PM

Previous topic - Next topic

Dino Boreanaz

On the advice of several members here I am going to try converting the preamp in my Marshall 3505 to the preamp of the Marshall 5002 in order to get a more guitar-like tone from this bass amp.  I've attached the schematics of both amps for reference (the 5002 is at the top of the multi-amp drawing).  This will be the first time I've tried anything like this so I have a few questions and concerns:

Can I desolder one leg of the input R1 and R2 on the 3505 board and jumper these to a prototype board so that I can build the 5002 preamp on a prototype board?

Can I desolder one leg of C3 on the 3505 board and jumper this to the wiper of VR2 (volume pot) for the output back to the power amp?

Can the +ve, ground, and -ve be jumpered to a prototype board from any convenient connection point on the 3505 board or should these come directly from the diode bridge and transformer?

Should I be concerned that the volume pot of the 3505 preamp is 4.7k while the volume pot of the 5002 preamp is 1M?  This seems like it would result in a very different input level to the power amp.

Will 1/4 watt, carbon film resistors be OK for all resistors in the preamp?

What type of capacitors are required for the various preamp capacitors?  Disc, electrolytic, mylar, silver mica, orange drop?

What voltage rating do these preamp capacitors require?

Do any (or all) of the 3505 preamp components need to be removed from the 3505 board in order to test the prototyped 5002 preamp?  In other words, can I simply connect the prototype 5002 preamp in place and effectively bypass the original preamp?

Just out of curiosity, on the 3505 schematic what do the arrows on one side of ZD2 & C13 mean?  Does this indicate a connection to ground?  These two components are at the top, centre of the 3505 schematic, but I've seen arrows like this in other schematics and wondered.

Sorry for the long post, and thanks in advance for any feedback and input you can offer.

phatt

Just be careful as I think the input stage of the 5002 is drawn wrong.
The output of U1 is grounded and looks like pin 5 of U2 is also grounded.
if you follow the ground (circuit common) path it wraps up around the top left corner and back to r3 ?? I might be missing something that better minds can see but to me the signal would be shorted after U1 so as drawn would not work. :loco

If it was me I'd just bread board a simple circuit and plug it straight into the input of the 3505 which will save you a whole lot of messin and give identical results,, give or take an electron of mojo. 8|
Phil.

Dino Boreanaz

Thanks for the reply, and I see what you mean about the ground path.  Is it possible that the ground path from pin 5 around the top-left corner is open when something is plugged into one of the inputs?  The reason I ask is that this portion of the 5002 preamp appears VERY similar to that of the 3005 preamp which is a much clearer schematic directly from Marshall (I'll attach it for comparison).

Since my intention would be to modify my 3505 with the preamp from the 5002 whose tone I want to mimic, I was thinking of bread boarding a replacement for the 3505 preamp and running this into the 3505 power amp ... I'm just not clear on how to actually make/break the necessary connections.  I don't want to go the route of using additional pedals or boxes, so I want to ensure that the existing 3505 preamp is entirely out of the signal path.  I didn't think this would be possible if I plugged straight into the input of the 3505.  Is there a way I can do this with minimal disturbance to the existing board and components?

phatt

Quote from: Dino Boreanaz on January 18, 2018, 01:30:04 PM
Thanks for the reply, and I see what you mean about the ground path.  Is it possible that the ground path from pin 5 around the top-left corner is open when something is plugged into one of the inputs?  The reason I ask is that this portion of the 5002 preamp appears VERY similar to that of the 3005 preamp which is a much clearer schematic directly from Marshall (I'll attach it for comparison).

Arrh yes you are right :tu: that now makes sense,, the circuit is muted when you unplug.
Phil.

phatt

#4
Quote from: Dino Boreanaz on January 18, 2018, 01:30:04 PM

Since my intention would be to modify my 3505 with the preamp from the 5002 whose tone I want to mimic, I was thinking of bread boarding a replacement for the 3505 preamp and running this into the 3505 power amp ... I'm just not clear on how to actually make/break the necessary connections.  I don't want to go the route of using additional pedals or boxes, so I want to ensure that the existing 3505 preamp is entirely out of the signal path.  I didn't think this would be possible if I plugged straight into the input of the 3505.

Well lift the input end of C3 (on the 3505 circuit) which is directly after the volume wiper and connect the new preamp output right there.

Quote from: Dino Boreanaz on January 18, 2018, 01:30:04 PM
Is there a way I can do this with minimal disturbance to the existing board and components?

Well No tiss not a simple task for the novice :-X
You will have to build the whole circuit with tone controls on a BBoard then break into C3.
The advantage of Bboarding the 5502 preamp into front end of the 3505 circuit is that you will have a whole new set of tones and distortion to muck around with and you may find an even better sound. Even just building the first stage will give the amp a different sound/ tone/distortion. you would have two tone sections and far more drive. Hint. 8)
Of course front panel space will dictate what is possible. xP

IME of friggin around with circuit ideas (which is many years now :duh)
I wanted for years to have it all in one Amp rig but for the sounds/tones folks want today I'd say you have a better chance of being hit by lightning. :lmao:
I've reverted to a simple Amp and all the fancy stuff is done with pedals. Multi FX boards are way to complex you are better off with dedicated pedals. Of course that comes with the pain of carrying the extra gear and you then need floor space on stage which can be a problem.
Phil.

Dino Boreanaz

Hey Phil, thanks so much for your feedback.

Just to clarify though, I'm not planning on running both preamps or trying to make some ultra-versatile, do-it-all amp with two complete sets of controls.  If I like the sound of the prototyped 5002 preamp, I'll make a version of the preamp that I can permanently replace the existing 3505 preamp with.  So the front panel would remain the same as it is now with its two inputs, gain, volume, and tone controls.

Any thoughts on the type of resistors and capacitors I need for the 5002 preamp?

What about the huge difference in the volume pot values?  Do you think this is simply due to the difference in the impedance of these two different preamp circuits and the net effect is that each feeds the power amp with approximately the same signal level?

phatt

That's OK,, I'm just giving you options.
Unless I've missed something,,,where your plan will come undone is that there is only ONE pcb which contains both preamp and pwr amp and all pots are pcb mounted
So if you wish to use a new preamp you have to make a whole new PCB.

Which is why I'm pointing out to just BB the front end of the preamp you want and plug into the front of the other. then consider your options before you mess up a working amp.


I quote you from the other thread;
Quote from: Dino Boreanaz on January 06, 2018, 01:38:38 PM
I'll go through the component numbers in detail later today, but is it possible that this accounts for the differences in component numbering between the 5502 and my 3505?  If so, would it be possible to make the 5502 preamp by substituting the appropriate components on my 3505 board?

Short answer; A BIG NO!!  For the reasons above. :duh
IF the two circuits used the same tone circuits then it would be possible but you would have to cut a lot of tracks to do it.

If there was enough room inside the chassis then you could build a new preamp PCB and use the same holes for the pots but from the pictures it looks like there is no room left for the original PCB to be moved back.
Phil.