Wow, thank you all for the responses!  I didn't expect to get this much input on a rather oddball question.

This is intended only for home use, it's not like I'm going to be using this amp to play with a band.

I have a Marshall 3505 MicroBass (that I was never really happy with) that is rated at 30W into a total load of 4 ohms and uses the same output transistors, but with different heat sinking.  I've attached a photo for reference, but it doesn't seem radically different to that of the Lead 12.

Wondering if you think there would be a benefit to trying to swapping the heat sinks for the ones in the 3505?  Due to the slightly different orientation of the transistors on the board, the heat sinks might require slotting the mounting holes.

Another potential option would be to fabricate my own custom heat sinks.  That would allow me to keep the hole locations to suit the existing component orientation, but add more fins and make them much taller as there's plenty of room in the cabinet.

Should I be concerned about the transformer?  The 3505 transformer is larger than that of the Lead 12.  Would it be worthwhile swapping it?

Thanks again.

I've just purchased a Lead 12 head (3005) to use as a bass amp into two 8 ohm cabinets for a total load of 4 ohms.  I know it works and there is no immediate damage because I used it for about 30 minutes today.  It wasn't too loud as I was just playing around to see what tones I could get and I was a little unsure of whether I would hurt anything, but it sounded just as I had hoped it would.  Everything I've read on this site about this amp leads me to believe that the power section of this amp can handle the lower load, so is it OK to use this head (rated for 8 ohms) into a 4 ohm load for extended periods of time at louder levels?  If so, are there any things I can (or should) do in order to improve the long-term durability while running into lower load?

Yeah, I wasn't sure what to make of the Vac readings either.  I'm using a digital multimeter, nothing fancy, but it has always given reliable measurements.  I can try re-measuring some of these values.

Based on the difference in the resistor readings, I'm suspecting bad caps.  I've probably gone as far as I can without taking some components out of the circuit, so this may have to wait a little while before I can devote some more time to it.

I measured the DC voltage before the 2.7k resistors and both + and - were nearly identical at about 22.7V to ground, 45.3V between them.

I could not measure a stable AC voltage before or after the 2.7k resistors.  The readings continuously ranged from a few hundred volts down to less than 0.1V.

The DC voltage after the 2.7k resistors measured the same 13.9V & 10.2V as I had previously measured, but this is in-circuit so I don't know whether it's due to the zener "dumping" too much voltage to ground or the resistor dropping too much voltage before it gets to the zener.

So I also measured the voltage drop across the 2.7k resistors and this accounts for the difference.  The voltage drop across one is 8.8V and the other is 12.5V.  These add up as you'd expect: 13.9V + 8.8V = 22.7V and 10.2V + 12.5V = 22.7V.

Then I measured the resistance of the two 2.7k resistors again and found that the resistance of the one on the side with 13.9V started significantly lower than 2.7k, but slowly rose to over 2.6k.  The resistance of the resistor on the side with 10.2V read a very stable 2.63k immediately and its reading did not change.  Does this indicate that the capacitor on the 10.2V side is bad?  Since these measurements are all made in-circuit, I would assume that the measured resistance of the resistor should change as the meter charges any connected capacitors.  Does this sound right?

I am planning on putting the 5002 preamp into the 3505 chassis, but it will require a new board since the circuit is quite different.  So I can't put the new components into the existing 3505 board and there will be some additional length of wire to connect between boards and from the new preamp board to the pots.  I'll have to try moving things around to see whether the squeal changes as I move the long jumpers that I currently have running between the breadboard and the existing 3505 board.

As for the op amp supply voltages, the zeners are in place.  I've tested the resistors in-circuit with a multimeter and the 2.7k resistors seem to measure right around 2.7k.  With the power on, the two 15V zeners measure 13.9V and 10.2V and the other 9.1V zener measures 8.6V.  It does seem like one of the 15V zeners is faulty; what effect would this have on the sound of the amp?

On the advice and encouragement of members here, in an attempt to get a more guitar-like tone from my bass amp, I have prototyped a Marshall 5002 (Lead 20) preamp on a breadboard and run this into the power amp of my Marshall 3505 (Micro Bass).  The result is everything I hoped it would be ... brighter tone, clearer mids, and a really nice overdriven distortion at higher gain settings.  Due to parts availability, I used slightly different pot values from those shown on the schematics.  I used 25k instead of 22k pots for the gain (VR1) and mid (VR4), and a 250k instead of a 220k pot for the treble (VR3).  I've attached the schematics for reference - the 5002 is the top portion of the hand-written schematic showing multiple amps.

There is just one issue that I need to fix before I make this a permanent modification.  There is a high-pitch squeal when I have the gain, treble, and mid all at maximum (10).  The squeal changes slightly in pitch and loudness as these three controls are slightly lowered, and is eliminated at gain settings below about 8.  By changing the capacitor in the second-stage feedback loop (C2) from 220pF to a much higher value like 0.01uF (10,000pF), the squeal is also eliminated, but all the treble is gone and the amp sounds very muddy.  I've tried adding capacitors in parallel to gradually increase this value in hopes of finding a compromise between eliminating the squeal and retaining some brightness, but it seems that any values that are high enough to eliminate the squeal also take too much treble out of the tone.

I'm assuming a stock Marshall 5002 does not squeal at any combination of gain, treble, and mid settings.  Is there some interaction between the 5002 preamp and the 3505 power amp that is different than the stock 5002 which would cause this preamp to behave differently in front of the 3505 power amp as compared to its intended combination.

Are there circuit changes (different value components, additional components, etc.) that I could make to eliminate the squeal while retaining the current clear, bright sound at the highest gain settings?

One final question, possibly unrelated to the squeal issue.  I measured the supply voltage to the op amp and found that it measures -13.9V and +10.2V.  These values seem quite different than the +/- 15V specified on the 3505 schematic.  Should I be concerned about this even if it is not related to the squeal?

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?

I had an amp with terribly scratchy controls that I was able to clean up with DeoxIT and now it works great.  If the crackling isn't too bad, you may be able to get rid of it simply by rotating the knobs back and forth a bunch of times as they tend to get scratchy when they're not used regularly.  If it's worse try spraying some DeoxIT into each pot and then rotating the knob back and forth a few times.  You don't need to spray very much into each pot and be sure to clean up and dry off any residue or overspray.  If that still doesn't fix it, you may need to replace the pots.

As for the orange spots, it's tough to say without seeing them, but it sounds like it could just be some rosin flux residue from the solder.  If that's the case, I would not consider it to be corrosion.

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?

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.

After initially thinking there was something wrong with this amp, it does seem to be working just fine - it just doesn't have any breakup at reasonable home volumes.  So I'm looking for your thoughts on whether it is possible to get a more over-driven sound (distortion, fuzz, dirt, overdrive, gain) from this amp.  If so, what specific changes would you recommend this electronics novice start with?

The only control that I did not previously have set to maximum was the volume, but with everyone out of the house this evening I plugged in a guitar with a humbucker, turned the volume up to full, and played around a bit.  I was quite surprised at how much difference it made!  Although certainly not high-gain, there is a nice, gritty breakup that was not present at lower volume.  So I don't think there is anything wrong with the amp and I now understand how the tone in the video was achieved (exactly like he said it was!).

I asked Marshall for the schematic for this amp and they provided it to me, so I'd like to get some feedback on whether it would be possible to get some more distortion at lower volumes from this circuit.  I'll create a new topic with the schematic so as not to confuse members with my (now pointless) comparison to another amp.

The "multi drawing" is the Marshall 5502 Schematic PDF file attached to Reply #2.  It includes the schematic of the 5502 which has been reported as being the same as the 3505, as well as a few other related amps.

The 3505 preamp schematic is one that I drew myself simply copying the schematic of the 5502 preamp, but re-numbered using component numbers from the board of my 3505 and their component values.  I've checked the connections on my board according to this schematic and it all appears to match.  This is what led me to conclude that my 3505 preamp is the same as the 5502 preamp, I just cannot explain the lack of overdrive in the 3505 as compared to the video of the 5502.

Any thoughts or feedback are very much appreciated.

Enzo, thanks again for spending so much time going through this with me.

It does seem that the preamp of my 3505 is the same as that of the 5502.  Even the component values "seem" to be the same from what I can make out on the handwritten schematic.  In fact, that is what does NOT make sense to me ... if the preamps are the same, why does my 3505 sound SO different than the 5502 in the video?  I can understand that my perception of the treble content can be affected by a multitude of things such as speaker, microphone, room, post-processing, etc, but my amp simply has none of the overdrive of the 5502 in the video even though the preamps appear to be the same.  This, more than the treble content, is what I would like to change about my 3505.

I guess I was hoping that some component value changes could get a similar degree of overdrive from my amp.  I'm afraid that even with your encouragement to "live dangerously", I'm still too much of a novice to take on a conversion to the guitar preamp ... maybe some day, but I'm not there yet!

As for the case of the missing ZD1 & ZD2, while it makes sense that the power amp is not really contributing to my tone issue, I didn't find these two in the other power amp shown on the multi drawing either.  I did, however, notice that they are shown on the schematic of Marshall's 3510, which is a 100W amp from the same product line as my 3505, and they are exactly where I thought they might be (in parallel with the two capacitors at the transition from preamp to power amp).  Just out of curiosity, what function or effect would these two zeners have in this circuit?

I've re-numbered the components from the 5502 schematic to match my 3505 and checked all the connections on my board with a meter.  As far as I can tell the attached PDF is an accurate schematic of the 3505 preamp.

Based on this, do you have any thoughts on how to increase the overdrive and perhaps add some treble?

Should I be looking beyond the preamp to achieve this?