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OK to run Marshall Lead 12 into 4 ohm load?

Started by Dino Boreanaz, November 25, 2018, 07:24:54 PM

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Dino Boreanaz

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?


The output transistors are ok with the higher power, but the heatsinks are a little small.
All I can suggest is try it and see how hot it gets.  If you cannot hold your fingers on the heatsinks for a couple of seconds then they're too hot.


If the specs reads 8 Ohms then,, Yes it's ok right up until it burns out. :lmao:

As you mentioned BASS then even more reason to heed the label as Bass puts a lot of stress on power devices. :-X
Yes the power transistors are way over spec but as *Tonyharker* noted the heat sinks are NOT!!  xP
It's your Amp,, so you roll the dice.  8|

Jazz P Bass

It's a 12 watt amp into 8 ohms.

You're really going to play a bass guitar through that?

Into 4 ohms, the amp will be expected to supply 24 watts into the load.
IF you play it all the way up, it will fail.


Agree with the others, the heatsinking is probably not adequate for sustained high volume use at 4 ohms.
If nothing else, a fan blowing on the heatsinks with some vents to let the air out of the chassis should help.  The gamble would be whether it helps enough.
If it were mine, I'd build a cable to run the 2 cabs in series for a 16 ohm load.


As to running beyond the specs...  Getting away with something does not make it a good idea.

Dino Boreanaz

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.


If your amp has 2x8ohm speaker outputs which are paralleled, it will be certainly OK to use 4ohm load. However, it'll be dangerous to use 4 ohm load on amplifiers which have only ONE 8ohm output-The current going thru the transistors will be doubled and they'll overheat and, plausibly, burn themselves down under such a current while using original cooling systems so using larger heat sinks and adding fans might be the first thing essential to be done. Replacing a transformer may help to prevent overloading and burning among the area with mains voltage but NEVER use any transformers with higher output voltages. Use ones with the same voltage and higher output current instead. In addition, don't forget to check the rectifier circuit and the fuse to prevent potential overloading.


Quote from: 66cccfff on November 28, 2018, 08:44:51 AM
If your amp has 2x8ohm speaker outputs which are paralleled, it will be certainly OK to use 4ohm load.
Not when the minimum load is 8 ohms.  Often the output jacks have 8 ohms written above but they have 2 jacks so that you can use two 16ohm speakers.
That is the case here.  Above the speaker jacks it says 8 ohms.
Below the jacks, it says "Do not use loadspeakers of less than 8 ohms.  IE. - 1 x 8 ohm or 2 x 16 ohm."

When there are parallel speaker jacks, and only a single impedance stated, assume it refers to the minimum total load impedance, not individual speakers (unless stated otherwise).

Dino Boreanaz

Quote from: g1 on November 28, 2018, 11:05:30 PM
That is the case here.  Above the speaker jacks it says 8 ohms.
Below the jacks, it says "Do not use loudspeakers of less than 8 ohms.  IE. - 1 x 8 ohm or 2 x 16 ohm."
This is correct for the Lead 12 that I'd like to use.

The rear panel of the 3505 head says ""Do not use loudspeakers of less than 4 ohms.  IE. - 1 x 4 ohm or 2 x 8 ohm."  So I'm just wondering what I can do to modify the Lead 12 head to work with the output impedance that the 3505 is designed for.  The heat sinking is the obvious difference and I'm certainly planning on improving that area.

Thanks also for the caution about swapping the transformer and the suggestions to check the rectifier and fuse as well.

I'll compare the two schematics again to identify any differences, but from what I recall the Lead 12 and 3505 have extremely similar power section circuits and components.

Dino Boreanaz

I've done some more investigation that I'd like to get your feedback on.  I'm thinking about making changes to the power stage of the Lead 12 so that it matches the power stage of the MicroBass since the MicroBass is intended to be used with a 4 ohm total load.  On the attached schematic of the power stage I've shown the component differences between the Lead 12 and several 20W and 30W Marshall solid state amps.  Some things came to my attention that led to some questions:

- In some cases (C8, R12, C14, & C15) the two Bass amps use one component value while the two Lead amps use another component value.  Is it reasonable to think that these differences are related to the "tone" rather than the power output?

- I measured and compared the secondary voltage of my Lead 12 and MicroBass amps.  The MicroBass is about 17% higher.  Is it correct to assume then, that the MicroBass power stage operates at rail voltage that is about 17% higher than the +/- 19V shown on the Lead 12 schematic (about 22.2V)?

- The schematics of the 20W and 30W amps all show the same T4937 transformer while the Lead 12 uses the smaller T4942 transformer.  Is it the higher rail voltage that results in the greater power output?  Considering these four amps as "known" quantities, the lower rail voltage of the Lead 12 into an 8 ohm load produces 12W, the higher voltage of the Lead 20 and Bass 20 into an 8 ohm load produces 20W, and finally the higher voltage of the MicroBass into a 4 ohm load produces 30W.

- Where the Lead 12 uses 1K resistors from the +/- 19V rails the others all use 2K7 resistors.  This difference seems to correspond very closely with the different voltage drop if I assume the power stages operate at 22.2V and drop to the same 16V preamp rail voltage as the Lead 12.

- R18 has me a little puzzled.  Here the two Bass amps use the same component value, but the two Lead amps do not.  Furthermore, the Lead 12 uses a lower resistance than the Bass amps, while the Lead 20 uses a higher resistance than the Bass amps.  So I'm not sure how to determine an appropriate value here.

- There are two component differences between what is shown on the MicroBass schematic and my MicroBass amp.  While the schematic shows the use of a 500mA fuse (same as the schematics for all of these amps) the board of my MicroBass states that it should be a 1A fuse.  And the schematic shows a W005 rectifier while my amp has a 2W02 rectifier.  So I'll probably go with the higher voltage 2W02 rectifier and the 1A fuse.

- Lead 12 schematic shows that C17 & C18 are 25V electrolytic capacitors.  Is this too close to the higher 22.2V rail voltage with the 30W transformer?  Should I replace these with higher voltage caps if I swap the transformer to produce the higher rail voltage in trying to mimic the power stage of the MicroBass?

Thanks as always for everyone's input.  I'm really enjoying the learning process with your guidance and advice.


Without getting into the component values, the one thing you have overlooked is the current capability of the power transformer.  That is where the extra power must come from, the little bit of extra voltage will not do it without extra current capability to drive the lower impedance load.

Dino Boreanaz

That's what I suspected and that's why I've been looking into using the transformer from the 3505 MicroBass as it's intended to produce 30W with a 4 ohm load.  It's also the same transformer shown on the schematics of several other Marshall solid state amps in the 20W to 30W range some of which drive 8 ohm loads and others that drive 4 ohm loads.  Do you think my intention of trying to replicate this power amp for use with my 4 ohm load makes sense in this case?

Dino Boreanaz

Just wondering now ... would it be better to replace the Lead 12 transformer with a higher VA rated unit,  but the same secondary voltage (so as to avoid changes to the rest of the power stage circuit) along with improving the heat sinking of the output transistors?

Jazz P Bass