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Author Topic: Marshall 3005/Lead 12 Power Trans Question  (Read 22139 times)

J M Fahey

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Re: Marshall 3005/Lead 12 Power Trans Question
« Reply #15 on: August 14, 2015, 04:05:41 PM »
Quote
I'm seeing more & more posts in various forums about owners in search of replacements for whatever reason.
Not surprised at that.
People has itchy fingers and want to  "do something" , problem is the longest living part in those amps is the transformer (unless amp shorts and fuse is replaced by a nail ) , it's also properly rated so there's nothing to gain by going to a larger one.
Also does not wear, so maintenance is out of the question, longevity is granted unless grossly nuked the way I mention above.
Transformers are one of the very few examples of a perfect machine designed and built by Humans.

Spend that money in a 4 ohm Jensen MOD1050 speaker instead.
http://www.musiciansfriend.com/accessories/jensen-mod10-50-50w-10-replacement-speaker

Or similar 10 inch Eminence or Celestion if available in 4 ohms.


Clyde

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Re: Marshall 3005/Lead 12 Power Trans Question
« Reply #16 on: August 14, 2015, 06:59:02 PM »
I have a 5005 combo that has a Celestion G10N-40 in it.  I wouldn't change a thing on that amp.  Love the sound. 

But now a question regarding speaker impedance.  The 5005 combo & the 3005 head/stack versions of these amps profess to have 8 ohm output load (the micro stack being 2 ea 16 ohm/parallel).  I was always of the understanding that in the world of solid state amps one never went below the 'rated' output impedance, lest the output transistors/power amp become toast.  These amps are ok at 4 ohms?

J M Fahey

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Re: Marshall 3005/Lead 12 Power Trans Question
« Reply #17 on: August 14, 2015, 10:41:52 PM »
Sorry, i should have been more detailed in my answer.

In 90% of SS amps, it's not safe to go below rated impedance (some have an extra power  and thermal reserve as in  Peavey) because it may put out some extra power if the supply allows so, but transistors can't handle it.
One big problem is that above rated current, transistor gain Hfe drops like a brick so asking double colector current may ask for 4 or 5 times more base current and you typically nuke the drivers ... milliseconds later the power transistors also die.

That said, in *this*  particular "12W" amp they used a 100W power stage  :duh

See for yourself:

Marshall 2200 a.k.a. Lead 100
100W RMS into 4 ohms, +/-40V rails, beefy enough to drive an output autotransformer , a single complementary pair TO3 metallic Motorola MJ2501/3001 (10 ampere, 150W 80V):


They used basically the same circuit , definitely same transistors in the 50W amp, then in the 30W into 4 ohms amp and this latest one was loaded with 8 or 16 ohms to tame it down to nominal 12W.
So one reason why this "little 12W amp sounds so BIG" is that it has a supercharged V8 engine under the hood ;)

30W @ 4 ohms:


so yes, the incredibly overbuilt "12W" amp , in this case, can easily drive 4 ohms loads and I always use them that way, sorry I forgot to explain why.
See that both use the exact same supply filters (2200X25) and roughly same voltage rails, the 12W uses +/-19V , the 30W unspecified but I guess 20/22V rails:


Also heatsinking is about the same:



EDIT: in a nutshell:
a) they originally use 8 ohms speakers, (or 2 x 16 ohms)  so if you have original speakers or replaced them with other brand 8 ohms ones, fine, you have changed nothing except improve sound quality.
b) the amp design and parts used stands way more, I have personally loaded them with 4 ohms and got around 20V with no ill effects.
The transformers I tested were fine, warm but not *hot*  .
Same with heatsinks.
c) in theory you can turn that amp into a 50W or more one, *if* you provide better supply (beefier transformer and higher voltage electrolytics) and mount transistors in a "real" extruded finned heat sink, and adjust dropping resistors values to still feed +/- 15V to preamp.

I haven't modded original ones but straight built new ones, with the beefed up supply and heatsinking I mentioned, for killer 50/60W combos.
« Last Edit: August 14, 2015, 10:58:45 PM by J M Fahey »

Clyde

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Re: Marshall 3005/Lead 12 Power Trans Question
« Reply #18 on: August 15, 2015, 12:30:33 AM »
And here I thought I knew something about these.  Many thanks for taking the time to post the details.  I never grasped the design relationship of these to their 50/100w cousins.  Supercharged indeed. 

Beautiful explanation.  I remain in awe...but learning.

Thank you sir.

J M Fahey

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Re: Marshall 3005/Lead 12 Power Trans Question
« Reply #19 on: August 15, 2015, 09:24:44 AM »
You're welcome :)

I got *very*  intrigued when I found two TO3 metallic 150W transistors in a 12W rated amplifier, my only explanation is that they bought a Million for a very good price and then the 50/100W amps didn't sell, so they had to get rid of them one way or the other.

Or same happened to Motorola/ON and *they*  gave Marshall very good price and conditions.

The market for TO3 metallic Darlingtons dried up when excellent TIP142/147 became massively available .
Basically same specs, except somewhat less dissipation, they became the muscle in tons of Valvestate/Fender/Laney/H&k/Crate/Ampeg amplifiers.
I also use them ;)

just gear gas

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Re: Marshall 3005/Lead 12 Power Trans Question
« Reply #20 on: October 19, 2017, 03:43:14 PM »
Hi J M Fahey,
I came across this old thread and saw your knowledge about these amplifiers so maybe you can help me with one question:
I have a Marshall Reverb 12 that I want to modify to a head because mine comes withe  the 8" Marshall S300 Speaker wich sucks... The reverb 12 has you know shares the same basic circuit of the lead 12 with the addition of the reverb circuit but unlike the lead 12 is rated output is 12 watts into 16 ohm, I´ve  compared the schematics of the Lead 12 and the reverb 12 and, with the little knowledge that I have. the only difference I see is that r11 and r6 of the power amp section on the lead are ratted 1k and on the reverb (R35 and R36) are rated 220 on the Lead ther is also a condenser C6 ratted 22/25v that I cant find on the Reverb... Also the lead transformer voltage is 14,3 V AC  and the reverb is 21 V AC acording to both schematics.
 I would like to modify the Reverb circuit to 8 Ohm speaker output, since I only have 8 ohm cabs and I will be using the amp at full blast not bedroom level I want to make sure that I will not blow anything...
Can you help me on this?
Thank you

just gear gas

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Re: Marshall 3005/Lead 12 Power Trans Question
« Reply #21 on: October 19, 2017, 03:46:33 PM »
Hi J M Fahey,
I came across this old thread and saw your knowledge about these amplifiers so maybe you can help me with one question:
I have a Marshall Reverb 12 that I want to modify to a head because mine comes withe  the 8" Marshall S300 Speaker wich sucks... The reverb 12 has you know shares the same basic circuit of the lead 12 with the addition of the reverb circuit but unlike the lead 12 is rated output is 12 watts into 16 ohm, I´ve  compared the schematics of the Lead 12 and the reverb 12 and, with the little knowledge that I have. the only difference I see is that r11 and r6 of the power amp section on the lead are ratted 1k and on the reverb (R35 and R36) are rated 220 on the Lead ther is also a condenser C6 ratted 22/25v that I cant find on the Reverb... Also the lead transformer voltage is 14,3 V AC  and the reverb is 21 V AC acording to both schematics.
 I would like to modify the Reverb circuit to 8 Ohm speaker output, since I only have 8 ohm cabs and I will be using the amp at full blast not bedroom level I want to make sure that I will not blow anything...
Can you help me on this?
Thank you

J M Fahey

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Re: Marshall 3005/Lead 12 Power Trans Question
« Reply #22 on: October 20, 2017, 03:21:23 AM »
Just saw this when going to sleep (4:20 AM here) so too foggy to answer, but will do tomorrow.
Take care  :)

just gear gas

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Re: Marshall 3005/Lead 12 Power Trans Question
« Reply #23 on: November 03, 2017, 10:33:14 AM »
Hi J M,
If and when you have the time to address my question I would much appreciate it, you are the only Marshall lead 12 expert I came across that could really help me...
Thank you

orjfor

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Re: Marshall 3005/Lead 12 Power Trans Question
« Reply #24 on: November 16, 2018, 04:41:35 AM »
Hi I guess this thread is dead since no one has written in for at least a year but i give i a try.

I have a Marshall 12 Micro stack and I thinking about changing speakers in it and also add the capacitor mod in it. I also got an idea to put a clipping stage in it by adding two diodes and a switch to handle it with. Is it possible to add it after the R8 resistor? See attatched Pictures of the pre stage and a clipping stage. I know that it is clipping stages in other Marshall SS amps

phatt

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Re: Marshall 3005/Lead 12 Power Trans Question
« Reply #25 on: November 17, 2018, 01:48:36 AM »
I guess it's possible but you will likely effect the max volume available.
It would likely need a recovery stage. And still may not be what you want.
Frankly just get a dirt pedal in front would be a lot easier and there are many to choose from. 8)
Phil.

Synkopy

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Re: Marshall 3005/Lead 12 Power Trans Question
« Reply #26 on: January 04, 2020, 10:10:36 PM »
EDIT: in a nutshell:
a) they originally use 8 ohms speakers, (or 2 x 16 ohms)  so if you have original speakers or replaced them with other brand 8 ohms ones, fine, you have changed nothing except improve sound quality.
b) the amp design and parts used stands way more, I have personally loaded them with 4 ohms and got around 20V with no ill effects.
The transformers I tested were fine, warm but not *hot*  .
Same with heatsinks.
c) in theory you can turn that amp into a 50W or more one, *if* you provide better supply (beefier transformer and higher voltage electrolytics) and mount transistors in a "real" extruded finned heat sink, and adjust dropping resistors values to still feed +/- 15V to preamp.

I haven't modded original ones but straight built new ones, with the beefed up supply and heatsinking I mentioned, for killer 50/60W combos.

Sorry to revive an ancient thread. But I came into possession of one of these amp heads recently and wanted to thank you so much for this. As I’ve managed to do just what you’ve described. Upgraded electrolytics, bigger transformer, upgraded heat sinks, and rectifier, and adjusted drop resistors. Also made a small capacitor change to tame the fizzy top end and added an op amp socket for good measure. This little dude can easily keep up with my 60W blackstar now!

pinkjimiphoton

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Re: Marshall 3005/Lead 12 Power Trans Question
« Reply #27 on: August 31, 2021, 01:02:18 PM »
zombie bump!!.... errr... zombie woof?

hi guys
got one of these on my bench. someone tried to "upgrade" it with an oversized xformer, and appears to have messed this up into the proverbial soup sandwich.

i have the amp semi working... the power amp, anyways. i can get a bit of a low-volume honk out of the speaker if i touch one of the leads at c8.
i replaced all the electros, and checked the diode bridge. seem to be getting about 19v/-19v at the power rails coming off the rectifier...
but that's it. there's not much in there, really... so i tried audio probing it last nite.
i have sound up to the input of the 1458... but that's it.
since i can't seem to get a reading for any voltages at pins 4 and 8, and no output after the 1458, should i assume the opamp is toast? a shorted opamp would make it so i get no voltage readings anywhere, wouldn't it?
just thought i'd ask someone with better knowledge before desoldering it.

one of the two power transistors in the actual amplifier seems to get warm after a few minutes too, the one on the negative rail.... i'm assuming a shorted opamp may make that happen?
i ordered fresh bc148's for it, fresh zener etc etc and a PROPER power xformer for it, a hammond 166j28 28vct transformer that will actually fit the footprint inside the chassis with proper foot spacing for the mounting holes.... and look forward to getting it running.

i appreciate any thoughts, guidance, and advice! peace!
jimi

DrGonz78

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Re: Marshall 3005/Lead 12 Power Trans Question
« Reply #28 on: August 31, 2021, 03:21:08 PM »
since i can't seem to get a reading for any voltages at pins 4 and 8, and no output after the 1458, should i assume the opamp is toast? a shorted opamp would make it so i get no voltage readings anywhere, wouldn't it?

Actually I would think just the opposite and if the opamp had shorted internally then it would lead to unexpected voltages present on many legs of the IC. I would think there is actually an open condition that exists in your circumstance. Meaning the missing bi-polar power supply that feeds pins 4 and 8 points to a power supply issue.

P.S. Why not start a fresh thread for this repair? It will make the repair easier to follow along as well.
“A person who never made a mistake never tried anything new.” -Albert Einstein

pinkjimiphoton

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Re: Marshall 3005/Lead 12 Power Trans Question
« Reply #29 on: August 31, 2021, 03:31:41 PM »
hi gonz,
mainly cuz this thread was already a wealth of good information. it looks like someone tried to take jm's post about beefing the power supply and apply it, but they didn't do too well. :duh
data sheet says a max voltage of 18 v for a 1458, and right now its reading about 20v/-20v at the power rails. i wouldn't think that small a voltage increase would pop it, but ... the power amp IS working. the preamp is completely dead.
i've got signal from the output of the MV on. nothing past pin 3 on the 1458, so, to my rookie eye it seems like the jelly bean took a hit from this... the transformer was only connected on one side, and had ripped off some of the wires from the power supply jack as it flopped around, so it may well have damaged something else, or shorted somewhere at some point and popped it.
if i don't get it, i'll start a new thread. just seemed relative to this one.
thanks man!