Marshall EL34 100/100 mod for 4 ohm output

[adlerburg]

I have a Marshall EL34 100/100 monobloc power amp that comes stock with a 2 way selector switch for output resistance... 16 or 8. In the user manual, it indicates if you want to run it at 4 ohm you need to contact an authorized Marshall agent. I'm a musician who is quite competent at wiring up my own electronics. Is this something I can do at home? I have all the schems. If not, could someone please point me to a good tech in my area (Woodstock NY or Albany NY).
I'll obviously need a good 3 position switch.. but what and where to get it is a question Id like to put out to the forum.
Thanks for any help
Regards,
-Mick

[Chuck H]

AFAIK a DP3T switch that can handle the current of that circuit and fit in the space simply doesn't exist. Certainly Marshall could have commissioned an appropriate switch but for whatever reason they didn't. Trust me, you can look high and low for a switch that will meet a specific criteria and NEVER find it. And "my" experience says you'll never find that one.

If they offer that an authourized service center can wire it for 4 ohms then I would guess that there is:

1) A proper way to wire it to a 4 ohm secondary (which a schemaic may not even reveal) or

2) A circuit that Marshall reccommends to accomodate 4 ohms but actually reduces output power.

If you post the schem we can look it over and offer more info.

Chuck

[adlerburg]

Thanks... here are the schems...
Marshall 100100 I-O.pdfMarshall 100100 Fuse bd..pdfMarshall 100100 Driver.pdfMarshall 100100 PA.pdfMarshall 100100 PS.pdf

[Chuck H]

Ok... There IS a 4 ohm tap in there. Looks like it's wired to an isolated connection to keep it out of harms way. This is an old Marshall practice. It should be easy to find. It's the OT secondary tap that's connected to a point that's connected to nothing (an actual lead wire coming out of the output transformer). You should be able to simply replace one of the other OT secondaries at the impedance switch with this lead and the one you replaced will become a 4 setting (though it will of course be labled incorrectly at the switch unless you change that too). It looks that simple.

Chuck

[adlerburg]

Thanks Chuck. I see that it is referenced on dwg 100100-65-02. You can see on the output transformers where there are options for 16/8/4 ohms. On the trannys, the 4 ohm wire is connected to "b6 and a9". I'm assuming these are the isolated taps. Is it as easy as connecting the 4 ohm wires coming off of both trannys to where it's associated 16 ohm connections are (3 and 2)? Looks like it. to make it simpler, I could splice the 4ohm lead wire onto the 16ohm wire (after cutting that of course) that goes to the switch, and just cap the 16 ohm lead that comes off the tranny.. this way i just have to solder the wires together and put a small piece of shrink tube, thus eliminating the risk of overheating the switch. If it is as easy as that, this would be my 2nd choice. My 1st choice would be to have a 3 way switch and have all impedances 16/8/4 available to me. Somehow I get the feeling I'll have a very hard time finding that switch.
This amp touts each section is 100w at both 16 and 8 ohm. Is that possible? and by going to 4 ohm, won't I essentially be doubling the power output? So will it now be a 200w amp? How does that work? Thanks

[Chuck H]

I can't see the amps innards so I can't comment on the logistics of how to implement the connections. But what you propose sounds fine. It looks to me on the schem that the secondaries are independant taps (they don't need to be hooked up in some whacky series or parallel to achieve different impedances). Hook it up how ever you see fit for convenience. It's just a matter of replacing the 16 ohm leads with the unused 4 ohm leads.

And you won't be increasing power. Power is volts*amps and at 4 ohms the voltage drops considerably while the amps go up. In the end it's a wash. SS devices work differently and some can be run into different impedances with different resultant wattage outputs. Not so with most transformer coupled devices like tubes and early transistor power amps. The exception being that you can the amp into an incorrect impedance for a DECREASE in output. Another thing to consider is that the tubes in any guitar amp are already set up to deliver the most they can (max dissapation...for a given plate voltage) and nothing will change that limit. The tube would melt or otherwise fail if pushed beyond that limit.

Chuck

[adlerburg]

Thank you for your help. Being that I'm not a pro technician, I don't have the fancy diagnostic equipment... is there an easy way to confirm and test that the output is correct at 4 ohm when I'm done? I do have a decent multimeter. Thanks again

[Chuck H]

If you have a signal generator (or a keyboard, low voltage AC wall wart, any means of generating a low AC voltage) then there is a way to confirm the OT is hooked up correctly.

Do the mod. Then, with no tubes in, the amp off and unplugged lift the center taps for the OT primaries. Then put the low AC voltage into one of the amps output jacks and measure the voltage going in. It is important that you measure the voltage as it is during testing. Write this voltage down. Then measure the voltage across the plate pins on two tube sockets on either end of the OT primaries. Do this test four times, Once with the impedance switch on the 8 ohm setting and again with the switch in the new 4 ohm setting for each OT. Be sure to measure the input voltage seperately for each test. Now...

Divide the voltage that appears at the primary by the voltage going into the secondary. This is your OT's turns ratio. Square the numerator and multiply by the ohm setting. This will be your OT's primary impedance. In all four instances the primary impedance should be the same. Probably between 1.8k and 3.3k but all four tests should produce the same number. If this is the case then it's probably hooked up correctly. Resolder the center taps and with an appropriate speaker load on both outputs and power it up. Check voltages at the power tube sockets to be extra sure there are no dangerous errors.

Chuck

EDIT: If this sounds complicated, it's not. Lift the center taps on the OT's, inject the signal and take the measurements. The rest is very simple math.

The hardest part for you will be locating the correct test points. With the schem you should be able to do it.

[adlerburg]

Thanks for the detailed instructions. I wish you were within driving distance. I'd GLADLY pay you to do it. It does seem a bit complicate (not too), and here i am on my kitchen table with a multimeter and soldering iron.... I feel a bit ill prepared. Do you know of anyone in the Woodstock or Albany NY area that is a good amp tech? Also, any idea where I could get the 3 pos switch so I can have the 16/8/4 ohm selection available?

[Chuck H]

It's like I said, if the switch were redily available Marshall would have included it as part of the design. It's very telling that they don't have the three position switch but they do have a 4 ohm tap from the OT's. I don't think you'll ever find that switch. Sorry. You could call an "authorized Marshall service center" and ask them about it. If they can't get a switch no one can.

There have to be techs in Albany. I'm on the other side of the continent and have never been to the east coast (though I would like to go some time) so I can't suggest a tech.

FWIW I build a couple of amps a year for customers. Not so much a business as a hobby. I don't have a shop and have been known to build at my kitchen table. Usually I do it in my office with all my day job crap shoved off to one side.

Chuck

[Slayer]

Did you ever get someone to make your amp go to 4ohms? I sent your schems to my tech and he says the amp isn't like the drawings. Was curriuos if you ever got this done and by who so I could contact them for info. I realize this is an old thread but hopefully you will see this reply.

[Chuck H]

Did you ever get someone to make your amp go to 4ohms? I sent your schems to my tech and he says the amp isn't like the drawings. Was curriuos if you ever got this done and by who so I could contact them for info. I realize this is an old thread but hopefully you will see this reply.

adlerburg's last activity on the forum was a little over nine years ago and his participation here pretty much ended with this thread. When your membership allows you might try private messaging him. If he used his real email address for his membership he should get a notification. I see you've done the smart thing and posted this in a new thread as well. That's probably your best bet. I'm going to check out that thread now. Welcome to the forum.

[Dave H]

I'm on the other side of the continent and have never been to the east coast (though I would like to go some time)

Did you get to the east coast Chuck?
I've been to both coasts and I live in the UK

[Helmholtz]

I've been to both coasts

....Of what?

[Chuck H]

Did you get to the east cost Chuck?
I've been to both coasts and I live in the UK

Never made it to the east coast. I don't travel much so it's more of a bucket list thing. I did get as far as the Carolinas a few years ago and that was an eye opener. There's a weird energy in the older settlements of the south. Maybe I'll get all the way to the coast some time in the NEXT ten years

Oh, and I hope you liked the west coast here. That's my home and I've lived up and down it. No history to speak of. Unless you go back to paleo times But we do have a unique culture anyway.