Marshall EL34 100/100

[Slayer]

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. This has been posted about along time ago but couldn't get the answers I need. I did download the schems from the thread. Has anyone actually had this done? My tech says schem doesn't show what he needs. I was hoping someone had some experience with this.

[olddawg]

You can’t just rewire the cabs?

[Pedro Vecino]

It´s likely that there are no mentions of that connection in the schematics. In any case you can locate the output transformers and identify the four wires of each secondary: common, 4, 8 and 16. One of them (4 Ohms) must be isolated and not connected under the rear board, I imagine.

[g1]

It looks to me like the 4 ohm tap is going back to the preamp driver board for feedback. (page 1 & 2 of driver board schem.)

[Pedro Vecino]

In that case, taking the feedback of 4 Ohms the wire cannot be isolated and disconnected.
Where can you see the schematics?

[g1]

Where can you see the schematics?

https://music-electronics-forum.com/...l=1#post180488

[Chuck H]

Perhaps your tech is having trouble interpreting the schematics? I don't know how long Marshall made that unit but there probably aren't a lot of variations. If the schem shows a 4 ohm tap then there should be one. It's a matter of getting it to the output circuit. There's a switch. So perhaps it could be substituted for one of the other impedances but that may depend on whether the secondaries are straight wired or convoluted like some Hammond models. If you desire a three way selector to ADD the 4 ohm option as well as the existing 8/16 options that may require a very special switch that may or may not exist.

[The Dude]

You shouldn't really even need a schematic. Just look at the OT wires. It either has a 4 ohm tap, or it doesn't. If it does, just rewire the switch, or add another jack. This shouldn't be that difficult for an experienced tech.

[Chuck H]

You shouldn't really even need a schematic. Just look at the OT wires. It either has a 4 ohm tap, or it doesn't. If it does, just rewire the switch, or add another jack. This shouldn't be that difficult for an experienced tech.

This^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

[Chuck H]

A recent post added to the necro thread on this topic solicited by the OP here.

Looks like there's three secondary taps to me.

I have an EL34 100/100 on the bench.
Here's a pic from the OT.[ATTACH=CONFIG]56917[/ATTACH]

[g1]

The top terminals are the secondary. One of them is the 4 ohm tap and is going only to the feedback connections in the driver circuits.
One of the others would need to be disconnected and added to the 4 ohm tap, so you would end up with 2 wires on the 4ohm tap and no wires at one of the currently occupied terminals.
The 2 empty terminals on the bottom row in pic remain unused.

[g1]

Ok, in the picture, white is on 4ohm tap, blue on 8, yellow on 16.
The white is the feedback wire and stays where it is.
The yellow gets moved to be with the white.
Re-label your rear panel so it reads 4 ohm where it used to say 16, at the impedance switch.

It's a stereo amp, so do the same for both channels.

[Chuck H]

Well it looks like the OP dropped this question here, another tacked onto the necrothread about this amp and hasn't been back since. No activity on his account since he clicked "post". The good news is that if he does check back in he has the easy and appropriate info for his problem as kindly posted by g1.

Unfortunately Slayers account is too young to allow private messaging, which would alert him via email (provided he used a functional email address for his membership). I'm going to attempt to send him an email personally (even though I'm pretty sure that's not protocol) in the hope that his "tech" can get this straightened out for him without a problem.

[loudthud]

What type of impedance switch does this amp have ? Is it the same kind of PCB mount slide switch that Marshall uses on the back of speaker cabinets that allows the 4x12 to be split into stereo ?

If you can find one, there is a Philmore (brand) three way toggle switch No. 30-135. It is an ON-ON-ON switch setup as a SP3T. It looks like a large 20A toggle switch. Rated at 20A-125V AC. The body is roughly 1.3 X 0.8 inches so it's as big as a large DPDT toggle.