SOB's useless Limit knob

[es336td]

Ok... I've used the Limit knob on my SOB 100 watt head, but it's fairly useless after 2. I am also a complete idiot on what can and can't be done with electronics. I recently got the bright idea, they have the THD hotplate and the like... would it be possible to replace the Limit circuit with something along those lines, or take one apart and incorporate it? Remember, I've already admitted to being an idiot. If that would work, it would be a little more useful IMHO.

Also... what mods have people done to this head to make it better?

Thanx

L

 

[Monsta-Tone]

I don't see the Limit pot on the drawing, but I've seen it in all of the pictures that I have of the SOB.

Maybe someone here has a better drawing.

As far as placing the Hot Plate circuitry inside your amp, I think that's a bad idea.
A really great idea to make an amp more manageable, volume wise is to use a Pentode/Triode switch. This will cut the volume in half. I do this to all of my Mesa amps that have a 60/100 watt switch because I really don't like the tone of a 100 watt amp that is only using 2 of it's power tubes.

Sorry I can't help you more, but if someone has a good drawing (not the hand drawn napkin that I have), I would be happy to look at it for you.

As for mods, what is it that you don't like about the amp?
It is extremely easy to add more gain, mids, bass, etc. But, it is a really good idea to have a starting point and a direction that you want to go.

 

[es336td]

I don't see the Limit pot on the drawing, but I've seen it in all of the pictures that I have of the SOB.

Maybe someone here has a better drawing.

As far as placing the Hot Plate circuitry inside your amp, I think that's a bad idea.
A really great idea to make an amp more manageable, volume wise is to use a Pentode/Triode switch. This will cut the volume in half. I do this to all of my Mesa amps that have a 60/100 watt switch because I really don't like the tone of a 100 watt amp that is only using 2 of it's power tubes.

I became enamored with the SOB, when I used a friend's 60 watt combo and I loved it. I used it for over a year when we played in a band together; he played bass and I played guitar. Years later, I found this on craigslist, or somewhere... I figured a 100 watt would be all I'd ever need. I could get a 4X10 or 4X12 cabinet and use the 2X10 I have now and be set for about any venue. I got it and have only used it twice because it's too loud. I normally just run it on the 60 watt setting. How does the mod you mention change things?

Sorry I can't help you more, but if someone has a good drawing (not the hand drawn napkin that I have), I would be happy to look at it for you.

This the one you have?

As for mods, what is it that you don't like about the amp?
It is extremely easy to add more gain, mids, bass, etc. But, it is a really good idea to have a starting point and a direction that you want to go.

Back in the early 70s, I acquired a Marshall Major head and simply loved it... however, for most all venues it was WAY TOO LOUD! Sold it... traded it for a 66 Fender Tremolux and a Super Reverb; the Tremolux was NOS in the original shipping containers... the SR had been modified to a 1X15 and had "issues" to say the least. My wife bought me a Boogie DC-5 head in the late 90s... I ran it on a custom made cabinet with 2 E110-8 JBLs. I LOVED this amp set up. It did everything I wanted... in 1998 it was stolen. I bought a .50 Caliber combo to replace it; it sucked. Sold that. Bought a Line 6 AX2 and used that exclusively for 10 years. Now, I have a tweed Blues Jr. with Bill M mods, and the SOB with a 2X10, Weber California customs, Tremolux clone. I think the main issue I have with the SOB is the last time I took it out, the 2X10 sounded like a 4X12... all boomy... finally tweaked it and got the sound I wanted, but it scared me. I wanted to make the Limit control a little more useful. I have also heard about Presence mods... effects loops... etc. No real need now, just wanting something more. I guess I should sell this beast and buy a Mark III or DC-5 head... Sorry for the rambling.

 

[Monsta-Tone]

Yeah, that's the one that I have.

Here's a little info on the Pentode/Triode switch:
http://www.torresengineering.com/triodpen12po.html


http://www.tone-lizard.com/Marshall_Myths.htm (scroll down toward the middle of the page)

A better idea came later when some Marshalls (and Fender's The Twin) featured what is usually called by everyone except the manufacturer a Pentode/Triode switch (which doesn't really switch the output tubes between pentodes and triodes, but that's a long story). 'Triodes' will have a much lower plate-to-plate impedance, and not even the average tube 'guru' was brazen enough to state that the impedance selector must be adjusted whenever the amplifier is run in the 'triode' mode. As a side note, those Marshall amplifiers with 'High' and 'Low' power switching (JCM900) have given service technicians very little trouble, while JCM2000 amplifiers (without High/Low switching) are the source of many output transformer headaches. Draw your own conclusions.
Speaking of 'triode mode', there is much confusion about the 'proper' way to connect the output tubes in a 'triode mode'. While you aren't necessarily really connecting the tube as a triode, my own preferred method is simply to use the Screen-Grid resistor already there. Quick, easy, and requiring only a switch and a little bit of wire, the modification works well. You need a resistance here because of the poor quality of EL34s available today, none of which can handle the considerable current that would be on the Screen Grid with no limiting resistor. Below is a simple diagram for achieving the modification. Amplifiers with four EL34s require a little more thinking, but switching can be accomplished with a simple DPDT setup as well. Of course you could also have only the outside or the inside pair of tubes on the switch shown below with a 100 watt Marshall, and this idea can even be further worked with a DPDT/Center 'off' switch for three way switching. An added safety feature here is that if the switch ever screws up, you'll just 'shut off' the output tubes.

The 'Pentode-Triode' switch, while not named correctly, is still very useful in high-powered amplifiers.

Basically, it lowers the output of the amp by about 1/2. I use this on all of my amps that have been modded and it is a standard feature in our custom amps.
It lowers the headroom (amount of clean volume) and some of the upper highs, while warming up the tone a bit.
You may have to re-EQ the amp to bring out some more of the highs. Other than that, you won't notice an extremely huge difference in tone, just volume.

 

[es336td]

Here's a little info on the Pentode/Triode switch:
http://www.torresengineering.com/triodpen12po.html

< snip >

Basically, it lowers the output of the amp by about 1/2. I use this on all of my amps that have been modded and it is a standard feature in our custom amps.
It lowers the headroom (amount of clean volume) and some of the upper highs, while warming up the tone a bit.
You may have to re-EQ the amp to bring out some more of the highs. Other than that, you won't notice an extremely huge difference in tone, just volume.

Very interesting sites! Thanx! I think this mod would be the one to get... I suppose the 1st thing would be to get an accurate schematic before I attempted to get this mod done?

 

[es336td]

I don't see the Limit pot on the drawing, but I've seen it in all of the pictures that I have of the SOB.

Maybe someone here has a better drawing.

How about this one? http://www.schematicheaven.com/boogieamps/boogie_sonofboogie.pdf

 

[Monsta-Tone]

That one shows the Limit.

What a funky design! No wonder they weren't very popular.



As for the Triode, it really works quite well. I like it a lot.
If you can't find anyone near you to do it, I'd be happy to.

 

[es336td]

That one shows the Limit.

What a funky design! No wonder they weren't very popular.

Since I'm schematic illiterate... how does this differ from the original Mark I?

As for the Triode, it really works quite well. I like it a lot.
If you can't find anyone near you to do it, I'd be happy to.

So that would/could replace the 60/100 watt switch? Are all 4 tubes needed? Sorry to be so dense.

I think I know someone that can do it... do I just take it to him and show him the design, or what? I don't see a prefab kit or anything like that for this model.

 

[Monsta-Tone]

I didn't look at the rest of the circuit, but the Phase Inverter is dramatically different.

It most likely would be pretty easy to convert it to the Mark I specs.



As for the Triode switch, all that is needed is a good, sturdy, UL approved Double Pole, Double Throw switch and some wire.
The fact that the kit I linked you to is $45, is simply because you are also paying for the knowledge. Oddly enough though, that knowledge is abundantly available for free just about everywhere else.

Make sure your friend follows the drawing that is on the Tone Lizzard page, or looks up a better one.
Also make sure he is competent. You are messing with high voltages and your power amp circuit. A wrong move could potentially melt your amp down.

It's not that hard to do, but he has to be careful.
And, yes, it could easily replace the 60/100 watt switch.
All 4 tubes are needed still.

 

[es336td]

I didn't look at the rest of the circuit, but the Phase Inverter is dramatically different.

It most likely would be pretty easy to convert it to the Mark I specs.

How involved would it be to convert to Mark I specs? Can the Pentode switch be added also? I know, if I wanted a Mark I why didn't I buy one? Money! It's easy to justify $700. to the finance committee (i.e.- wife) instead of twice that... I guess my ultimate would be Mark III on, but... Mark IIC+ is way out of my price range. Might as well try and get a Dumble!

Thanx for all the response. I'm seriously contemplating taking it to my amp guy and having him put in the switch... just would like to be able to say, "also do this..."

 

[Monsta-Tone]

I sent you a PM with the price and my phone number.

Let me know if you have any questions.