My new L100W Custom Mk3 and Hello from a newbie

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My new L100W Custom Mk3 and Hello from a newbie

Postby ThorntonReed on Thu Jan 30, 2014 9:42 am

Hello
Well I seem to be one of the growing numbers of people realising how good these amps are. I just bought a new L100 Mk3 and I'm impressed. Big Gilmour fan and this is what drew me to the Sound City. Yes, I'm sure they are no Hiwatts but I can honestly say that tonally it's probably the best amp I've ever played. My other amps are a Laney VC30, Marshall JMP SLP from 1978 and a few other smaller wattage amps. The sound is just so full. I love it.

So my one seems to be a bit of an odd ball too.
4 power amp tubes but not the extra 2 holes as some have.
5 B9A sockets (No tube in the middle one but it is wired up) What would happen if I put a valve in here?
So why is it some seem to have 5 B9A sockets and some only 4?

Here's some pictures of mine https://www.dropbox.com/sc/z96antyz39dzcge/vXoYeO7RDG
Here's a video I did very briefly just to give an idea of the tone of this thing. (Sorry for the poor quality, Not meant to be a display of my abilities :P )


Not really interested in modding it, I like it just how it is. Really surprised by the amount of overdrive that's achievable with this as well. I prefer it to my JMP. Putting up the mids seems to increase the gain too, is that normal?

Well any interest in mine and some questions answered would be appreciated.
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Re: My new L100W Custom Mk3 and Hello from a newbie

Postby Papa Dog on Fri Jan 31, 2014 8:39 pm

Welcome to the forum
the chassis with 4 power amp holes is early.
I believe it is the same chassis used for the 200 Mk3.
Later chassis for the 100 mk3 were the same as the chassis for the later 120 Mk4Please go to The SOund City Site and take a look at the schematics.
compair that to the schematics of a HiWatt.
You will find that there is little, if any difference between a CP103 and a Mk1 L105, nor is there any difference between a DR103 and a Mk3 100.
IN fact, up until Mk4 the only differences which I have been able to document are in lead dress (DR made all his mil spec and sound city always seems to look a little more haphazard) and that a few componenets might have been "second tier" as opposed to DRs use of only "top shelf" parts.

re: the middle pre-amp socket. It looks like the only thing which is "wired up" is the heater. You could put any sort of tube which has the same pinout as an ECC83 or 12at7 and all it would do is light up.

The overdrive is because of the "preset" trimmers on the cathodes of V2. I cannot see in the pictures if they are both still there, but if they are you can set one for dirty and one for clean, set the volume controls to the same volume (but not the same position) and switch between them with a simple A/B switch.
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Re: My new L100W Custom Mk3 and Hello from a newbie

Postby partsstan on Sun Feb 02, 2014 1:12 am

Welcome to the flock. I agree with Papa dog you have a very early mk3. The fact that it doesn't have an effects loop indicates an early model along with the 4 hole output tube chassis. :D :D :D :D :D
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Re: My new L100W Custom Mk3 and Hello from a newbie

Postby ThorntonReed on Mon Feb 03, 2014 12:58 pm

Interesting information there. Thanks very much.
I didn't quite understand your last paragraph Papa Dog. Are you saying that it's possible to affectively change the amount of gain produced? using the trimmers?

So how early would early be? I thought the Mk3 ran from about 1968?
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Re: My new L100W Custom Mk3 and Hello from a newbie

Postby Papa Dog on Mon Feb 03, 2014 3:58 pm

ThorntonReed wrote:Interesting information there. Thanks very much.
I didn't quite understand your last paragraph Papa Dog. Are you saying that it's possible to affectively change the amount of gain produced? using the trimmers?

So how early would early be? I thought the Mk3 ran from about 1968?

'68 would be "early" for the Mk 3. They only ran that series for c. 3 years.
seems that they would develope a new amp to fit on an existing chassis. When the next series was indevelopement they would get the new chassis into production before they were ready or willing to release the new circuit, so they continued building the old amps on the new chassis.
They did it again transitioning between the Mk 4 120+ and the Tour Series.

Yes, you can control the gain of a channel by changing the cathode trimmer on V2 for that channel. It is a very mimitted amount of change, however. It is the interaction between the volume of V1 and the "pre-set" gain on V2 that gives a degree of pre-amp overdrive or cleanness.

Hope that helped.
There Ain't No Such Thing As A Free Lunch.

"The time has come," the Walrus said,
"To talk of many things:
Of shoes--and ships--and sealing-wax--
Of cabbages--and kings--"
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