Metropoulos Forum

Brontosaur wrote:Mark, your layout made it quicker, hats off !

Brontosaur
Are you from Australia as well ?
if so where abouts are you ?

Icarus wrote:Brontosaur
Are you from Australia as well ?
if so where abouts are you ?

Not quite.... I'm french ! Sorry, mate !

I serendipitously found the source of this mod, and it is a lot less exotic then you guys think!!! It Actually comes from the Crunch channel of the Marshall 6100 30th Aniversary models! nearly Verbatum, except for one huge detail!

joey wrote:I serendipitously found the source of this mod, and it is a lot less exotic then you guys think!!! It Actually comes from the Crunch channel of the Marshall 6100 30th Aniversary models! nearly Verbatum, except for one huge detail!

Nice find! If I had to guess at the missing part implication, I would say it's the -15v bias shift attached to the 3K3 resistor. This effectively drops the OPs schematic ground connection to ~ -5V. This drastically reduces the gain of the stage as well as increasing the current through the stage from .5mA to about .8mA. It also changes the clipping characteristics because of the bias q-point. If I guessed wrong then....carry on

Groovenut wrote:

joey wrote:I serendipitously found the source of this mod, and it is a lot less exotic then you guys think!!! It Actually comes from the Crunch channel of the Marshall 6100 30th Aniversary models! nearly Verbatum, except for one huge detail!

Nice find! If I had to guess at the missing part implication, I would say it's the -15v bias shift attached to the 3K3 resistor. This effectively drops the OPs schematic ground connection to ~ -5V. This drastically reduces the gain of the stage as well as increasing the current through the stage from .5mA to about .8mA. It also changes the clipping characteristics because of the bias q-point. If I guessed wrong then....carry on

Wrong! but very close!, you were in the right mind frame just missed a detail. -15V I think is wrong on the scheme. the 3.3K resistor according to the scheme looks actually to be connected to one of the outputs of the Octal Latch, which really only deals with bistable logic conditions, so it will either have a logic high or logic low state, or +5V/0V. It is a selectable mode, and the 3.3K forms a potential divider with the 2.2K. It is raising the cathode reference voltage a about 2 volts biasing the stage warmer.

joey wrote:

Groovenut wrote:

joey wrote:I serendipitously found the source of this mod, and it is a lot less exotic then you guys think!!! It Actually comes from the Crunch channel of the Marshall 6100 30th Aniversary models! nearly Verbatum, except for one huge detail!

Nice find! If I had to guess at the missing part implication, I would say it's the -15v bias shift attached to the 3K3 resistor. This effectively drops the OPs schematic ground connection to ~ -5V. This drastically reduces the gain of the stage as well as increasing the current through the stage from .5mA to about .8mA. It also changes the clipping characteristics because of the bias q-point. If I guessed wrong then....carry on

Wrong! but very close!, you were in the right mind frame just missed a detail.

Back to studying for me I guess I cant elevate myself from newb just yet!

Groovenut wrote: Back to studying for me I guess I cant elevate myself from newb just yet!

In all fairness you were technically right, it was the -15 that was wrong, but you would have to realize the effect of that amount of over biasing to question the schematic. You have come a long way in this past year though

joey wrote:

Groovenut wrote: Back to studying for me I guess I cant elevate myself from newb just yet!

In all fairness you were technically right, it was the -15 that was wrong, but you would have to realize the effect of that amount of over biasing to question the schematic. You have come a long way in this past year though

Thanks Joey! I appreciate the encouragement!

joey wrote:I serendipitously found the source of this mod, and it is a lot less exotic then you guys think!!! It Actually comes from the Crunch channel of the Marshall 6100 30th Anniversary models! nearly Verbatim, except for one huge detail!

Seems there's a 22uF in parallel with 1K5 at V1B cathode ... Hmmmm ... I'll give it a try ...

joey wrote:I serendipitously found the source of this mod, and it is a lot less exotic then you guys think!!! It Actually comes from the Crunch channel of the Marshall 6100 30th Aniversary models! nearly Verbatum, except for one huge detail!

6100? Priceless.

Well spotted, Sherlock.

Brontosaur wrote:

joey wrote:I serendipitously found the source of this mod, and it is a lot less exotic then you guys think!!! It Actually comes from the Crunch channel of the Marshall 6100 30th Anniversary models! nearly Verbatim, except for one huge detail!

Seems there's a 22uF in parallel with 1K5 at V1B cathode ... Hmmmm ... I'll give it a try ...

V1b is only for the clean channel. the crunch channel which this mod was taken from only utilizes V2A/B, and V6A/B. The one huge detail is what Groovnut, and I have been discussing. The person who did the mod most likely didn't understand how it was really working in the 6100. If it by chance sounded good the wrong way, well then......

demonufo wrote:

6100? Priceless.

Well spotted, Sherlock.

What better way is there to Mod a Marshall amp, than mod it using another Marshall circuit.

Yeah, I laughed when I saw it too, and even harder when I noticed the key missing ingredients. The other funny thing is that the preamp supply voltage for the 6100 is a bit higher too, the original owner, or tech who did the mod was probably pretty mad that it didn't sound the same as the 6100's crunch channel.

joey wrote:V1b is only for the clean channel. the crunch channel which this mod was taken from only utilizes V2A/B, and V6A/B. The one huge detail is what Groovnut, and I have been discussing. The person who did the mod most likely didn't understand how it was really working in the 6100. If it by chance sounded good the wrong way, well then......

I'll have a closer look and try to get close to V2 &V6 values. Sounds more 'hifi' to me as it's now. And the pots values after V1 are far from the average marshall ones. I suppose that's for tailoring the clean channel. Thanks Joey !

Brontosaur
His is it sounding for you so far and how are you running the amp ?

Did you disconnect the low input like you were talking about in your other thread ?
If so why did you ?


One other thing you might want to try if you gigging around

I did with the amp when I had the mod .When I needed more sounds
Because of the mod the Low input has more gain as well but a much fuller sound.

I plugged into the low input re EQ'd the amp with more treble and used a (two in one) overdrive pedal so i could have three sounds .

Pedal off was a slightly broken up clean tone (with single coils )
Hit the OD pedal to mild gain & level with Humbucker for a crunch tone
Then Hit the OD pedals second channel more level & gain for a high gain lead with a good lift in volume
( I like my lead sound allot louder)

This concept didn't work in the High gain input because its allot brighter & with the gain on tap would compress & not
lift in volume as well.
But worked very well in the low input !
because the amp was running semi clean