Metropoulos Forum

That will work differently from the common 5k / .68u or .1u with DC voltage on the pot, since it should be wired / connected differently. There should be no DC on the pot using the 4k7 NFB split off of the PI tail resistor. I'd use a min of 10k pres pot or 25k, so the range is not limited. 5k pot with 4k7 works okay if you like the range. 4k7, 4k3 or 5k pot with nor resistor alters the NFB some comparatively. The 25k circuit (wiring, not pot value) lowers gain some, adds top end and texture. It will shock you at first. Marshall must have ran short on 25k pots and 4k7 resistors at some point. There are anomalies through the years.

Found a schematic that showed this combo, but off course it's not wired like the schematics either......

I will see if I have a 4k7 resistor and a 22 or 25k pot and do a little presence wiring test.

5k pot with .68
5k pot with .1 (Scandinavian wiring?)
5k pot + 4k7 with .68 (British wiring at the time?)
25k pot with 4k7 with .68
25k pot with 4k7 with .1

Anything I’m missing?

Looks complete. Keep in mind you have the 5n bright cap along with added brightness, more tightness, less clarity, and added texture coming into play with some of those presence circuits. Could need to adjust, or might be perfect for you.

gluke wrote: ↑

Wed Jun 30, 2021 11:37 am

Opened up the ‘76. It has stock .68 presence. 5k pot with a 4k3 resistor?!?!?!?

Were they all over the place in 76-77 or is this how that wired the .68 presence?

I just retubed and recaped a JMP 77 and the Presence its 5kb/.68uf Mustard with 4k7 resistor

The 78 JMP was 25kb/.1uf Mustard

All 2203 circuits

snakepit86 wrote: ↑

Fri Jul 02, 2021 7:22 pm

I just retubed and recaped a JMP 77 and the Presence its 5kb/.68uf Mustard with 4k7 resistor

The 78 JMP was 25kb/.1uf Mustard

All 2203 circuits

Damn, now it is very very tempting to just swap the whole pot with my buddy's amp, ha ha ha. Would that be to rude? I am rewiring his to .1 anyway.

You could make a 25k pot 5 or 10k with a parallel resistor. 25 -10k is fine since the circuit only uses one element of the pot. 5k pot can limit the high end at max clockwise rotation.

@gluke have you ever considered trying the 39 mod?

dukeamps wrote: ↑

Tue Jul 06, 2021 3:57 am

@gluke have you ever considered trying the 39 mod?

Not really, no. It's usually what I or my HRI buddy Marsa wants to try. Last year it was Jose, this year it this AFD thing and now he wants to try VH next.....

I thought about trying mk7 in my 100W Metro which is currently set up as Angus Young's live amps.

gluke wrote: ↑

Tue Jul 06, 2021 6:52 am

dukeamps wrote: ↑

Tue Jul 06, 2021 3:57 am

@gluke have you ever considered trying the 39 mod?

Not really, no. It's usually what I or my HRI buddy Marsa wants to try. Last year it was Jose, this year it this AFD thing and now he wants to try VH next.....

I thought about trying mk7 in my 100W Metro which is currently set up as Angus Young's live amps.

ok no worries, i think its a great mod too, I have done so my Jose specs great fun also

dukeamps wrote: ↑

Tue Jul 06, 2021 9:50 am

ok no worries, i think its a great mod too, I have done so my Jose specs great fun also

Yeah, part of this is general interest in the 80 LA mod scene and the timeline of mods. I think there is great info about the scene in the Lee Jackson interviews and videos. Especially stuff like when he talks about going to NAMM late 70s early 80s and discussing mods with the guys from Marshall. It's not like they were disconnected and it was a full blown hot rod scene at the time with support from the Marshall tech guys. And when I hear those songs on the radio now is and think about how I always felt they had much cooler sounds than we got from our stock gear, well yeah, they had much cooler gear, ha ha ha...... And this whole SIR #39,#36, #34 story fits right into the whole picture as SIR was one of the hot spots of the scene.

Crickets... (again)...

Hi Gluke,

I really like the tone of your mod

According to my understanding you have posted the values for each stage of your mod. Unfortunately I am not able to match to which "basic" schematic layout it refers to I started reading from beginning of the thread. Are the ones on page 140 matching your schematics? Would be really interested in the #36 preamp circuit and correspondig power supply...

Best regards,
Tom

Hi Gluke,

I am following some time your YouTube sound samples. The tone you have achieved is really great in my ears

I would be really interested in a #36 preamp. You posted your stage configurations, but I am not sure which is the basic circuit it refers to. Is it the one on page 140 or is it different. Also about supply voltage for each stage is not clear to me

Can you give me a hint?

Greetings,
Tom

I am back at work......

Home office was much better for coffee break modding session.

Changed all my resistor wattages to the same as those in the Mk7 schematic. Changed all my pF caps to silver mica. Moved the 1st stage grid stopper to the jack so the hot shield works like the mk7. Changed the PI tail resistor to 15k (from 12k). Changed all diodes to UF2008.

1st stage: 33k, hot shield, 2k7/680n, 100k/220p, 22n, 1n/1M pot, 100k/150k voltage divider.
2nd stage: 1k5/100n, 100k/470p, 22n, 1M pot
3rd stage: 10k, 120k, 22n, 470p/470k/470k
4th stage: 820R/470n

https://soundclick.com/r/s8ikol