Schematic of Skyliner stack and Bluesmaster PI?

[greiswig]

Hi, all,

I've tried using the search function, but I can't seem to find where/if there might be schematics of these. Are there such things?

[Fischerman]

The BM PI is the one used here and the Skyliner stack is the one in the ODS-101 schemos. Although I thought the 'correct' Presence pot was 25K-L in the BM PI.

You might notice that the BM PI is very close to a Marshall PI with the 'later' Marshall presence circuit.

[odourboy]

Although I thought the 'correct' Presence pot was 25K-L in the BM PI.

You might notice that the BM PI is very close to a Marshall PI with the 'later' Marshall presence circuit.

FWIW, I got a more usable range from a 10K-L pot in my Skyliner/BM PI build.

[kleinm]

I had a 25kL pot with a 39k draped across it for about 15K. Worked great for me.

[ayan]

Although I thought the 'correct' Presence pot was 25K-L in the BM PI.

You might notice that the BM PI is very close to a Marshall PI with the 'later' Marshall presence circuit.

FWIW, I got a more usable range from a 10K-L pot in my Skyliner/BM PI build.

Careful there. When I had the BM PI, I tried a 50K RA pot (readily available) which I believe read about 7.5K from wiper to input at the halfway up setting (and decreases from there). I found that the pot started to work at about 10 o'clock. I am afraid I don't how much resistance it read, but I know it must be somehwere between 50K and 7.5K, probably around 20K or so? Wild guess.

Bottom line is that maybe 10K is already equivalent to having some of the highs onthe NFB loop being dumped to ground, i.e., a brighter sound than the amp would get with a 20K pot turned all the way down. I liked the feel of the 50K RA pot a lot, by the way -- superior to the regular 20KL I had tried as well.

Gil

[odourboy]

I really wanted to like a higher value but kept finding too little usable room on the dial. Note that in the otherwise equivalent presence circuit, Marshall uses a 5K pot.

[mdroberts1243]

I had a 25kL pot with a 39k draped across it for about 15K. Worked great for me.

I didn't have the right pot in stock so I ended up with 15k and I'm not gonna change it... presence comes alive around 6.5-7 on the dial, so even lower would work for me.

[mlp-mx6]

Note that in the otherwise equivalent presence circuit, Marshall uses a 5K pot.

Are you sure? They have a 5K pot in parallel with a 4.7K resistor? I don't recall seeing that before. I've seen the 5K pot all by itself, though. I have also seen the 25K with a 4.7K in parallel in a Marshall schematic.

[odourboy]

Note that in the otherwise equivalent presence circuit, Marshall uses a 5K pot.

Are you sure? They have a 5K pot in parallel with a 4.7K resistor? I don't recall seeing that before. I've seen the 5K pot all by itself, though. I have also seen the 25K with a 4.7K in parallel in a Marshall schematic.

Check out the 50 and 100W 'Master' models (pages 589 and 595 of Aspen's Tube Amp Book V4.1 for reference). PI, NFB and Presence pretty much identical to the BM... except for the 5K pot.

[Fischerman]

odourboy,
Unless I'm misunderstanding you, I think we are talking about two different Marshall presence circuits. I think you are referring to the 'old' one that can have a bit of a scratchiness when turned due to DC current through the pot. The 'newer' Marshall Presence circuit is like the BM and has no scratchiness because no DC current flows through the pot.

EDIT: Like this JMP schemo.

[odourboy]

Nope.. same circuit. Just a different pot. Like this:
http://www.kbapps.com/audio/tubeamps/marshall2204.html

Anyway - let's not flog this to death. It's just another tweak.

[Fischerman]

No biggie odourboy...just discussin' amps.

But that schemo you posted shows a 5000pF treble cap (should be 500pF), the 'second' grid on the PI has a .022uF cap (should be .1uF), and shows 6550 power tubes with 220K bias splitters (should be 150K splitters). Just at first glance it doesn't look right (and doesn't look like an 'authentic' Marshall schemo...looks like somebody drew it themselves).

[odourboy]

Well my original reference was from Aspen's book which is a copy of the authentic schematic. So I put it back on you to check it out.