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PostPosted: Mon Jun 08, 2009 7:17 pm 
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Was looking over schematic on my new 410H and trying to figure out the changes that are switched in/out when changing modes in the different channels? I'm pretty sure the differences between the channels are pretty much just additional gain stages being switched in but the modes are a little different. I've built tube amps before (nothing this complex though) and have created different gain levels on the tube gain stages by switching in/out different values for the cathode bias resistors and bypass caps. I don't see that in that being done in the 410H circuit, so was wondering how they do it here.

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PostPosted: Tue Jun 09, 2009 12:14 am 
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I had posted this a few months back:

If you look at the schematic, this is the signal path I see, which I believe is mostly correct:

Input>
>1st stage of preamp V6b (2nd half of V1)
>out on pin 1 of connector 7 to gain control circuit
>returns from gain control circuit on pin 3 of connector 7
>splits, where one path goes to V7b (2nd half of V2), and the other path to V6a (1st half of V1)
Note: at this point, V6a (1st half of V1) and V7b (2nd half of V2) are switched in and out of the circuit depending on the channel/mode selected
>V7a (1st half of V2)
>V8b (2nd half of V3)
>out on pin 1 of connector 201 to tone control circuit
>returns from tone control circuit on pin 3 of connector 201
>V8a (1st half of V3)
>continues on to effects and reverb section mixing circuits...

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Amps:
Marshall JVM 410H, Mods-3H Choke, C83 (switchable), Negative Feeback (pot), Treble circuit mod, Clean gain boost mod
Marshall YJM100
Marshall JMD:1
Splawn Quick Rod
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2 Marshall 4x12's/Avatar Hellatone 60L's
2 1x12/EV Force
Preamp/Fx:
Axe Fx II
Guitars:
Gibson SG 61 RI
Epiphone SG Copy (Kahler Trem)
Schecter C1 Standard FR
Kramer American Pacer


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PostPosted: Tue Jun 09, 2009 8:34 am 
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Marsh,
Yeah, that's definitely the signal path I traced to. I figured CLEAN just uses the first amp stage V6b into its tonestack, then V7a for gain. The CRUNCH, OD1, and OD2 channels just change the tonestack to the normal Marshall configuration (after the gain stages) and add additional gain stages (V7b and then V6a) for the higher gain channels. What I am curious about is the different gain levels between modes and how these are switched in. You're saying that it's done by different configurations in the gain stage chain through the use of the switches. I'll have to take your word for it, cause I can't see any other method being used. Maybe Santiall can chime in on this at some point.

Thanks

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Gibson Les Paul Studio Lite '91
Music Man Axis Super Sport
VH 5150 Musikraft clone
Edwards John Sykes Les Paul Custom
Marshall JVM 205H
18W Marshall clone
6V6 Plexi Jr SE
Bogner 2x12 cab


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PostPosted: Tue Jun 09, 2009 9:45 am 
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From what I see, that's pretty much it, and of course the components associated with the switched gain stages would result in a slightly different character for the channel/mode.

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Amps:
Marshall JVM 410H, Mods-3H Choke, C83 (switchable), Negative Feeback (pot), Treble circuit mod, Clean gain boost mod
Marshall YJM100
Marshall JMD:1
Splawn Quick Rod
Cabs:
2 Marshall 4x12's/Avatar Hellatone 60L's
2 1x12/EV Force
Preamp/Fx:
Axe Fx II
Guitars:
Gibson SG 61 RI
Epiphone SG Copy (Kahler Trem)
Schecter C1 Standard FR
Kramer American Pacer


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PostPosted: Sat Jun 13, 2009 11:15 am 
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Ok, I found this home-drawn(I think) block diagram on another forum, which pretty much answers my question.

Image

Looks like the GAIN on/off switch in the plate circuit of V2a (V7b on schematic) switches a 1M resistor in/out of
a 1 uF cap to ground. Anybody know how this affects the gain? I'm thinking it changes the bias of that stage, but
I usually do that by changing the cathode resistance.

For those who don't have a schematic, here's an accurate version of the V2a circuit (can't post the schematic as I'm
sure it wouldn't be legal).

Image

_________________
Charvel San Dimas
Gibson Les Paul Studio Lite '91
Music Man Axis Super Sport
VH 5150 Musikraft clone
Edwards John Sykes Les Paul Custom
Marshall JVM 205H
18W Marshall clone
6V6 Plexi Jr SE
Bogner 2x12 cab


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PostPosted: Sat Jun 13, 2009 11:55 am 
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The block diagram is from the JVM owners manual (page 52 in the 410H manual). Someone just added the "Valve Chart" drawing in the lower right corner.

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Amps:
Marshall JVM 410H, Mods-3H Choke, C83 (switchable), Negative Feeback (pot), Treble circuit mod, Clean gain boost mod
Marshall YJM100
Marshall JMD:1
Splawn Quick Rod
Cabs:
2 Marshall 4x12's/Avatar Hellatone 60L's
2 1x12/EV Force
Preamp/Fx:
Axe Fx II
Guitars:
Gibson SG 61 RI
Epiphone SG Copy (Kahler Trem)
Schecter C1 Standard FR
Kramer American Pacer


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PostPosted: Sat Jun 13, 2009 12:03 pm 
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Ah, that's what I thought. Seemed too nice to be home-grown and I had heard of a block diagram in the manual (don't have one cause bought mine used). Thanks for that correction, Marsh.

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Charvel San Dimas
Gibson Les Paul Studio Lite '91
Music Man Axis Super Sport
VH 5150 Musikraft clone
Edwards John Sykes Les Paul Custom
Marshall JVM 205H
18W Marshall clone
6V6 Plexi Jr SE
Bogner 2x12 cab


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PostPosted: Wed Jun 17, 2009 12:09 am 
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both valve chart and block diagrams are in the handbook.
The valves were renumbered at some point and V1 became V6 and so on but I guess this was already discussed before.

cheers
Santiago

EDIT: that relay changes the AC gain without changing the bias point ;-)


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