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Old 26th October 2011, 09:11 PM   #1
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Default Marshall JCM900 2500 preamp

Hi

I am trying to make sense of the preamp for a the marshal JCM900 2500 (http://www.drtube.com/schematics/mar...d0109-iss9.pdf). I have a few questions:

- why the two zeners at the input?
- One leg of c14 is connected to gnd and the other to the top of the input connector through something called "link". Is this link a contact? If not, what is c14 actually doing?
- VR1 is connected to the Cathode of V1b through c15 and to the grid of V1b. What exactly is it doing to the tube? Also, D1 is part of the same circuit. That is operating as a limiter correct?
- Can someone explain to me what kind of feedback C16 creates?
- I read on a bunch of places that Vr2 "bypasses" the rectifier BR1. I am assuming that basically, on its lowest setting it attenuates the signal enough so that the bridge will not clamp it. Is that assumption correct?
- lastly, and i am ashamed to ask this, but what does c6 does to the circuit on V2a?

Thanks all
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Old 26th October 2011, 11:03 PM   #2
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hpupo,
I'll have a quick go before the mods move this to the instrument forum page.
1) Zeners at input are simply clamps - probably protection against DC on the output of effects boxes/pedals.

2)C14 is the "Bright" Cap for "Gain 1" Control - it increases the treble at low Gain 1 settings. This is common , the way it's been drawn is confusing.

3) The "Link" is likely to be a wire link on the PCB. What is'nt clear fron the schematic is that it connects that signal point to ground (mute function) when nothing is plugged in. That little arrow on the top of the input socket is saying that that connection is broken when you plug in a guitar cable.

4)VR1 is the "Gain 1 control - it sets the amount (level) of signal into the second gain stage

5)C16 provides a HF roll off to stop the amp sounding harsh and agressive at higher gain settings.

6)VR2 is the Gain 2 Control - it sets the amount of signal into the 3rd stage AND therefore the amount of clipping provided by BR1, BR1 will have a fixed "threshold", when you set VR2 for more than that threshold the signal starts to clip on the peaks.

7)C6 is another HF roll off to stop the sound becoming too agressive, this time it is used to increase grid capacitance, it works against the surce resistance which means that it is dependent on the Gain 2 setting to some degree. It may also be helping to suppress any diode switching noise from BR1.


Hope this of some help.
Cheers
Ian
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Old 27th October 2011, 01:14 AM   #3
dgta is offline dgta  United States
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1) C2 takes care of any DC. Not sure why input is clipped.
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Old 27th October 2011, 02:22 AM   #4
kevinkr is offline kevinkr  United States
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Guitar amp threads belong in Instruments & Amps per forum policy. Moved..
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Old 27th October 2011, 04:35 AM   #5
Enzo is offline Enzo  United States
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Those back to back zeners at the input are 9v types, so they only clip if the input is in excess of 9 volts - not likely from a guitar. But they do clamp any input transients at that level. Personally I expect them more in solid state amps to prevent inputs from exceeding op amp rail voltages.

Now that I look, the channel does change over to op amps after V2, so perhaps they prevent spikes that could get that far.

I could be wrong, but at only 10pf, negative feedback cap C16 looks more like it is about stability and suppresssing RF than tone shaping.

I would agree that the link is just a wire on the circuit board. They wouldn't normally note such a thing on the schematic. My inner sense tells me they probably note it there because in an earlier design revision level, it may have been a cap. If the cap was later deemed unnecesary, then they replaced it in the schematic with "link." That is just an opinion. Revisions 7, 8, and 9 noted in th drawing legend are not specified, but could contain such changes.
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Old 27th October 2011, 09:17 AM   #6
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Quote:
Originally Posted by Enzo View Post
Those back to back zeners at the input are 9v types, so they only clip if the input is in excess of 9 volts - not likely from a guitar. But they do clamp any input transients at that level. Personally I expect them more in solid state amps to prevent inputs from exceeding op amp rail voltages.
I would imagine, along with the 10K resistor, they are for when you accidentally plug the speaker output of your PA in to the preamp input

A simple brute force protection measure against a silly mistake.

Quote:

I would agree that the link is just a wire on the circuit board. They wouldn't normally note such a thing on the schematic. My inner sense tells me they probably note it there because in an earlier design revision level, it may have been a cap. If the cap was later deemed unnecesary, then they replaced it in the schematic with "link." That is just an opinion. Revisions 7, 8, and 9 noted in th drawing legend are not specified, but could contain such changes.
Presumably it's an option, depending on the specific input socket fitted - presumably it's for a socket with an extra switch, so it shorts the grid of the second triode to chassis to keep it quiet with no input - as opposed to the normal short before the first triode. It's a cunning plan, it will make it a LOT quieter with nothing plugged in.
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Old 27th October 2011, 10:13 AM   #7
Enzo is offline Enzo  United States
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Marshall does that on a lot of models. The input jack has a shorting contact to ground the tip, but the sleeve ground cutout also grounds off the signal path a couple stages inland. In fact on certain models when someone asks what to do about the dead OD channels I tell them to follow that mute line to the op amp in the middle of that channel and change it. ANother static victim. They use both, not generally one or the other choice. They generally are the CLiff jacks.
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Old 27th October 2011, 03:14 PM   #8
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Thanks everyone. Now what about C15, R6, D1? I can kinda see what they do, but how exactly does it work? Why are they tied into the cathode?
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