Marshall 4104 reverb mod

69xlch
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« on: July 13, 2009, 12:57:44 PM »

Hi, I am an experienced amp tech. I am trying to add reverb to a 4104
I have installed the reverb transformer and 2 9pin sockets to the chassis
I'm using a typical black face fender reverb circuit.
I am having trouble finding where to tap into the preamp for the reverb input.

When using only the HI input(J1) on the 2204 circuit
I am using the point where C3 J2 R5 C4 join for the reverb input
and point "Y" (input to the phase inverter)
to bring the reverb output back into the circuit.

The reverb works, but the tone stack does not. When I take the reverb out of
the circuit the tone works again so I know this is not right.

Any Ideas ?


« Last Edit: July 13, 2009, 01:00:54 PM by 69xlch »	Logged

wyatt
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Re: Marshall 4104 reverb mod

« Reply #1 on: July 13, 2009, 11:22:37 PM »

You tapped too soon. On a Blackface it's sent and returned to the Reverb right before the PI. No wonder you tone stack isn't working, you are creating a parallel route that bypasses it entirely.

Looking to the 2204 schematic, Reverb Input AND Reverb Output should come after the Master Volume pot (or if using a PPIMV, after the Treble pot). You may want to try both before C9 and try after (I would start with before C9, which is where people tap for FX loops). Fender connected Reverb Input and Output with a 3.3M resistor, that creates the parallel signal (Reverb would be too wet in series), you may have to tweak values. Skip the 10pF cap that Leo put in parallel with this resistor, that cap is a bright cap and is a big part of Fender's shimmering reverb tone, but I think will sound out of place on a Marshall.

Now that I hopefully have helped some I'll just note the great lengths people will go through to add an almost useless feature. Guitarists who try and use onboard reverb live always end up sounding like crap because the amp's verb and the room's natural reverb make everything too wet. In dead room's a little slapback delay will add all the life necessary and retain a ton more of the amp's clarity and definition. And when recording, effects are also added after the fact to a dry track, that way you never have to record for every trial setup. So, the only time onboard reverb serves much purpose is sitting around the living room (and i do keep an outboard for that purpose).


« Last Edit: July 13, 2009, 11:24:11 PM by wyatt »	Logged

69xlch
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Re: Marshall 4104 reverb mod

« Reply #2 on: July 14, 2009, 02:10:37 PM »

Thanks for the response, but what is ppimv ? I have already tried to tap after the treble , and after the master volume as well as several other locations and the only place that somewhat worked is where I stated I am using. I even tried a 2.2k and 270 ohm resistor on the feedback circuit like fender does for a line out as the input to the reverb. all it does is ocillate where ever I try. I think maybe it has something to do with having 4 gain stages , but as you can see, I am not an expert. Posting here is what I did after all else failed. I will try tapping around C9 and see what happens.

As far as the rest of your comments go, all I can say is

Just looking for something that nobody else I know has.
Different strokes for different folks!

Thanks Again


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wyatt
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Re: Marshall 4104 reverb mod

« Reply #3 on: July 14, 2009, 10:10:34 PM »

Post phase inverter master volume.

The most common place for a master volume is right before the PI, and that is were it is in a stock 2004. But the boutique trend is to move the MV after the PI, which allows more power amp crunch, or so they say. I mentioned it above since you were modding the design, I didn't know if you had changed it.

Here is how Marshall put reverb on JCM 800's...
http://www.schematicheaven.com/marshallamps/jcm800_splitch_rev_50w_4210.pdf


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69xlch
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Re: Marshall 4104 reverb mod

« Reply #4 on: July 15, 2009, 02:18:56 AM »

Thanks again, I have looked at that drawing many times trying to solve this problem. This drawing is newer then the circuit that I have, and uses transistors, diodes, and/or optoisolaters that "I will not use" ! I am not looking for HI LO input switching. The LOW input has been removed. The circuit is hardwired so that C3 R5 C4 join, bypassing J2 and keeping 4 gain stages with the tone stack at the end of the circuit as it would be if you just plugged into the HI input.

I am starting to think that I am going to change the circuit to an older 2204, tap the reverb input before C9 and use the low input J2 for the reverb return instead of bringing it back in at C9


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wyatt
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Re: Marshall 4104 reverb mod

« Reply #5 on: July 15, 2009, 08:19:14 AM »

I'm grasping here.

Is the reverb tank properly match for incoming impedance?


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69xlch
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Re: Marshall 4104 reverb mod

« Reply #6 on: July 16, 2009, 11:19:26 AM »

The reverb tank is matched to the reverb transformer if that is what you are asking.
I don't know why it would work in a fender and not a marshall. 12AX7's in both preamps. The tube manufacturers set the specs a tube needs to work, not fender or marshall. If there is an impedance mismatch, I don't know


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