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Question - adding spring reverb to my 2204.

Motterpaul

Tone is in the Ears
Messages
13,080
I jut acquired a Marshall JCM800 2204 - about '77. It is a single channel and has no reverb.

I have a spare reverb tank and the ability to build a preamp to push it. I would like to find the right spot in the 2204 circuit to add the reverb input so I can feed it signal and blend the output in with the overall signal. I do not want to drill any holes or make it permanent. I am guessing it would pretty much go in the same space as a LarMar loop - correct? (but it also should be parallel, not series).

My idea is to just add a couple of solder points for the send & return, and to put all the electronics in the separate box hidden away inside - with some pots to drive the reverb and send it back to blend with the output.

Anyway - if anyone has ever done this and knows the process - PLEASE let me know, I don't need or want a full loop mod, this is nm all original amp and iI want to keep it that way as much as I can. But I really miss having the reverb and I don't want to put it upfront with a pedal (though I know I could).
 

walterw

Platinum Supporting Member
Messages
38,595
i don't really see it;

just bite the bullet and get the metro amps loop added, at which point you can do whatever you want with the amp. verb is cool and all (and would sound good in there), but the big advantage of the loop is that it would allow for volume boosting after the preamp overdrive.

the metro kit is way less invasive than trying to invent some sort of outboard reverb tank to splice in.

it's also the best thing i ever did for my '78 2203.
 

J M Fahey

Member
Messages
2,692
Agree that adding a loop is the simplest mod, of course you will have to drill 2 jack holes, no big deal, it can be done tastefully; and has other uses.
Through them you can add whatever you want, probably some digital reverb which lately are getting very very good.
Or a classic outboard Fender Reverb .

Now if you wanted to add a spring tank bolted to the roof of your enclosure, plus some SS electronics to drive it (all tube will take too much space and complicate it), yes, it can also be done, don't know your degree of commitment and available tools.

Doubtlessly it will be quite more complex.
 

amphog

Member
Messages
4,253
Do the loop, then build your reverb. You can put all of it inside the amp, but noise may be a issue.
 

zenas

Member
Messages
8,859
One of these ?


Seems like the easiest thing, no mods required and according to many it's the best reverb going.
(use a Holly Grail myself)
 

J M Fahey

Member
Messages
2,692
^^^^^^ This one * is* the Holy Grail ;)

The OP does not want to use it up front, as a pedal, reasonable because heavy distorting it will probably turn gorgeous Reverb into mush, but it will work very well, if added after the Master Volume, and I guess the Metro Loop can be wired that way.

And if not, it should :)
 

Motterpaul

Tone is in the Ears
Messages
13,080
I actually did envision using an approach very similar to the loop. I have had them added to previous 2204 amps so I know how they work (not great, actually, but that is a different story). I found the "metro loop" didn't work too well for volume control, like a factory loop does, but I was trying to use the amp (with a ppmiv) and add a clean boost in the loop - that was problematic (it did not boost much), but this situation is different.

I have now moved to using drive pedals up front into a relatively clean preamp, but I do want to the "loop" just for the reverb. I own a spring tank and I actually have great soldering skills for pedals and a layout for a spring reverb pre-amp here:

larger image of this here: http://4.bp.blogspot.com/-wV6M1ZTYv...w/9MjNgnnpGZI/s1600/GF-Cook-Spring-Reverb.png

Now, of course, I need to make sure I have the right voltage to drive this (9v) but the loop is made to drive 9v pedals anyway. So, I should just need the loop circuitry AND this and I should be good. The only thing is that I want to install the loop with no drilling, I don't want to ruin the amp. I would like to just "tie in" at the right spots and maybe put the loop and this preamp in a small enclosure that I can attach to the bottom. I also could opt to build a digital reverb with a Belton brick which would also be 9v compatible.

I am a little surprised this is not kind of a standard thing.
 

EADGBE

Senior Member
Messages
12,339
They've got reverb pedals that sound really good. Are you sure you want to mod such a collectible amp?
 

J M Fahey

Member
Messages
2,692
The only thing is that I want to install the loop with no drilling, I don't want to ruin the amp.
I never cease to be amazed by this fear or "ruining the amp"
Ruining ..... really ........ ?
It's a guitar amplifier, which somebody bought in a store to play.

It's not the Marshall prototype built in 1963, which should be kept in a Museum behind bulletproof glass or something, but a standard amp made at least by the thousands, if not tens of thousands.

As said above, I trust you can do a neat job, drilling 2 aligned jack holes in the back panel. :)

Back to the Tech problem, there's not a practical way you can add a Loop in a PPIMV amp and expect it to do its main task, which is to add the "clean" effects *after* the distortion, because it's too late in the chain for that, sorry.

You are already inside the power amp !!!!

A loop *can* be added just after the standard MV, because signal there is 1 or 2 Volts audio, rack/studio type effects can handle that, and something like the Metro loop probably attenuates that to pedal level (say, 100-200mV RMS) and then boosts it up again, but at a clipping PI we are talking scary 200Vpp or so, with the actual signal reaching power tube grids some 30-40V RMS (at least Vbias*0.7) ... and to boot you have two such signals, out of phase ..... impossible to drive any effect with that, even if you could attenuate and regain level ... once mixed for the effect, you can't split them again.

If you got so close to the end, plain let the power amp clip, attenuate that, add a loop and drive another power amp.
There are a few commercial and custom made systems like that, but I guess it goes beyond what you are trying to achieve.

In a nutshell: if you want to use power tube distortion, or the closest thing: PPIMV distortion, it's too late, signal level or power is way too high; and if you add your effects before that point, you are using them before distortion, unfortunately you can't escape that.
Only option is to add them after a conventional MV, simply because distortion (or most of it) has already been achieved.
Even if you let the power amp clip something, mainly to round sound up and add balls and fatness, then you won't compromise your reverb that much.
 

Motterpaul

Tone is in the Ears
Messages
13,080
JM Fahey... I found a schematic for the 2144 which is a 2204 with reverb. You're so right about the voltage. It is about 253V coming off V3 - they put a choke there to go to the reverb and another one to bring it back. So, I am not going down that path (too much trouble). So, I'm back to the loop.

BTW: in reference to the PPIMV, MY problem was that I was trying to use the amp for the high gain tone, and tp use the PPIMV to set the overall loudness, and to use the loop to insert clean boost to put my leads over the mix. BUT what I should have realised is that a PPIMV is feeding the Output tubes, and if you have them nearly maxed already, you are not going to get much more out of them. Now, I don't know how they make loops differently in other high gain amps, but I can put a boost in my DSL100, or my EVH and get as much added volume as I want, but it didn't work with the ppimv. I am not (yet) an amp builder, but I can tell something is different with these two scenarios.

Anyway, I am back to the loop, and no, I do not want to drill holes in a perfectly stock '77 2204. Other people can do that, but I am pretty sure the value remains higher the less you molest them. I did it to a cheaper 2204 I had before and it was fun, but the loop and ppimv really did not work out so well together, not like modern amps can do it.

EDIT: also - in this case I am NOT trying to use any distortion in the 2204. I am using pedals into clean sound. The 2204 is just a great amp for pedals so that is the deal. So, I definitely won't be putting in a ppmiv, but the loop looks pretty inevitable.
 

amphog

Member
Messages
4,253
There must be a hole somewhere in the top of the chassie that you could slip some shielded cable through.
 

Motterpaul

Tone is in the Ears
Messages
13,080
There must be a hole somewhere in the top of the chassie that you could slip some shielded cable through.
Yeah, or I will just keep it internal.

BTW, for the metro loop I have to be able to measure the B+ voltage at the PI supply point. They show how to do that with the turret build, but not with a PCB 2204. Can someone please show me where that is on the PCB?

Here is a link to the metro loop instructions: http://home.comcast.net/~jbjdav26/Loop instructions Rev 1B4.pdf
Here is the link to the 2204 schematic: http://mhuss.com/MyJCM/JCM800_2204.gif
 

pdf64

Member
Messages
7,661
Instead of messing with the 2204 signal path (especially so for a parallel loop), how about using a small solid state combo as a reverb amp, as per the old Leslie system?
Mic the 2204 speakers / use a direct box on its speaker output, and feed that signal to a reverb fx set to 'wet' only, then feed that to the combo.
If the combo has a decent reverb built in, and can be set 'wet only', then even simpler.
 

Motterpaul

Tone is in the Ears
Messages
13,080
Not a bad idea - but let me tell you what I am doing. I am designing guitar effect pedals, so I already have the head going to an ISO-cabinet which is miked. I then monitor the sound through studio monitors. My goal is to hear how poedals sound recorded (and by the way - they sound great - through an amp, but I get recorded tones I never got using just a tube amp).

I could just add some reverb to the DAW. That would work.

But, I hve to say that I also plan to use this head as my gig rig someday myself, and I really like reverb. so.... Right now I have a "holy Grail" pedal hooked up just before the amp and it actually is not too bad, but I am also still looking into the metro loop.
 

RussB

low rent hobbyist
Messages
11,158
Yeah, or I will just keep it internal.

BTW, for the metro loop I have to be able to measure the B+ voltage at the PI supply point. They show how to do that with the turret build, but not with a PCB 2204. Can someone please show me where that is on the PCB?

Here is a link to the metro loop instructions: http://home.comcast.net/~jbjdav26/Loop instructions Rev 1B4.pdf
Here is the link to the 2204 schematic: http://mhuss.com/MyJCM/JCM800_2204.gif

It's a piece of cake on a horizontal input 2204,






 
Last edited:

Motterpaul

Tone is in the Ears
Messages
13,080
It's a piece of cake on a vertical input 2204,






@RussB - I JUST found an old thread by you. You describe this whole process. Can you please tell me where to measure the B+ voltage so I pick the right decoupling resistor when I install MY loop. You say something about having changed your voltage by changing out a 10k with a 20k Resistor, so I can't rely on your choice of resistor (although it is probably close).

Thank you!!

With the rest of the instructions I have I think I can get by. By the way - for those looking here is a good schematic on how to do this: http://home.comcast.net/~jbjdav26/4010 later PCB with ZL loop.png
 




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