What is this mod to 2204

[Icarus]

Hi
I bought a great sounding 1977 JMP 2204 a few years back . One of the best sound amps I have ever heard
I found out it was modded , luckily I had a guy trace the a schematic .

Getting it serviced out of town (on tour) a tech removed the mod without me asking
I didn't have time to argue but always wanted similar mods done to the amp & other amps I have
I'm attaching a copy of what the original tech drew up on the mod
As im no tech im hoping you guys can tell me what was done

[Groovenut]

The plate resistor on the first gain stage (GS) has been changed from 100k to 220k adding more gain. The first coupling cap (CC) has been reduced from 22n to 10n reducing the amount of low frequency going to the second GS. The cathode resistor has been changed to 1.5k and bypassed with what looks like a 33u which is almost the standard Fender value. Its a bit more center biased and full range than the traditional Marshall 2k7 // .68u. The treble peaker 470k // 470p has been removed and replaced with a 100k. A 100k grid stopper has been added to the second GS as well as a 1M grid resistor. The grid resistor has the effect of changing the output impedance of the 1st gain stage as the gain is turned up and reducing bass a small amount. This is where things get really interesting. The second GS has been altered to roll off high frequencies with the 470p // 100k plate resistor. The standard 10k cathode resistor had been bypassed with a 2.2u and a 33k or a 3.3k // 1u has been placed in series with it. This cold biases the stage so that it clips hard on one side, but has full signal gain because of the bypass caps. I am pretty sure it's supposed to be a 3.3k otherwise the bass would be overwhelming. The treble peaker has been removed and there is an additional 100k in // with the usual 470k giving a total series resistance of 82k. I cant really tell what the shunt resistor is there. If it is a 470k or a 100k. I would guess a 100k this would decrease the bass from the second stage. Everything looks standard after that. Very interesting mod. Probably had a ton of gain.

[Icarus]

Thanks for the info
Oh year allot of gain & super tight with lots of bottom end very punchy
it sounded awesome
I was shocked when the tech removed the mods

[Brontosaur]

Thanks for the info
Oh year allot of gain & super tight with lots of bottom end very punchy
it sounded awesome
I was shocked when the tech removed the mods

What kind of style or sound was it best at ?

[Icarus]

Its hard to say because everyone hears things differently

But for example gain on about 4/5 it reminded my of Accept "Balls to the Wall" very clear & tight
dialed back it was a cleaner very tight punchy honk that hits you in the chest.

Good for an AC/DC type sound , but much tighter more bottom end than all of my amps without any flubb
Think "Get it hot " or "beating round the bush" of Highway to hell.

Gain on about 8 it compressed allot and was pretty thick & juicy with a warm sustain
I wouldn't say like Santana type sound
but sounded like allot of the 80's Hard Rock (maybe Jose or Lee Jackson ) modded amps

But not Hi Fi and sterile sounding like some amps can get.
It was pretty shit hot sounding

[Brontosaur]

Ok I may give it a try as it's the sound I'm looking for : I'll tell you how it sounds to my ears !

[Icarus]

Sure give it a go, hope it sounds as good as my 2204 did !
Im no tech but might get someone to do the mod to the amp or another one of my amps one day

One thing I was going to try
with this mod the Low input could be worked a bit to sound like a stock 2204 and the high input is pretty hot

I wonder if you could use an A/B Box to switch between the two giving you a
Rhythm - Lead footswitchable function. ?

[HTH]

I wonder if you could use an A/B Box to switch between the two giving you a
Rhythm - Lead footswitchable function. ?

not possible with the configuration of the hi/lo inputs on a 2204.

also, if you didn't ask the tech to remove the mod, take the amp back and get him to put it back the way it was - he's taking the piss.

[robert]

+1!
Since you have the schematic of the mod, it shouldn't be a problem for this tech to rewire it.

BTW: don't forget to ask him why he made the decision to delete the mod!
Maybe he did it for electrical safety (e.g. dangerous sloppy wiring of the mod)

Regards

Robert

[Groovenut]

I wonder if you could use an A/B Box to switch between the two giving you a
Rhythm - Lead footswitchable function. ?

The switching is possible but not with an external A/B box. The high input channels output would have to be attenuated to the level of the low input channels output at the master volume. This is easily done with a relay, but requires some circuit modification. KOC has a pretty good write up of this in TUT 1. I believe he uses FETs in his, but the method is the same.

[Capi]

The cathode resistor has been changed to 1.5k and bypassed with what looks like a 33u which is almost the standard Fender value. The second GS has been altered to roll off high frequencies with the 470p // 100k plate resistor. The standard 10k cathode resistor had been bypassed with a 2.2u and a 33k or a 3.3k // 1u has been placed in series with it. This cold biases the stage so that it clips hard on one side, but has full signal gain because of the bypass caps. I am pretty sure it's supposed to be a 3.3k otherwise the bass would be overwhelming. The treble peaker has been removed and there is an additional 100k in // with the usual 470k giving a total series resistance of 82k. I cant really tell what the shunt resistor is there. If it is a 470k or a 100k. I would guess a 100k this would decrease the bass from the second stage. Everything looks standard after that. Very interesting mod. Probably had a ton of gain.

The first cathode resistor is 1.5k bypassed by 2.2uf ( not 33uf wich is too high value for this mod ), 2nd 3.3k//1uf in series with what looks 10k//2.2uf

[Groovenut]

The cathode resistor has been changed to 1.5k and bypassed with what looks like a 33u which is almost the standard Fender value. The second GS has been altered to roll off high frequencies with the 470p // 100k plate resistor. The standard 10k cathode resistor had been bypassed with a 2.2u and a 33k or a 3.3k // 1u has been placed in series with it. This cold biases the stage so that it clips hard on one side, but has full signal gain because of the bypass caps. I am pretty sure it's supposed to be a 3.3k otherwise the bass would be overwhelming. The treble peaker has been removed and there is an additional 100k in // with the usual 470k giving a total series resistance of 82k. I cant really tell what the shunt resistor is there. If it is a 470k or a 100k. I would guess a 100k this would decrease the bass from the second stage. Everything looks standard after that. Very interesting mod. Probably had a ton of gain.

The first cathode resistor is 1.5k bypassed by 2.2uf ( not 33uf wich is too high value for this mod ), 2nd 3.3k//1uf in series with what looks 10k//2.2uf

Yeah the chicken scratch on that schematic is pretty hard to read... not wanting to guess what the intent was, I read it as what it looked like. However, your observations make perfect sense given what the OP says it sounded like. I still cant figure out what the shunt resistor after the second stage is maybe a 150k and the input resistor looks like an 82k as well. I would assume the tonestack resistor is a 33k.

[joey]

The cathode resistor has been changed to 1.5k and bypassed with what looks like a 33u which is almost the standard Fender value. The second GS has been altered to roll off high frequencies with the 470p // 100k plate resistor. The standard 10k cathode resistor had been bypassed with a 2.2u and a 33k or a 3.3k // 1u has been placed in series with it. This cold biases the stage so that it clips hard on one side, but has full signal gain because of the bypass caps. I am pretty sure it's supposed to be a 3.3k otherwise the bass would be overwhelming. The treble peaker has been removed and there is an additional 100k in // with the usual 470k giving a total series resistance of 82k. I cant really tell what the shunt resistor is there. If it is a 470k or a 100k. I would guess a 100k this would decrease the bass from the second stage. Everything looks standard after that. Very interesting mod. Probably had a ton of gain.

The first cathode resistor is 1.5k bypassed by 2.2uf ( not 33uf wich is too high value for this mod ), 2nd 3.3k//1uf in series with what looks 10k//2.2uf

Yeah the chicken scratch on that schematic is pretty hard to read... not wanting to guess what the intent was, I read it as what it looked like. However, your observations make perfect sense given what the OP says it sounded like. I still cant figure out what the shunt resistor after the second stage is maybe a 150k and the input resistor looks like an 82k as well. I would assume the tonestack resistor is a 33k.

looks just like another 100K shunt, with the 470K bypassed with another 100K. fist cathode bypass is 2.2uf, and second is the 1uf, and 2.2uf Capi saw.

interesting..........

[rockstah]

whats interesting is that the second image , closer view i guess, doesn't include the input grid resistor value... does that say 82k?
as well second closeup doesn't show the 1000p cap on the volume.

after reading the thread i decided to edit the schematic.


Mark

[joey]

whats interesting is that the second image , closer view i guess, doesn't include the input grid resistor value... does that say 82k?
as well second closeup doesn't show the 1000p cap on the volume.

after reading the thread i decided to edit the schematic.


Mark

I think you are looking too deeply into this guys. The iput grid stopper looks to be 82K. the second picture is probably just detailing the input stage.