2203 mod help

[krokit]

Hey guys Ive been modding my 1978 2203 by following the suggestions over at Tales from the tone lounge.
Ive removed the bright cap, removed the treble peaking circuit and replaced it with a 68k resistor which is then connected to the wiper on the volume control for more 'garlic'. This really improved the tone of the amp but it still needed more gain so I changed the cathode resister as suggested in point 4 and replaced it with a 4K7 But now when I turn up the preamp volume to full, the amp cuts out! When I turn it back down it comes on again but as I turn it up over 5 the distortion sounds like a fuzz box and it sounds worse the higher I turn up untill it cuts out completely! Any idea why this is happening? The amp was sounding really good untill this stage which is a bit worrying!

All the best

 

[charveldan]

You molested your amp, it doesn't like you.

 

[NewReligion]

You molested your amp, it doesn't like you.

LOL, be nice Dan.

Reverse the last mod. In other words return it to stock, take out the 4.7k and put the 10k back in and see if the problem is resolved before irreversible damage occurs. It is easy to get in over your head if your not an Experianced Amp Tech.

I am not a TECH: But I have recently experimented with some of the same "mods". As I have been told by those who are Electronic Technitians who specialize on Amplification, Proceed with CAUTION! Just because it is on the internet does not make it true/right/correct what have you. In addition there are other factors to concider such as has your head been modified before, does it deviate from the stock schematic.

Good Luck.

David

 

[krokit]

LOL, be nice Dan.

Reverse the last mod. In other words return it to stock take out the 4.7k and put the 10k back in and see if the problem is resolved.

Sounds like a plan

Cheers bud.

 

[blues_n_cues]

LOL, be nice Dan.

Reverse the last mod. In other words return it to stock take out the 4.7k and put the 10k back in and see if the problem is resolved.

I am not a TECH: but I have recently experimented with some of the same "mods". As I have been told by those who are Electronic Technitions who specialize on Amplification, Proceed with CAUTION! Just because it is on the internet does not make it true/right/correct what have you.

yeah you crossed something on the preamp. why not just buy a tubescreamer or od? i have a 2204 but i never run my pre past 7 & use the guitar volume(lp)back about 1/2 to get clean blues or country & then roll up & od it for 80's priest kind of metal or w/out the od it's ac/dc.

what kind of tone are you looking for?

 

[krokit]

yeah you crossed something on the preamp. why not just buy a tubescreamer or od? i have a 2204 but i never run my pre past 7 & use the guitar volume(lp)back about 1/2 to get clean blues or country & then roll up & od it for 80's priest kind of metal or w/out the od it's ac/dc.

what kind of tone are you looking for?

I tested the amp after each stage of the mod and all was well untill I changed that cathode resister! Thetone was there but just needed a little more gain. I thought the 4K7 was the sweet spot!

Cheers

 

[blues_n_cues]

I tested the amp after each stage of the mod and all was well untill I changed that cathode resister!

Cheers

so change it back & rock & roll man....

 

[krokit]

so change it back & rock & roll man....

Ive had a couple of cold beers so it'll have to be tomo!

 

[NewReligion]

Ive had a couple of cold beers so it'll have to be tomo!

Good Call!

 

[krokit]

Good Call!

Be interesting to know why that final step turned the amp into a big muff??

It actually sounds really good with the pre at 5 but after that....!!!:Ohno:

 

[NewReligion]

Be interesting to know why that final step turned the amp into a big muff??

It actually sounds really good with the pre at 5 but after that....!!!:Ohno:

Yeah, it's the "but after that" part that will cause you problems if it's not resolved.

David

 

[krokit]

Yeah, it's the "but after that" part that will cause you problems if it's not resolved.

David

Absolutely! Cheers.

 

[MajorNut1967]

If your amp is truly a 1978 2203, the standard JCM 800 mods won't work well and that you get a mush sounding amp: can you give us the serial number or some inside guts shot photos?Is the amp farting or motor boating also?

 

[krokit]

If your amp is truly a 1978 2203, the standard JCM 800 mods won't work well and that you get a mush sounding amp: can you give us the serial number or some inside guts shot photos?Is the amp farting or motor boating also?

Hey thats interesting.. So the 800 mods wont work well! Its def a 1978. The serial no. is 00539k and the test date is 17/1/78. Here's some pics of the inside and outside. I can take some pics of the internals when I reverse stage 4 of the mod which is putting back the 10k cathode resistor.
Not sure what you mean by motor boating but when the pre is at 5 its not to bad but as its turned higher its sounds like a fuzz face with the battery running low, and then finally cuts out when it gets between 9 or 10!

Cheers for the help

Here ya go.

http://dl.dropbox.com/u/1404714/Marshall 100w 2203 1978/SDC15038.JPG
http://dl.dropbox.com/u/1404714/Marshall 100w 2203 1978/SDC15039.JPG
http://dl.dropbox.com/u/1404714/Marshall 100w 2203 1978/SDC15040.JPG
http://dl.dropbox.com/u/1404714/Marshall 100w 2203 1978/SDC15044.JPG

 

[MajorNut1967]

Hey thats interesting.. So the 800 mods wont work well! Its def a 1978. The serial no. is 00539k and the test date is 17/1/78. Here's some pics of the inside and outside. I can take some pics of the internals when I reverse stage 4 of the mod which is putting back the 10k cathode resistor.
Not sure what you mean by motor boating but when the pre is at 5 its not to bad but as its turned higher its sounds like a fuzz face with the battery running low, and then finally cuts out when it gets between 9 or 10!

Cheers for the help

Here ya go.

Okay well it's definitely an early model 2203, which is very good by the way. Now these amps in those schematics between the JCM 800 and the JMP 2203 are very similar the JMP's just don't seem to sound right with too many gain or garlic mods as the tone lizard puts it. I would definitely put the 10k back in the Cath resistor and if you're going to remove the first trouble peeking circuit just use one 68K on the input side of the pot and do not put one on the wipe. Now as anyone changed the plate resistors on the first two valves? Because if they are changed up like to a 220 K, you need back down to 100 K. if you need a little extra push you can bypass the 820 ohm resistor with a .68 MFD capacitor. I really don't know what type of sound or tone you trying to achieve but those amps have their own character and it's a very desirable, but it is your amp. One final question did that amp originally have 6550 power tubes? And if so did you do change over, because there's some bias circuit changes at that need to occur and the bias split resistors may have to be changed from 150 K. to 220 K. but better yet some inside gut shots would be great! Oh motorboating is when you have gain modded or turn up on the moon and the amp starts going "PUT PUT PUT" or farting noise! LOL

 

[krokit]

Hey thats interesting.. So the 800 mods wont work well! Its def a 1978. The serial no. is 00539k and the test date is 17/1/78. Here's some pics of the inside and outside. I can take some pics of the internals when I reverse stage 4 of the mod which is putting back the 10k cathode resistor.
Not sure what you mean by motor boating but when the pre is at 5 its not to bad but as its turned higher its sounds like a fuzz face with the battery running low, and then finally cuts out when it gets between 9 or 10!

Cheers for the help

Here ya go.

Okay well it's definitely an early model 2203, which is very good by the way. Now these amps in those schematics between the JCM 800 and the JMP 2203 are very similar the JMP's just don't seem to sound right with too many gain or garlic mods as the tone lizard puts it. I would definitely put the 10k back in the Cath resistor and if you're going to remove the first trouble peeking circuit just use one 68K on the input side of the pot and do not put one on the wipe. Now as anyone changed the plate resistors on the first two valves? Because if they are changed up like to a 220 K, you need back down to 100 K. if you need a little extra push you can bypass the 820 ohm resistor with a .68 MFD capacitor. I really don't know what type of sound or tone you trying to achieve but those amps have their own character and it's a very desirable, but it is your amp. One final question did that amp originally have 6550 power tubes? And if so did you do change over, because there's some bias circuit changes at that need to occur and the bias split resistors may have to be changed from 150 K. to 220 K. but better yet some inside gut shots would be great! Oh motorboating is when you have gain modded or turn up on the moon and the amp starts going "PUT PUT PUT" or farting noise! LOL

Hey thanks for the info. Ill put the 68k back on the pot as you suggest..

As far as I know the amp has always had el34's..

I can only describe the sound when the pre is turned up over 5 as a hyper compressed/ thin, fuzzy sound that gets thinner and thinner untill it cuts out!!

Ive taken a couple of pics of the insides so hope they help.

Thanks again.

http://dl.dropbox.com/u/1404714/SDC16668.JPG
http://dl.dropbox.com/u/1404714/SDC16674.JPG

 

[krokit]

Ive put the 10k cathode resistor back and the amp sounds great again! Then I took the 68k resistor off the wiper and back on to the input side of the pot and it sounds even better, so many thanks for the suggestions MajorNut1967
Ill leave the treble peaking circuit and the bright cap off as it was way too bright with them on but Im really happy with the way the amp sounds now

What do you think of the internals. Anything look dodgy?

Thanks again

 

[charveldan]

Notice the wires going to the "turrets" at the right end of my board, both my 78's have the turrets, wonder why yours doesn't?

Also your screen grid resistors are discolored from over heating, i'd have them replaced & filter caps if they're original.

 

[krokit]

Hey cheers bro.. yeah, that's odd concerning the turrets.. wonder why mine doesn't have them?
I noticed the heat damage to the screen grid resistors when I first decided to try the mod but did'nt have anything to compare them with to know if this was normal or not on amps this old, so cheers for highlighting that.
I guess I was expecting to have to renew stuff like the caps and stuff as Ive never had it serviced, and I got it about 93/94, so thats cool.
I'm gonna check the bias as Im curious to see how thats looking, but apart from that its sounding pretty good :cool2:

Thanks again bud

 

[charveldan]

Hey cheers bro.. yeah, that's odd concerning the turrets.. wonder why mine doesn't have them?
I noticed the heat damage to the screen grid resistors when I first decided to try the mod but did'nt have anything to compare them with to know if this was normal or not on amps this old, so cheers for highlighting that.
I guess I was expecting to have to renew stuff like the caps and stuff as Ive never had it serviced, and I got it about 93/94, so thats cool.
I'm gonna check the bias as Im curious to see how thats looking, but apart from that its sounding pretty good :cool2:

Thanks again bud