JCM 900 4100 HGDR - JMP Transformer and other possible mods?

[Bad.Seed]

Anyone familiar with the JCM 900 4100 HGDR circuit? I bought this one last year and immediately noticed the amp had been modded with a JMP style output transformer. I've always felt this amp sounded a lot fatter with a lot less gain and buzz than a regular HGDR model, but I have no idea what may have been done to the circuit before I got it. If anyone is familiar with these, would love to pick your brain!

 

[JackBootedThug]

That’s pretty interesting dude.

 

[JackBootedThug]

I have always liked 900’s also. Call me crazy….

 

[Rex Rocker]

I highly doubt the output transformer has any influence on how gainy those amps get. If there is less gain, I pretty certain there is another mod someplace else.

 

[Bad.Seed]

I highly doubt the output transformer has any influence on how gainy those amps get. If there is less gain, I pretty certain there is another mod someplace else.

Right, I know the transformer wouldn't affect preamp gain which is why I was curious if anyone could spot any obvious mods to the rest of the circuit.

This JCM 900 sounds much fatter and less buzzy than many I've had before it so I just found it interesting

 

[Rex Rocker]

Right, I know the transformer wouldn't affect preamp gain which is why I was curious if anyone could spot any obvious mods to the rest of the circuit.

This JCM 900 sounds much fatter and less buzzy than many I've had before it so I just found it interesting

Maybe something related to the clipping diodes? Fatter, less buzzy, and less gain seems like that might be related.

I can't quite tell from the pics you posted, though. They all seem to be there. On the second pic, there seems to be something up with the solder joints? Maybe?

 

[Al Chapo]

Here's my '00 model for reference. Only mod here is the reverb tank.

 

[vultures]

I did a double take, I thought you picked up my old JCM900 4100. I had one with a JMP100 Output Transformer, but from June 2003. The Transformer is spec'd the same and commonly used in these. Apparently the output transformer dying is common in the JCM900 4100.

R33 is changed on the LT power supply to a vintage style looking 270Ω Carbon Comp 1W Resistor. The schematics call for a 270Ω, no big deal.
R28 looks like it has a resistor piggybacking off it. Too blurry to tell.
Looks like power diodes might have been replaced.

The one I had sounded massive and thick, the only problem is that JMP100 transformer is heavy as a fucking brick and will destroy that amp unless you pack it with foam to hell and back. I bought mine, extremely cheap, off my buddy because it got damaged in shipping, the JMP transformer ripped out of the chassis, bashed some tubes up pretty bad. I shaped the chassis back to square, mounted the transformer back much more securely, traced the circuit, found absolutely nothing else wrong with it or the JMP transformer, sold it back to my buddy at the same price. I know you ship amps a lot, so just be careful if you decide to ship that one with that transformer.

 

[Racerxrated]

I did a double take, I thought you picked up my old JCM900 4100. I had one with a JMP100 Output Transformer, but from June 2003. The Transformer is spec'd the same and commonly used in these. Apparently the output transformer dying is common in the JCM900 4100.

R33 is changed on the LT power supply to a vintage style looking 270Ω Carbon Comp 1W Resistor. The schematics call for a 270Ω, no big deal.
R28 looks like it has a resistor piggybacking off it. Too blurry to tell.
Looks like power diodes might have been replaced.

The one I had sounded massive and thick, the only problem is that JMP100 transformer is heavy as a fucking brick and will destroy that amp unless you pack it with foam to hell and back. I bought mine, extremely cheap, off my buddy because it got damaged in shipping, the JMP transformer ripped out of the chassis, bashed some tubes up pretty bad. I shaped the chassis back to square, mounted the transformer back much more securely, traced the circuit, found absolutely nothing else wrong with it or the JMP transformer, sold it back to my buddy at the same price. I know you ship amps a lot, so just be careful if you decide to ship that one with that transformer.

View attachment 253634

I think those amps had the runaway bias thing that plagued the 6100 anniversary series; and would also plague the first DSLs when they came out.

 

[TotallyRadGuitars]

The first thing I notice is that your middle two power tubes don't have their grid stopper resistors (1k5's)
You're also missing the fuses for the heaters on the power amp board (side facing the speaker jacks)

You do have the correct 4 red LED's for clipping, and it *looks* like the slope resistors for the EQ and the pre-boost EQ are correct. Those would have a pretty huge effect on the sound if any of them were changed, but it is hard to tell the colors from the picture (for example, 3x orange + gold 33k is way different from 3x red + gold 2.2k). These amps actually have two treble/middle/bass EQ's, one is the knobs you control on the front panel, the other is only active on the boost channel and is a "pre" EQ that shapes the tone before it goes through the LED clipping stage, and can't be adjusted - fixed values. So if a previous owner wanted to modify the sound, it'd be pretty easy to swap some resistors in there and have a noticeable affect on the sound - check R41, R40, R12, R7... slope is R43, boost channel only.

Like many Marshalls, a lot of them went through little revisions over time, for example you have a lot of those big white 2k2 resistors on the power board like the '92 schematic, but my Reissue 900DR has 470's there (like the '02 schematic). Not sure what difference that makes as far as the sound, but my point is only that especially for an amp that has clearly been repaired at some point (likely blew up some power tubes and took the transformer with it), whoever repaired it may have been working from who knows what version of the schematic, so you may have some small differences that way. For example, if your amp is from 1993 but the repair person used a schematic from 1997 to replace damaged components from whatever took out the original transformer.

 

[Bad.Seed]

I did a double take, I thought you picked up my old JCM900 4100. I had one with a JMP100 Output Transformer, but from June 2003. The Transformer is spec'd the same and commonly used in these. Apparently the output transformer dying is common in the JCM900 4100.

R33 is changed on the LT power supply to a vintage style looking 270Ω Carbon Comp 1W Resistor. The schematics call for a 270Ω, no big deal.
R28 looks like it has a resistor piggybacking off it. Too blurry to tell.
Looks like power diodes might have been replaced.

The one I had sounded massive and thick, the only problem is that JMP100 transformer is heavy as a fucking brick and will destroy that amp unless you pack it with foam to hell and back. I bought mine, extremely cheap, off my buddy because it got damaged in shipping, the JMP transformer ripped out of the chassis, bashed some tubes up pretty bad. I shaped the chassis back to square, mounted the transformer back much more securely, traced the circuit, found absolutely nothing else wrong with it or the JMP transformer, sold it back to my buddy at the same price. I know you ship amps a lot, so just be careful if you decide to ship that one with that transformer.

View attachment 253634

Who makes these transformers? Honestly after hearing how much bigger and louder this thing sounds compared to the other 4100's I've had, I want to grab one of these transformers for my slx as well!

 

[vultures]

Who makes these transformers? Honestly after hearing how much bigger and louder this thing sounds compared to the other 4100's I've had, I want to grab one of these transformers for my slx as well!

Best guess is a Dagnall C2668. Maybe an older Heyboer MOP100 that they make for Mojotone. Mine never had any manufacture marking, just that big generic sticker.