Bought a “modded” Marshall…what is this??

[Fusionbear]

I’ll also add one more thing. It makes the preamp tubes more susceptible to damage/premature wear if you wall voltages are too high. There’s less B+ drop going to the preamp tubes and if your wall voltage is 123-125, your preamp tubes could be seeing 75V or more than intended by design. Just keep that in mind.

JTM45/100 Preamp tubes sometimes see 425+. Yes they sometimes wear quickly, but they sure sound amazing in those amps. I use Chinese or NOS in those circuits. This amp is still pretty low voltage comparatively.

 

[Fusionbear]

Good to know! I always use a Brown Box with my Marshall’s…down to around 115V

This amp is a modern replica designed for 120-125VAC so a brown box is not necessary. But, with that being stated, you could use a variac down to 90VAC and rebias for some other less punchy but delicious "brown sound" tones if you like....

 

[FourT6and2]

He was having trouble with it being too middy and mushing out too quickly.

Ah that explains the 56k slope.

 

[SLOgriff]

Howdy!

Yes, I remember that amp. I modified it for a buddy who plays in a Deep Purple/Rainbow Tribute band. He was having trouble with it being too middy and mushing out too quickly. So, I converted it to Bass spec with the higher voltages in the preamp to get closer to the Blackmore preamp voltages in the Marshall Major amps. I used a jumper on the 10k dropper resistor so that if he didn't like it, I could easily remove it to make the preamp voltages stock. I'm not sure who added the KT88's. I think it came stock with 5881's. I know he got into some financial struggles and sold most of his gear. I offered to restore the amp to stock for free, but he had already sold the amp. I'm no longer working on amps, but it would be easy for a competent tech to restore. I pulled the board out to change the treble cap and slope resistor, and PI feed coupling caps to make it look clean and professional.

It's a loud amp!

Hope this helps...

Thanks Carl, I appreciate the information! And YES, very clean work for sure!

I haven’t checked the bias on these KT88’s yet…do the bias resistors appear stock? I’m quite sure all these newer 1987X’s come loaded with EL34’s….

Thanks
Bryan

 

[Fusionbear]

Thanks Carl, I appreciate the information! And YES, very clean work for sure!

I haven’t checked the bias on these KT88’s yet…do the bias resistors appear stock? I’m quite sure all these newer 1987X’s come loaded with EL34’s….

Thanks
Bryan

So it looks I might have set it up for 6550's or KT88'S. The bias resistors at the apex of the bias feed are 100k's and the bias feed at the bias circuit is also adjusted. This is in line with what my friend wanted. More headroom. T use EL34'S, you will have change the 100k's to 220k and return the main bias resistor to stock...

 

[SLOgriff]

So it looks I might have set it up for 6550's or KT88'S. The bias resistors at the apex of the bias feed are 100k's and the bias feed at the bias circuit is also adjusted. This is in line with what my friend wanted. More headroom. T use EL34'S, you will have change the 100k's to 220k and return the main bias resistor to stock...

Great, if you feel confident you made the changes for KT88’s, then I’m just leaving as is!