Bought a modded Marshall - some weird stuff inside...

[RedPlated]

GC had a 75’ JMP 50 watt local. Actually a pair of them, a 74’ and a 75’ which appear to have had identical mods to them. I played the 75’ and it sounded really great. Couldn’t figure out who modded it but I picked it up and have 72 hrs to return it.

After getting it home, I’m shocked at how good the amp sounds. It can do low gain, 800 type stuff and gets pretty high gain with tight bottom end. Like a Friedman HBE level of gain.

The amp has a master volume where “volume 1” used to be and gain control where “volume 2” was. There are two knobs in the left inputs. Top one seems to add upper mids, some gain and aggressiveness. The lower one adds some sort of presence. The right top input is higher gain, lower right is slightly lower gain with a bit different feel.

The amp has an extra preamp tube and an fx loop that actually sounds really good.

I looked inside and there’s some stuff I can’t explain. On the left side of the PCB, there are multiple black “pucks” with caps on them. They're glued to the board and seem to run to the extra knobs. Anyone have any idea what these are and/or who may have modded this thing?

I’m still really surprised at how good the amp sounds. It sustains forever and harmonics just fly off the fret board. I have until tomorrow to return it. I’m torn because it sounds so good but I have a ton of amps right now. Plus, the mods are unknown and just a little unconventional.

I’ll attach a few pics.

 

[skullfxr]

Personally I don't buy modded amps unless they were professionally modded. I much prefer modding them myself. Then I know that I did it right.

 

[FourT6and2]

I think those might be variable capacitors? Like a potentiometer, but for capacitance instead of resistance. Pretty cool. Extra tube appears to be for the loop.

 

[RedPlated]

Interesting....I've never seen or heard of variable capacitors. I've been going crazy trying any Google search I can come up with to figure out what these things are. Whoever did the mod must have had to remove all the caps and resistors on the board below those black pucks. So maybe they are also replacing that part of the circuit as well?

Yeah, extra tube is for the loop. The loop sounded a bit thin when I plugged fx in at first. I threw an EHX 12ax7 in it and it seems to be better, actually very transparent. The EHX probably has more gain for recovery than the 60's tube that was in there. The amp has all 60's 12ax7's in it. The Amperex in V1 is pretty legit. I A/B'd it against a lot of CP tubes and it held it's own. A little on the lower side of gain but very clear. This amp has plenty of gain either way.

 

[Recon]

Hi,

Maybe these are some kind of inductors for additional tone shaping options? Kind of RLC filtering option.

 

[skullfxr]

Those are likely variable caps, i learn about them back in college, but I must say that I don't think that I have ever touched one.

 

[cardinal]

That's some crazy stuff!

Looks like he took off a lot of the V1 stuff from the board but did it point-to-point on the tube socket and over the top of those hockey pucks. A lot of the rest of the board was reworked too.

My guess would have been inductors for high pass and low pass filters?

 

[311boogieman]

but I must say that I don't think that I have ever touched one.

Don't do that without one hand in your back pocket if it is in an amp j/k

 

[mooncobra]

if it sounds good and you like it, keep it!

 

[glpg80]

They look like either puck diodes or variable caps, but I am leaning towards variable caps.

Variable caps were invented to tune heterodyne mixers that have an intermediate RF frequency stage so that it that aligns with standards for AM radio. They don't really go up large in value enough for bassband, so there are a lot of them wired in parallel to get the value up large enough to tune for a specific value by ear. You don't see them in builds because they are normally impractical for audio, very lossy, very large, and expensive.

 

[Wizard of Ozz]

Some sort of variable capacitors. Never seen anything like it used in a tube amp. The quality of the work whatever they are is trash. Gut it... and repopulate the board with the correct size/value caps. The globs of silicone and bare/exposed wiring make it look like a middle school science fair project gone bad. Even the solder joints look nasty.

They were probably going for some sort of interpretation of a dampening/feedback/resonance mod. Tighter/looser bottom end etc.

The discs almost remind me of the impedance/voltage selectors on older Marshalls from the '70s and '80s.

 

[kdmay]

Guys I am pretty confident they are inductors. I have used ones like these before in my attenuator build. Could be a Diezel/Uber type NFB setup, but I’d need to spend a little more time trying to trace these two knobs out to the circuit.

 

[skullfxr]

Some sort of variable capacitors. Never seen anything like it used in a tube amp. The quality of the work whatever they are is trash. Gut it... and repopulate the board with the correct size/value caps. The globs of silicone and bare/exposed wiring make it look like a middle school science fair project gone bad. Even the solder joints look nasty.

They were probably going for some sort of interpretation of a dampening/feedback/resonance mod. Tighter/looser bottom end etc.

The discs almost remind me of the impedance/voltage selectors on older Marshalls from the '70s and '80s.

Exactly. That's why I don't buy modded amps, especially ones that are botched abortions of shoddy lowquality work. Keep the yellow caps though as long as they are good and have the necessary value. If they the original Marshall ones (can't tell on phone) they have mad Mojo to them.

 

[RedPlated]

Some sort of variable capacitors. Never seen anything like it used in a tube amp. The quality of the work whatever they are is trash. Gut it... and repopulate the board with the correct size/value caps. The globs of silicone and bare/exposed wiring make it look like a middle school science fair project gone bad. Even the solder joints look nasty.

They were probably going for some sort of interpretation of a dampening/feedback/resonance mod. Tighter/looser bottom end etc.

The discs almost remind me of the impedance/voltage selectors on older Marshalls from the '70s and '80s.

Yeah that was basically my concern. I don’t want to keep it if it’s a hack job. I’ll likely return it today. For what I paid, it doesn’t justify spending more on fixing the issues. Kind of sad because it really does sound good.

 

[WarHed]

Yeah those are inductors not variable caps.. Also you'll notice they are in series, capacitors in series here would be ... yeah.
Metroplex circuits had inductors on the cathodes at one stage

 

[Wizard of Ozz]

Yeah those are inductors not variable caps.. Also you'll notice they are in series, capacitors in series here would be ... yeah.
Metroplex circuits had inductors on the cathodes at one stage

If so... those are some big @$$ inductors for a tube amp. I've seen inductors in amps before... but much smaller ones. A side shot might help confirm... if there is wire wrapped around a core.

But on the tops there are tabs and slots, looks like they were meant to sit inside of something and be adjustable/rotated?

 

[WarHed]

Rivets/center screw aside, we can see they are wired in series which you wouldnt do with a variable cap. We can also see those caps on top which appear to be wired in parallel with the [things] - inductors in parallel with caps on the cathode is what Metroplex used to do.

They sure are big, but I think whoever modded this has just picked some crap out of the parts bin that came from something else, so they could be 10A rated inductors (small value, physically huge) and put them in series to get the value they wanted...

 

[RedPlated]

I just went out and jammed on the amp. I’ve only had it a couple days so I don’t have a ton of time on it. This thing smokes. Seriously sounds great. Gets super heavy for drop D stuff, yet cleans up and can play low gain blues even. Those frequency control knobs wired to whatever those black pucks are work outstanding. It cuts or adds high and lows when raising or lowering the gain. Keeps the feel right you want it. I literally have a few hours to return it but I think I’ll just keep it. I know the circuit looks a little strange but I guess it doesn’t really matter if it sounds great.

 

[RedPlated]

This amp also has 6550’s which I’m liking. Tight bottom. They’re Sovtek, and who knows how old. They are biasing within 4ma but I’d like to put something fresh in there. I have a spare set of Ruby 6550aSTR. I tried them and it tightens the bottom a lot. Super tight. But they’re mismatched pretty far out. I was looking at the Tung sol 6550 reissue. Anyone have opinions on the best 6550 for these modded Marshalls?

Here’s a quick and dirty iPhone clip at very low volume

 

[japetus]

. I know the circuit looks a little strange but I guess it doesn’t really matter if it sounds great.

Until it shorts out in a few weeks and you have to spend $400+ to take it to a tech to fix the sloppy work. And then you are pissed off and the seller says " " sorry.