Modding JVM410HJS (Negative Feedback, AFD / RR selector)

[FarlsUK]

Hi all!

I sold my original JVM410HJS about 7 years ago, and have regretted it ever since; and as of yesterday, I'm now the proud (re)owner of one; they're bloody hard to track down!

I wasn't going to mod it, but I just couldn't help myself.... so today I spent a few hours applying the negative feedback mod and the Rhoads / AFD mods. Only trouble is, the original forum with the guide shut down.... and the only post I could find was this a couple on here using photos of my original mods from thefretboard forum. Thankfully I had the original photos on my phone, and could figure out the wiring etc. Easily the most stressful part was soldering across that resistor on the board; a little bit of poo came out, especially as I melted the cap next to it a bit!

Check out the negative feedback dial and the Rhoads / AFD switch on the right:

Anyway, here are the mods for anyone else interested. Obviously, do this at your own risk:

Negative Feedback Mod

Instead of placing another resistor in parallel with R58, I cut the 3rd blue cable at the NFB block - the 3 blue leads on the board, and the cable to cut handily has an arrow above it on the PCB - and put a 1Meg log pot between the cables. You'll need to obviously extend the cables by soldering extra lengths to them.

This has the result of allowing you to change the negative feedback; fully left (or '0') is stock NFB - which I actually prefer - and moving clockwise gradually increases the resistance for the NFB. This has the effect of 'opening the amp up'; you'll notice the depth and presence don't have much impact (if any) when the new NFB pot is fully clockwise.

AFD / Rhoads Mod

This was the scariest mod to do due to having to solder to an existing resistor on the PCB, and it's a VERY tight fit.

To do this, get an On-On-On switch. For the middle two arms, wire a length of cable to each side. For the top two, connect a 3k7 resistor (I had to use a 2k7 and a 1k in series). For the bottom, connect a 100nf / 0.1uf cap in series with a 2k7 resistor.

Now for the shit-your-pants bit.....

Find resistor R161 on the PCB; it should be around the 2nd power tube socket on the left of the board. You need to solder one of the ends of lead you connected to the middle connections on the switch to one side, and the other cable to the other side. You probably want to discharge the filter caps before doing this! I didn't but I'm stupid.

The point of this mod is that when you have the switch in the middle, it is the stock JVM410HJS gain stages. Flip the switch up, and you get the AFD mode; seems to have more mids and bit more girth and gain. Flip it down, and you get the Rhoads mod - pure filth. More gain, more body and more sizzle.

Hope that helps anyone else looking to do these - I'll try and record some vids soon.

Don't ask me 'how' they work, as I have no idea.....!

 

[ElKabong]

The only problem I see is your negative feedback pot and what I've always thought of as the Plexi mod are reversed.
I gotta dig out my schematics on the capacitors I used on the 3 way. Two fun mods to play with.

From the pictures, you do nice work.

Now you shame me into getting a proper pot-knob.

 

[lespaul6]

good job on the mods... the color of that new knob would aggravate my ocd traits every time I looked at it!

 

[ElKabong]

good job on the mods... the color of that new knob would aggravate my ocd traits every time I looked at it!

I never look at it while I turn it

 

[FarlsUK]

The only problem I see is your negative feedback pot and what I've always thought of as the Plexi mod are reversed.
I gotta dig out my schematics on the capacitors I used on the 3 way. Two fun mods to play with.

From the pictures, you do nice work.

Now you shame me into getting a proper pot-knob.

Cheers!

Not quite sure what you mean by the Plexi mod, and then being reversed though…?

 

[ElKabong]

Cheers! I'm Jim

As I recall, the Plexi mod was a couple of capacitors wired across a 3 way switch. I think 1 was a 47mf, I'd have to find my markups for the other... and what it was wired to. Middle position was stock JVM. I did it on a pair of 410's that I love. It's been a long time since I made those mods...

The 'reversed' part was kind of a joke. I put my negative feedback pot above the 3-way switch, and covered it with an ugly knob from a junk guitar.

Your finished version looks much better than mine.

 

[spacerocker]

Negative Feedback Mod This has the result of allowing you to change the negative feedback; fully left (or '0') is stock NFB - which I actually prefer - and moving clockwise gradually increases the resistance for the NFB. This has the effect of 'opening the amp up'; you'll notice the depth and presence don't have much impact (if any) when the new NFB pot is fully clockwise.

Firstly - congratulations on having completed the mods successfully! I am a little surprised that you went for the negative feedback mod on a HJS, as the HJS already has the negative feedback reduced (compared to a standard JVM) to around the same level as a 2203. However the mod does allow you to go further, to "Vox" levels if required...but that could explain why you said you prefer the mod on "0" , or at stock level....

Negative Feedback Mod

Instead of placing another resistor in parallel with R58, I cut the 3rd blue cable at the NFB block - the 3 blue leads on the board, and the cable to cut handily has an arrow above it on the PCB - and put a 1Meg log pot between the cables. You'll need to obviously extend the cables by soldering extra

It's a good job you didn't put a resistor (or a pot) in parallel with R58 (actually R132 on a HJS), as this would have had the effect of INCREASING the amount of negative resistance - which is not what most people want...However by fitting the pot in line with the cable, you put the pot in SERIES with R132, which gives the desired effect of reducing the amount of negative FB as the pot is turned clockwise

AFD / Rhoads Mod

This was the scariest mod to do due to having to solder to an existing resistor on the PCB, and it's a VERY tight fit.

To do this, get an On-On-On switch. For the middle two arms, wire a length of cable to each side. For the top two, connect a 3k7 resistor (I had to use a 2k7 and a 1k in series). For the bottom, connect a 100nf / 0.1uf cap in series with a 2k7 resistor.

I think you probably used an "ON-OFF-ON" switch (typo?)....An "ON-ON-ON" switch actually isn't a switch at all!

That is a MASSIVE cap you have used there! What voltage is it? 400V, 600V? You only needed a 63V Cap for that mod, which would have been considerably smaller... The problem with big, heavy components like this is that the legs can eventually fatigue with the weight of the component flapping around, and can become disconnected over time....

good job on the mods... the color of that new knob would aggravate my ocd traits every time I looked at it!

I agree! - however the slightly paler knob colour fitted to the earlier JVMs is now almost impossible to source. Marshall changed supplier in 2018 and since then they have used the slightly darker gold......

 

[Hackingthestrings]

I am considering doing the negative feedback and AFD/Rhoads mods to my JVM 215c, will these work on this model also? I will probably just use a 250k pot however.

I think I may do the choke mod and the OD2 to OD1 mods along with these. Can anyone help with the schematics for these please? Heck even the schematics for the AFD/Rhoads and neg feedback mods?

Thanks for the help!

 

[stickyfinger]

No way a cap and resistor magically makes the JVM a AFD or RR.
Most of those JVM forum mods were duds to begin with.

 

[Scholly]

Most of those JVM forum mods were duds to begin with.

No.
And where's your evidence for that anyway?

My 205 has a few of these mods, and I A/B'ed it to a stock one. Totaly different sound and feel.

 

[Hackingthestrings]

What mods did you make?

 

[Scholly]

Lemme see...
Added a choke, OD2 to OD1 (but only changed the resistor, so the bass-response stayed tight and punchy), and lowered compression.
And not realy a mod, but I put in KT77s. Gives a bit more headroom, but doesn't change the sound. I all so put in 5751s in V6 and V7, to bring the gain down and make it a bit more useable.
Those mods made the amp sound super aggressive and snarly.
Dunno what stuff you play, but if you don't play heavy stuff, you might wanna do some of the other mods.