Marshall JVM 205H factory mod on V8 preamp tube?

[sedzep]

I just bought a used Marshall JVM 205h head from the original owner. He bought it new a couple months ago and it is dated 20th week of 2022 on the bar code sticker. The circuit board issue number is 06 and the board is dated 35th week of 2021. On the V8 preamp tube (see attached pictures}, there is a mod done on 2 of the pins with a resistor (looks like 470 ohm and a diode}. Any idea what this is for? The original owner has no idea that it was even there because he never opened up the amp. I can find no info or other pictures anywhere about this. Just wondering if any of you guys know anything about this mod because it appears to be factory. Thanks again, Dave.

 

[rocker68]

Interesting. Know nothing about 205H's or that mod . Looking for a schematic . How's it sound ?
Not certain that's a factory mod ? Wondering what that is accomplishing ?

 

[Whizzercone]

Doesn't look factory. Looks like someone was wrong or lying IMO.

 

[sedzep]

The amp sounds good, no problem with the sound. I just never seen that on a Marshall preamp tube before. I was just wondering what it's for. I'll just leave it in until I find out what the purpose really is. Thanks for responding.

 

[Leonard Neemoil]

Interesting.

Maybe move this to the workbench section so more techies might see it.

 

[dtier]

It is likely an arc suppression network to protect the tube on cold start up. Marshall may have added it, at the factory, because of warranty issues with that tube stage (direct coupled cathode follower,) and the run of tubes they were using. It should have no affect on tone.

 

[sedzep]

Thanks everyone for the great insight. The forum wouldn't let me link this post to the workbench. I'll just keep the mod in for now, thanks.

 

[marshallmellowed]

The design engineer is a member here (Santiago), he may be able to shed some light. Doesn't look factory to me, unless they just don't clean the excess solder rosin from the board after making a change.

 

[spacerocker]

The design engineer is a member here (Santiago), he may be able to shed some light. Doesn't look factory to me, unless they just don't clean the excess solder rosin from the board after making a change.

I came across this factory mod on a 2023 JVM I was working on. I asked Santiago about it and he confirmed that it is a factory mod. It is done to provide extra protection for the cathode follower stage. Some brands of ECC83 valves can be vulnerable to the large voltage that occurs between these pins on power up and arcing can occur inside the valve. This mod limits the voltage between the pins and allows a greater choice of valve brands to be used in this position. Hope this answers the question!

 

[Timo V]

I came across this factory mod on a 2023 JVM I was working on. I asked Santiago about it and he confirmed that it is a factory mod. It is done to provide extra protection for the cathode follower stage. Some brands of ECC83 valves can be vulnerable to the large voltage that occurs between these pins on power up and arcing can occur inside the valve. This mod limits the voltage between the pins and allows a greater choice of valve brands to be used in this position. Hope this answers the question!

This. It's Merlin's famous cathode follower protection mod.

The Valve Wizard -Cathode Follower

 

[rocker68]

Thanks for the update . Good to know / share .

 

[sedzep]

Wow, all you guys are great. Thanks spacerocker for solving this mystery. I'm definately leaving the amp as it is. This is a great forum and I'll definately be checking in more often. I recently retired and I'm catching the bug again for playing along with a good dose of G.A.S. Thanks again and God bless you all, Dave.