Mark IV Rhythm 2 mod for more gain?

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Markedman

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Can a Mark IV be modded for more gain on Rhythm 2? My IV-B is in the shop getting a check-up and was wondering if anyone here has modded their amp to up the gain? I spent a few hours on this site and other sites and I couldn't find any info other information than "RHY2 sucks!" I like RHY2 and use it more than RHY1 or Lead but if I had more gain I would use it as the knob is @ 10 now. I have the JP2C and like the two lead channels and the Mk.4 would be more to my liking with just a little more gain on RHY2 like the JP2C.
If anyone knows of a way to do this, please let me know so I can pass it along to my tech Andrew @ Bell Audio in Brookfield, CT, he said he can't get to my Mk.4 for two weeks so I have time to research here and other sites, thanks, Markedman.
Also, if I put the Mark IV up for sale, would the mod be a desirable upgrade by you, my potential purchaser?
 
I don't know if it works or not. If you get it done I would be very interested in the results. Please post them in this thread....good, bad or ugly.
 
MESA says;

Kris Dilbe
Kris Dilbeck (MESA/Boogie)
May 16, 13:26 PDT

Greetings
Thanks for your inquiry and unfortunately, we do not have a mod that will increase the amount of gain or drive out of the Rhy2. I am sure a 3rd party tech can experiment for you, but we don't have an official say as we have not tested that amp with any mod like that.

Thanks

Kris
 
How much gain are you trying to get? Here's my Mark IV (A) on Channel 2 (Crunch), straight into the amp - no boosts or pedals. It's just a crappy iPhone clip, but pop on some headphones and you can get an idea:

https://www.youtube.com/watch?v=aOVZGzAHqBE&feature=youtu.be

I think it has plenty for a basic crunch. It's obviously not going to have Lead channel levels of saturation, but can definitely do a crunchy Marshall vibe.
 
RHY2 is the only channel on any amp where I can crank the treble without it getting too bright. My treble and gain are 10...while I would love to have the gain of LEAD, it has its own unique character that I love. If you have a successful mod, please let us know!
 
Mike B. modded my R2 on my IVa for more gain. Basically made it so when R2 pull fat was engaged the treble knob had more range. It did give it more gain, but honestly the tone still wasn't great. I can email you a pic if you PM me.
 
Last month I sold my IVa and replaced it with an early Rev G blackface triple recto. I couldn't be happier with the trade. :D
 
My copy of the Mark III schematic is so fuzzy I can't read some component values on it. But on V2A there's some components that are switched when you engage rhythm 2. One of those is the cathode resistor. Whatever its value, if you increase it, you'll get more drive out of it. Assuming a plate resistor value of 100K, a normal Mesa cathode resistor value is 1.8K for a gain stage that is not a high gain stage. The higher that resistor value, the more you rebias the tube for cold clipping (biased to cutoff) which gives more distortion. For reference, the Soldano cold clipper stage runs a 39K cathode resistor and that is also the value used in the clipper on a Dual or Triple Recto.

To find the value you really like, I'd consider sticking a 50K pot in the circuit, adjust it until the distortion level is right where you want it, then measure the pot value at that setting and then replace the pot with a fixed resistor of that same value.

Another way to increase drive is to increase the value of the plate resistor. Usually it's around 100K, and even on a clean silverface Fender amp, increasing that from 100K to 220K makes for a pretty rich crunch with just one resistor changed.

However, when you increase plate resistor values, what you are doing is increasing the voltage gain of the tube. It will require less input voltage change to drive the tube to cutoff or saturation. It will also go into compression more easily.
 

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