Updated Mark III+ Info

Yeah that's correct the bus wire from volume 1 to the treble pot is replaced with a 100K resistor. It's a hangover from when the RP10 IICs had the + conversion done to them as Mike B leaves this resistor in place.

I should point out that the 100K on its own does NOTHING. But when combined with adding a 15uF on V2B cathode (the one with the 1.5K cathode resistor - the signal goes V1A > V1B > V2B > V2A > OUTPUT on the IIC+) it weirdly creates an additional gain boost. Can be used as a switchable ++ easily enough by switching the 15uF on V2B on and off with the 100K resistor hardwired in. Easier than adding in the relay and all the additional extras, but you lose the 3.3M/10 or 20pF addition. There are also some hard wired in changes to the lead circuit in a switchable ++ mod but I'm not sure on the exact effect of these. I normally keep it simple. Switchable 15uF off V2B is enough for me

See details and discussion of this fire breathing dragon mod here:

http://forum.grailtone.com/viewtopic.php?f=27&t=40649#p292545

Oh and following up from this old thread this is the latest info on the III+ for all stripes

https://forum.grailtone.com/viewtopic.php?f=27&t=74714&start=60#p521928

J

Err where to start...

.Anyway, that 15u , like the some other bypass caps, is it a 10v tantalum?

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Yes. Usually a T110. Could be a 20V or higher though. Or a yellow bullet T322. Or whatever they had spare at the time...

I'm a little tech savvy but obviously not enough

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I don't want to stop you tinkering, but on a IIC+ if you don't know the circuit or electronic design or what you're doing 100% I would leave everything alone or send it to Petaluma for servicing or any upgrades if you can. They're too precious an amp to be playing around with IMO.

Could you please explain hardwireing in the 100k and to make it switchable. Does that mean I can activate this extra gain whenever I want?

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Again read what I wrote before. It's not the 100K that becomes switchable... it's the 15uF on V2B. It's a poor man's ++ but easy to do. Not something offered by the factory. ALL the info is there... https://drive.google.com/open?id=1ieTnMLeSiNDoXu_9Or0ky2RIDwbLAcUQ
http://forum.grailtone.com/viewtopic.php?f=27&t=40649#p292545

You can switch the 15uF on V2B ON/OFF using a 10K push/pull pot from a Mark III. But you need to mod the circuit area to make it fit. Or you can make it footswitchable easily as it's literally an ON/OFF switch. Or both! If you don't know how to do this, don't ask. I am not responsible for people tinkering and destroying their rare expensive amps and harming their value. Seriously if I was buying a IIC+ and I found out someone other than Mike B had done ANY work I would be very suspicious and could massively harm the value. Just something to be wary of.

So, the 100k is useless between the pots if there is no bypass cap on V2 B?

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Yes... IT'S A HANGOVER FROM A IIC! Read the topic I sent you about it. Factory IIC+s don't have this. Only C++s have a cap on V2B... the 'fire breathing mod' isn't anything offered as a factory mod. It's a poor man's ++ but works and works well!

My amp came with a 1.5k resistor there. I read where Mike put other values there

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It's always been a 1.5K as far as I know.

I seen some gut shots of a C+ with that 100k tween pots but when I zoomed in, I didn't see any cap on V2b

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This 'fire breathing dragon' mod isn't anything offered at the factory, just something mentioned on here. It won't have a cap on V2B unless it has a ++ mod done to it, along with a load of other stuff. It will have a 100K resistor if it started life as a IIC. Usually an RP10 or SP10 board but there are some IICs with RP11 or SP11 boards. The 'fire breathing dragon' is a poor man's ++ mod essentially, but you can make it switchable at any time easily and I like the boost.

I have a Simul and that 3.3m/ 10pf circuit, all schematics that I've seen say 20pf on Simul.

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You also have the cap present around the 270K resistor on the lead circuit right? PM me some images of the power board and we'll go from there. Are you sure it's a simul? Some are labelled up as such but are actually 100W amps...

Basically all IIC+ work should go to Mike B unless you live in some far away country.

If you could delete those other posts on here about the 100K resistor that'd be great as hopefully I have answered enough info on here as to not muddy the waters more about the murky C+ info out there. It's not good practice to post off topic or on old threads generally.

Cheers,

Jon