IIC+ with extra triode

[user 41014]

I'm hoping some of the plus officiandos or even comancharos can chime in on this "barn find".

Can someone let me be lazy and list the IIC+ stages? It is my understanding that V1 is the clean gain and V3 for lead gain and V5 is of course the PI. That leaves reverb on V4 along with the loop which I'm guessing is V3 and V4 unless it was the reverb that uses separate tubes for out/in. It also leaves V2 for the suspected tone stack and it is that theory on V2 that pertains to my best guess at the extra triode use.

As you can see the added 9-pin socket is not original by color. Only 1 triode is being used.

On the board side two things stand out for the +1 (pun intended). The grid appears tied to the DC rail on the board so it's not a typical gain stage and the coupling cap is on the cathode. The other thing of note is that if I have the signal path in my head correct it is feeding into the grid of what I think is V2A.

All that said, does this mean we are looking at a cathode follower to buffer a clean gain stage for a high performance driving of the tone stack given the lower impedance going into the TS?

I would almost think a cathode follower would be better served on the lead channel but putting it on the front end effects everything down river. I haven't looked at the original stages well enough yet so there may already be a cathode follower somewhere in the amp.

I was told this amp is blowing fuses. None of the caps are shorted. The power pin sockets looked stretched to me and after my recent experience with a rectoverb cutting out and spitting at me I can't help wonder if the pin sockets are the low hanging fruit.

 

[JD]

Interesting have you tried it with the extra 12AX7 in and out how does it sound

 

[user 41014]

I was told it was blowing fuses and have a few open projects so I've held off on this one. I need to pick up amps from Mike soon and this one may get dropped off for a total once over given it's modification. I doubt anyone has a IIC+ like this to weigh in on it.

I did a quick check for shorts on all the caps and that is as far as I got. It could of course just be a bad tube but I bought a few amps from the seller and all the tubes went into a box for the journey and are mixed.

I think next is tightening up all the pin sockets. Most if not all the tubes are old 415's so I need to get my Weber out and see if any n the bunch need to be trashed. But I'll stick some 420's or something in there to test and keep an eye out for redplating even tho the coupling caps on the PI are not shorted.

 

[xdg999]

I have a c+ with 6 12ax7's. Mike B. said the cathode follower is a buffer stage, no gain. Just tends to make it feel likes it's got more push and headroom.

 

[user 41014]

I have a c+ with 6 12ax7's. Mike B. said the cathode follower is a buffer stage, no gain. Just tends to make it feel likes it's got more push and headroom.

Awesome. Do you have a pic of yours to see if was implemented the same way?

 

[xdg999]

Here's some pics. Hope it helps.

 

[FedWar]

@electric mayhem you can find the best unofficial schematic here
https://boogieforum.com/threads/mark-iic-schematic-correct.74680/
But I think you already knew this, so I write it here just in case.

What preamp PCB revision is this? (RP11A, RP11, RP11 etc..)
Does it have any additional knob like the "limit" on the back?

The only buffer (i.e. cathode follower) that are standard on mark are the return of effect loop, and in the coliseum I think also the limit circuit (which I haven't seen any schematic of)

 

[user 41014]

Here's some pics. Hope it helps.

It more than helps- it makes me very comfortable knowing an exact brother of this one exists. One off is cool but built to standards on a machine blowing fuses eases my head. I can proceed without the design being in question.

This is 1386*, not the latest but one of the later ones. How common are the 6-tubers?

What preamp PCB revision is this? (RP11A, RP11, RP11 etc..)
Does it have any additional knob like the "limit" on the back?

The only buffer (i.e. cathode follower) that are standard on mark are the return of effect loop, and in the coliseum I think also the limit circuit (which I haven't seen any schematic of)

Thank you for letting me be lazy. This is RP11A (1st pic). I'll look at that schematic to train my eye.

There is no limit board in or knob on the back of the DRG. I thought they were only on the very late IIB/IIC/IIC+ coli's.

I have not seen a 7 pre tube coli with this tonestack driving buffer (just an inactive 6FQ7 buffer on the PI I never really figured out). But speaking of 6 tubers there is a limit on my late II coli as you see here with the late boo bear

 

[FedWar]

How common are the 6-tubers?

I would dare to say "unicorn-like"

The tone stack is pre distortion, it is plate fed and it has a recovery stage afterward, so I don't really understand what's the point of having a cathode follower here.

 

[Jay Omega]

Interesting that these are different.. If I'm looking at these correctly, @electric mayhem - yours looks like it's a buffer for the FX loop send. @xdg999 - I can't quite make out what yours is doing with the supplied pictures, but it looks like it's post effects loop

 

[JD]

Interesting that these are different.. If I'm looking at these correctly, @electric mayhem - yours looks like it's a buffer for the FX loop send. @xdg999 - I can't quite make out what yours is doing with the supplied pictures, but it looks like it's post effects loop

Exactly Electric mayhem's version runs off V2b and xdg999's runs off V2a, same place just 1 before fx loop and other after I will try it with mine and see what it does. Very Interesting

 

[JD]

Interesting that these are different.. If I'm looking at these correctly, @electric mayhem - yours looks like it's a buffer for the FX loop send. @xdg999 - I can't quite make out what yours is doing with the supplied pictures, but it looks like it's post effects loop

Exactly Electric mayhem's version runs off V2b and xdg999's runs off V2a, same place just 1 before fx loop and other after I will try it with mine and see what it does. Very Interesting

 

[quad48uF]

It more than helps- it makes me very comfortable knowing an exact brother of this one exists. One off is cool but built to standards on a machine blowing fuses eases my head. I can proceed without the design being in question.

This is 1386*, not the latest but one of the later ones. How common are the 6-tubers?


Thank you for letting me be lazy. This is RP11A (1st pic). I'll look at that schematic to train my eye.

There is no limit board in or knob on the back of the DRG. I thought they were only on the very late IIB/IIC/IIC+ coli's.

I have not seen a 7 pre tube coli with this tonestack driving buffer (just an inactive 6FQ7 buffer on the PI I never really figured out). But speaking of 6 tubers there is a limit on my late II coli as you see here with the late boo bear

That's sweet! Would love to hear that amp.