Re: the pimp / mod the Poor Man 660 thread
« Reply #200 on: June 21, 2011, 12:40:07 PM »

I have go dig around to find the internal pics. These are all I could find. I took the second pic about five years after I built the thing.

Nice pictures!!, really good unit!
how about the schematic for this unit?....do you have a copy?

I love the Vaccum Tubes Sound!!!


how to add the M/S to the pm670?
« Reply #201 on: December 26, 2012, 11:04:47 PM »
HI All,
i like to know if it's possible and how to add the M/S to the pm670 like the original fairchild?
I use the edcor transformer.

Re: the pimp / mod the Poor Man 660 thread
« Reply #202 on: December 27, 2012, 12:06:54 PM »

My 6k4p tubes just arrived :)
Please help me how to connect the pins to triode mode!
I've read the post about it, but not 100% clear...
Pin 1 is for grid, 3-4 heater, but dont know the others... :-\



There's been no mention of stereo linking method to date, and it's not on the PM schematic. The Fairchild simply ties the two control voltages together.  Here's some food for thought.

Many limiters use diode isolation to ensure dual mono and stereo operation match in timing. The Fairchild fails to correct for this, for better or worse. As is, in the original design, if you toggle between linked and unlinked the time constants will change because of the nature of paralleling networks. We do not have the complexity of time constant switching found in the Fairchild, so it's not as if I'm suggesting we alter something that already mimics exact original operation. As far as I can tell, the only thing possibly changed by addition of diodes is the relatively minor need to overcome the diode voltage drop through additional drive on the threshold control. 


Here's a comparison of the mono Collins 26U-1 with the stereo 26U-2 for example.  This is not to be taken as a cookie-cutter drop-in, given the differences in design.  The Collins being the more common type of side chain driven from a high impedance source with fairly high bias voltages present, and the PM660 being a side chain driven from a low impedance source with fairly low bias voltages present.   The Collins manual describes the 1N459's as preventing feedback into the 1 mfd caps, to avoid increase in attack time from effective lowering of 1 mfd value. 

The unattached connection from the 0.01 cap below runs to the link switch, then to the other identical side chain and audio channel. 

Could this idea be brought in to a pair of Federal am864 builds?  Ive been thinking about adding a link function and not sure of the best way to implement it.  Any thoughts would be great!!!

Re: the pimp / mod the Poor Man 660 thread
« Reply #204 on: December 09, 2018, 06:41:52 PM »
At the moment I've got on channel modified and the other stock for a quick possibility to compare.
I wil certainly stop my research now for today (I've got children to pimp ... sorry, to wash  ;D)
So, about 8ms attack time and 300ms release time : remove R17, shunt (or not it's not important here) R18, shunt C4 and change the pot (RV7) for a fixed 1M resistor.  :D

Listen, compare, it's really different !!! A lot quicker than stock, and maybe clearer, but I'm not sure I must perfect my settings between the two... I want to be sure they sound close without compression but I won't have time enought today to do this...

See ya later  ;)

Hey Lolo-M,

 So after using tons of bus compression, I am pretty sure the quoted mod is what I am after - about 8ms of attack time, and 300ms of release. I am, however, still a bit of a newb when it comes to the lingo of things. How exactly do I shunt C4? Would I take it out and replace it with a wire?

 Also, I am curious if there is a way to have about 8ms attack with variable release?
Justin Heron
Artist / Recording Engineer


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