Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: PharmRock on February 06, 2021, 11:19:33 pm

Title: Switchable 2204/1987 using 2 inputs and DPDT
Post by: PharmRock on February 06, 2021, 11:19:33 pm

So I have the Hoffman Plexi-100 board that will be modified for a 50 watt amp.  Another thing I've been contemplating is whether or not I should stay with the 1987, or perhaps go the 2004 route.  I stumbled upon a couple of options, one being Sluckey's Dual 50, but I didn't want to go with adding another preamp tube, and it would take too much to modify the board I already have.  After more research on the Metro forums, I found a layout for "2203 or 1959 Bright -- Neil Special".  This layout has the 1959 Bright channel hi and low inputs, and a 2203 low input.  A DPDT switch allows the 1959 Bright Channel to cascade into what would be the second stage "cold clipper" of the 2003.  If you plug into the 2203 low input, the signal is just like a regular 2203 low input, with really nice cleans.  With the DPDT switch in the 1959 position, you get a regular old 1959 Bright Channel.

I've modified this circuit based on my limited knowledge of schematics and after taking a crash course in DIYLC.  I've attached the DIYLC file and PDF to this post and was hoping some of you "more smarter" guys than me can provide me some feedback whether or not I have this right.  From V2 afterward, its just like a 2204 circuit, including a master volume.  I just didn't draw it out.

One thing I changed from the "Neil's Special" is remove the 1959 (1987) low input, since I doubt it is something I would ever use.  I also made the 1959 bright cap on the volume pot switchable.

Thanks for taking a look.

Title: Re: Switchable 2204/1987 using 2 inputs and DPDT
Post by: PharmRock on March 04, 2021, 10:14:41 am

just bumping this thread up to see if there are any concerns with this schematic I'm proposing.  Forgive the "nonstandard" schematic layout...first time trying to do this stuff. 

I'll be using an old Hoffman Plexi 100 board and will change the "plexi" V1a cathode resistor/cap to "JCM800" 10K without a bypass cap.  The switching will be done with a mini DPDT on the front panel.  The 2204 "Low" input jack will have the treble peaker 470k/470pf on the jack.  Essentially I'm trying to reproduce this layout in schematic form (at least V1), with 1 fewer input jacks (minus the 1959 low sensitivity jack). http://www.pleximods.com/images/neilspecial.pdf (http://www.pleximods.com/images/neilspecial.pdf)

Thanks for any feedback and suggestions.