John Sherwoods se108it Mono Block SE 300B amps

ksys1.jpg (94570 bytes)

Visit John's Homepage for the most up to date info

After using the wonderful (but now discontinued) Welborne Labs V-35 amp for a year or so, I decided I needed another project which might take me even closer to sonic perfection. After hearing a Cary 300-B amp driving some decent speakers, I said to myself,  "self......this is where I need to be going !"   So..... on with the show.

I looked at schematics until I was blue in the face.  I finally decided on a topology proffered by Simon Shilton of SJS Electroacoustics. It's figure #5 on the SJS website.

I have chosen to construct this amp as two monoblocks, each vertically arranged with the power supplies on a bottom deck and the audio deck on the top.  The "rack" is of sweat soldered copper plumbing pipe in 3/8" and 1/2" diameters.  All power paths from the power supply to the audio deck are carried (properly insulated) THROUGH the copper tubing.  No wiring is visible!

Contacting Simon, I discussed his transformers with him and decided to go for it.  The transformers and chokes (88lbs) arrived safely and they are of good quality.  Of course one could build this thing by purchasing the parts helter skelter, and maybe saving a buck or two.....but these are tailor made for the application.   I believe that to implement his design, one would have to get several power transformers to accommodate the various voltages required for the three power supplies. As far as component choices go, I'm not all that picky about audiophile grade stuff. I did think it would be advantageous to use the Cerafine Capacitors in the power supplies.  The resistors are a hodge-podge of brands, but nothing exotic.  Maybe when it's up an running and well burned in and I'm attuned to it's sonic character, I'll play with some Kiwame's and such. 

The 300-B's and ceramic sockets, I purchased from ANGELA INSTRUMENTS and the GZ-378's, 6CA7's, and 5842's I purchased from that pillar of the tube community, Ned Carlson at TRIODE ELECTRONICS

kamp1.jpg (102519 bytes)
The adjustable equipment rack (left) is homemade also.


May / 29 / 1999

As of Today, both channels are completed and making music...... good music to my ears.

Everything seems to be working perfectly with no explosions, meltdowns, or noxious fumes!  It's been run 10 hours and I have just the slightest perception of some break-in improvements.  It's hard to say.  After they've broken in a bit, I'll try to a/b the thing with the V-35 amp which I've become so accustomed to over the last 3 or 4 years.

I've added a set of "Decco" lookin' meter panels, which are at present not hooked up.  'Glad the thing was breadboarded because now I see many esthetic things I want to change in the final version.  The power supply is slowly evolving to settle down the various voltages.  It makes no sense to play with "mods" until it's broken in with Simon's values in place.   Stay tuned !

OK, So here is th' beast ........ Well, the right channel, as it sits. As you can see, the bottom "deck" is all power supply, with a CV378/GZ37 for the B+ and one 6CA4/EZ81 for the plate of the WE417/5842 and one for neg. bias on the grid of the 300B.  

The top deck is: input jack to 5842 to IT (hidden behind the JJ 300-B)  and on to output xfmr.   Only 2 tubes, 2 trannies, 5 resistors and 1 capacitor on that top breadboard........... that's it.   

When all is completed and sounding good, I intend to machine an aluminum chassis plate and maybe finish and red anodize it.   Either copper or wood sides.  Don't know yet.   When finished, I'll experiment with lowering the audio deck a few inches...... it sorta' looks gawky at that height.   I think a  polished copper heat deflector or maybe a screen/shield under the audio deck is in order also.   The "BIG" tube will be centered over the meter in the final version.

JS Pictures