SIT on it

Back in my younger days in Milwaukee I used to hang out with Arthur Fonzarelli drinking Shotz beer talking about music and all of the cool things in life.  Today we have the interwebs and these conversations now occur worldwide.  One of my good customers in Hong Kong was interested in some SIT (V-fet) amps and with curves like a triode I had to wonder why I have yet to hear a V-fet amp with a nice DHT sound.  I offered to do a pair of nanocrystalline cored transformers for the Tonkin 2SK180 and then things got a bit out of hand.  Here are three pair nano of outputs ready to be packed up.

I had the idea that maybe much of the lacking sound I have experienced from my limited exposure to amps with these "triode-esque" devices had to do more with the with the circuit than the devices. We figured the first place to start was the power supply which needed to be choke input with amorphous cored 250mhy 2.5A chokes.

Next came the issue of driving the ungodly 2.5nf of input capacitance these devices have.  Historically an active buffer and fixed bias is used and I figured it was best to go with what was known to work and suggested a 417A into a 49% nickel 4:1 stepdown transformer.  Then it was suggested that maybe a Telefunken RE404 could be used and of course 80% nickel too.  I had spice crunch some numbers and sure enough for this particular case the DHT would do it.  The problem became actually getting a wide bandwidth 200hy @10ma  4:1 done on an 80% core.   Since there will be Coleman regs on the filaments I figured why not run that 450ma of high impedance current through a tertiary winding on the 4:1 to null the current through the primary?

The resulting 4:1's have a 200hy inductance with a window of operation of 7-11ma and when all put together this amp should have -1dB points of just under 20hz to around 40Khz.   Bias for the 2A of output device current will be in the form of a 7-8 ohm source resistor and bypass cap.  In the future this can be compared to fixed bias through the transformer secondary.

Stay tuned...

GM70, Silver and Nanocrystalline

Just finished these up for a GM70 amp Jeffrey is building for a customer.  They are wound with 0.9999 pure silver wire on big nanocrystalline cores.

The silver winding wire is brought out for direct connection to the circuit.  While I am a firm believer that it is the amp designers responsibility for the ultimate packaging aesthetic, I do have some 125mm cubed aluminum covers that will cover them if needed.

After many years of insisting that doing multiple taps only adds to the compromises I want to avoid in a design, I have finally come up with results that I deem acceptable.  Below are the plots of a similar transformer wound with the ability for 4, 8 and 16 ohm taps.

For behavior below 100hz I look at the inductance plots to make the best choice of inductance vs. current for the intended application and set the air gap accordingly.  The plots below were taken with the equivalent of a 16W excitation voltage @ 50hz.  Pink would be suitable for a parallel 300B,  Brown an 845 or GM-70 and something inbetween green and blue could be chosen for a 211.

dave