F6 Clone Convertible PCB blog

Here is the F6 board rendered by CRT and some help from others.




It's convertible premise is to be able to swap out transformers using daugter cardsand be able to swap out variations on the schematics Nelson Pass presented at the 2012 Burning Amp Festival.
Additionally, places for two pairs of 2SK/2SJ are here to allow GR's to be used to allow for higher than 8ma IDSS without degeneration on the front end. If you have 8ma IDSS BL grade for use here, than your all set.
You can use standard power resistors or 5MM thin film type.



Here are some of the daughter cards that mount to the main board. They are available for Jensen, Cinemag and Onetics Quadfilar transformers.
These are mounted with 2 2x10 @ 2.54 mm pins. Note the PC PINs of the transformers are all keyed, with pin 1 and 2 typically being closer together.



Here is how to mount the female parts to the board. Make sure they are flush, and solder appropriately. Be certain to get a good joint where the traces meet the pins.



Currently, there are some photos of the pre-production F6, the production one shown above is a bit smaller.
Here is the 'female' connectors mounted to the main board. Note the small circle in the upper left. This needs to match with the Daughter cards circle.


Here I do a test without the Fets mounted. You can test some basic voltages from here. Note with IRFs. I needed to change to two Blue LEDs to get adequate bias.



Here is a unit using IRF240's and Onetics transformer. The distance between FETs holes is 120MM, and the width is enough that it fits into the DiyAudio store's 'big chassis'. A small one was not available to test.



Here is the Cinemag transformer running on a perf board. (Cinemag boards now available) We distanced the mounting pins on the main board so one can hook up there own transformer if it's not one of the above.



The picture below shows a fully 'jumpered' board.

The input is jumpered directly to the transformer. Also note the Zen Pots are jumpered in the photo with the middle and pin towards the source pins jumpered.
The source resistors for the R100's are jumpered - here the source resistors for the Jfets are not jumpered (R11+12) because of the input directly to the transformer. This 'Unbuffered' schematic setup did not work well in my system. I got a bit of motorboating with the speakers. Then I went with just input and shunt resistors. But there was still some noise. I went for the Figure 13 buffered setup, and this worked well.

Note for SemiSouth's one pair of LED's are jumpered. For IRF240's I used two blue Leds to get higher bias. Either way, I recommend Blue LEDs with a Vf of 3V.



Distortion Figures without Source resistors or Zen Pots (Figure 13). These are done at 1W across 8R at 2.83v output using a 1K sine wave. You can see the 2K bump which is visual second harmonic of 1K source.

Here are some distortion figures for the Jensen transformer. Note less db on H2 means more 2nd harmonic distortion

Here is the Onetics in same channel.


Here is the Cinemag in same channel. All with no degeneration of Zen Pots


Notes on building the power supply board, and taking bias current measurements.

First, the F6 needs a dual rail supply.
The Power Supply board some bought is multi-purpose, so one must jumper the two halves together to connect the grounds. Note the jumper on the left, next to the letters G and G. Jumber and solder this.



When taking current measurements for a dual channel in one chassis amp. Here is what I recommend. Take a multimeter with clip leads attached and attach across the .1R resistor in power supply.
Either side will do, as it draws current symmetrically. Now we want to bias until the voltage gets to be .150v.
To do our math,
I = E/R
voltage .150/.1 = 1.5
You should start by half that if your uncertain.
When playing with Zen Pots with MOSFETS, be very careful, you don't want source resistors to go below .47R, so overall current measured at the PS needs to remain stable. I would leave it jumpered and add resistors if the harmonics do not measure well.

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