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F6 Clone Convertible PCB blog

Posted 27th January 2013 at 12:48 PM by Tea-Bag
Updated 29th June 2013 at 07:30 PM by Tea-Bag

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

Click the image to open in full size.


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.

Click the image to open in full size.

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.

Click the image to open in full size.

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.

Click the image to open in full size.

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.

Click the image to open in full size.
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.

Click the image to open in full size.

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.

Click the image to open in full size.

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.

Click the image to open in full size.

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.

Click the image to open in full size.

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.

Click the image to open in full size.

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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Comments

  1. Old Comment
    buzzforb's Avatar
    Very nice Tea.
    permalink
    Posted 27th January 2013 at 02:53 PM by buzzforb buzzforb is offline
  2. Old Comment
    PKI's Avatar
    Wow, thanks for the blog! If I buy jfets from Spencer and they are calmed to be BL, I can trust and use just one( n and p) of the, right?
    permalink
    Posted 27th January 2013 at 07:55 PM by PKI PKI is offline
  3. Old Comment
    Nice job TB - I like your distortion figs
    permalink
    Posted 6th February 2013 at 10:10 PM by flocchini flocchini is offline
  4. Old Comment
    Tea-Bag's Avatar
    PKI, yes you can use 1 BL < 8ma, or two GR's which typically go no higher than 6ma. This is for situations where you dont but in a source degeneration resistor (10R) and have it jumpered as I do in these pictures. If you got >8ma IDSS small Toshibas then add the degeneration resistors. I recommend going without degeneration if possible. Just less stuff in the circuit.
    Over 8ma on one jfet with no degeneration will exceed their .4w rating. This nelson discussed in the 6moons article. Using two GR's, I can get 12ma total over .8w dissapation total with no degeneration.
    permalink
    Posted 7th February 2013 at 12:19 AM by Tea-Bag Tea-Bag is offline
    Updated 7th February 2013 at 12:22 AM by Tea-Bag
  5. Old Comment
    "Note the jumper on the right, next to the letters G and G." This should be the jumper on the left, correct?
    permalink
    Posted 7th February 2013 at 03:03 PM by bugzbunny bugzbunny is offline
  6. Old Comment
    Tea-Bag's Avatar
    Quote:
    Originally Posted by bugzbunny View Comment
    "Note the jumper on the right, next to the letters G and G." This should be the jumper on the left, correct?
    Yes correct. I will edit.
    permalink
    Posted 7th February 2013 at 07:34 PM by Tea-Bag Tea-Bag is offline
  7. Old Comment
    6L6's Avatar
    To clarify, with R100's and no degeneration you want the K170/J74 to have an Idss of 6ish?

    And with R100, the bias LED should be (1) blue with about 3Vf
    permalink
    Posted 9th March 2013 at 11:09 PM by 6L6 6L6 is offline
  8. Old Comment
    Tea-Bag's Avatar
    Quote:
    Originally Posted by 6L6 View Comment
    To clarify, with R100's and no degeneration you want the K170/J74 to have an Idss of 6ish?

    And with R100, the bias LED should be (1) blue with about 3Vf
    For no degeneration on the small JFETs, use <8ma - else will need degeneration there. Otherwise 2x GR grade will do 2x dissapation and these are usually 5-6ma each.

    permalink
    Posted 10th March 2013 at 01:09 AM by Tea-Bag Tea-Bag is offline
  9. Old Comment

    Images

    Unable to see the images. Anything I should do to access?
    These were viewable about ten days ago
    permalink
    Posted 12th June 2013 at 10:23 AM by bravi bravi is offline
  10. Old Comment
    Tea-Bag's Avatar
    the pics are currently down in all my blogs. They are all links to the site itself, and probably went down after there upgrade.
    I will ask about it.
    permalink
    Posted 12th June 2013 at 01:32 PM by Tea-Bag Tea-Bag is offline
  11. Old Comment
    Great work mate.
    permalink
    Posted 5th October 2013 at 10:41 AM by 2 picoDumbs 2 picoDumbs is offline
  12. Old Comment
    Where is the bom for this amp??
    permalink
    Posted 4th November 2013 at 10:16 PM by mk57 mk57 is offline
 
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