aviel

Building the Fetboy Thread
« on: December 17, 2018, 05:46:55 PM »
Hey,
I decided to start a new thread to  share my building process and hopefully get some insights.
this is actually my first "complete" diy project (i.e - not a kit).

i built 2 fetboy gain modules on veroboard, i plan to create a "modular" preamp where i can change the gain modules to test different flavours. this all should be on a 500 series style board.

i just finished the 2 blocks,  i connected them, wired up an OEP A262A3E transformer, and started expiremnting.
i dont plan on using an outputput transformer, but i might add a blanced line driver.

i do have some  thoughts, maybe you guys and gals can shed some light on:
1. what should be the resistor value connected beetween the secondaries of the transformer?  the gain block itself has input impedance of 10M..
2. i noticed the darlingtons  are getting quite hot, even though only ~30mA goes through them, should i worry?
3. i plan on adding a That 1646 line driver after the second gain block, with a volume control similiar to the one between the 1st and 2nd blocks, to get a trimmer kind of behaviour. what do you think?


aviel

Re: Building the Fetboy Thread
« Reply #1 on: December 17, 2018, 05:47:48 PM »
attaching more pics

My3gger

Re: Building the Fetboy Thread
« Reply #2 on: December 18, 2018, 06:53:57 PM »
We have Fetboy build thread with lots of ideas and answered questions, you might want to check there. I only see block schematic of Hamptone HJFP, what's needed is circuit schematic. HJFP isn't exactly the same as Fetboy, later has more common transistors and a few differences for simple build.
Recheck values if darlington gets really hot. Iirc, HJFP doesn't have termination resistor across input transformer secondary.


Re: Building the Fetboy Thread
« Reply #3 on: December 18, 2018, 07:55:01 PM »
Its me again  ;)

1) The HJFET preamp has ~ 200k over the 1:10 input tx secondary, so the input impedance at primary winding is 200k/ (10x10)=2k. If you want to establish the same situation with your transformer you have to do the math. 2k x (6.45x6.45) = 83,33k. If you are heading for better squarewave response you could google "Zobel network".

2) it is warm there, some have used heat sinks

3) I think it is a good idea.
« Last Edit: December 19, 2018, 03:45:37 AM by rock soderstrom »

My3gger

Re: Building the Fetboy Thread
« Reply #4 on: December 21, 2018, 01:48:44 PM »
You are correct, i looked at my 2 series HJFP modules where OEP has secondary loading resistor of about 80kohms.
The only unloaded mic input in my gear is Chicago 1:10 into tube preamp RCA BA-2. Also tried various loading resistors for modern 1:10 Cinemag trying to compensate falling highs from 17kHz in OT, only to find out IT works best with suggested value... I will check once more why some older tube preamps don't need sec. loading.
When Scott Hamptone shared schematic he suggested certain manufacturer for 2N5457. Mine also has MPSA14 darlington, ZTX653 and LM137 reg as per schematic. I didn't have to use heatsinks, about every 2nd Fet was ok without changing values.
Here is HJF schematic with explanation and thread about biasing values:
https://tapeop.com/interviews/37/diy-jfet-mic-pre/
https://groupdiy.com/index.php?topic=1184.20
It is very refreshing seeing someone doing HJFP on veroboard, not another kit :) I suggest playing with a bit NFB and resistor values because modules have a range of useful sounds.

aviel

Re: Building the Fetboy Thread
« Reply #5 on: December 22, 2018, 06:19:18 AM »
Scott suggested 200k. The reason i was curious is because in fabios schematic he wrote 27/50/100k.

I went with the 200k for now. Will se how it goes.
 
After trying to change caps values and playing with the trimmers, i could barely hear any change when adding\removing the 220uF bypasd capacitor...

I also used mpsa14 and ztx and trimmed the current to 40ma as suggested. They get HOT.!!

aviel

Re: Building the Fetboy Thread
« Reply #6 on: December 23, 2018, 07:31:29 AM »
here are some more pictures of work in progress build.
it is not pretty at all  :-[

everything seems to work untill now. i am going to add now the That 1646 line driver with trimmer knob.

I've noticed a strange behaviour when scoping the signal at the input of the 2nd block:

seems that if i lower the gain, i get more clipping on the input of the 2nd stage. that doesnt make sense at all, the output of the 1st stage sees constant 100k and doesnt change no matter where the pot is, but for some reasone the input signal to 2nd stage is clipped! this dissapears when i maximize the gain...  ???





 

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