"Fender Blackface" FET preamp - anyone done this?

[Robin755]

No problems with running it on the +side of the supply,

the tonestack, it's ok I guess, not that experienced on that level,
if you don't like it, you can still change it by another (like the marshall one on the page)
but with my tele, it sounds quite good, nice clean tone, and good distortion,
I'm really happy with the result I'm getting,

Robin

[J M Fahey]

Nice work, congratulations. Running your preamp from "only" the +B supply is no problem, you still have a lot more than enough signal to overdrive your 1875. I think you´ll have a smoother EQ range using Log pots (what Fender uses).

[armstrom]

I have a question for anyone who has built this circuit... Is the schematic 100% correct? There are two crossing nets that do not show a connection on the schematic but I want to make sure they're really not connected...
I'm talking about the crossed nets right above R5 and R8 in the schematic.

So, if the schematic is correct and there is no connection at this point, how does the input signal reach the gates of Q2 and Q4? Or is this an AB style amplification stage where Q1/Q3 amplify the positive half of the input signal and Q2/Q4 amplify the negative half?

Also, if I don't care about having the low-gain input I can just ditch J1 and R2, correct? Will R3 still be required?
-Matt

[teemuk]

It is actually a class-A push-pull amplifier circuit, commonly known as “SRPP”. The drive signal for the upper FET is coupled from the drain of the lower FET. In this case, capacitively.

J1 and R2 can be ditched but R3 forms a RC filter together with the gate capacitance. Its effect can be extremely negligible, though, so you can toss out the resistor and see if it makes any difference.

[armstrom]

Great, thanks for the information. I like to understand how a circuit works before I build it. So far all of the circuits I've worked with (in distortion pedals) only make use of a single fet for each gain stage in a common source configuration.
-Matt

[elberto]

finally got this all together.  Something missing from the preamp schematic (that I needed at least) was a current limiting resistor on the supply rail.  I stuck a 1k 2w in series and it's working great.  lots of clean headroom, no farting out even with my humbucker equipped guitar strung with 13's, and there is a pleasant, mild grind with the volume all the way up.  as I mentioned earlier I went with a modified big muff tone control instead of a TMB tonestack, and I used J201 FETs.

[armstrom]

Do you have any sound clips? Also, what kind of power amp are you using?
-Matt

[elberto]

no sound clips unfortunately, my recording rig is inoperable at the moment.  I'm using an lm1875 power amp (kit from qkits.com).

I've been using a fender 5F4-ish amp (simpler one stage preamp and one knob tone control) and the sound is interestingly very similar through the same speaker (celestion gt-100).  I would say the 5F4 has slightly rounder break-up characteristics, but I'm really digging the lm1875 + mu-amp combination.   

[jthank]

Hi elberto,

I am planning to build the preamp with J201. Just wondering if any change in the values is needed in order to use J201. Would you give some advice? Thanks.

[Superfuzz]

Hey I was searching a thread like that from ages! I've came across that blackface with fet schematic so many times and I've never heard anyone who built it! There are some mods I'd like to do to it too! the first one would be to add a resistor on the upper fet source , they says it makes the output inpendance much lower (so you can drive a tonestack in a better way) then I'd put another resistor+bypass cap on the lower fet source (just for a sake of imitation of the real thing)..another nice thing to do would be adding a silent biasing network instead of the two big resistors ..asymmetrical biasing could be good too!
oh man I just want to be a better DIYer to make my first amp for real!

[adrianhertz]

Hello everyone i came across this post long time ago when i was building this preamp, after some time is complete, so heres a little video i recorded, not great quality but you can hear it, no effects or anything just guitar and preamp, i"ll upload some quality record later...


Sent from my iPod touch using Tapatalk

[Loudthud]

The same circuit was looked at in this thread:

http://www.ssguitar.com/index.php?topic=3373.0

[J M Fahey]

Thanks for posting