Welcome @jaylow!Well,
nothing at all coming out is actully better than "slightly muzzy" or whatever.
{Looks at
Prof Tweed circuit}
AAARRRGGHHhhhhh... OH NOEZ, not THIS :duh thing AGAIN. I do wish
runoffgrove.com would withdraw this abomination.}
We have already had several threads about this so-called "design". You may notice that in the "Baby Bassman" NOT ONE of the anode loads is a trimpot. The reason for this is that the person who originally designed the Bassman,
Leo Fender, actually knew something about electronics.
The long and short of it is, if you were going to make
anything adjustable to compensate for FET spread it would be the Source resistor, not the Drain, because the Source resistor only effects the DC conditions, while trimming the Drain changes both AC and DC conditions at the same time (and incidentally allows the Drain load to be "adjusted" to zero which is an absurd situation in a preamp).
So, what can be done?
Set all trimpots to the maximum resistance (lowest Drain voltages).
In each stage reduce the trimpot value until the Drain is at half the supply.
Check that the Source voltages on each FET are a fair bit lower than the Drain voltage (i.e. the FET isn't saturated).
Connected to an amplifier you should be able to hear crackles and pops when you probe around the output with your meter. Follow the circuit back towards the input probing at each Drain and Gate point to confirm you are getting pops and crackles. Where this stops happening you have something wrong, a wiring mistake, a missing strip cut, a FET in backwards, a solder dag shorting tracks, etc etc.
There is no way to predict the overall gain because the "design" is totally at the mercy of the FET spread, when a good design is independent of component spread.
This is my take on a FET preamp - no trimpots at all.
(http://s1341.photobucket.com/user/Roly49/media/Solid%20State%20Guitar%20Amps/FETPreamp121003_zpsc44f4831.gif.html)
http://www.ssguitar.com/index.php?topic=2912.15 (http://www.ssguitar.com/index.php?topic=2912.15)
Thanks for the debugging tips! Hopefully I can find the problem and fix it up. It's my first foray into DIY circuits.
I've had to put my project on hold. Turns out I had a bunch of bad transistors.
Quote from: jaylow on May 01, 2013, 08:05:29 PM
I've had to put my project on hold. Turns out I had a bunch of bad transistors.
That's quite unusual unless you got them second hand.