Author Topic: Professor Tweed build report and capacitor issues  (Read 3034 times)


Professor Tweed build report and capacitor issues
« on: January 01, 2008, 06:57:41 AM »
I just finished my first Professor Tweed.
A very nice and sofisticated circuit with a lot of classic sounds available.
When I was biasing it the first time I got a warm, full sound with the typicaly airy jfet sparkle but not so much of the twangy character I had expected.
Looking closely I noticed that I had mistakenly put in all 22n caps instead of the last pair of 2n2 to ground right before the output.
I quickly replaced them with the correct values and was actually a bit dissapointed with the result at first.
I have a Tweed Champ replica (5f1) which is almost identical to the Princeton (minus the tone control) so I was able to compare the pedal with the real thing. With larger caps (the 22n's) I got the typical warm punchy growl with the gain at full tilt, but when I changed to the correct value caps this nuance was lost to a much harder, raspy and IMO not as authentic kind of overdrive. The sound also went from a kind of  12" combo character with lots of bluesy sparkle, to a somewhat louder 8" trebley twang. I hope you get the picture  :icon_wink: By all means; it sounds very cool and it is probably perfect for those who play surf, but some of the bass content is lost compared to the real thing, as is much of the mentioned "airy" sparkle which, by the way, is not present in my Tweed Champ unless I switch off the feedback loop (a neat little mod for those who own a Champ and want more distortion out of it).
For those of you who plan to build this circuit and are after a somewhat "warmer" tweed sound, I'd suggest putting in some caps of higher values than the suggested 2n2 - allthough not as high as 22n. Maybe somewhere in between. I guess the pair of 10K resistors might need to be changed as well. Personally I'm going to put sockets in mine and experiment some more. The "wrong" values I used first rendered the tone control quite useless. But I guess this can be fixed by tweaking the tone control caps.

Thanks to the guys at ROG for making these incredibly interesting emulations! Will we see other amp emulations from you in the future ? I sure hope so.  :icon_biggrin:
« Last Edit: January 01, 2008, 07:51:52 AM by Krinor »

Uma Floresta

Re: Professor Tweed build report and capacitor issues
« Reply #1 on: March 26, 2009, 10:34:51 AM »
There are lot of posts like this throughout the forum. They may not get responded to, but they ARE helpful. I saved this information a while back, and last night finally played around with the output "speaker emulation" caps. I tried a number of values, and also played around with the coupling cap between Q2 and Q3, and the cap in the feedback loop. I finally settled on the following configuration:

As you can see, I put quite a large coupling cap between Q2 and Q3 to allow more bass to pass through. Conversely, I've got a very small cap in the feedback loop. I tried many values, from ridiculously large (100uF) to moderate (.022uF) to none at all, but the little 20pF cap sounded best to me. I'm not really sure why - I just went with what sounded best. The 20pF cap has almost the same effect as no cap there at all (more gain), but I liked the tone a little better than with no cap at all.

I tried a few combinations on the output lowpass RC filters. I didn't try 22nF caps - two 4.7nF caps already cut too much treble for my tastes, so I tried changing only the last cap to 4.7n, and that sounded just right to me.

Also, I found that individual JFETs vary WILDLY in gain. I'm using all MPF102s, but the one I've got in Q1 has quite a bit of gain - more than a J201 that I tried putting in its place! I've tried other MPF102s that had so little gain that I could barely coax any overdrive out of the PT. So, it's a good idea to try out a number of JFETs until you find the ones that give you what you're looking for.

I also found biasing by ear gave me better results than trying to get exactly 4.5 volts. In fact, I've got the Q1 trimpot wired as an external control because I like to play with that one a lot.

The stock Professor Tweed actually sounds great, but I wanted to modify this one to better suit the style of playing I've taken up lately. I used to use the PT a lot on the neck pickup or both pickups on my Tele, for a chimey, just breaking up sound. Now I find myself playing a lot of surf, instrumental rockabilly and spaghetti western type music, and I wanted the PT to sound better using my bridge pickup. Stock it was too brittle for that. Now it still cuts some bass, but all brittleness is gone when using the bridge pup. It's got a very nice, meaty sound, maybe closer to a 5E3 Tweed Deluxe than a Princeton. I'm really happy with the way it sounds on all pickup positions now.

« Last Edit: March 26, 2009, 10:37:25 AM by Uma Floresta »

Uma Floresta

Re: Professor Tweed build report and capacitor issues
« Reply #2 on: April 08, 2009, 12:02:04 PM »
I'm continuing to play with the circuit, trying to get more of a 5E3 Tweed Deluxe sound out of it. This time I changed the first and third 22nF coupling caps to 100nF as in the 5E3 schematic, and returned the second one to a stock 22nF. Definitely more bass in there now. I ran out of time, but I'm going to try removing the cap from the feeback loop again, and see how that sounds. Also, I'm going to put a 100k pot in place of the final resistor for increased control of the post-gain treble. There's so much you can tweak on this little guy.