Professor Tweed help please!!!

[RoCe]

Hi everybody, I'm Roberto.
I have a question about Professor Tweed's jfet. I buy 4 mps102 and I build Professor. Then I set all drain voltage like ROG, but source voltage are different 2-2.2 V. I test professor into clean channel of my tube amp, but the sound with the volume knob all way up, isn't so dirty like ROG sample! I think that I have jfet with wrong Vgs and Idss!!! Should I make a selection of mpf102? How jfet are best suited for professor???
Thanks a lot!!!

[petemoore]

J201
 NTE458 [highest gain IIRC]
 these transistors are higher gain than the MPF102, and will make the circuit distort more...[if everything else is 'right'].

[RoCe]

I don't think that's only a gain problem.
If I want to obtain a response  similar to ROG with mpf, which mpf I must use? Which features mpf trannies should have?
Thanks...

[petemoore]

They say try different ones in Q1 position, that's where you'll notice the gain differences of the Q's more. Always Reset the bias for each Q swap.
 After finding a loud ones in Q1, use the loudes ones for the transistors in the Cct. or adjust to taste.

[RoCe]

They say try different ones in Q1 position, that's where you'll notice the gain differences of the Q's more. Always Reset the bias for each Q swap.
 After finding a loud ones in Q1, use the loudes ones for the transistors in the Cct. or adjust to taste.

Thanks petemoore!!! But what is the transistors in the Cct."?

[petemoore]

Sounds like you're using MPF102's but I suppose any Jfet that meets the needs would work.
 MPF102's are low gain devices compared to J201's or NTE458's which have the highest gain of the three.

[RoCe]

Hi,
I resolve the problem, I change all tranny with j201 and I have intresting sound up to abount 3/4 of volume knob.
Furthermore I do some simulation whit ltspice and I notice that the feddback capacitor alter a lot the frequency responce in particular the slope cut of at bass freq (about 30Hz). Indeed the princeton have a more smooth slope (about 20dB decade) while professor with a capacitor in ROG advised rage have a more high sloope (about 40dB). With a capacitor in pF rage (120pF) I obtain that the two responce are more likely.
What do you think about it?
Thanks a lot!!!

[onboard]

The Q1 source resitor bypass capacitor make alot of difference. You simulated it with a pF range cap and got a slope like the Princeton? Cool. Did you try a sound test?

The actual Princeton cathode bypass cap is 25uF  - I thought the JFET emu technique used a 10:1 rule for cap values - guess there are no rules :wink:

[RoCe]

No the feedback cap!
The sound is more full, but is also more overdrived because I think that feedback is strongly reduced! Anyhow the sound with the vol knob at an half is in my opinion very nice, specially with a Les-Paul style guitar.
Thanks all, in particular at ROG crew!!!

[petemoore]

Oh yeaH
 Sounds like you got sounds like you like.
 Messing with them Caps that Make a Dif'...you bet man !!! I think I even know which one you're talking about, but absolutely...
 Sometimes switching capacitance values does wonders, source bypass...for revoicing/boosting...
 tube to Fetsim circuits?>  messing around with the gain levels on each stage, and the voicing...maybe even to the point of adding/subtracting values or caps themselves, even whole gain stages...might get some extra mileage for those who choose to test them out.
 Like most things you think might work or work better... [and these are Fet ampsims are relatively new], you'll never know unless you try.