BIAS issue, not sure what to do

[droneshotfpv]

Good morning fellow enthusiasts! I have an issue with setting the BIAS on my newly built Mofeta pedal. So here goes......

First, I have built other pedals with BIAS settings ( i.e. Blue Shoe Gai Pan) without any issues. I used the same "model" trimmer pots as suggested, the 3362p's from Tayda, etc. What I did notice on the Blue Shoe Gai Pan was similar to this, but not as extreme, and I will explain.
The issue, first and foremost is that when using the trimmer, rolled ALL the way back to the left, it will not go lower than 11.2v on Q1, 11.35v on Q2, and Q3 not less than 11.1v. On the Blue Shoe Gai Pan all trimmers went to around 3.8v - 4.1v except the furthest right one, which only went down as far as 6v'ish, but that was WAY better than these.
I used 20k trim pots as suggested on the BOM provided by @PedalPCB , and as the others, I ordered them from Tayda in the 3362p format.

Suggestions? Did I screw up somewhere? Also, just as an FYI, it sounds pretty good as is, not gonna lie. I am fine with how it sounds right now, but I have not had a chance to do more than a fast play through of a couple sounds / turns of the dial as I was in a hurry this morning, but it just bugs me as I saw in a previous thread that 4.5v was recommended as a starting point.
I am concerned that with that high of voltage that it might not sound like it should, or that at higher volume levels it might clip out hardcore, but to be honest, I am not REALLY certain what high voltages do in this case... maybe it's a good thing? lol
Anyway, I am open to whatever I need to do, or I will just leave it be if that is what I should do.

Thanks in advance!

 

[Robert]

Where did you get your 2N5457s?

 

[droneshotfpv]

Where did you get your 2N5457s?

I think Tayda, but it could have been mixed in with a batch I got from Amazon (which was quite some time ago). Is there any tests I could perform to verify their legitimacy? They are stamped properly, but we all know that could be easily cloned. I have a TC1 tester and of course my DMM's

 

[droneshotfpv]

Where did you get your 2N5457s?

So I combed through my purchases, and apparently I only bought my 2n5457's from Amazon, not Tayda. I purchased them almost 2 years ago ish it seems, and this is what was listed as the specs (didn't verify it at the time, as I had no clue what I was doing)

Part NO.:2N5457
Package:TO-92-3
Description:JFET N-CH GP 10MA TO-92
Supplier:SICSTOCK
Current - Drain (Idss) @ Vds (Vgs=0):1mA @ 15V
Drain to Source Voltage (Vdss):-
Current Drain (Id) - Max:-
FET Type:N-Channel
Voltage - Breakdown (V(BR)GSS):25V
Voltage - Cutoff (VGS off) @ Id:500mV @ 10nA
Input Capacitance (Ciss) @ Vds:7pF @ 15V
Resistance - RDS(On):-
Mounting Type:Through Hole
Supplier Device Package:TO-92-3
Power - Max:625mW


If this info is correct, I will check the ones I have against this info and go from there.

 

[Robert]

These are the measurements from one of MMBF5457's pulled from my prototype build. (DCA75)

Vgs(off)=-1.52V at Id=5.3µA
Vgs(on)=0.13V at Id=3.55mA
gfs=3.3mA/V at Id=2.1mA to 3.6mA
Idss=3.09mA at Vds=3.00V
Rds(on)=280.2Ω at Id=3.6mA and Vgs=0.0V

 

[zgrav]

I used SMD 2N5457s from Tayda. Had no trouble getting the bias voltage down below 4.5v. I think I have them dialed in to about 4.75 now and plan to leave them there.

 

[Robert]

Same here, mine came from Tayda as well.

 

[droneshotfpv]

Same here, mine came from Tayda as well.

If my hunch is correct, and I got fakes from Amazon (shocking, right? :-D ) I will probably order the SMD versions and just carefully solder them on instead of the stuff on there now. I do intend to put it through it's paces though to see the tones / sounds I get "as is" , because so far, at low volumes and a super quick test, it sounds pretty good.

 

[droneshotfpv]

here is what my tester reads.. I don't have a DCa75 (yet) but I have one of those ATMEGA328 / TC1 testers. The results are just a couple tested from the batch, and the final 3 results are the ones IN the pedal:
id=0.19ma @vg=134mV
0.18ma @vg = 126mV
0.22ma @ vg = 151mV
0.23 @ vg= 159mV
IN THE PEDAL:
1.2ma @ vg=15mV
1.2ma @ vg= 27mV
Can't get the reading off of Q3 with it in the pedal, will have to pull the board and test.

 

[droneshotfpv]

Just an update. I pulled the board out of the pedal, and tried to test the Q3, and got the same reading I was getting with it IN the board. It comes back as "2 diodes" instead of what the others were reading. Is this potentially the issue?

 

[droneshotfpv]

Just an update for information. I was talking with several members here last night, and went ahead and tried some 2n5485's (@Chuck D. Bones , sorry, I thought they were 2n5458's, I transposed the numbers slightly lol) in this circuit just to see if I could get the voltages to cooperate.
The short answer, yes... I can now bias them between 0.785v and 16.1v. I was able to set the voltage to the suggested 4.5v for this circuit, and it sounded awesome! Thanks to a very generous member here, I have some Genuine 2n5457's inbound, with which I will test this circuit out (my 2n5485's are spoken for, otherwise I would stick with them) and verify voltages, as well as sounds, and do some comparisons with other JFETs just for information sake since I socketed my JFET holes (after this fiasco lol That was harder than it should have been... if you're gonna socket for test purposes, do so before hand, don't be like DroneShot kids! lol)

Let this be a lesson... While I know better NOW, in my earlier days of ordering, I thought Amazon was a trustworthy source.. I have received some genuine parts from them / the sellers on there, but it's a serious gamble, as in the case of these 2n5457's I received. They are either non genuine, or potentially so badly spec'd / rejects that they aren't useable for the needs of this circuit.

 

[bilisnegra13]

I know is an old thread.. but i just build one Mofeta and i cant get below 11.2V for bias... Im also confused why people talk about 4.5V for bias and others 9V.... Since it has a charge pump to 18v it supposed to be biased on the half ( roughly) which is 9V i suppose... Please help and enlightenment on this ! Cheers