C414 EB P48 build

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Spencerleehorton

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I have noticed on the data sheets that both 2n5457 and 2n5484 have 10ma forward gate current and only give 25v gate to drain, compared with j201 50ma and 50v?
Will this make a big difference?

If so might just use 2 x 2n5457 and see what I get?
 

Gus

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You are looking at the wrong specs

The first stage JFET drain is protected being the bottom 1/2 of the cascode

IMO you are changing to many things. Get the minimum circuit to run in cardiod first outside of the microphone with a test signal at the input.


 

Spencerleehorton

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Sorry gus, dont really understand your last msg?
I am only changing the source resistors R8 and r5 and using j201 in T3 and 2n5457 in T1.
I dont have any switches and only have cardioid.
Could you explain what you meant in a bit more detail please? cascode?

I dont know what im aiming for here voltage wise, the voltages ive found before were from a work inprogress post and havent been confirmed as correct?

I understand these transistors have to be biased correctly and the FETs need correct voltages to operate properly, my knowledge is pretty low on all fronts with these, so yeah im stabbing in the dark really until i get some firm voltages.

looking at T4, i seem to get 43-44v on collector and when i twiddle R5 or R8 i can get 20-32v sort of range on base and emitter.
ideally i need to know what T3 should look like voltage wise, is 2.69v on gate correct? along with 3.11v on source or are these voltages not really important and the emitter of T4 and drain of t3 important?
there is then exactly the same problem on the other two T2 and T1?
testing at c15/r15 i am getting 41v which does drop to a steady 39v.

I found c17, c18 and c12 where the wrong capacitance, had 220nf in rather than 22nf!!!
also c8 and c9 were 100nf rather than 10nf!!

transformer seems to check out fine, 60R on secondary and 130R on primary, good inductance on it as well.

capsule ive taken out of a working fet47 which sounded fine, so no issues there, but still the same low output on one version with j201 and 2n5457 in it, on another version its just got j201 in it and its much louder!!??

i just cant work out why there is such a volume difference?

both of them sound pretty trebly as its obviously not biased properly.

i will put a 1khz signal in and see where the gain changes, T4 emitter around the 22v mark seems to sound better.
 

Spencerleehorton

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Comparing VGS on PF5102 and J201 and they look similar 0.7 - 1.6v on pf5102, 0.3 - 1.5v on j201,  GSS for both is 40v, so for T1 it seems a close replacement.
For T3 vgs on 2sk30a is 0.4 - 5.0v GSS=30v and 2n5457 is 0.5 - 6.0v GSS=25v.
I'm gonna to put in 68pf as capsule and power with the two 6.8k and 50R is replacement of the transformer and see what I get.
Is this info ok or am I looking at the wrong stuff?
 

Gus

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With the cascode the JFET drain voltage will be the NPN emitter voltage so the gate to drain voltage is in a safe area for most JFETS.

If you search for cascode you should find a lot of results

IMO you are changing to many things at one time

In one of your post the output voltages and calculated current looked good
I would work on the input stage

the 1 meg, 121k, 121k  voltage divider string sets both the NPN base voltage(1meg 121k node)
and the fixed voltage bias part for the JFET(121k 121k node)

The source resistor sets the source bias part of the JFET bias

So hopefully you can adjust the bias with the source resistor value
however if you can't you can readjust the 121k, 121k part by keeping about the same total resistance and increasing one in value and decreasing the other value to change the voltage at the center node

What company did you get the JFETs from?
 

Spencerleehorton

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i bought the j201 from ebay, im sticking with the transformer ive wound and the edge terminated capsule now, i have put in the j201 as T1 and BC550c as T2, T3 is 2N5457 and T4 is BC550C.
output wise, im getting a very good output volume but voltages are way off and ive just recorded a bit and pretty much all the waveform is in the top half of the waveform, so lacking correct bias.
R5 ive chaned from 12k to 15k and hasnt really done much to correct the voltage along with R8 ive changed to 8.2k from 6.8k.
Voltage on drain of T3 is at 27v so i need to readjust R8 and R5 and see if i can get better volatges.
I feel im so close now to get this working correctly, also i think the output is a little too loud now as i suspect i have it step up rather than step down.
Let me tear it apart one more time and correct!!!
 

Spencerleehorton

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adjusting R5 and R8 with J201 in T1 and 2n5457 in T3, i cant seem to get close with voltages, think it worked better with PF5102 and J201 in!!?
Transformer is now correct way round, just need to choose correct FETs!
 

Gus

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If you look at forums like https://www.diystompboxes.com/smfforum/index.php?board=2.0

One thread
https://www.diystompboxes.com/smfforum/index.php?topic=121553.0

You will find threads about counterfeit out of spec JFETs.
Check that you have JFETs
and that the IDSS(one of the easier things to check) is at least in the range

9VDC battery
Gate connected to source
Meter set to mA between +9VDC to drain
Source/gate grounded
read the meter
for the J201 is it between the min a max IDSS mA?
Check all of your JFETS

I would search and build one of the simple JFET testers you can find on the web
 

Spencerleehorton

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Ah ok so I could have bought a duff set of fets, I think I've built one of these before, will try and dig out or build a new tester, then test, thanks again for your almighty patience!!! I really hope to get one of these sounding good!!
 

Spencerleehorton

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finally made some headway with this:
have left R8 at 6.81k and only changed R5 to 11k.
capsule voltage at c15/3.3uf starts at 55v/53v for split second then goes down to 38/39v.
T1 is J201
E-6.45v
B-3.53v
C-4.10v

T2 is BC550C
E-6.55v
B-7.01v
C-22.80v

T3 is 2N5457
E-22.57v
B-2.64v
C-2.80v

T4 is BC550C
E-22.40v
B-22.74v
C-43.4v

have recorded a bif of drums through it at the back of the room and its sounding pretty good, have applied a bit of top cut and compression.
wanting to make two of these so i can Glynn Johns the drum sound.
At least now the voltages arnt perfect but i am getting a good signal and waveform.
 

Spencerleehorton

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Hopefully i'm starting to understand this a bit better, adjusting the source resistor so i can achieve 0.45ma on T3 and 0.350ma on T1, did i get this correct?

if voltages dont change then i need to adjust R8 to 6.2k and R5 to 11.6k, obviously if the volatges move i need to keep working the change in resistance out to achieve the required milli amps.
 

Spencerleehorton

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Have wound second transformer to exactly same spec as first working transformer and made changes to source resistors to get 0.45ma and 0.35ma but still getting low output from second build?
So must be either capsule or something in circuit, will trace signal and see if I can find what gives!!!
Capsule worked in mic it was taken out of so I'm pretty sure I've made a mistake in circuit.
 

Spencerleehorton

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working mic as follows:

T1 uses J201
D=6.45v
R2-R3=3.53v
S=3.92v with 11k resistor

T2 uses BC550C
E=6.55v
B=2.64v
C=22.8v

T3 uses 2N5457
D=22.57v
G=2.64v
S=2.61 with 5.8k resistor

T4 is BC550C
E=22.4v
B=22.74v
C=43.3v

have changed T4 as emitter and base dont seem to have the 0.6v difference but it doesnt change?

Is the second version im using same everything apart from T1 is 2N5457 rather than J201.

heres the voltages of the second version:

T1 = 2N5457
D=6.83v
R2-R3=3.83v
S=3.89 with 11k

T2 = BC550C
E=7.10v
B=7.55v
C=21.75v

T3 = 2N5457
D=21.35v
G=2.42v
S=2.62v with 5.8k resistor

T4 = BC550C
E=21.25v
B=21.79v
C=40.2v

anything jump out as wrong?
 

Spencerleehorton

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have just changed 3.3uf/c15 noticed it was only 35v, changed for 50v and checked other components on second version, plugged it back in and im getting good voltages and output is good output!!!???
must have been a dodgy cap!

Just the 3rd version to get working now!!
 

Spencerleehorton

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The 3rd version has a dodgy capsule as I measured it and it was down to 37pf!! I tuned it up to 62pf and it now works fine but this is a terrible capsule so I have ordered another edge terminated version single sided.
Sounds pretty good on guitar amps!!!
Got my head around these now!!!
Now at least I can test different caps and see how they sound.
 
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