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Re: BJFE - Emerald Green Distortion Machine

PostPosted: 13 Jun 2018, 17:42
by Manfred
That wouldn’t reduce the gain much more, you’d be better off increasing the 2k resistor on q2’s emitter as well


I think there is a need for changing the gain of this stage, because the output level of it is limited by the clipping diodes.

After initial doubts about the AC128 stage of The Keeley Germaium Amplifier which has no base resistor,
but in that case the resistor is not needed because the biasing is achieved by the Emitter leakage current in combination with the Emitter resistor.

I still have my doubts as to the oparation without the 470k resistor.
I had thougts about and made some trials and found that the FET without the gate resistor acts as a voltage controlled resistor,
this explains why there is no stage gain in that case.

Re: BJFE - Emerald Green Distortion Machine

PostPosted: 14 Jun 2018, 05:34
by Seiche
marshmellow wrote:Yes, at that point you can just take out the cap and resistor completely. To further reduce gain you can put in a series resistor with the 22µ on Q2 emitter or take it out as well. Like Belanger said increase the emitter resistor or increase the 47k in series with the potentiometer, which limits the gain range.

But would that not reduce the distortion? I want to keep the amount of distortion. The issue is turning up the gain increases the volume too much instead of just increasing distortion. I would expect the diode clipping to limit this. I have triple-checked my diodes are properly connected to GND.

Manfred wrote:I think there is a need for changing the gain of this stage, because the output level of it is limited by the clipping diodes.

Do you mean there is no need?

Belanger88 wrote:
Seiche wrote:[...]I have also upped the 2k to 20k, but I think it should be increased even more (maybe also the 8k2)

That wouldn’t reduce the gain much more[...]

why not?

Re: BJFE - Emerald Green Distortion Machine

PostPosted: 14 Jun 2018, 09:27
by Manfred
Manfred wrote:
I think there is a need for changing the gain of this stage, because the output level of it is limited by the clipping diodes.

Do you mean there is no need?


I estimated the overall gain for the first two stages of 20 to 25, which is not much.
It was presupposed that the Drive pot is in maximum position and the 22uF capacitor is connected to ground by the Voice pot.
Could you remeasure this gain in the real circuit?

At the moment I can see no way forward at all to me, so I must reorder the FETs and build the circuit for myself.

Re: BJFE - Emerald Green Distortion Machine

PostPosted: 14 Jun 2018, 17:27
by Seiche
marshmellow wrote:Yes, at that point you can just take out the cap and resistor completely. To further reduce gain you can put in a series resistor with the 22µ on Q2 emitter or take it out as well. Like Belanger said increase the emitter resistor or increase the 47k in series with the potentiometer, which limits the gain range.

I actually just bypassed everything after the clipping diodes and the 47n cap directly to the 50kA vol pot. Sounds great, and the gain is more manageable. So apparently the last transistor has to have negative gain (less than 1)? How does that make sense? The tone is not much different. So is it only an impedance thing?

Re: BJFE - Emerald Green Distortion Machine

PostPosted: 15 Jun 2018, 00:19
by Belanger88
I have a few NSC J202’s. I could send you a handful if your willing to put the work in like you always do :D

I appreciate what you do for the forum a lot,

Manfred wrote:
Manfred wrote:
I think there is a need for changing the gain of this stage, because the output level of it is limited by the clipping diodes.

Do you mean there is no need?


I estimated the overall gain for the first two stages of 20 to 25, which is not much.
It was presupposed that the Drive pot is in maximum position and the 22uF capacitor is connected to ground by the Voice pot.
Could you remeasure this gain in the real circuit?

At the moment I can see no way forward at all to me, so I must reorder the FETs and build the circuit for myself.

Re: BJFE - Emerald Green Distortion Machine

PostPosted: 15 Jun 2018, 16:14
by Manfred
I estimated the overall gain for the first two stages of 20 to 25, which is not much.


Sorry I was wrong, I calculated the 2nd Stage as FET-stage not as BJT-stage.
The BJT stage has a gain of ~100, therefor the maximum total gain could be ~ 250.

Re: BJFE - Emerald Green Distortion Machine

PostPosted: 15 Jun 2018, 21:43
by Manfred
I have a few NSC J202’s. I could send you a handful if your willing to put the work in like you always do


Thanks for your offer but I already ordered 2N5952 FETs

Re: BJFE - Emerald Green Distortion Machine

PostPosted: 28 Jun 2018, 13:49
by Manfred
I bought eigth 2N5952 transistor.
Here the data from the transitor with the average values:

N-Ch JFET
Vgs(off)=-2,23V at Id=5,3µA
Vgs(on)=-0,23V at Id=5,01mA
gfs=3,9mA/V at Id=3,0mA to 5,0mA
Idss=6,00mA at Vds=3,00V
Rds(on)=177,7Ω at Id=5,0mA and Vgs=0,0V

2N5952 ID_UD.jpg


2N5952 ID_UG.jpg

Re: BJFE - Emerald Green Distortion Machine

PostPosted: 05 Jul 2018, 10:57
by Seiche
so if these are average values, what is the variance?

Re: BJFE - Emerald Green Distortion Machine

PostPosted: 05 Jul 2018, 21:38
by Manfred
Seiche wrote:so if these are average values, what is the variance?

measurement results of eight pieces 2N5952
2N5952_Measured_Data.jpg

Re: BJFE - Emerald Green Distortion Machine

PostPosted: 28 Jul 2018, 21:45
by Manfred
Measurement results of eleven pieces CV7351 (Military Version of 2N1308)
CV7351.jpg

Re: BJFE - Emerald Green Distortion Machine

PostPosted: 29 Jul 2018, 11:51
by Manfred
Here some curve traces of the CV7351 with the average hFE value of 128@5Milliamps
The parameters for tracing were chosen for the interesting operating point.
An Hfe@0.6Milliamps ~110 and a forward transconductance S~4 Milliamps/Volts can taken from the curves.
CV7351_Traces1.jpg

CV7351_Traces2.jpg

Re: BJFE - Emerald Green Distortion Machine

PostPosted: 29 Jul 2018, 20:18
by Manfred
I built the DC-circuits on a bread board and read the values.
The transitor used are both these with the avarage values.
The values shown in brackets are the read values from the post further up.
The next task is the AC-measurement for each stage seperatly.
Emerald_DC_Measurement.JPG

Re: BJFE - Emerald Green Distortion Machine

PostPosted: 31 Jul 2018, 17:45
by Manfred
The next task is the AC-measurement for each stage seperatly.


It takes time, because I need a new digital oscilloskop, the current one just broke.

Re: BJFE - Emerald Green Distortion Machine

PostPosted: 15 Aug 2018, 15:29
by Manfred
Seiche wrote:
I'm not using 2N1308, I'm using MP38A, I don't know if that makes a difference, the MP38A are very low leakage.


I bought a bunch of twenty MP38A transistors, one of them was defektive.
Measurement results of nineteen pieces MP38A:
MP38A_Measurements.jpg


Here some curve traces of the MP38A with the average hFE value of 86@5Milliamps
The parameters for tracing were chosen for the interesting operating point.
Compared to the CV7351 it was required to douple the base current trace range, because the MP38A has less current gain.
MP38A CurveTraces_1.jpg

MP38A CurveTraces_2.jpg

Re: BJFE - Emerald Green Distortion Machine

PostPosted: 15 Aug 2018, 16:17
by Manfred
I read the values using the MP38A transistor in the same circuit.
The read values using the CV7351 are shown in brackets, these are very close together.
Emerald_DC_Measurement_2.JPG

Re: BJFE - Emerald Green Distortion Machine

PostPosted: 16 Aug 2018, 05:40
by Seiche
How did the mp38a measure (Hfe and leakage)?

Re: BJFE - Emerald Green Distortion Machine

PostPosted: 16 Aug 2018, 09:56
by Manfred
I am used the PEAK Atlas Pro DCA 75 device for measuring the values and tracing the curves.
https://www.peakelec.co.uk/downloads/dca-pro-user-guide-en.pdf
You find the measurement procedures in the user guide.
The IcLeak means IcE0.
Therefore follows that IcB0 = IcE0/(hFE +1)
There was a fault in the chart, the unity of VceSat is Volt not Milliamps.
Herr the corrected chart:
MP38A_Measurements.jpg


BTW, the forward transconductance of a transistor is is independent of type and model.
for this reason it is for any transistor S = Ic/UT whereby UT ~ 26 Millivolts
and thus the open loop voltage gain of the stage Au0 = S*Rc
In these cases:
For the VC7351: Auo = (0,63mA/26mV)*8.2kOhms ~ 199
For the MP38A: Auo = (0,6mA/26mV)*8.2kOhms ~ 189

As already mentioned, at the moment I can't do the AC-measurements because my digital oscilloscop was sent to be repaired.
I bought a low-cost chinese oscilloscope for the meantime, but I sent it back, because the graphic resolution was too bad.

Re: BJFE - Emerald Green Distortion Machine

PostPosted: 31 Aug 2018, 21:38
by Manfred
My oscilloscop is now repaired, so I could start the AC-measurements of the device.
First the Germanium transistor stage, with simultaneous comparison using the CV7351 and the MP38A transistors.
Only the output curve traces are shown.
MeasurementCircuit.jpg

GermaniumStageInput10mV.jpg

GermaniumStageStartPointClipping.jpg

GermaniumStageInput25mV.jpg


The Voltage ranges are different, the Vrms is the same:
GermaniumStageInput50mV.jpg

Frequency response:
GermaniumStageLowCutFrequency.jpg

GermaniumStageMidFrequency.jpg

GermaniumStageHighCutFrequency.jpg

Re: BJFE - Emerald Green Distortion Machine

PostPosted: 01 Sep 2018, 05:18
by jalmonsalmon
So ...
:scratch:
MP38A's work for this and sound more or less the same as the CV7351's?
Like blind fold comparison test? :secret: