SOLVED Episode Boost biasing

[EGRENIER]

I just completed the episode boost. Now I need to bias it with the trim pot. I’m not sure what I’m supposed to look for in terms of value or even sound.

I assume I’m biasing Q1 (2n5457) based on the trim pot location. I read in some Other thread that I should aim for half of the voltage. So when the pedal is on 9V should I read 4.5V on the collector ?

Right now full, open gives me 9.22V and if I turn the trim pot CW I can drop it to 7V. I checked and I have the right trim pot 50K.

So when facing the flat side starting from the left I read:
with pot CCW all the way
9.22V, 229.5 mV and 0

with pot CW all the way
7V, 218 mV and 0

Any suggestion ?

 

[spi]

However, based on the design of the Episode Boost using a 50K trim pot, it is meant to be above half.

I can't claim it was designed this way. More likely, the jfet is out of spec and is not biasing in the range.

If you're happy with the way the pedal sounds as is, you probably don't need to be too concerned about how it biased. If you're experience some unpleasant distortion or compression, then you can either try to find another jfet, or try a 100K trimmer and see if that helps.

 

[Big Monk]

Well that's interesting...

The Episode boost as 2 mode, 9V and 18V. I measured 9.22V with the trim pot full opened, if I turn on the charge pump with the 9V/18V switch I measured around 17.4 (If memory serves, would have to double check that).

So based on that 65% rule:

9.22V should be biased at 6V (5.992)
17.4V should be biased at 11.3V

However the 50K trim pot won't let me go there... Hmmm !

Where are you measuring voltage?

 

[EGRENIER]

Where are you measuring voltage?

View attachment 23303

Huh not in this section... On the board, I measure on the drain side of the JFET...



Which I believe is more here in the schematic:



Am I measuring this wrong ?

 

[EGRENIER]

I can't claim it was designed this way. More likely, the jfet is out of spec and is not biasing in the range.

If you're happy with the way the pedal sounds as is, you probably don't need to be too concerned about how it biased. If you're experience some unpleasant distortion or compression, then you can either try to find another jfet, or try a 100K trimmer and see if that helps.

I have 2 more JFET, so I can could try swapping them and see if that change anything, however I understood that what I measure is what I feed the JFET no ?

 

[Big Monk]

Huh not in this section... On the board, I measure on the drain side of the JFET...

View attachment 23304

Which I believe is more here in the schematic:

View attachment 23305

Am I measuring this wrong ?

I meant where are you measuring the source voltage?

 

[spi]

I have 2 more JFET, so I can could try swapping them and see if that change anything, however I understood that what I measure is what I feed the JFET no ?

Yes, but what you feed to the jfet will be impacted by the jfet itself (and if it weren't, there'd be no reason for a trimmer).

 

[EGRENIER]

I meant where are you measuring the source voltage?

OK let me recap this, the only measurement I've taken are on the JFET pins:

So with the pedal set to 9V and when facing the flat side starting from the left I read.
With pot CCW all the way:
Drain: 9.22V
Source: 229.5 mV
Gate: 0V

with pot CW all the way:
Drain: 7V
Source: 218 mV
Gate: 0V

The source is VCC before the trimpot no ? So I didn't check that but I would on the 9V/18V switch if I need to...

If you tell me what you need from the board, I can pull the value quickly...

 

[Big Monk]

OK let me recap this, the only measurement I've taken are on the JFET pins:

So with the pedal set to 9V and when facing the flat side starting from the left I read.
With pot CCW all the way:
Drain: 9.22V
Source: 229.5 mV
Gate: 0V

with pot CW all the way:
Drain: 7V
Source: 218 mV
Gate: 0V

The source is VCC before the trimpot no ? So I didn't check that but I would on the 9V/18V switch if I need to...

If you tell me what you need from the board, I can pull the value quickly...

I'm just curious what your Vcc values are in both positions.

 

[Chuck D. Bones]

Your JFET is too cold (Idss & Vp too low). All too common to find those on eBay. Probably the leftovers from someone else cherry-picking the "good" ones. Take solace in the knowledge that your cold 2N5457s will be useful as subs for J201 in some pedals. Ideally, you should find a hotter JFET. Doesn't have to be 2N5457. Did you solder the transistors into the sockets? What' the point of using sockets if you're gonna solder them in? Your other alternative is to reduce R4 (try 2.2K) or change the BIAS pot to 100K.

A brief JFET lesson:
JFETs run in constant-current mode, which means once you've picked a device and a source resistor, the DC drain current is constant and does not change if Vcc or the drain resistor (\trimmer) are changed. The only exception is if the JFET saturates (Vd = Vs) and we don't want that. I bring this up because you can dial-in the drain voltage to 50% or (65% or whatever) of Vcc at 9V and if you change to 18V you won't be anywhere near 50% of Vcc anymore. In my mind, this means that there is no "magic" bias setting, only the one that sounds acceptable to you at both 9V or 18V.

 

[EGRENIER]

Your JFET is too cold (Idss & Vp too low). All too common to find those on eBay. Probably the leftovers from someone else cherry-picking the "good" ones. Take solace in the knowledge that your cold 2N5457s will be useful as subs for J201 in some pedals. Ideally, you should find a hotter JFET. Doesn't have to be 2N5457. Did you solder the transistors into the sockets? What' the point of using sockets if you're gonna solder them in? Your other alternative is to reduce R4 (try 2.2K) or change the BIAS pot to 100K.

A brief JFET lesson:
JFETs run in constant-current mode, which means once you've picked a device and a source resistor, the DC drain current is constant and does not change if Vcc or the drain resistor (\trimmer) are changed. The only exception is if the JFET saturates (Vd = Vs) and we don't want that. I bring this up because you can dial-in the drain voltage to 50% or (65% or whatever) of Vcc at 9V and if you change to 18V you won't be anywhere near 50% of Vcc anymore. In my mind, this means that there is no "magic" bias setting, only the one that sounds acceptable to you at both 9V or 18V.

Oh boy, now I look like a fool

Yes I did tack the middle pin to the socket. I used socket to easily swap the transistor and usually put a tack once I confirm the pedal works. I'm always afraid that the transistor may come loss if the pedal is tossed around. I can probably swap the JFET easily, I have two more, hopefully not also "too cold"...

If the JFET is working fine, what value should I read of the drain pin ?

 

[EGRENIER]

I'm just curious what your Vcc values are in both positions.

OK I check the middle pin of the on/on switch, I get 9.22V and 17.85V (Based on if I'm on the 9V or 18V setting.)

 

[Chuck D. Bones]

You should be able to get 1/2 Vcc without running the trimmer to the end stop. Doesn't mean that's the idea value, but it's a good starting point.

If you have a breadboard, you can pre-test your transistors there, then solder them in and you don't have to mess with sockets.

 

[EGRENIER]

You should be able to get 1/2 Vcc without running the trimmer to the end stop. Doesn't mean that's the idea value, but it's a good starting point.

If you have a breadboard, you can pre-test your transistors there, then solder them in and you don't have to mess with sockets.

Don't have a breadboard

But ok, that 1/2Vcc is what I would measure on the drain pin correct ?

I will try to swap the JFET and see if one works better...

 

[Chuck D. Bones]

If you're going to build with JFETs or germanium, a breadboard is highly recommended. This is all you need to get started:
https://www.amazon.com/gp/product/B07LFD4LT6

FYI, I just tried four random in-spec 2N5457s with Vcc = 9.08V, R4 = 4.7K and Rd (trimmer) = 47K. The Vd readings were between 6.63V and 6.93V. These parts were all at the lower end of the spec range. For S&G I also tried PF5102, J112 & 2SK193. They could all be dialed-in to 4.5V with the trimmer somewhere in the middle. Electronic Goldmine sells 2SK193 4ea for $2.

 

[EGRENIER]

OK will check the other 2 I have and go from there...

Could Amazon be trusted ?

(20PCS) 2N5457 JFET N-CH GP 10MA TO-92, Pipe Cutters - Amazon Canada

(20PCS) 2N5457 JFET N-CH GP 10MA TO-92 in Pipe Cutters.

www.amazon.ca

I can get a pack of 20 delivered tomorrow

 

[Chuck D. Bones]

Mmmmmmmmmmmmaaaaaaaaaaaaaaybeeeeeeeeeeeeeeee.

Test 'em all for Vp & Idss when you get 'em. If any are out of spec, then you can decided whether to keep them or return for refund. The pic in the advert has all identification obliterated; that's not encouraging. Read the first review, they do look fake. Do ya feel lucky?

 

[EGRENIER]

Do ya feel lucky?

Not after reading the reviews LMAO....

Oh well, my other options are J112 from Mouser at 20$ shipping or Electronic Goldmine, if I can figure out 15$ worth of stuff to buy from them

 

[Big Monk]

Not after reading the reviews LMAO....

Oh well, my other options are J112 from Mouser at 20$ shipping or Electronic Goldmine, if I can figure out 15$ worth of stuff to buy from them

Let me see what I have.

I believe I have some leftovers from my Aion Ares build. Are you in the US?

 

[EGRENIER]

Let me see what I have.

I believe I have some leftovers from my Aion Ares build. Are you in the US?

Nah, I'm in Canada... I appreciate the offer tho...

Tayda has some J112 in stock, I guess it's time to put, yet, another order in... DHL kills me on shipping but if I load up on stuff it make it worth it...

Anyway, first thing first, let's see what the other 2 are like...

Appreciate all your help, I'll keep you posted.

 

[Chuck D. Bones]

Get on EG's mailing list. They put stuff on sale every week. It's surplus, so stuff constantly comes and goes. They have some generic Vactrol-type devices; a good deal when they're on sale. You have to test 'em 'cause there are usually a few duds. At 12 for $3, I don't mind having to toss a few out.

J112s have a large spec range for Idss & Vp, so be prepared to test & sort them. You might want to get some 2N5458s from Tayda as well. They are like 2N5457s, only hotter.