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]

So when the pedal is on 9V should I read 4.5V on the collector ?

Usually you bias jfets on the drain.

 

[EGRENIER]

Usually you bias jfets on the drain.

Thanks, but talk to me like a 5yo, what’s the drain and how should I read it ?

 

[EGRENIER]

Usually you bias jfets on the drain.

Ok after checking the 5457 specs, the left pin when facing the flat side is the drain. The trim pot is indeed controlling the value on the drain from about 7V to 9V…

 

[music6000]

Ok after checking the 5457 specs, the left pin when facing the flat side is the drain. The trim pot is indeed controlling the value on the drain from about 7V to 9V…

Definately 503 written on the side of trimmer?

 

[EGRENIER]

Definately 503 written on the side of trimmer?

Yes, I double check that, melted it a little, not proud of it but that’s a no issue.

 

[music6000]

https://resistor.cherryjourney.pt/ , Cick on Bands for 5 colours:

Can you confirm R2- 33K, R3 - 1M, R4 - 4K7 ???

 

[EGRENIER]

https://resistor.cherryjourney.pt/ , Cick on Bands for 5 colours:

Can you confirm R2- 33K, R3 - 1M, R4 - 4K7 ???

R2 - 33k
R3 - 1M
R4 - 4k7


Confirmed.

 

[music6000]

R2 - 33k
R3 - 1M
R4 - 4k7

View attachment 23286
Confirmed.

Only other thing I can think of, Do you know if you have a Legit 2N5457, there are Fakes out there???

 

[EGRENIER]

Only other thing I can think of, Do you know if you have a Legit 2N5457, there are Fakes out there???

Funny you ask, they came from @Dali, not sure where he sourced them.

Tayda was out, so I asked him if he had spares. He gave me 3.

For the record, the pedal seems to work fine, I’m just wondering what should I aim for with the bias (V) value or in term of sound. I played around with the trim pot by hear, and I notice a change when I trim it down, but not sure what I should aim for.

Do you see something wrong whit the value I posted ?

I do feel like I have a ground issue, but that’s for later. I have a fair bit of background noise that goes away if I touch the pots metal knobs.

 

[BuddytheReow]

If you touch the enclosure and noise goes away that means the enclosure itself is not grounded properly. Crank up the temp on your iron and reflow the ground connections on your jacks.

 

[spi]

BTW, doesn't this have a voltage booster? 1/2 of your voltage may be more than 4.5 V.

 

[EGRENIER]

BTW, doesn't this have a voltage booster? 1/2 of your voltage may be more than 4.5 V.

It does have a charge pump but to boost the voltage to 18v. If I switch it on, all the values double up. At least on the drain side.

 

[Dali]

Funny you ask, they came from @Dali, not sure where he sourced them.

I think the origin was eBay since the bag you got isn't pink...

I don't even remember for which pedal I got them.

 

[EGRENIER]

I think the origin was eBay since the bag you got isn't pink...

I don't even remember for which pedal I got them.

No worries, I think they are working fine, I'm just trying to understand how the bias needs to be used on the boost.

I was hoping to confirm that the ranges I'm getting with the trim pot are normal from 7V to 9V and 14V to 18V when I engage the chargepump.

Not sure why @music6000 thought there may be an issue with the transistor..

 

[music6000]

No worries, I think they are working fine, I'm just trying to understand how the bias needs to be used on the boost.

I was hoping to confirm that the ranges I'm getting with the trim pot are normal from 7V to 9V and 14V to 18V when I engage the chargepump.

Not sure why @music6000 thought there may be an issue with the transistor..

Basically, the 50K trimmer is not enough to Bias 9 VOLTS @ 4.5v on the Drain.
You wouldn't use a trimmer to only adjust 2 volts!
Either the Design is wrong or you have an out of spec 2N5457.
Increasing the Trimmer to 100K will get you there but is it going to work efficiently????

 

[spi]

The recent 2n5457s I just bought measured out of spec--that seems to be going around a lot. Hard to find any source for decent ones now.

Biasing at the midpoint gives the maximum headroom to the signal before compressing or distorting. In your case keeping it at the lowest point is probably best. However sometimes that compression sounds good in which case tuning by ear is appropriate.

Do you notice any audible difference when you move the bias point? If so, just go with whatever sounds the most pleasing to you--in the end that's what matters.

 

[Big Monk]

As a starting point, the EP-3 had 14.4 vDC off a 22 vDC source at the drain of the JFET in question.

That's 65%.

So measure the voltage coming into the circuit and set the drain to 65% of the source. Then tweak from there.

 

[EGRENIER]

The recent 2n5457s I just bought measured out of spec--that seems to be going around a lot. Hard to find any source for decent ones now.

Biasing at the midpoint gives the maximum headroom to the signal before compressing or distorting. In your case keeping it at the lowest point is probably best. However sometimes that compression sounds good in which case tuning by ear is appropriate.

Do you notice any audible difference when you move the bias point? If so, just go with whatever sounds the most pleasing to you--in the end that's what matters.

OK so if I get this right, most common bias value of a JFET is half the voltage or thereabout. However, based on the design of the Episode Boost using a 50K trim pot, it is meant to be above half. In my case it measure at 7V up to 9V. The more voltage to the drain, the more it will reduce headroom and add compression and even distortion.

In my case, this is a combo build (Episode and Dirty Sanchez), so I'm looking more for headroom than distortion (The DS can take care of that).

Therefore I should set it as low as possible to maximize headroom and raise it slowly until I like the "compression"...

Do I get this right ?

 

[EGRENIER]

As a starting point, the EP-3 had 14.4 vDC off a 22 vDC source at the drain of the JFET in question.

That's 65%.

So measure the voltage coming into the circuit and set the drain to 65% of the source. Then tweak from there.

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 !