starlight fuzz transistor bias

[harrisxr650]

Hi everyone ,

question might be silly

Vero is from http://dirtboxlayouts.blogspot.com and its verified

I think something is off with my build.
Pots working and do a change except the gain pot which just cuts volume when turned clockwise, doesnt seem to reduce amount of gain.

Note that i used a 220kA pot for gain instead of 250kA
and 22kB instead of 25kB for tone

I cannot bias Q1 & Q5 Drain @4,5v

can someone provide a guide on how to bias the transistors as i'm not sure if i did it right?

Measuring 2n56457 drains and

Q1 i cant get it over 4.25v
Q2 @ 4.5v
Q3 @ 4.5v
Q4 @ 4.5v and trimpot seems to affect the gate of Q5
Q5 @ 9 (?)

[antonis]

Q5 is OK..
(it's a Source follower..- No DRain resistor hence no voltage drop..)

Q1 is also OK but if you like to go up to 4.5V, lower 20k Drain reistor value..

[duck_arse]

please advise the what value cap markings you have fitted here.

[pacealot]

Nice spot there, my gluteal/Anatidian friend. If that's as large a cap (in value) as it visually appears to be (in size), that would certainly have the effect OP describes...

[antonis]

It indeed looks like a 1μF box cap,,
(instead of 1nF..)

[harrisxr650]

please advise the what value cap markings you have fitted here.

It seems like 1uf but its 1nf.

[antonis]

Not relevant to your issue but there is a discrepancy between schematic & layout concering reverese polarity protection diode..

[harrisxr650]

Not relevant to your issue but there is a discrepancy between schematic & layout concering reverese polarity protection diode..

Diode is reversed?

[antonis]

Diode is reversed?

Not exactly..

In layout is set in series with +9V where in schematic in set between +9V & GND..
(both should work fine with the former suffering from a diode forward voltage drop..)

[duck_arse]

It seems like 1uf but its 1nf.

I have lost all faith in WIMA caps, now. I'm almost tempted to go and look at datasheets to see what series it "fits".

[duck_arse]

the more I think about that cap, the more I think it is not of WIMA manuf, not unless they lost all their engineering specs and moved to a backyard in chynah. I've never seen a fat wima like that, not a red/black print one, not a 1000pF/100V unit one.

but I suppose if it measures 1nF, it doesn't really matter.

and that I should stop thinking about it.

[harrisxr650]

the more I think about that cap, the more I think it is not of WIMA manuf, not unless they lost all their engineering specs and moved to a backyard in chynah. I've never seen a fat wima like that, not a red/black print one, not a 1000pF/100V unit one.

but I suppose if it measures 1nF, it doesn't really matter.

and that I should stop thinking about it.

Its definitely 1nf. but what you say about manuf troubles me, its supposed to be a 100pf/100v FKS2 cap.

[duck_arse]

.... it[']s supposed to be a 100pf/100v FKS2 cap.

1000pF or 100pF? also, no, Sir! from the FKS2 datasheet -

Marking:
Colour: Red. Marking: Silver.

1000pF is range minimum. dims should be  - 2.5W    6.H    7.2L   

FKP2 would be red body black markings. that 1000pF would be 4.5mm wide.
FKS3 would be red body black markings, range minimum is 1000pF. dims should be  - 3W   8.5H    10L

[anotherjim]

I wouldn't worry too much about those JFET drain voltages. Anything 3v to 6v is good.
Q5 must have 9v on the drain, it is wired to it. It is a source-follower/buffer and it's the source voltage that tells you how it's biased.

The Gain pot is misnamed IMHO. It is a volume control for the preamp output. I would call it Drive. It doesn't change the gain of the circuit amplification stages. If clockwise reduces volume have you got the wires to lugs 1 & 3 swapped over?

[harrisxr650]

.... it[']s supposed to be a 100pf/100v FKS2 cap.

1000pF or 100pF? also, no, Sir! from the FKS2 datasheet -

Marking:
Colour: Red. Marking: Silver.

1000pF is range minimum. dims should be  - 2.5W    6.H    7.2L   

FKP2 would be red body black markings. that 1000pF would be 4.5mm wide.
FKS3 would be red body black markings, range minimum is 1000pF. dims should be  - 3W   8.5H    10L

sorry for the late reply, its 1000pf for sure,
i check all components with dmm before soldering, but since you mention it i de-solder  it and check it again.

whats not sure is if its a wima or not