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Author Topic: starlight fuzz transistor bias  (Read 299 times)

harrisxr650

starlight fuzz transistor bias
« on: January 21, 2023, 03:52:03 AM »
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 (?)












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antonis

Re: starlight fuzz transistor bias
« Reply #1 on: January 21, 2023, 04:29:48 AM »
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..
« Last Edit: January 21, 2023, 04:33:57 AM by antonis »
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duck_arse

Re: starlight fuzz transistor bias
« Reply #2 on: January 21, 2023, 09:59:12 AM »


please advise the what value cap markings you have fitted here.
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« Last Edit: Soon at 09:27:12 by someone sensible »

pacealot

Re: starlight fuzz transistor bias
« Reply #3 on: January 21, 2023, 01:31:30 PM »
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...
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antonis

Re: starlight fuzz transistor bias
« Reply #4 on: January 21, 2023, 03:50:44 PM »
It indeed looks like a 1μF box cap,,
(instead of 1nF..)
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"I'm getting older while being taught all the time" Solon the Athenian..
"I don't mind  being taught all the time but I do mind a lot getting old" Antonis the Thessalonian..

harrisxr650

Re: starlight fuzz transistor bias
« Reply #5 on: January 21, 2023, 05:23:02 PM »
Quote from: duck_arse on January 21, 2023, 09:59:12 AM


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

It seems like 1uf but its 1nf.



« Last Edit: January 21, 2023, 05:34:08 PM by harrisxr650 »
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antonis

Re: starlight fuzz transistor bias
« Reply #6 on: January 21, 2023, 05:40:29 PM »
Not relevant to your issue but there is a discrepancy between schematic & layout concering reverese polarity protection diode..
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"I'm getting older while being taught all the time" Solon the Athenian..
"I don't mind  being taught all the time but I do mind a lot getting old" Antonis the Thessalonian..

harrisxr650

Re: starlight fuzz transistor bias
« Reply #7 on: January 21, 2023, 05:50:38 PM »
Quote from: antonis on January 21, 2023, 05:40:29 PM
Not relevant to your issue but there is a discrepancy between schematic & layout concering reverese polarity protection diode..

Diode is reversed?
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antonis

Re: starlight fuzz transistor bias
« Reply #8 on: January 21, 2023, 06:09:28 PM »
Quote from: harrisxr650 on January 21, 2023, 05:50:38 PM
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..)
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"I'm getting older while being taught all the time" Solon the Athenian..
"I don't mind  being taught all the time but I do mind a lot getting old" Antonis the Thessalonian..

duck_arse

Re: starlight fuzz transistor bias
« Reply #9 on: January 22, 2023, 09:14:27 AM »
Quote from: harrisxr650 on January 21, 2023, 05:23:02 PM

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".
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« Last Edit: Soon at 09:27:12 by someone sensible »

duck_arse

Re: starlight fuzz transistor bias
« Reply #10 on: January 23, 2023, 08:02:22 AM »
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.
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« Last Edit: Soon at 09:27:12 by someone sensible »
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