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Pass Citation 12
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Old 5th November 2013, 03:50 PM   #51
electrochap is offline electrochap  Canada
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Here are some pictures of my recently acquired CT11 and CT12. I'm almost shy putting the iron on these... They are both completely original, not a single wire moved... Both no rust, corrosion, oxydation, nada... Working like new; not a scratch, pots are clean.. etc...
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File Type: jpg photo 2.JPG (105.4 KB, 474 views)
File Type: jpg photo 3.JPG (137.2 KB, 469 views)
File Type: jpg photo 4.JPG (91.0 KB, 462 views)
File Type: jpg photo 5.JPG (105.2 KB, 457 views)
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Old 29th October 2015, 06:36 PM   #52
cucuclock is offline cucuclock  France
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Pass Citation 12
Hey Electrochap,
this is quite old thread but maybe I have an answer here. I'm using a couple of Tazz board for C12 Mod and Bias setting is always a scaring point for newbie, at leat for me..on your schematic and pcb it seem clear but I would like to be sure:
If I understand Right, At the initial Turn On the screw of the variable resistor R17 must be in COMPLETE COUNTERCLOKWISE position?
This is the schematic. Thank you
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Old 29th October 2015, 08:09 PM   #53
electrochap is offline electrochap  Canada
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Hey CUCUclock, actualy I've never built the CT12 with Tazz boards even if I have the whole kit (including the Coroidal trannys, FETs, caps everything) in a box on a shelf ...

The one I've built uses old Hitachi Mosfet (J50, K135) which are laterals and are not biased like IRFs.

If I were you, I would just start at midpoint with a meter watching and adjust wherever you have to go. Slowly. But I'm sure there are way more knowledge people on that Forum (like Tazz and others) who will answer your question knowing exactly where to start... Sorry, I leave this one to others.
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Old 30th October 2015, 10:23 AM   #54
cucuclock is offline cucuclock  France
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Thank you Electrochap, it's a good starting point
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Old 25th November 2017, 08:38 AM   #55
cucuclock is offline cucuclock  France
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Screen Shot 2017-11-25 at 10.37.19.png
Quote:
Originally Posted by juma View Post
To lower the bias current, you'll have to lower the value of R14 to 4k7 (or to 5k by adding another 10k in parallel to R14).
If you want to take out the Vbe multiplier circuit (Q4, R14, R17) and use a single resistor to bias the output stage just put a 1k or 2k multi-turn pot in parallel with C5 instead. Don't forget to set that pot to 0 Ohm before turning the amp on.
Also, check the current through Q3 (should be about 5 mA) by measuring the voltage across R13. If that's so, you'll have the output stage bias at 150-200 mA with pot set at 400-500 Ohms.
it seems the opposite that Mr Pass says.....if I follow him for a project, I can't see why don't follow him on the Bias...
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Old 25th November 2017, 09:50 AM   #56
juma is offline juma  Germany
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Quote:
Originally Posted by cucuclock View Post
it seems the opposite ...
Sure.
The first quote is about the amp version that uses vertical MOSFETs and I was commenting the version with Lateral MOSFETs (k135/j50) in the output stage.
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Old 25th November 2017, 12:21 PM   #57
cucuclock is offline cucuclock  France
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Thank you Juma, I could finally had some confirmation about Bias setting !!!!
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Old 25th November 2017, 12:58 PM   #58
juma is offline juma  Germany
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It's easy - if you you use vertical MOSFETs you need Vbe multiplier to control the temperature drift and bias adjusting begins with low/zero current through output stage and, at the start, the pot has to be set to maximal resistance value.

If you decide to use Lateral MOSFETs you don't have to worry about thermal drift so you don't need Vbe multiplier and instead of it, just a single pot between the gates is all you need. With such a configuration the Id adjustment should be started at minimal/zero pot resistance setting.
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Old 25th November 2017, 04:55 PM   #59
Nelson Pass is offline Nelson Pass  United States
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Pass Citation 12
Quote:
Originally Posted by cucuclock View Post
it seems the opposite that Mr Pass says.....if I follow him for a project, I can't see why don't follow him on the Bias...
Totally depends on where the pot is.

Collector to Base - go for 0 ohms.

Base to Emitter - maximum ohms.
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Old 29th November 2017, 05:23 AM   #60
cucuclock is offline cucuclock  France
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thank you so much !!! a bit of light in the darkness
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