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Old 4th February 2013, 09:06 AM   #11
iso is offline iso  Finland
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Default 6BQ5 amp with Tannoy

When you use different amplifiers with negative feedback, sound quality of different speakers seems to be quite unpredictable.

Crossover network and loudspeaker seems to form some kind of energy storage that feeds feedback loop of the power amp. As time constants of this energy storage are different at different frequencies, amplifier or loudspeaker reviews can be quite misleading if you do not use exactly the same amp and speaker combination.

Best Regards

Kimmo Salminen
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Old 25th February 2013, 07:30 PM   #12
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Well I've completed my MF Citation 12 but have a problem with biasing.... Can't get lower then 350ma.

I'm using the K135 and J50 old Hitachi TO3s and BC139 BC140 everywhere else.

Cant find transconductance neither bias info on the datasheet for the Hitachis.

Playing with the BIAS pot don't seems to have any effects. Even removing the 4.7K in parallel don't have an effect.

Can anyone help me?

I've verified the schematics on my original "modified" boards and everything is right, as the "Revised Mosfet 12 schematic, complementary version" shows.

Can K135, J50 be wrong for this application?

I've done all verifications, got the amp to clipping around 20 RMS volts on both channels. Both channels have the exact same problem; bias around 350ma, even higher at start up going a bit lower after.

Thanks anyone!
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Old 25th February 2013, 11:49 PM   #13
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Quote:
Originally Posted by electrochap View Post
Well I've completed my MF Citation 12 but have a problem with biasing.... Can't get lower then 350ma.

I'm using the K135 and J50 old Hitachi TO3s and BC139 BC140 everywhere else.

Cant find transconductance neither bias info on the datasheet for the Hitachis.

Playing with the BIAS pot don't seems to have any effects. Even removing the 4.7K in parallel don't have an effect.

Can anyone help me?

I've verified the schematics on my original "modified" boards and everything is right, as the "Revised Mosfet 12 schematic, complementary version" shows.

Can K135, J50 be wrong for this application?

I've done all verifications, got the amp to clipping around 20 RMS volts on both channels. Both channels have the exact same problem; bias around 350ma, even higher at start up going a bit lower after.

Thanks anyone!
If you want to use lateral mosfet for citation 12, you must modify the VBE multiplier resistor value since they are intended for Vfets which has around 4v for Vgs. You can also omit the VBE multiplier and replace with just a low value resistor since laterals don't need thermal compensation.
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Old 26th February 2013, 12:26 AM   #14
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Quote:
Originally Posted by electrochap View Post
Well I've completed my MF Citation 12 but have a problem with biasing.... Can't get lower then 350ma.
If you check the Gate to Source voltages on the outputs, I'm sure you will
find it high. And what's wrong with 350 mA? Too hot?

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Old 1st March 2013, 12:34 AM   #15
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Gate to source voltage is at +/- 14 volts.

Hmm... what's wrong with 350ma of bias voltage... gets hot but I've seen worse. I was just trying to follow the original plan. I should get both clipping starting at the same voltage hey? Means I should replace 5K pot by a higher value as this one seems to have no effects on the current...

I've been trying to work on that Citation12 project for a few years now. I even bought 4 IRF130 at the time ($15ea!). I've never noticed it was an article you wrote at the time Mr Pass. Really interesting project to convert quite regular architecture for a more modern plan...

What do you all think should sound better: IRF130, Hitachi J50 and K135 or IRF240 and 9240?

Thanks all!
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Old 1st March 2013, 12:45 AM   #16
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The Hitachis ought to be conducting at around 1V Vgs, so the complementary
pair of followers would have a bias circuit around 2V. At the voltages you
mention, they shouldn't work at all. What are we missing here?

How about the pinouts? Aren't these laterals with Drain and Source pins reversed?

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Old 1st March 2013, 12:48 PM   #17
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Default Dooohhh!

I knew about Drain/Source reverse pin out and swapped these 2. And verified, verified, verified,...

I've had this Citation 12 Mosfet project for a few years now and never had a chance to get into it until a few weeks ago. I then noticed it was one of your (N. Pass) projects and thought "that is why it is interesting; taking a quite regular architecture audio amp plan and change it to a way more modern one using more up to date discrete components". So a little note here to thank you giving so many interesting projects using none conservatives thinkings.

OK. I did bought 4 IRF130 ($15ea!) at the beginning with some regular 5% resistor at $0.25 for 5 pieces at the corner electronics store...

When I've started to work on it, noticed there were supposed to be 2% metal resistors 1/4 watts. SO I did order some 1% metal resistors. I also thought I had some of these old Hitachi Mosfet that has a quite good reputation (they were on one side of a H&H 800 watts amp).

Anyhow... I'll try to add a few pictures of my project to this post. If that can help a bit... You'll notice one red jumper that will on one the front side of the board on one side and on the back for the other... Didn't noticed there were wholes...

Well thanks for helping!
Attached Images
File Type: jpg backboard.jpg (383.4 KB, 345 views)
File Type: jpg closeup.jpg (186.9 KB, 332 views)
File Type: jpg frontboard.jpg (286.7 KB, 332 views)
File Type: jpg under.jpg (414.5 KB, 325 views)
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Old 8th April 2013, 09:19 PM   #18
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Default Got it!

Finally I'm now listening to this CT12 MOSFET using K135/J50. Amazing sound. I'm using Mark Levinson ML1 and Tracer speakers. I did try Bryston 2B amp; very detailed but too dry. Tried Dynaco ST70; not bad. Warmer but less details. Tried my Heathkit EA-1; around the same, nicer mids & highs,nice female voices. But now, this CT12 is really the total all of them; very well detailed in the mids/highs with full bass. Note that I've replaced the 4 6kuf by 8200.

And better soundstage as well!

I'm really happy with the sound!!!

Only odd thing I would like to do is decrease the bias voltage a bit. It gets really hot after 1 hour or 2. Can't leave my fingers on the heathsink...

I don't know how to calculate this VBE multiplier or how to replace by a single resistor....? Help anyone?

I can measure anything... I'm an telecom tech fooling around in vintage audio.

God it sounds good... If I lower the current, will it sound different?

I have +/-45 volts as rails.

If needed, I can measure at what voltage each FET starts conducting...
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Old 8th April 2013, 10:21 PM   #19
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post exact schematic you're using
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Old 9th April 2013, 02:53 AM   #20
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Where did 45 volt rails come from?

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