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Old 7th February 2006, 05:55 PM   #21
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I will draw an acurate schematic now so that you have a better idea of whats going on, I never thoguht that one throguh properly. I'll check the other thread out aswell.
Many thanks,
Owen
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Old 7th February 2006, 05:58 PM   #22
anatech is offline anatech  Canada
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Hi Owen,
Yes, we are pretty much starting over.

-Chris
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Old 7th February 2006, 06:13 PM   #23
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ok.. new schematic, hopefully there arent any errors in it.
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Old 7th February 2006, 06:37 PM   #24
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Quote:
Originally posted by DoomPixie
ok.. new schematic, hopefully there arent any errors in it.

You have the VBE transistor shorted , you need one cap from the tube to the collector and another cap to the emitter of Q1 (the VBE transistor).
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Old 7th February 2006, 07:10 PM   #25
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i just did that liek you said and the resistor (R03) burns out.. tried it twice.. changed back to the single cap and it runs again? :S could Q1 be fried?

the tube stage is exactly as it is in that diagram now.

EDIT: forgot to mention it made a loud buzzing / screaming sound straight away when i turned it on and before i managed to turn it off R03 had burned out.. after i changed back to the single capacitor and replaced R03 it runs again..
Owen
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Old 7th February 2006, 08:08 PM   #26
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i replaced the two drivers with BD139/140 and it seems to run with the two capacitors now.. it is getting better.. at full volume the sound is almost perfect now! and it makes sound with the volume lower than it used to, but it still needs some work, i will work on the css and current sink tommorow and then i will add a stage.
Owen
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Old 7th February 2006, 08:41 PM   #27
anatech is offline anatech  Canada
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Hi Owen,
Now that you've got beyond fire and fuses, it's time to fix you bias circuit.

Ilimzn is quite correct in saying yours has a problem. Put the variable control in the B-E leg. If it goes open or has a bad contact, the bias will drop to zero instead of going wide open. Include a series resistor to increase the usable range (by excluding the cutoff region).

-Chris
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Old 7th February 2006, 09:05 PM   #28
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Hi Chris,
i'll have a look at the bias circuit tommorow, it isnt MY bias circuit.. the entire output boards are from a fisher amp, i cant take any credit for their poorly designed bias circuit.
When i add the CSS and Current sink should i use variable resistors in them so that i can adjust the current? Or roughly how much current should i set it up for?

With the amp on full volume It is surprisingly good sounding amp at the moment, no doubt it's sound will change but hopefully for the better. it is already my favorite amp, it's just a shame it only works at full volume..lol

Many Thanks,
Owen

EDIT: Added updated schematic
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Old 7th February 2006, 10:06 PM   #29
anatech is offline anatech  Canada
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Hi Owen,
You are set up for 1.7 mA now, so keep it the same With the extra gain stage you could drop that to 1 mA and have lots left over. I wouldn't worry about making them adjustable. Use an LED reference for you current source to make life easy (1K2 emitter resistor).

-Chris
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Old 8th February 2006, 12:28 AM   #30
ilimzn is offline ilimzn  Croatia
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And BTW your 47uF output cap is NOT ok unless you are driving just a tweeter with it. For an 8 ohm load it presents a high pass at 423Hz. So, this amp would hardly be capable of bass.

You also need to check bias of the output stage, you can completely disconnect the tube stage for this. Tell us what voltage you measure on the output transistor emitter resistors.

The screeching sound you get is probably a combo of oscillation and one of your original BC drivers going into secondary breakdown when the input cap is charged, but I would bet on oscillation as being the original culprit. You may be missing an output Boucherot cell or Zobel network. Start with the first for now, 10 ohms 2W in series with 0.1uF together in parallel with output. This is only a quick and dirty thing, it will need proper implementation later. This may have been present in the original amp, just not on the amp board (usually it's somewhere near the protection circuitry ir even on the output posts).

You can test your output stage by conencting your source to the input cap, instead of the tube stage. The amplitude will be very low but you should hear undistorted sound if all is well. Before you get to that point, I don't hink it makes sense to connect the tube stage.
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