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Old 4th May 2009, 03:06 PM   #1
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Join Date: Apr 2009
Default diy Ampeg preamp

Hi all,

I want to make a hybrid amplifier for bass guitar. I want to make the preamp of an "Ampeg Classic".
For the poweramp, I want to use the UCD400 module from Hypex. (I will also start a new thread about this in class D subforum)

I am studying electronics, and I already built a guitar amplifier, based on the Marshall 18W. But that was just following the schematic.
Now I have a few questions.

- What's the main difference - soundwise - between channel 1 and 2?

- In stead of 2 jacks for bright and normal, I assume that I just can use one jack and a switch?

- How do I connect this preamp to the poweramp? I would think: with connection 1 and 4 (ground). But the UCD400 has a differential input.

- Is the "mid-range"-coil available in shops? Or do I have to wind one myself? How to do this? (I read somewhere that the value of the coil is: 0-100-300-800 mH)

- I want to delete the polarity switch. That's no problem, I think?

- The switch of the "mid-range select"; is that a 2P3T (2-pole 3-throw)?

- The 600V caps (above the fan). They are electrolytic, I assume? What is there polarity? Why don't they just use one?

- Is 1/4 Watt enough for the resistors?


I hope you guys can help me out a bit. Thank you very much.


Here is the schematic of the preamp:
http://members.lycos.nl/carrejans/temp/AmpegPreamp.jpg

Here is the schematic of the ampeg poweramp (I'm not using this, but it might be handy to look where the connections go to):
http://members.lycos.nl/carrejans/temp/svt6550.gif
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Old 9th May 2009, 05:35 PM   #2
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Join Date: Apr 2009
Can anyone help me out with some of these questions?
I will apreciate it.
Thank you.
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Old 9th May 2009, 07:30 PM   #3
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Join Date: Jun 2004
Location: Indiana
Well I have built exactly one guitar amp but since no one else is stepping up let me see if I can help at all.

Yes you could switch the bright and normal with a single input jack. I did something similar on mine but it was an 18W output with plexi'ish front end so a little different (see attached PDF). I fed both the bright and normal channels the same signal all of the time and used individual level controls to mix the two channels together before the tone stack and PI stages so an infinitely variable tone was possible.

Not familiar with this design but there should be some references online somewhere. Not sure whether the second channel is trem or different EQ characteristics or what.

I would say 1 and 4 also. I don't know anything about that module (not into SS). I think we would need to see the data sheet or something.

As to the choke why not call Ampeg and order one? You have the part number.

You should delete the polarity switch and redesign for three wire grounded power connection IMO.

No not electrolytic. With a value that small I would be using polyester film or polypropylene film for that. I have no idea why they would use two caps like that. It is sometimes done to increase the voltage capacity but that is a kludge. No idea why they would do it there.

Personally I would use 1/2W metal film for most of the resistors. Check load resistors (I^2*R) in the plates of CC stages and cathodes of CFs to see what the quiescent dissipation is and then at least double it. For example the 6C5 datasheet indicates that it can take cathode current of 8mA. With that 47K load resistor you would be dissipating 3W at 8mA. Chances are they have it biased cooler than that however and if it were say 1.5mA then you would be at 1/10W so out 1/2W would be fine. So we need to get a handle on the currents involved. I would guess they are at 1 or 2 mA in that circuit but I haven't found a data sheet with the transfer characteristics yet and I am not good at visualizing the bias from the plate curves.

That schematic is rather complicated due to the way they chose to implement it. You might want to consider boiling the circuits you want down to the essentials and build up the features from there. Just a thought.

Hope this helps.
Attached Files
File Type: pdf wesampfinal.pdf (32.1 KB, 159 views)
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