Alembic F2-B project gone cookoo - diyAudio
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Old 5th April 2009, 04:35 PM   #1
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Default Alembic F2-B project gone cookoo

I have to revive the old Alembic thread by Phil Mailloux, but this time on my own.

For those of you who tried to help me out, I'm still stucked with two non-working units.
I followed these schematics for both preamp and power supply:
http://moosapotamus.net/IDEAS/F2B/alembic.htm

I only made two changes to the preamp. I left out C8 and used a 10k VR3 mid pot instead of 250k.

My problem is that both identical units emits a squeal in the signal.
At first I thought it was some kind of feedback in the tonestack as it seemed to disapear when I bypassed it.
I now know that I made a mistake by connecting pin 6 of the tube via a capacitor directly to pin 2, overriding the volume pot too.
That made the squeal disappear, sending a weak signal to the power amp.
Thinking it over, I realized that leaving out the vol pot left me with no added gain in the second triode.
The weak signal must then have been only from the first triode.

But today I made some surprising meassurements.

I connected pin 6 to one side of the vol pot via a 0.1uF capacitor, the other side to to ground and the wiper to pin 2.
When the wiper on the vol pot was turned to ground I meassured 0 volts AC from 'out' to ground. As expected.
But as I turned up the vol, the output quickly raised to app. 35V AC about 1/3 way up and then slowly decreased to app. 6V AC at full gain.
This was with no signal applied to the input.

With that amount of signal, no wonder the power amp was moaning.

With these new informations, does anyone have an idea of what's going on here?


Greetings,

Henrik Nordskov
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Old 15th April 2009, 04:01 AM   #2
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Hmmm. Hmm..... I say, hmm indeed.

Well for one, I have to ask, are the DC voltages OK everywhere? That's the first thing I would check, because if they are not then you can't debug the AC behaviour of the amp if the DC parameters are not right, or unstable. But I guess you know this already.

Secondly, if you say both units are behaving the same way (forgive me I did not read the original thread, but I will), are both units making this "noise" (you said "amp was moaning" lol) in the power amplifier? Do both units have C8 not installed? Why did you decide to remove it?

What happens if you disconnect the top side of the Volume pot? I mean remove the signal from the first tube, just leave the volume pot's top side floating. then vary the volume control and see what happens at the output.

Lastly, there is also a possibility that the tube is oscillating because of the interaction between your 0.1uF cap (that's pretty big) and the 1Meg volume control.

I await your feedback or comments from other members. good luck!
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Old 9th June 2009, 02:44 PM   #3
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Thanks for your respons gain-wire.

I'm sorry I haven't replied to your post untill now, but the darned amp have been put on the shelf for a while and I forgot to check this forum for replies.
The problem have being solved by a wiring diagram from Groundhanger. I rewired one of the amps and there it was. All cleared up.
I still don't know what the problem was, as the fault were in the tone stack which I did bypass.

But thanks anyway.

Nordskov
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Old 17th June 2009, 09:03 PM   #4
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And, I think that 250k mid pot is a mistake in the schematic, in RG Keens original schematic (which he traced from an original unit or redrawn from the factory schematic, one or the other), it says 10k. I myself used 25k, and that works fine, I think the tone-stack is a worthwhile addition, by the way, very useable for bass (and probably guitar as well, as it is a fender blackface originally, and Dave Gilmour uses the unit, albeit modified).
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