Alembic F2b preamp clone

[michal_k]

got it! thanks a lot, I'll use your layouts for sure and post results on that forum. hope it's gonna be sooner than somewhen in next decade 

[Lorcan]

Actually it's a quite a simple build for a tube amp. For me the hard part was gathering all the info and finding the parts, but you don't to do that again 

[jstack]

Hello, I've been having troubles with my f-2b lately. The output volume gradually drops. Ive measured the B+ and the 12vdc heater line and their voltages don't fluctuate much. When I measure the voltages at the tube pins I get this:
Normal output (when its first turned on)
1 180v
2 6mv (climbing)
3 1.8v
4 11.3v
5 11.3v
6 181v
7 50mv
8 1.8v
9 0v
B+ 300v +/-2
Heaters 11.1v

Low output (after a few minutes of playing through it)
1 159v
2 269mv (climbing)
3 2v
4 13v
5 13v
6 113v
7 1.6v
8 2.8v
9 0v
B+ 300v +/-2
Heaters 12.8v


Its a brand new JJ 12ax7 tube. Does anyone have any ideas? Is it the tube?

[jstack]

It was the tube 

[R.G.]

It was the tube 

As I tell people when I advise them on fixing tube amps, there is a reason those things are in sockets. 

[jstack]

Blast!
Turns out it wasn't the tube after all, I've opened up a new thread...
http://www.diystompboxes.com/smfforum/index.php?topic=79839.0

[Lorcan]

Files moved here http://www.lorcan.me/projects/tube-bass-preamp/

[Tubebass]

When operating heaters on 12 volts, the 12v should be connected between pin 4 and 5, with pin 9 left unconnected. You're frying your tube!

[Tav]

A friend of mine built a double channel F2b clone, he also installed a Tube cathode follower circuit on both channels in order to lower the output impedance, a good mod, I can drive effects without any problems now. A master volume was also installed to tame the massive ouutput of the preamp.
I use this clone ala Gimour in the Hiwatt power section, for guitar use.
There is way too much bass response though, I must keep the bass pot completely off in order not to have my amp rumble in an unpleasant way, considering I'm using 4x12" closed back cabinets.
What caps should I try to replace to reduce the low frequencies response?

[Jarno]

Well, if it's the tonestack, I think something must be of a wrong value in there, either resistor or cap. If it's more of a guitar application you are looking for, you might want to restrict the low-end response by lowering the first cap. Alternatively, how are the tube stages bypassed? What value electrolytic is on there?

[Tav]

Well, if it's the tonestack, I think something must be of a wrong value in there, either resistor or cap. If it's more of a guitar application you are looking for, you might want to restrict the low-end response by lowering the first cap. Alternatively, how are the tube stages bypassed? What value electrolytic is on there?

I have replaced both the 50uF/25V Electrolytic caps with 25uF but still it has too much bass response. I was suggested to change the second one with a lower value cap, like 4,7uF or 1uF, could that help?
The preamp has no errors in the construction, it works flawlessly and is extremely silent, only way too much bass response

[Jarno]

You can also just clip one lead of the electrolytic, removing it completely from the circuit, that's the easiest way to try if it helps (and easiest to restore if it doesn't).
Come to think of it, if you do this on the first stage, you'll move the eq points of the tonestack because the output impedance changes (lower), right?
What size is your input cap? 100nF? Maybe lower this to 22nF, are you using the unit for guitar?

[d@vide]

less value...   0.47uF or 0.22uF

[TobiasD]

People,

I'm sorry for kicking up such an old topic.
I'm also trying to build a F2b. I used Lorcan's schemes and Jarno etched the PCBs for me.
But now i was wondering how i should connect the pots (high, treble, and bass). Could you help me with this?

Tobias

[delaystomper]

hi I'm very interested in your tread . I'm looking to build a single channel mainly for home recording o computer software ( protools) through my fast track or other device in general
what is the best schematic to use for a single channel including the power supply side. and whiter the things to leave off from the to channels not needed for a single channel ?
thanks