Alembic F2-B

[Jarno]

Originally Posted by oatmeal769

Hello, great thread so far!
The last few years I've noticed the sound seems to have progressively dulled and lost it's life. The unit also has a little hum, and is pretty noisy. I was wondering if anyone could take a look at a picture and / or suggest the items I can replace which will restore it, maybe even make it sound better than new. All the resistors, etc. came with it originally, and it may have been re-capped back in the 80's but nothing has been done to it since then.

Here are the things I'd like to add or update with it:

It has an old 2 prong electrical cord with no safety ground - That can't be good when beer gets spilled on it.

I'm feeding the input with active EMG pickups. They have 200 ohms output impedance. Should I put a resistor on the input? I've read the F2-B really only likes passive pickups.

I'd REALLY like to make the main output a balanced 3 conductor connection instead of the unbalanced 1/4" one currently there, is that possible? All of the other gear in my rack is balanced in/out, and I use the rig in some really noisy environments!

I'm feeding a balanced TRS 10K ohms input (a compressor) from this unit. I don't really think I need more gain, I'd just like to ensure no hum from lighting, electrical, etc.. As well as giving the compressor the drive it wants to operate well.

All excellent suggestions so far, with regard to the balanced out, you can either connecto a (good quality) DI box on a very short lead. Or buy such a box and put the transformer hardwired in the 19" enclosure of the F2b, an easy enough fix.
With regard to the gain of the unit, I am only usig active basses and when I originally built mine I noticed it had waaayyy to much gain. So when I rehoused it (see earlier in this thread) I added an L-pad input attenuator (two resistors and a switch), and I also tacked a 200k resistor over the 100k plate resistor of the second stage, that tamed the gain excellently!
And the tubes make a big difference as well, I tried a couple of different ones, thinking that it shouldn't make that much of a difference, but it really does. I am very happy with the ECC803S which is in there now.

[Jarno]

Originally Posted by jstackbass

The problem is the output volume is slowly falling after nearly a minute of being powered up.
When I probe around with a meter, I find that the voltages on the Anode are falling rapidly while the Grid and Cathode voltages are rising (the Grid fluctuates far more than the Cathode though).
Also slightly more strangely, the volume pot at power up has a normal taper with no noise or scratchyness, but as soon as the voltages start fluctuating ~1min it starts getting this prominent scratchyness and the taper seems to change greatly.

Are you sure all of your tone caps are rated for at least 300 volts? If not, you could put a 100n coupling cap from plate of the first stage to tonestack, I think I did this too, not sure anymore.

Another change, which is not related to your problem, is adding a gridstopper for the second tube. Sometimes I notice a very loud hum when I have used the unit for a bit and then close the volume control all the way. I think (but I'm not sure) a grid resistor would remedy this.

[jstackbass]

Why would my tonestack caps need to be rated for 300vdc? When I measure the voltage after the 100k resistor its well under the 200vdc they are rated for... I'm confused...

[jstackbass]

Aahhh, after some reading I see now the importance of their being a higher value. Thanks for the suggestion, I'm off to get some 300+ volt rated caps!

[oatmeal769]

Originally Posted by jstackbass

Aahhh, after some reading I see now the importance of their being a higher value. Thanks for the suggestion, I'm off to get some 300+ volt rated caps!

Where did you do that reading??? I just ordered mine...

[Jarno]

In some of the schematics, there is no cap (other than the tonecaps) between the plate of the first stage and the pots for the tonecontrol. If the tonecontrols are not rated for the full voltage, than there will be DC on the pots. An alternative solution to this would be to add a 100nF cap right after the plate of the first stage put before the tonestack.

[oatmeal769]

I've done all the power supply caps and resistor upgrading, The amp subjectively sounds quieter and much more punchy to me now.
I'm going to put the 100nF cap in soon, I'll report back, it's a really interesting idea.

I think I'll also be doing a small choke in the power supply in place of the first 1.5K resistor.
It's plenty quiet already, but now my obsessive nature has taken over! LOL

[Jarno]

LOL, yeah, sometimes you just have to give in to that OCD. Mine's got 3x 450uF on the B+, perhaps also a bit excessive for a single tube :-)
Rectified and regulated heaters are a must, IMHO.

[oatmeal769]

Are you using that cap as a 'choke'? I'm guessing that would roll off some lows, or something wouldn't it? I don't want to change the tone, just use a choke to perhaps quiet any ripple that still gets through.
Thoughts?

[Jarno]

Well, it does help to get rid of ripple, but it's not a choke, it's a CRCRC filter. And it's on the B+ high voltage supply, so no signal passes through it. I also needed to drop quite a bit of voltage, because of the transformer I had, so the series resistors between the caps are something like 10k and 15k.