Purchased a Used F2-B with Issues - Seeking Advice

[caseycayce]

Hey, Alembics;
 
I just received an F-2B I purchased through eBay. It's my first sojourn into the F-2B, and it's what might be called Vintage, serial 686. I'd like to know if any of the problems I'm experiencing would somehow be considered normal, easily fixed, or if I need to send it back. These are the issues:
 
1: When I plug it into my Mesa 50/50 stereo amp, with nothing in the F-2B inputs, all the controls backed off to zero, it hums pretty loudly. Normally, all controls at zero, there shouldn't be any hum, especially so loud.
 
2: The A Channel is picking up some kind of consistent static interference.
 
3: When I switch either of the Bright toggles, there's a microphonic switching sound coming through the amp speakers. By microphonic, I mean it's not the sound of electrical current switching, but the sound of the physical switch action resonating through the chassis. When diagnosing further, I notice that tapping on the chassis anywhere causes the same microphonic noise. In other words, the whole chassis is alive, like a microphone or like tapping a live guitar pickup.  
 
In an effort to troubleshoot the problem, I tried several different patch cords and plugged it into a completely different electrical zone to eliminate the possibility of Hz interference. There was no change.
 
Any feedback would be hugely appreciated.
 
Thanks.
 
Cayce

[mica]

Cayce,  
 
First, fix the 60Hz hum, see resoldering or replacing filter capacitors, which is the primary service needed on older units. You will be able to tell if the foam is deteriorated and if the filter caps have suffered with a visual inspection.  
 
You need to fix this first and retest, because some of the other problems may be related to this primary issue.  
 
(I'm going to move this over to the  section, so look for it there shortly)

[edwin]

These are fairly easily fixed and not uncommon for preamps of this vintage. As Mica pointed out, the filter caps are the first stop. I had this problem on both of my vintage F2Bs and it behaved just as you say. The repair took less than 20 minutes. New filter caps after all these years isn't a bad idea anyway. Electrolytic caps go bad over time.
 
 
The microphonics might be related to the caps, might be a tube, or might be a loose solder joint somewhere. Pretty easily fixed.
 
If you don't have experience with tube electronics, and their associated high voltages, get a tech to do it. It should be quite simple to get it all sounding great once again. Unless you paid a really high price, I would keep it.  Love the old ones!

[caseycayce]

Again, I'm impressed with the feedback from this Alembic group. You guys are great, an example of what a forum should be.
 
Let's say I wanted to give this F-2B a thorough refurbishing. Would the parts be available through Alembic? I'd want to stay as close to original as possible. I'd like to replace:
1: Filter Caps
2: All control pots (some are pretty scratchy and Ch. A volume has a dead spot at the end).
3: Tubes.  
 
All of this, of course, is subject to your advice. I'm especially interested in tube suggestions.
 
Also, would it be advisable to install a three conductor, chassis-grounded power cord?
 
I'm comfortable doing the work myself, having been doing my own guitar and equipment repairs & mods over the past 20 years or so. If there are schematics and any other good procedural resources, that would be helpful as well.
 
Thanks again for the great help.
 
Cayce

[lbpesq]

Cayce:
 
Before you replace the pots, you might want to try exercising them.  Sweep each pot back and forth, from 0-10-0, quickly 30-50 times.  This often fixes scratchy pots.
 
Bill, the guitar one

[tubeperson]

Yes, the old you scratch my pot, and I'll scratch yours.  Excellent suggestion BTGO!

[edwin]

Filter caps are pretty generic. Just get ones that have the right values and form factor. Of course, Alembic would be willing to sell you some if they have them in stock.
 
I agree with Bill about the pots. Don't write them off until they are well exercised. They are sealed pots of high quality, but if they sit for a while, they can get scratchy.  
 
Tubes are a matter of preference. It's worth trying whatever you can get your hands on. I don't recall what's in my preamp, but I was using 5751s for a while. A bit cleaner with lower gain. Hm, maybe I'll go back to those. They were a cheaper alternative with mil-spec construction designed for hard use, but now they've gotten pricey. But all 12AX7 type tubes will work. Select for tone and low microphonics.
 
All that said, I've found that the differences between tubes can be somewhat subtle compared to the difference between the F2B and other preamps, such as Eden, etc.

[keith_h]

The big thing with tubes is ensuring the tube will fit inside the shield. Not all manufacturers have the same size envelope with some being bigger than others. I'm not sure if the F2B is shielded like the F1X but is something to look into to so you can take the shield with you when tube shopping.  
 
Keith

[gtrguy]

JJs are great and priced right.

[caseycayce]

Ok... the plot thickens.  I decided to open the case on this F-2B and see if there was anything obvious. There was... is. Apparently there's been a half-hearted attempt at a home remedy on a filter cap sometime in this preamp's recent history. I've attached two images, worth approximately 2000 words, if legends holds true. F-2B_Image_1 is exactly how it looked when I lifted the cover. F-2B_Image_2 is where I've lifted the filter cap out. Looks like the cap separated from the board at some point and was bridged with a length of typical lamp wire.  This is a "What would You Do" kind of question. Should I replace all 4 caps? If so, where can I get the closest equivalent (or better) to the Sprague 39D 35 UF-450VDC 7617 L?  Thanks again.  Cayce  

 

  (Message edited by davehouck on April 01, 2015)

[edwin]

If it were me, I wouldn't get an axial cap like the originals. I'd get a radial cap (both leads on the same end) and replace them all.  
 
Something like this would work fine:
 
http://www.digikey.com/product-detail/en/UCY2H390MHD/493-6915-ND/3664475
 
You can go as high as you want on the voltage and 35 mfd to 50  mfd (for a little extra, you probably don't need it but it wouldn't hurt) as long as it fits. Caps have gotten smaller and more reliable since the F2B was first produced. Just put in some new weather stripping to hold the caps in place. As long as you get a good brand, it's fine, Spragues aren't particularly magical and these caps are not in the signal path.
 
Other than the caps, it looks like it's in good shape. You could also change out the plate resistors (100k) with metal film to reduce the noise level while you are in there (I do this with all my Fender style preamps, as they can contribute to the bacon frying sound once they start to go bad), but it's probably not necessary.
 
 
Hm, maybe it's time to check out the filter caps in my F2B....

[slawie]

I would replace all 4 capacitors. The electrolyte inside does degrade over time and also if not used for a period of time.  
Small investment for peace of mind.

[sonofa_lembic]

Just a note:  NOT all 12AX7's will fit Alembic preamps.  There are variations in the size of the blown glass bulb, and the preamps have metal surrounds for the tubes.  Mesa, Ruby, and many other 12AX7's will fit, but certain Russian tubes such as Electroharmonics will not fit in the metal surround as they are too large in diameter.

[sonofa_lembic]

Just a note:  NOT all 12AX7's will fit Alembic preamps.  There are variations in the size of the blown glass bulb, and the preamps have metal surrounds for the tubes.  Mesa, Ruby, and many other 12AX7's will fit, but certain Russian tubes such as Electroharmonix will not fit in the metal surround as they are too large in diameter.

[mica]

I spoke with Casey yesterday and he's sending it in for recapping and testing.  
 
Trevor is correct about the fat tubes. We're using the JJs now, and anything that will fit Marshall amps should work - they use the shielding sleeves too.