Alembic F2-B Pre-Amp Restoration project

[oatmeal769]

Hello friends,
I have a 35 year old Alembic F2-B Pre-Amp.  I play bass through it, and nothing else sounds as good to me.  I've been using it so long, I don't know that anything ever will.

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 Schematic (I have both) and 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, especially while I have my lips stuck to a metal microphone.

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.

=====================================

* Here is some advice I've been given so far elsewhere, on another forum.  Can anyone add to or suggest changes? I'd really like balanced but it's starting to sound beyond my abilities.:  *

" ... Tubes and caps are the most likely for hum.
The orange ones are not electrolytics and do not need replacing. All cracked resistors obviously need replacing

Every one of the electrolytics needs replacing. The yellow ones and the silver ones.

If you want to quiet the noise, replace one of the resistors in the PS with a cheap choke -- it'll do wonders. - Replace the first 1K5 resistor with something like a hammond 154G

Replace the power cord! Particularly since you plug an instrument that you touch in to this, that should be fixed asap.

If you want a balanced output, use an output transformer -- Edcor would be good here, something like the XSM10K/10K would likely work well and could just be bolted to the output.
there is an issue which is that the Z out of the preamp is like 50K and won't likely do a good job driving the transformer's inductance. I think you should either work another tube in there (6T4 perhaps?) to lower the Z out so it can actually drive something like a cable, or the next stage, or you should switch from the 12ax7 to something with a lower rp -- maybe a 12ay7 or 12at7. The former will increase the signal a bit -- a 0.1u coupling cap will be too small in either case.
Assuming you do lower the impedance, on the primary you would connect out to + and - to ground (or the cathode if you like), and leave the input ct unconnected. For out, the "right" way to do it is to connect the + and - transformer taps to hot and cold, and connect the chassis (now grounded to safety ground) to the "ground" pin on the xlr.. ... "

Thanks for any input!

[RicharD]

Sure does look like a Fender front end don't it? 

Replace all the electrolytic caps.  I'd go ahead and replace the diodes too with 1N4007.  That would be a dollar well spent, especially if they're old germanium diodes which I'm fairly certain they are.  That should fix the hum.  A grounded power cord is always a good idea.

Zout is roughly 37K.  It's also coming off a plate.  Not exactly ideal for driving a loZ output but you might have luck simply hanging a transformer off the back end.  Adding a cathode follower stage between the existing output and the loZ transformer would be the right way to do it.  The Cinemag CM-2810 is the bomb when it comes to loZ output transformers.  Don't forget a polarity switch and a ground lift switch if you tread into these waters.

-Richard
 

[tubesornothing]

...It has an old 2 prong electrical cord with no safety ground

...it may have been re-capped back in the 80's

Like buttery says,  cap job and grounded cord

The last few years I've noticed the sound seems to have progressively dulled and lost it's life.

and... have you ever replaced the tubes?

The unit also has a little hum

hopefully that will go away with a cap job, wait and see

and is pretty noisy.

replace it all with metal film resistors will cure it (you may loose some mojo though...)

That can't be good when beer gets spilled on it,

My god man,  that's beer your spilling!  Lick it up off the amp before you kiss the mic

[RicharD]

I wanna toss out that I question the correctness of the schematic I posted.  I've never seen, much less been inside of a F2-B, but I have protoboarded this circuit when developing the Panna Maria.  At the input, the 1M grid leak would be found in front of the grid stopper in all Fender amps.  The 68K grid stopper produces a f-3dB of roughly 30kHz.  During my experiments, I changed this to a 33k which brightened thangs up, but my customer quickly dismissed the circuit for having excessive gain and for sounding like a Fender.  Another thang that bothers me is the output coming off the plate.  Not good for driving a cable more than 2 feet long.  None the less, the F2-B has good collector's value.  Personally I wouldn't hack on it.  Give it a cap job, new diodes, and put a grounded cord on it and see where she stands.  You might need to change a plate resistor or 2 if it's crackling.  Check the power supply resistors too and make sure they are within tolerance.

Please post pictures.

Oh yeah... welcome to the forum.

-Richard

[oatmeal769]

Sure does look like a Fender front end don't it?

Yep, it was adapted straight from the front end of a Fender Showman Blackface.

Replace all the electrolytic caps.  I'd go ahead and replace the diodes too with 1N4007.  That would be a dollar well spent, especially if they're old germanium diodes which I'm fairly certain they are.  That should fix the hum.  A grounded power cord is always a good idea.

Which caps are electrolytic caps, and what should I replace them with?  I was told "Replace Everything ORANGE" (see pictures)
Oh, I forgot to mention - I'm really good with a soldering iron, but I know next to nothing about circuits and how they work.  I know how to point out certain components, and how not to get electrocuted but that's about it.  I'm a tinkerer, but not all that knowledgeable about the subject.you gotta dumb it down for me somethimes...

Zout is roughly 37K.  It's also coming off a plate.  Not exactly ideal for driving a loZ output but you might have luck simply hanging a transformer off the back end.  Adding a cathode follower stage between the existing output and the loZ transformer would be the right way to do it.  The Cinemag CM-2810 is the bomb when it comes to loZ output transformers.  Don't forget a polarity switch and a ground lift switch if you tread into these waters.

I'm driving a balanced TRS 10K ohms input (a compressor) from this unit.  The cord goes from the unbalanced Alembic main output to the balanced input of the compressor.  The cord I'm using is an unbalanced shielded cable about 1 foot long... Do I really even NEED to balance?  It sounds like more trouble than it's worth, and as you say, it does have collector value.  But I do want to be sure I'm feeding a quiet signal at the exact right amount that the compressor would like to see.

I wanna toss out that I question the correctness of the schematic I posted.  I've never seen, much less been inside of a F2-B, but I have protoboarded this circuit when developing the Panna Maria.  At the input, the 1M grid leak would be found in front of the grid stopper in all Fender amps.  The 68K grid stopper produces a f-3dB of roughly 30kHz.  During my experiments, I changed this to a 33k which brightened thangs up, but my customer quickly dismissed the circuit for having excessive gain and for sounding like a Fender.

It's known to some that the commonly available schematics on the web contain an error or two, the 68k thing sounds familiar...  But the schematic looks like a lot of chicken scratch to me.  Again, I can kinda identify components, and see the routes but schematics give me a headache.

Another thang that bothers me is the output coming off the plate.  Not good for driving a cable more than 2 feet long.  None the less, the F2-B has good collector's value.  Personally I wouldn't hack on it.  Give it a cap job, new diodes, and put a grounded cord on it and see where she stands.  You might need to change a plate resistor or 2 if it's crackling.  Check the power supply resistors too and make sure they are within tolerance.

If I replace one resistor, I may as well do all, right?  They aren't expensive, even the fancy ones, right?

Please post pictures.

I'll post some now for analysis, and then after to compare.

Oh yeah... welcome to the forum.

Thanks guys for a great welcome!  I look forward to learning a lot.

One other question:

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.

.

[oatmeal769]

have you ever replaced the tubes?

Yes, I have a stable of some old Mullards and Telefunkens, as well as some 70's RCA's that sound great.

hopefully that will go away with a cap job, wait and see

I was always told that hum could likely be Caps.

replace it all with metal film resistors will cure it (you may loose some mojo though...)

Have you a link?  I don't understand the difference, I just know the ones in there now look ancient.  I certainly don't want to lose mojo, my playing needs all the help it can get!

My god man,  that's beer your spilling!  Lick it up off the amp before you kiss the mic

Sir, I don't know you but are you accusing me of a spillin' BEER?? I'd never dump perfectly good beer into a useless piece of metal and silicon - even if it was foreign beer.  Heck, I'd lick it offa the circuits even if I'da done it.  Them's wrasslin' words.  Besides, who said it was MY beer that done got spilt??
J/K, I'm a wise ass!

[RicharD]

Your 4th picture, Caps.jpg are the caps you want to replace.  I notice 3 thangs:

These caps are 20uF at 450V and the schematic I posted shows 47uF.
The schematic shows 4 caps but yours has 5, one of which is mounted in an "after thought" fashion.
Seems like these caps should be radial leaded (both leads coming out 1 end) instead of axial leaded (1 lead out each end).

Apparently Alembic still makes this unit.  Wouldn't surprise me 1 bit if the later units have 47uF caps instead of 20uF caps.  Yours looks like a 70's vintage unit to me.  I can't really tell from the pictures why there's a 5th cap.  At a guess, I think somebody has done some mods.  That unit isn't supposed to have a loZ out, but yours does (last pic).  The loZ stuff looks after-market to me whereas the XLR jack is screw-n-nut and everythang else is pop rivet.  Looks like the input grid stoppers are in fact 68K and that they are in fact in front of the 1M grid leak resistors.  Hmm.... dual input jacks per channel.  Is there a 68K on each jack or only 1 per pair of input jacks?

There shouldn't be a problem using active pickups with this unit unless your pickups have DC at the outputs (which they shouldn't).  You could put a series bypass cap on the input, but that's gonna totally jack with the tone.

The diodes do not look like they're old germanium so don't worry about replacing them.  It wouldn't hurt to measure the 3 resistors  (red violet brown and 2 brown green red) in the cap picture.  You'll need to disconnect 1 end of each to accurately measure these.  I doub't there's a problem here since there is very little load on the power supply but.... it is the power supply and it's always a good practice to verify the power supply is healthy.  Look at the 20uF cap connections for any tell tale signs of replacement (ie the solder joints look messy compared to the rest of the board).

Personally, I'd replace as little as possible and make every attempt to keep it looking as original as possible.  A cap job and a grounded cord might offend some collectors but screw them.  A safe working unit is more important.  That's a cool box.

-Richard

[DummyLoad]

The schematic shows 4 caps but yours has 5, one of which is mounted in an "after thought" fashion.

the schematic seems to be something someone drew up, other than alembic - it is possibly a drawing of a later production unit, or unit someone already modified.

Seems like these caps should be radial leaded (both leads coming out 1 end) instead of axial leaded (1 lead out each end).


or perhaps not - look closely, there is a trace where the (-) of the caps lands on the PCB - in early 70's, axial units were much more common in HV. the dates codes are certainly early 70's - having seen how other alembic equipment was constructed around that time, id be willing to bet that unit is stock, other than perhaps the lo-Z out - still, it is possible that it's a factory hack too. i'm betting it is.

don't add a choke - leave it stock, but as buttery says, do check PS resistors and the plate resistors for drift - CC resistors will drift up in value, never down.

you do have a hum balance pot - check it as well.

before you start, be aware that 60-70s PCBs are usually really crappy, in that it's easy to lift the foil off the board and you could lose a pad or two - use a solder sucker, keep the heat time on the pads to a minimum, and DON'T roll the iron tip on the pads. use a quality low watt soldering iron suitable for PCB work, or soldering station. e.g. weller.

note the polarity of caps and diodes BEFORE you pull them. you already have pics for orientation of the caps and one of the diodes, so make sure at least you have a good pic of the other diode. you could also do it old fashioned way - mark the PCB or draw. 

at the very least, check the the PS diodes. personally, i'd replace them w/ 1N4007s. you'll need to lift one end of the diodes to check them. while you have the ends up of (or pulled) diodes, pull all the PS caps and measure the PS resistor chain, (the two 1.5k) if it's more than 3k (>10%) then assume one or both of the resistors has/have drifted - check the 270 ohm resistor as well.

while you have all the PS caps out, pull the tubes (moot) and measure the plate resistors, again if >10% they should be replaced. next, pull the bypass electrolytic caps, measure the cathode resistors.

now tack in all the new parts, double check your work.

the thing that will probably irk you the most: it is likely you'll have to open up the hole for the AC power cord entry, if you upgrade your unit to a 3 wire grounded power cord. suggest a uni-bit and clamp the unit down on a bench before drilling.

oh yeah, welcome! great bunch of folks here and a very gracious host.

PS. doug sells sprague atom-lytic caps (use the 500V caps) and the diodes - click here

[oatmeal769]

Holy smokes!  Richard and ISOTone,  I am SO SORRY for wasting your time!  Those are the wrong pictures.     I opened the wrong folder, and haven't seen the inside of mine in a while....  The next post will be pictures I just took today.

  Again, BIG apologies, I really value the suggestions you're making.

The blank output jack in my new pictures is a jury rigged jack for a footswitch on the outputs, (obviously added later) from which I removed the wiring to the outputs.

The caps have been replaced once before, as evidenced by different solder work.  

I think there's been some tinkering with resistors on inputs as well.  

The ones on the output have been replaced, but I don't know if they are correct or even belong there.

The 1/4" out jacks were all replaced by me - (late one night on a bus after a gig and a few beers if I remember correctly, LOL) because the stock ones had worn out.


Would there be any benefit to replacing the power transformer with a newer torroidal unit, instead of inserting a choke?  I think I read that torroids are much quieter?

Is the 'hum balanced pot' adjusted by a meter, or just 'by ear'?

Since it's already been modded, I don't mind modding it further as the goal is better quieter sound.  It's been with me on almost every gig and studio situation for 20 years, that's not gonna change, it's a work horse, not a showpiece.  It could likely be restored to original fairly easily as well.

Someone (not me) also had pulled out the grommet and pulled the AC cord through a few times forcefully, enlarging the hole.  I have the right tool - a panel punch - to make a clean one and replace it with a modern round cable.

[RicharD]

No problem.  (1)Notice that the power caps are magically 40uF now.  My guess is that this unit is a little newer but still 70's vintage.  From your pictures, it does not appear that the power caps have been replaced.  (2)The diodes look like 1N4007 so they're probably fine.  If you get in there to replace the caps, there's no harm in replacing those diodes.  They're cheap.  Be sure you pay close attention to polarity of the diodes and caps.  (3)These are the power supply voltage dropping resistors.  I'd flat out replace these with metal film instead of carbon comp.  They're in the power supply so flame proof is a plus and you won't lose any carbon comp mojo since they're not in the audio circuit.  (4)This is the hum balance pot.  It sets the point at which the filaments are grounded.  You set it by ear.  Typically centered is the place to be.  (5)Please fix this with a proper strain relief and grounded cord.  Take the green wire to the chassis right by the power transformer.  Now... the existing wiring is neutral fused which works but is not preferred.  I'd rewire as follows.  Take the white wire to 1 line lead of the power transformer (doesn't matter which one).  Take the black lead to the tip of the fuse.  From the side of the fuse, run a wire to the power switch.  From the other terminal of the power switch, take a wire to the remaining transformer primary lead.  (6)There is the possibility that the cathode bypass caps are failing.  They are electrolytic caps too.  The one furthest to the right looks suspicious on the + end.  This probably wouldn't cause hum but it certainly would cause a decrease in gain.  (7)Not that there is a 68K resistor on each jack which with 1 input will put them in parallel effectively creating a 34K grid stopper.  RG Keen's schematic is erroneous. 

As ISO noted, you can get all the parts you need right here from Hoffman.  Quickest shipper in the amp parts biz.  Another very important point he mentioned is the fact that it's super easy to accidentally pull up a trace on these old 70's PC boards.

I see no reason to change out that Stancor PT.  If you do, send me your old one.    

[oatmeal769]

This is excellent, I'm going to do exactly what you've specified, thanks very much!  I'm going to order all at once, so it might be a week or two until you hear from me again, need to wait for my next check.

Yes, this is a vintage '70's unit, about 1973-74 I think.  Serial number 712.

1.)  If I read your post correctly, the only way the input could be the 68k as posted in the schematic would be if both inputs were used at the same time.  when either is used singularly, the actual resistance is 34k?  How does the second input reduce signal by 6dB as it says in the spec's?

2.)  Right now, my caps are 40uf 450v, the schematics say use 47's...  What is the difference / benefit of either?  I have another picture of a re-cap job done supposedly by Alembic  which shows 47's  (below).   The pic probably isn't real recent though, because as you see, the power cord was not switched out.

3.)  Power Supply diodes will be replaced, at least one of them is a 4007, I looked today. I'll do the power supply resistors, and cathode bypass caps with metal films too.  as you say they aren't in the audio loop, and they're also only a buck or so each, so why not.  Can I just go through and replace the orange drop caps too while I'm in there?  They're also a buck each, and will take a couple minutes each to do.

4.)  I'm assuming that by leaving the carbon comp resistors alone, that will help retain MoJo? Can those drift also, or do they simply 'work or not.'

5.)  So between the Sprague Atom, and the Ill. Power Supply, that Doug supplies, the Spragues are better?  Is it true or possible that these caps can have effect on sound?  In 'audiophile' circles, they all seem to like Rubycon, etc.

6.)  Last Q., is a balanced out beneficial or even necessary?  I'm driving 8 inches of cable into a balanced TRS 10Kohms input.  At this point though I'm starting to think it would be cool to keep it as 'stock' as possible.




Picture is of a re-cap job on someone else's F2-B, supposedly done by Alembic.

[DummyLoad]

gotta love the foam under the caps to keep them rattling...  i'd have used goop - that way the next guy would cuss at me - a lot! j/k

a telefunken and a mullard... life is good.   :glasses9:

if you like the way it sounded, and it was quiet enough for you once upon a time; why mess with it? the toriod (or any mod other than for safety) is probably not a wise idea for that vintage piece of equipment.  just my 2 cents.

happy hacking!   

--pete

[RicharD]

1.)  If I read your post correctly, the only way the input could be the 68k as posted in the schematic would be if both inputs were used at the same time.  when either is used singularly, the actual resistance is 34k?  How does the second input reduce signal by 6dB as it says in the spec's?
2.)  Right now, my caps are 40uf 450v, the schematics say use 47's...  What is the difference / benefit of either?  I have another picture of a re-cap job done supposedly by Alembic  which shows 47's  (below).   The pic probably isn't real recent though, because as you see, the power cord was not switched out.
3.)  Power Supply diodes will be replaced, at least one of them is a 4007, I looked today. I'll do the power supply resistors, and cathode bypass caps with metal films too.  as you say they aren't in the audio loop, and they're also only a buck or so each, so why not.  Can I just go through and replace the orange drop caps too while I'm in there?  They're also a buck each, and will take a couple minutes each to do.
4.)  I'm assuming that by leaving the carbon comp resistors alone, that will help retain MoJo? Can those drift also, or do they simply 'work or not.'
5.)  So between the Sprague Atom, and the Ill. Power Supply, that Doug supplies, the Spragues are better?  Is it true or possible that these caps can have effect on sound?  In 'audiophile' circles, they all seem to like Rubycon, etc.
6.)  Last Q., is a balanced out beneficial or even necessary?  I'm driving 8 inches of cable into a balanced TRS 10Kohms input.  At this point though I'm starting to think it would be cool to keep it as 'stock' as possible.

1.)  Look at this schematic.  Pay special attention to how the jacks are switched.
http://www.schematicheaven.com/fenderamps/twin_reverb_ab763_schem.pdf
2.)  40...47uF... it's all good.  Back in the olden days, caps were expensive & bulky so they skimped.  A little more filtration is a good thang with ss rectification.
3.)  No reason to change the orange drops.  The likelihood of them being bad is slim to none.  Not worth risking damaging the PCB.
4.)  Carbon comps can & will drift.  That's why you either love them or hate them.  I'm a CC fan myself.  Leave em be.
5.)  I've used the cheap Illinois caps w/o issue a zillion times.  Sprague is a better cap.  For this unit, I'd do Sprague just cuz.
6.)  A balanced out would be beneficial if you intend to hit the snake at this point.  In your case (jumping to the next box in the rack), it's a total waste of time & money.

[FYL]

I've used the cheap Illinois caps w/o issue a zillion times.  Sprague is a better cap.  For this unit, I'd do Sprague just cuz.

Please read what has been published here and elsewhere re. today's Sprague Atom caps: they are bog standard elcos fitted in big cans (hence lower reliability because of thermal problems) and resold for two or three times the price of better performing modern caps.

[oatmeal769]

Butterylicious,
Awesome, thanks man!  Before I submit my parts order to Doug, I'll Post to be sure I didn't forget anything.

1.)  Look at this schematic.  Pay special attention to how the jacks are switched.

Not sure I understand the schematics, inputs, but I do see that the original uses two, rather than one 68k, I'm sure that has something to do with it...

2.)  40...47uF... it's all good.  Back in the olden days, caps were expensive & bulky so they skimped.  A little more filtration is a good thang with ss rectification.

Okay, I'll go for the 47's.  Maybe even 50's?

6.)  A balanced out would be beneficial if you intend to hit the snake at this point.  In your case (jumping to the next box in the rack), it's a total waste of time & money.

Makes total sense RE: Balancing.  I never have any intention of running this any more than a foot away to the next box.  I'm just hoping to keep the signal as quiet and noise free as possible.  I also want to be sure I'm feeding the 'optimal' level to the balanced 10K input.  It seems though from what I've been reading that as long as it's over 600 ohms, it'll be fine.

Please read what has been published here and elsewhere re. today's Sprague Atom caps: they are bog standard elcos fitted in big cans (hence lower reliability because of thermal problems) and resold for two or three times the price of better performing modern caps.

FYL,
What would you use or recommend??  Some Hi-Fi audio dudes elsewhere speak highly of Rubycon, but I know Doug doesn't sell them.  I have a high dollar 'boutique' headphone amp, all point to point using the 'best' components.  It uses some big oil, or P.I.O. caps.  It's been said those were the best 'sounding' ever.  Would any of these (Rubycon, P.I.O., or cheapies) even make a difference, since none are in the audio loop?

gotta love the foam under the caps to keep them rattling... i'd have used goop - that way the next guy would cuss at me - a lot! j/k

ISoTone,
LOL, seems like someone already had that idea in mine, I'll be cleaning all that stuff out during my restoration...

a telefunken and a mullard... life is good.

Not just any Telefunken, it's a '63 smooth plate. * Sounds of applause, fireworks, Ooohs and Ahhhs *  The Mullard is a '61 short ribbed plate.  I've recorded, level matched and compared.  The difference is ever so slight, but it's there.  The Tele is a little 'darker', the Mullard is a little bit more 'sparkly'  I still have the original RCA's that came with it, which sound markedly better than a couple new Russians I tried several years back.

if you like the way it sounded, and it was quiet enough for you once upon a time; why mess with it? the toriod (or any mod other than for safety) is probably not a wise idea for that vintage piece of equipment.  just my 2 cents.

That's a good two cents.  Based on how Butterylicious was salivating for the original Stancor that is in there, I think I'll keep it.  I was also going to install an IEC AC jack, but I think I'll go with a grommet / strain relief, and a plain black cable, about a foot long.  I think I will go ahead and keep the collector's value with it.  Someday, I'll die, and it'll be my legacy, LOL

Thanks for all the help again!

[FYL]

What would you use or recommend??

Illinois, F&T, Nichicon, MIEC, CDE/Mallory, TCI and many others  produce really good HV elcos. If you want 500V+, go for F&T or TCI; if you're looking for compact caps MIEC and Nichicon will fit the bill, if you're looking for big cans, CDE/Mallory has some really nice stuff, etc.

I have a high dollar 'boutique' headphone amp, all point to point using the 'best' components.  It uses some big oil, or P.I.O. caps.  It's been said those were the best 'sounding' ever.

Caps have no sound of their own. They can show defects when improperly used and thus impose their sonic signature, but no one - I repeat no one - has ever been able to find any subjective differences between two properly chosen and used capacitors in a double blind test.

Would any of these (Rubycon, P.I.O., or cheapies) even make a difference, since none are in the audio loop?

The power supply caps *are* in the signal path.

[oatmeal769]

Illinois, F&T, Nichicon, MIEC, CDE/Mallory, TCI and many others  produce really good HV elcos. If you want 500V+, go for F&T or TCI; if you're looking for compact caps MIEC and Nichicon will fit the bill, if you're looking for big cans, CDE/Mallory has some really nice stuff, etc.

I just need to keep them about the same size and shape, they need to fit just as they're shown in the pictures.

Caps have no sound of their own. They can show defects when improperly used and thus impose their sonic signature, but no one - I repeat no one - has ever been able to find any subjective differences between two properly chosen and used capacitors in a double blind test.

I think you mean OBjectively, right?   SUBjectively is based on listener opinion...  I'm mostly a fan of DBT's too, so point taken.  I'll get some 500V 47 or 50mf.    The Illinois one's are fine then, I take it...

The power supply caps *are* in the signal path.

But what you're saying is that it's irrelevant because the brand or type doesn't matter as long as the volts and farads are matched correctly, right?

.

[FYL]

The power supply caps *are* in the signal path.

But what you're saying is that it's irrelevant because the brand or type doesn't matter as long as the volts and farads are matched correctly, right?

As long as the caps are properly chosen - voltage ratings, capacitance, ESR, etc. - and used, yes.
We're looking at a small power supply for a couple of low-bandwith tube stages, basic Illinois caps will be fine.

[oatmeal769]

Awesome!  This project really isn't even going to cost much!!

[imaradiostar]

Just a thought- all this talk of caps and it's not going to make a huge difference. Correct value Xicons will work just fine.

If it were mine and I were driving a compressor I'd add a mosfet source follower to lower the output impedance just a little bit. I'm sure it could be done in a minimally invasive way- mosfet gate (thru a small stopper) would connect where the big coupling cap connects, 47-100k to ground from the source and drain to B+...up the value of the coupling cap for full effect.

That setup will even drive a pretty low Z out, should you want to.

Don't assume that cleaner is better- sometimes the imperfections are ideal for a given instrument.

I once built a bass DI for a guy that I'm sure wasn't 100% clean but it sure did sound good. It was a single 12ax7 in a pedal operating as a gain stage into a direct coupled cathode follower connected to a DI box transformer through a large value film cap. A second unbalanced out was taken at the cathode follower with another film cap. I made the 1st stage cathode bypass cap switchable for a "gain change" if needed. I'm sure it wouldn't have held up well on hi-end test gear but it made his bass sound great.

jamie

[oatmeal769]

If it were mine and I were driving a compressor I'd add a mosfet source follower to lower the output impedance just a little bit. I'm sure it could be done in a minimally invasive way- mosfet gate (thru a small stopper) would connect where the big coupling cap connects, 47-100k to ground from the source and drain to B+...up the value of the coupling cap for full effect...  That setup will even drive a pretty low Z out, should you want to....

Uhh... Will that make it go gooder?  I have NO idea what you just said, my knowledge is extremely limited beyond how to solder, and how to point out components.  You'd have to draw me a picture, LOL

Don't assume that cleaner is better- sometimes the imperfections are ideal for a given instrument.

Oh, not at all.  If I wanted ultra clean I'd go solid state.  I want that little bit of tube fuzz, and soft compression. - And the cool analog tone stack... But only that stuff, not hum and noise etc. introduced by worn components or incorrect usage.

[mcrracer]

Great project. Is it possible that you could post the B+ voltage and the AC voltage output of the transformer? Thanks

[oatmeal769]

Hey guys,
I'm ready to make a purchase of my components, but I have some big gaps in my electronics knowledge.

I can identify components, and more or less how a circuit flows.  I know how to test with a multi-meter, and I know how to solder but as far as which resistors, and capacitor sizing, schematics, etc. I'm clueless.  I need to be sure I do this right, so could you guys help me make a "shopping list" here on Hoffman?

For the moment, let's assume that all the CC resistors in the channels will be fine, I can always go to the electronics store and pick up 1 or two that I need.  I'd like to keep the mojo there if I can...

The big PS caps I already know are going to be 500v 50uF but the rest of the stuff I'm not sure how to select...

To replace the two silver looking electrolytic capacitors in each channel, I can see that they say Sprague 300V 50-160 TC-1160, but no idea what that means, or what to get.  I'm assuming they are the electrolytic cathode bypass caps.


Then I want to replace all the carbon comp. resistors in the PS with new metal film resistors.  I know which ones they are, but the voltages, watts, etc.?
 -
(2x) 1.5K  resistor (metal Film)
(1X) 270  resistor (metal Film)
(1x) 250  resistor (metal Film)
(1x) 68  resistor (metal Film)

Then I need 4 diodes (I think) for the PS as well.
(4x) 1N4007 Diode - again, which ones?

Any thing else I should add to the list?  I have a power cord, extra wire, solder equipment, meter, etc.

[mister ed]

Yes.. the differences in tone caps CAN be heard.. IF your playing solo.

[RicharD]

Parts list:

4 - 47uf 450V Electrolytic Caps
2 - 50uf 25V Electrolytic Caps
2 - 1N4007 diodes
2 - 1k5 1/2 watt metal film resistors
1 - 270 ohm 1/2 watt metal film resistor
1 - 250 ohm 1 watt trimmer

The other diode & resistor only feed the LED.  Not worth messing with.
For the big caps: 40, 47, or 50 uF is fine, 450 or 525V is also fine
For the little caps: 50uF at 25 to 50 volts is fine.  Even 100 volts is fine but would be a pain to install due to their size.  Doug sells a 50/50 Sprague, use that. 
The plain ol resistors are 1/2 watt.
The trimmer is gonna be hard to find.  It's only dissipating a little over 1/8th watt so a 1/2 watt would be fine.  I wouldn't risk a 1/4 watt.  I question if it needs to be replaced.  With the tubes pulled, sweep it while watching it with an ohm meter.  If it's not broke, don't fix it.
 

[oatmeal769]

Thank you  again Butterylicious!

Well, here's my parts order.  I will get the trimmer locally IF I need it.  I'd rather not de-solder it, etc. if it works.
Doug didn't have the 270 Ohm 1/2 Watt resistor, so I'll get that one here as well.

-------------------------------------------------------------------------------
Item ID........ Detail.... ...Qty ....Each ..Extend Description................
-------------------------------------------------------------------------------
1/2 watt metal. 1.5K......      2    $0.20    $0.40 1/2 watt metal film resistor
1N4007......... ..........      3    $0.25    $0.75 1N4007 diode...............
47/500v........ ..........      4    $6.25   $25.00 47/500v....................
50/50v......... Sprague...      4    $2.15    $8.60 sprague atom cap...........
Grommet 1/2.... ..........      1    $0.15    $0.15 rubber grommet.............
Grommet 3/8.... ..........      1    $0.15    $0.15 rubber grommet.............
cable.......... ..........      3    $0.32    $0.96 mini shielded cable........
fuse large..... .5 amp....      1    $0.80    $0.80 large Slo-blo fuse.........
strain......... ..........      1    $0.50    $0.50 power cord strain relief...
-------------------------------------------------------------------------------

His prices are reasonable, and NO TAX! 

[RicharD]

>
His prices are reasonable, and NO TAX! 

And his service is exceptional.  Warm your iron up for Wednesday evening.

[stingray_65]

as far as your unbal out.

weber sells the way cool tiny trannys https://taweber.powweb.com/store/balun.jpg

I've used it on a couple amps now and love it, I've started putting them in my speaker cabs as well, so any amp I plug into the speakers has a balnced out. I usually forego the R1 and put a 100K lin pot in there, it gives you a little more versitilty I feel.

only problem with these is they are so inexpensive, I have to wait till I make another order to get one, so I add 2 or 3 on each order I make from there.

IMHO I'd pass on the panel mount XLR they sell there, it's plastic, it works, but it wouldn't survive gigging, go buy a neutric or switchcraft metal one.

[oatmeal769]

Thanks for the suggestion, but I'm going to stick with unbalanced out, and keep the collector's value intact.  I'm never going to run more than an 18" cord anyway to the compressor right underneath it.  If I send any farther, it would be from the compressor's output, which is balanced.

[oatmeal769]

Got a package in the mail today, I should be able to start tomorrow.  I'll post pics, etc. when I'm done!

[mcrracer]

What series Orange Drop caps were in there? 715, 716 or PS6? Thanks Should be printed on side of the caps next to value.

[snoof]

FYI...

pointing out the error in V1b just so's everybody knows.  Plate and cathode are reversed.  RG must have had a brain fart when drawing up the schem.  I built one and looking at that error had me scratching my head for a moment...

[oatmeal769]

I completed everything so far with no problems, I'll post details and pictures soon. 
I just ordered all the 'Orange Drops' from Doug as well, I'm still getting just a bit of hum from one channel, especially if I put my finger near / on one of the Orange Drops.  BUT overall, it's much quieter, which is cool, and I think the punch and liveliness of the amp is back.  I really should have made level matched recordings of both before and after, But subjectively, it has a lot more punch than before.

[mcrracer]

What series Orange Drops were in the preamp when you started? There are three different kinds. Thanks

[oatmeal769]

What series Orange Drops were in the preamp when you started? There are three different kinds. Thanks

These are Sprague 6PS.  The two cap sizes are P10's and P47's.  Why do you ask?

[mcrracer]

Thank you very much I build  tube guitar amps and know that there are distinct differences in sound between the three kinds of ODs. I was just wondering which ones were giving the F-2B its' MOJO! I akso know that in certain amps, The TW Express comes to mind, that a certain cap is almost mandatory. Thanks again for the reply.

[oatmeal769]

Distinct differences?  Such as?  I was under the impression that the material they were made of was more to do with voltage and design.  IE all of them rated for 500V and over are polystyrene.  I thought also of swapping in some Mallory's as well. 
At the end of the day though, I'm becoming less and less convinced that individual components, etc. do much to 'change' the sound...

[mcrracer]

Your opinion and you are entitled to it, but that does not make it fact. I am not going to get into a long drawn out debate. Ny health will not allow it. If you are interested you may do the research that I have done to find the answer. The data exists.  Not onl choicr of componrnts but their physical location and wire dress also matters. Thanks again for the info.

[oatmeal769]

Not trying to get into a pissin' match at all.  I just wonder what tonal differences are said to exist?  I didn't know you could use more than one kind of orange drop with that voltage requirement.  Is there a site you can recommend I read?

[oatmeal769]

Alembic F2-B refit

Sorry for the long delay in posting pictures.

I'm pretty well done, I've decided to forgo the balanced output, but the rest of the suggestions made here I have implemented. 
The result is a unit which is much MUCH quieter - almost silent, even at extreme volumes / gain.  A bit of thermal tube noise, and maybe just a tiny bit from the remaining CC resistors, but a huge improvement.  No hum whatsoever.  The active electronics in my bass are now far and away the noisiest part of my signal chain.

It has also regained the punch and clarity I felt it had been lacking.

Here's a rundown, and pictures.

New wiring as needed.

ALL Capacitors replaced.

All new metal film resistors and diodes in the power supply.

Channel CC resistors replaced with MF resistors where needed.

New grounded power cord, re-wired for proper grounding.
 
Added choke between X4mr and P.S.

Added small caps between tube output and tone stack.

Tinned circuit board, & riveted to chassis.
 
Cleaned or replaced all jacks, cleaned chassis, tube mounts, new screws as needed.

[oatmeal769]

Here's another picture:

[RicharD]

I haven't been around much lately but I wanted to say congrats and nice work.  It looks really good.  I bet you're glad to have it back working all proper and stuff.  I love it when people pop in here and fix thangs themselves.  DIY baby.

[oatmeal769]

Thanks!  Indeed it is VERY satisfying.  I liked what I learned as well.
Hey, check this out -
I'm thinking of making another pre-amp, really more of a direct box, based on this circuit. More of a simplified reverse engineering job than anything...

What I'd like to do is make the same power supply, and then use a 12AX7 - and nothing else - as the preamp. No tone stack, extra caps, etc.
I'd just run the input jack straight to the first stage, and connect the pins to go directly to the second stage, and then straight to the output jack.

The only extra might be a first and /or second stage gain control. The tone would then be entirely based on the tube, (I think) and the bass plugged into it.

I'm a complete novice, but I think it will work (?) My curiosity as a 'minimalist' has had me thinking about this for some time now.
It seems my most satisfying 'mods' lately have been in streamlining how much circuitry the tone signal has to go through, and I'm really liking the results.  I disconnected the onboard E.Q., and bass/treb controls on one of my basses recently, and found I heard more punch and clarity. 

What do you think? Any suggestions?

[RicharD]

Less is more... Bauhaus

No doubt bypassing tone controls on a guitar/bass guitar will add punch.  Any tone control (except for an active) will have  some dB loss and obviously some frequency response attenuation.  Get rid of it, no losses = more punch.

Attached is an analysis of the AB763 preamp circuit which the Alembic is directly copied from.  The 2 gain stages are effectively the same.  (In actuality the 1st stage has about double the load {RL} on it but that's only <1dB difference so lets just count on thumbs and call them both 35dB, AV 50.  The EQ has a huge mid scoop so lets just round this stage to -15dB or a AV of 0.2
35-15+35=55dB  or 50*.2*50=500  AV of 500 is 54dB... close enough
A gain of 500 is ample... excessive even.  Now take out the negative factor in the equation above and you get AV=2500 & 70dB  You gotta have a volume pot and you prolly won't be able to turn it past 1 w/o overdrive.

Often less is still less.

[PRR]

> Attached is an analysis of the AB763 preamp

No actual attachment.... you mean your inline text analysis?

> The only extra might be a ....gain control

Agree with Butter's analysis. Gain will be in the 2,000+ range. This is probably TOO much for some situations. Output of 0.1V is common. Times 2,000, is 200V. Which is a little more than a 2V-in power amplifier wants to find on its doorstep.

But of course what really happens is the second stage clips at "only" 100V peak, dirts-up your sound, and is still too hot for the power amp.

You can turn-down between stages, indeed it is handy to have a knob right there near your plug-jack (albeit electrically between stages). But two cascaded 12AX7, and no other loss, you can never use the upper half of the pot. It's just too much.

It would also be nice to have a somewhat lower output impedance, in case you ever use a longer cord from preamp to power amp.

Which all suggests a different tube-type than 12AX7. But we won't mess with success.

Throw-away some gain and excess level at the second plate resistor. The 12AX7 still "feels" the 100K load we know works so well. The output gets 1/10th (or 1/5th) of the 12AX7's full swing. The output impedance is 9K (or 17K), which will drive 30 feet of cable to the top of the audio band.

Pick the first or second tap down in testing. (And if I'm full of snow again, you still got the 102K tap at the plate.)

[RicharD]

DoH!!!!!  OK it's attached now.   

Question for oatmeal:
When you use the Alembic, where do you usually keep the level set?
The reason I ask is because I have breadboarded this (Fender) circuit more than once and have even tried as a stand alone preamp where I found it to have waaay too much gain.

Here is a schematic for a bass preamp I designed/plagiarized about 6 months ago.
http://www.sotxampco.com/Schematics/STAC/Chanel-B-8.pdf
 

[oatmeal769]

A gain of 500 is ample... excessive even.  Now take out the negative factor in the equation above and you get AV=2500 & 70dB  You gotta have a volume pot and you prolly won't be able to turn it past 1 w/o overdrive.

So, maybe in keeping with the original design, maybe only use one gain control for the first stage and don't attenuate the second?

Which all suggests a different tube-type than 12AX7. But we won't mess with success.

Brainstorm! - I have thought of substituting a 6SL7 into the Alembic circuit before.  I read somewhere that this was the original impetus for a 12AX7.  There is some talk about it being a nice sound too, although more likely to be microphonic.  Would this knock the gain down enough? 
Another option might be to do it with a 12AU-7 or something with less mu ?

Throw-away some gain and excess level at the second plate resistor. The 12AX7 still "feels" the 100K load we know works so well. The output gets 1/10th (or 1/5th) of the 12AX7's full swing. The output impedance is 9K (or 17K), which will drive 30 feet of cable to the top of the audio band.

If I read this right, you're saying it becomes 'Low-Z' out, without even having to use a transformer - I.E., the perfect direct box!

When you use the Alembic, where do you usually keep the level set?
The reason I ask is because I have breadboarded this (Fender) circuit more than once and have even tried as a stand alone preamp where I found it to have waaay too much gain.

Gain is at 4 to 5 (out of 10)  I have used it as low as 2 or 3, which makes it VERY clean.  Beyond 5 or 6 and it drives whatever I have downstream (compressor or power amp) too hard.

Here is a schematic for a bass preamp I designed/plagiarized about 6 months ago.

plagiarism is the sincerest form of flattery!

[snoof]

There's Low-Z and then there's balanced Low-Z.  Most direct boxes are balanced.

[RicharD]

>There's Low-Z and then there's balanced Low-Z.  Most direct boxes are balanced.
>If I read this right, you're saying it becomes 'Low-Z' out, without even having to use a transformer - I.E., the perfect direct box!

There's more than a couple of ways to do this.  If you want to avoid costly and colorful transformers, one could wire 1/2 of a 12AU7 as a split load phase inverter and get your balanced out that way.  The other 1/2 could be a cathode follower for your unbalanced output.  Personally I happen to like colorful little transformers.

>Brainstorm! - I have thought of substituting a 6SL7 into the Alembic circuit before.
Go for it.  The 6SL7 is a schweet sounding tube.

>Gain is at 4 to 5 (out of 10)  I have used it as low as 2 or 3, which makes it VERY clean.  Beyond 5 or 6 and it drives whatever I have downstream (compressor or power amp) too hard.
This vallidates what PRR said about 2 - 1/2's of a 12AX7 cascaded together.Let's grab my numbers from before:
V1 * Tone Stack * V2 = VGain    50 * .2 * 50 = 500  or 54dB
Now lets insert a volume pot set at 50%.
v1*tone loss*vol pot loss*V2 = 50*.2*.5*50=250 or 48dB
Let's call this Maximum usable gain.  I think it's still on the high side.  Your milage may vary.
If we set the volume pot at 10% we get
50*.2*.1*50=50 or 34dB which is really a good number to shoot for.

>What I'd like to do is make the same power supply, and then use a 12AX7 - and nothing else - as the preamp. No tone stack, extra caps, etc.
There is a very good way to do this and limit gain and decrease distortion.  Instead of wiring the 12AX7 as a grounded cathode, wire it as a plate follower.  This is what I did on my preamp.  The major difference between a grounded cathode and a plate follower is negative feedback.  Yes you're adding a couple of resistors and a capacitor, but the payoff is much less distortion and much improved gain control.  I realize my schematic is hard to read.  The software I drew it in is "vintage" and it needed to be drawn complete since it's built onto a PCB.  I can redraw it in SCH, broken down into it's fundamental stages if you'd like (later when I have some free time).  I'll go ahead and explain it just for drill:

The 1st stage (V1b) is a plate follower.  Gain is set by the relationship between R4/R1 = 1M/100K=10 or 20dB
Next is the volume control.  It's gain varies between 0 & 1.  Let's assume it's at 50% so call it .5 or -6dB
Just in front of the volume control is a low cut switch RC network.  Ignore that for now.
The 2nd stage (V1a) is a grounded cathode.  Let's just call it's gain 50 or 34dB
Next is a James Tone Stack with a loss of 28dB (Vg=.04)
This is the basic preamp section.
Let's total this up:
Gain= 10*.5*34*.04=10
dB=20-6+34-28=20
20dB is a gain of 10
With the volume dimed, it's a gain of 20 or 26dB

Off the output of the tone stack, the signal splits to the 2 1/2's of the 12AU7.
V2b is a cathode follower.  It has a gain of less than 1, like .93 but for now just call it 1.  This feeds a 1/4" jack and is my unbalanced output with an impedance of about 450 ohms.
V2a is another plate follower with a gain of about 10.  This feeds an output transformer with a gain loss of about .1  V2a's gain was set to compensate for the loss of the output transformer, effectively = 1.  The secondary of the OPT hits a polarity switch and a ground lift switch, both quite necessary for a balanced output.

The power supply quite stiff (300uF).  It takes a while to charge which 1/2 the reason I put a 555 timer in.  The other 1/2 is to allow the filaments to heat up.  The timer is simply a 60 second delay keeping the outputs disconnected until the circuit is warmed up.  Since I already had 12VDC, it was obviously prudent to use this for filaments.


OK.... so you prolly don't need all the fluff and you said you want to take a minimalist approach using a 6SL7.  I'd suggest trying a bypassed plate follower shooting for a gain of about 30.  Insert a volume pot and use the other 1/2 as either a cathode follower if you want an unbalanced out or as a split load if you want a balanced output... although you might not get a low enough Z out with a 6SL7.

[RicharD]

Attached (hopefully) are copies of my circuit w/o the relays or power supply and other misc fluff.  I tried to draw it clearly to help you conceptualize a circuit that suits your taste.

-Richard