Help with an instrument pre-amp?

[oatmeal769]

I'm sorry if this isn't exactly the forum for this, but I think my amp is probably REALLY similar to a headphone amp, and I trust Hi-Fi heads more than musicians...

I have a 35 year old Alembic F2-B stereo pre-amp for instruments. I play my bass guitar through it, and nothing else sounds as good to me to this day. It is based on the input section of a Fender Showman Guitar Amp.

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 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, and it may have been re-capped back in the 80's but nothing has been done to it since then.

Not only that, but 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 really good with a soldering iron, understand very basic electricity etc. but stop WAY short of saying I know electronics. Schematics give me the willies, but I think I can get through this one... If I have to. It really isn't too bad. This would be my biggest electronics project though, so I kinda need some hand holding to get started

Also, 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. I feel like I could do some good by increasing the signal by 6dB (which I think is what the next item wants to see anyway - a balanced signal. Also, I use the rig in some really noisy environments!

If this isn't the place for this, N.P., but could someone suggest where a better place to post might be?

TVMIA !

[dsavitsk]

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

Tubes and caps are the most likely. It is possible that resistors have drifted, but it is doubtful they have drifted that much. Really, I'd guess you are just used to the sound and that it is not as good as you once thought Or, maybe your pickups are dying a slow death.

But, the schematic is too small to see clearly.

If you want to quiet the noise, replace one of the resistors in the PS with a cheap choke -- it'll do wonders.

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

particularly since you plug an instrument that you touch in to this, that should be fixed asap.

Also, 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. I feel like I could do some good by increasing the signal by 6dB (which I think is what the next item wants to see anyway - a balanced signal. Also, I use the rig in some really noisy environments!

You have a misunderstanding of what "balanced" means. All those amps and cd players that give you an extra 6db when balanced do so because they are really bridged which doubles the circuit. If you want 6 db more from this, buy another one and use them both at the same time. 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.

If this isn't the place for this, N.P., but could someone suggest where a better place to post might be?

The best guitar amp forum I know is http://www.ax84.com/ Diyaudio tube section is also pretty good.

[oatmeal769]

Tubes and caps are the most likely. It is possible that resistors have drifted, but it is doubtful they have drifted that much. Really, I'd guess you are just used to the sound and that it is not as good as you once thought Or, maybe your pickups are dying a slow death.

LOL, that could very well be too! Some of those old resistors though are brittle, and I think one or two might even be cracked. Has the color code changed in 35 years? Can I just match up the colors and replace any that are chipped?

But, the schematic is too small to see clearly.

I'll re-post it as two separate JPG files in this post, that should fix it.

If you want to quiet the noise, replace one of the resistors in the PS with a cheap choke -- it'll do wonders.

That is just placing a transformer in line with the PS?

particularly since you plug an instrument that you touch in to this, that should be fixed asap.

Not only that, but put my face on metal microphones - attached to completely different devices on different AC circuits, Amazing I'm not dead... Can I just solder or screw the third wire on the chassis?

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.

Outstanding, I didn't know it was that easy. The input side shows (+), (-) and (CT). CT would go to the chassis ground as well, right?

The best guitar amp forum I know is AX84.com - The Cooperative Tube Guitar Amp Project Diyaudio tube section is also pretty good.

Excellent, thank you, I will post there as well.

[dsavitsk]

LOL, that could very well be too! Some of those old resistors though are brittle, and I think one or two might even be cracked. Has the color code changed in 35 years? Can I just match up the colors and replace any that are chipped?

sure -- use carbon comps. Either vintage from AES or Ohmite Little Demons from Digikey.

That is just placing a transformer in line with the PS?

replace the first 1K5 resistor with something like a hammond 154G

Outstanding, I didn't know it was that easy. The input side shows (+), (-) and (CT). CT would go to the chassis ground as well, right?

err, well, 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 -- more than the 6db you were looking for, while the latter will lower it. You'll need to make some other changes to do the latter, however. Oh, and a 0.1u coupling cap will be too small in either case

But, 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.

[oatmeal769]

Great. I'm going to have to do a little research on understanding the last bit, but I understand the gist. I can do that on my own though, you've already been more than helpful!

I'm feeding a balanced TRS 10K ohms input from this unit. I don't really think I need more gain, I'd just like to ensure no hum from lighting, electrical, etc..

Also, I'm feeding the input with active pickups they have 200 ohms output impedance. Should I put a resistor on the input?


.

[kevin gilmore]

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

The orange ones are not electrolytics and do not need replacing.
All cracked resistors obviously need replacing.

[oatmeal769]

Thanks. At this point with everything else, it kinda makes sense to me to 'just do it' while I'm in there, even if some of them don't need replacing. Still though, between all that, the choke, and maybe the balanced out, I think I'll see some real improvement.