Can you guys tell me about a preamp by looking at its guts?

[highlux]

Was given this old rack tube preamp by a friend. It works...but the tone stack seems in op. Would be cool to learn on..I think.

Was curious if this was a good design.....or if I would be better off harvesting parts and starting from scratch?

My theory and design is non existent. So I draw off of your knowledge here.



Thanks in advance.

[nosaj]

Rather than harvesting why don't you learn about what you have? post a front shot also.
heres a sample of what I see when I look at the pic
thanks nosaj

[highlux]

I agree...I want to learn from it...fix it...should be a good primer on building an amp if I can understand this little preamp circuit. Is it of decent design though?

Being hand built...I'm up the creek without a schematic....lol.

[nosaj]

It looks ok to me. The great think about it is it can become whatever you want to change it into. Fix it first it may already be what you like.

First things first what does it do? Any lights, etc

nosaj

PS look back at my pic in previous post

[highlux]

Yes it lights up....it works. It boosts the signal....but....the tone controls (tonestack) seems to be not working.

[highlux]

Oh....and Thank you for helping!!


Dave

[Mike Sulzer]

Looks like somebody used a lot of parts that are available from Radio Shack.

Was given this old rack tube preamp by a friend. It works...but the tone stack seems in op. Would be cool to learn on..I think.[ATTACH=CONFIG]36408[/ATTACH]

Was curious if this was a good design.....or if I would be better off harvesting parts and starting from scratch?

My theory and design is non existent. So I draw off of your knowledge here.



Thanks in advance.

[highlux]

Well, radio shack does sell parts...is that your official assessment? Anyone who knows preamp circuits instead of stupid comments have anything to add?

[Enzo]

Hello highlux, welcome to the forum. let's keep it friendly, OK?

A lot of us here know a lot about preamp circuits. We can't really comment on the design much because we have no schematic to look over. Have you got one? or can you draw one from the circuit? it appears to be a single dual triode and a power supply, so that shouldn't be hard to do. If you are learning, that would be a good exercise in schematics.

As to construction, if it works, sounds OK, and isn't full of noise, then the layout is successful. That is about all I can say about design.

If I design something, I will have part values in mind. If I have to rely on Radio Shack for parts, their limited selection means I may have to use other values than I intended. So that means my circuit may not be as optimal as the design behind it. So it is a fair comment.

[highlux]

I have a call into the guy who built this 20 years ago. Evidently, he had a short run at building and selling amps/pre-amps. I found a business card inside the enclosure, and I tracked the guy down...lol.
He made this 25 years ago. So I don't know what parts were around then. Hell There was no internet then. Maybe he can produce a schematic. I guess it would be a good project for me to reverse engineer this and make the schematic myself.

Just figured someone would be interested in looking at this and critiquing the design or layout...other than just mentioning the parts. I suppose if I had went to school for electronics I could say things like that...ohhh look at those junkie parts....tsk tsk. But I am more into helping people and being kind rather than using my knowledge as a way to build my own self esteem by knocking others who simply have not learned what you have.

So...anyone who actually likes to help...maybe we can figure out why the tonestack is not working but the rest is...I would like to know...

[nosaj]

Then lets start at the input . Grab a pencil and paper draw the input symbol then the wires from it to the next part. It's like a big dot to dot. Please fill in values of components as you go along. There are guys here that will bend over backwards to help you. Don't cut off your nose to spite your face.(Sometimes post will need to be ignored if not helpful) A lot of it though will come down to you making the effort and showing what you've got or where you've gotten to and then someone pulls you along to you get the great AHA!. There are plenty that like to help but not many who want to do all the work for someone.
You have the part in hand and the best observation of it, whereas we only have a flat 2d model.
IT appears to be a very smiple preamp and for a first one not a complex one. Draw it up for us. You can do it I know you can.
(For what's it worth I do love gutshots of amps, but can rarely draw anything from one and not knowing exactly what you were asking in the beginning thats why I just labeled basic component sections in the pic.

Thanks,nosaj

[highlux]

Nosaj,


Thank you very much! I will draw this up today as my project. Sorry about being smart alec in last post. I have to realize that electronics wizards may not be the most socially engaging types...lol.
I have a friend coming from Illinois to pick up a couple amps I fixed for him (just clean pots...new 9 pin input jacks) So it will be later this afternoon. I need to get to guitar center as well...closest place for me to get deoxit...I'm out and in the middle of cleaning my mackie 16-8.

I guess I need to get thicker skin and ignore comments that are not helpful....great advice. I just wear my heart on my sleeve...so I take things a little harder than some.

I guess graph paper would be a good idea. I want to sandblast and powdercoat the enclosure after I have a nice schematic....then I can put it all back together. (I powdercoat stuff at home too!)

[nosaj]

I've been in your shoes and in the scheme of things I am still a newbie at this. These guys help me quite a lot, So I try to jump in and help when i think I can and help hone the helping skills. I do not know a tenth of what they know, but if you ask the right questions(or even the wrong ones) and respond to their questions with needed voltages values, etc. They will teach for sure.

I was never able to find anyone local who would help as much as this group or this group Antique Radio Forums ? Index page
Excelent guys and gals at both sites.

Thanks,
Nosaj

BTW where do you hail from? I'm in the Pensacola, FL area

[MarkusBass]

Just figured someone would be interested in looking at this and critiquing the design or layout...other than just mentioning the parts. I suppose if I had went to school for electronics I could say things like that...ohhh look at those junkie parts....tsk tsk. But I am more into helping people and being kind rather than using my knowledge as a way to build my own self esteem by knocking others who simply have not learned what you have.

I noticed two issues in the preamp:
- there is so called "death cap" used (from the mains power supply to the chassis). This should be most probably removed.
- there are two separate transformers used - one for the heaters and one for the high voltage supply. This is not a typical design. Usually such preamps have dedicated single transformer. Maybe that's why Mike mentioned Radio Shack.

So...anyone who actually likes to help...maybe we can figure out why the tonestack is not working but the rest is...I would like to know...

I'd like to help but you haven't provided any photo that has readable component values. The only component I could read was 22u/16V capacitor. You need to provide much better photos if you want us to help you. Apart from this, it's a very basic preamp - something like Fender Twin. When tone stack does not work, this can be caused by many reasons including: broken wire (to the ground), failed potentiometers, failed capacitors.

I suggest that you provide better photos of the tube area. Otherwise, we can only guess what is wrong with the preamp.

Mark

[highlux]

I gotcha.... I did tile/hardwood flooring for 25 years running my own business with 5 employees until I injured my spine in 2013. I have been trapped at home since, getting out maybe 2x week for short runs....I have had 2 multilevel lumbar fusions in the last year and a half. I have 18 screws...4 plates...2 rods an 3 spacers in my lower back. I am usually in pain. Probably why I am crabby at times....lol

I am bright and quick....I want to know it all at once...but I cant. Drives me nuts. I cant cram 25 years of experience in electronics into my head. I do many things well....but I hate being the noob...lol

Thanks for the link to the other site. I will register and check it out.

I will make the drawing today. I also have to make a video of the buzzing hotrod deluxe amp in other thread.

I have been probably overloading myself with books/videos/etc about electronics and amps. Probably have a terabyte of stuff gathered up. The suggestion about drawing this circuit....was the best thing I have learned so far....other than the safety stuff...I read and read and re read. I have rubber mats. I have chopsticks. I have a discharge lead I made with a resistor inline to drain caps. I keep one hand
in pocket while working live. Safety is number one for me.

I am determined and relentless. I will learn this...even if it means going back to School. (I think I can learn what I need to about amps and sound gear without resorting to that...but I will if I have to after all...just doing repairs....not designing.)