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

[highlux]

I will get way better photos as well as a hand drawn schematic today. That is my mission. Will be good learning. I had a feeling it was kind of hokey. I probed the housing before I touched it when I first plugged it in...I dont trust gear that is "broken". I am pretty new...but I knew it sure could have been a neater job for sure.

Thank you.

Dave

[MarkusBass]

Drawing the schematic of this preamp is quite easy. You have 7025 tube which is a double triode. Just find out what is the pinout of the tube and check what is connected to anode, grid and cathode of each of the triodes. It seems to me that both cathodes have 1.5k resistors (I'm only guessing) in parallel with 22uF capacitors (like Twin). The cathodes most probably have 100k resistors. So this is 30% of the task already done. You do the rest

Mark

[highlux]

Thanks man! I hate being at the bottom of the learning curve. lol.

double triode....is essentially 2 tubes in one right?

[DrGonz78]

double triode....is essentially 2 tubes in one right?

A 7025 is essentially the same as a 12ax7a or regular 12ax7. The difference is that the 7025 tube can reduce noise and hum. It has two separate gain stages being a dual triode, two different sets of grids, plates and cathodes. If it is an old vintage tube then it really is a nice tube to have on hand!

[66 Kicks]

I have drawn the power supply from as much as I can see. The power switch looks like a Carling DPDT, but I can't tell exactly how it is wired from the picture. I put my best guess on the schematic, but it could be wired differently or it just uses half the switch for connection points.

The transformers are almost certainly Magnetek Flat Packs -- I have one in my hand right now and it is identical. They have enough power to run six 12AX7's if you are so inclined.

[Mike Sulzer]

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

Actually, I did not say there was anything wrong with Radio Shack parts. I used to use them all the time for quick projects before the RSs where I live got closed down. I actually do know quite a bit about preamp circuits of various kinds, having designed such things since the mid sixties when I was just a teen. I was just trying to get a better feel for what this is, but, given your hostility, I think I will leave it to others to help out.

[big_teee]

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

Don't be so sensitive.
You will need to be thicker skinned than that to survive on the MEF.
Just some friendly advice!
GL,
T
**Please try to keep the blood out of the water!
This place can get hostile fast!

[Leo_Gnardo]

Actually, I did not say there was anything wrong with Radio Shack parts. I used to use them all the time for quick projects before the RSs where I live got closed down. I actually do know quite a bit about preamp circuits of various kinds, having designed such things since the mid sixties when I was just a teen. I was just trying to get a better feel for what this is, but, given your hostility, I think I will leave it to others to help out.

Heck even Dumble used Shack parts. And achieved shall we say, a certain level of success that others only dream of? It ain't where you get parts, it's what you do with 'em. C'mon, I'll buy you a drink Mike, I'm bugging out of this one too.

[Enzo]

if it were me drawing a circuit, since there is one tube, use the data sheet for it. I know it has two triodes, so on a blank sheet, I'd draw the two triodes a bit left and right of center. I'd then look at each pin of the socket and see what is connected there. For example pins 3 and 8 are cathodes, and will likely have a resistor connected, and also probably a parallel capacitor. Look at most any preamp circuit, yours will look similar, honest. The other end of those resistors probably go to ground, do they in yours? Pin2 is a grid, and a resistor and something else will probably connect. Pins 1 and 6 are plates and will likely have a high value resistor to B+, like 100k or 220k. Unless the second stage is a cathode follower, in which case the plate connects directly to B+.

Your tone stack is very likely just a Fender/Marshall clone, though others are possible. So look at one of those circuits and see if yours kinda looks the same. When tone stacks have no effect, the first place I look is at the MIDs control or resistor (looks like yours has a control) if it goes open or loses its ground connection, the tone stack stops doing anything.

A personal note. I will be direct with you, or anyone else, I won't candy coat anything, so please realize that criticism is not an attack. If I tell you something is wrong, it is not to puff up my ego, it is to tell you I think something is wrong. After all, even professional football quarterbacks have passing coaches, and professional golfers have swing coaches. I bet if you were doing flooring and a new hire did something wrong, you'd point it out, not just to feel better about yourself, but to instruct the man.