who is the amp guru

[dragonworks]

around here? HELP:

[Steenamaroo]

I'm not sure there is one.......

sup?

[jimmys69]

around here? HELP:

Someone who makes a bunch of money, recording great players, in a world, far far away.....

His name is.........

wait for it..........

Hell, I don't know.

[PRHunt]

around here? HELP:

Might help to spell out exactly what help you need???

[dragonworks]

I am trying to resurrect an old preamp and have very limited knowledge and unable to obtain a schematic.

[PRHunt]

I am NOT the guru, but do you have access to any test equipment - multi-meter or volt ohm meter for example?

Also, what is the preamp? And what is the problem - no power, no signal, distortion???

Hopefully someone like ecc83 will wander by and pick this up...

But the more descriptive information you provide, the better.

[Steenamaroo]

Yeah man, what's the preamp?

[dragonworks]

I hope this helps.

It is a teisco stereo preamp. Two channel, one clean, one with tremolo and reverb, both with bass, mid and high. I have a clean signal coming through both channels. No reverb or tremolo on channel two though. I have meters, scope, AF generator. I can see the signal on the plate and grid for the input to the reverb tank, I can see the signal at the input of the tank, I see a very weak and distorted signal coming out of the tank, it may not be a signal at all, but just eddy currents or gremlins maybe, I am new to all of this. I can see no signal at all at the output of the reverb tube, either on the plate or the grid. All plates etc. appear to have good DC voltage on them. There is zero DC on the grids I think. The input side of the rever tank also goes to a footswitch, the footswitch I think routes the clean signal back to the output? The output from the tank goes to the deapth control for the reverb, which then gets passed through some resistors and caps that are tied in somehow through the volume and tone controls? It goes to a rail and I am not sure how it gets passed to the out put grid of the tube? I am not sure the tank is any good. It is hard wired, three section it appears, the input comes into the right section and jumped to the other two sections then going to the footswitch. The output section on the tank has three outputs tied together that then goes to the depth control. I have tried the verb with the footswitch on and off. All three inputs and all three outputs also have individual grounds.
As for the tremolo circuit, I can see a signal, a strong one, but I don't know what it is supposed to look like on the scope? When I turn the depth pot I can see a change but not really with the speed control. Both plates for the tube are hooked up to one side of a small transformer. The other side of the transformer has two resistors wired to it which then goes to the output for channel two. I am not sure how the tremolo circuit is supposed to work yet.
There are five tubes total, it was given to me with five 12ax7s in it. Four out of the five tubes are using both sections, one tube is only using half the tube. I am pretty sure I have identified the four tubes that use both sections as channel one and two preamp, one tube for trem, one for verb.
That leaves the tube using only one half that I am not sure of what its purpose is? I can see a change in amplitude on the trem tube and the unidentified tube when turning the volume knob on channel two. I have a bunch of 12AX7s and have tried multiple tubes.
free dogman.

[ranjam]

..... I have a clean signal coming through both channels....

Meaning you can hook up the preamp to a power amp and get output from either 'channel'? With the tone and volume controls working? So far, so good.

......No reverb or tremolo on channel two though.

Meaning the controls do nothing, and no effect is heard?

.... I have meters, scope, AF generator. I can see the signal on the plate and grid for the input to the reverb tank, I can see the signal at the input of the tank, I see a very weak and distorted signal coming out of the tank......

Well, you have the right tools. Now to trouble shoot. Usually, but not always, the reverb is coupled to the pan via a transformer. Rarely is it a capacitor, but you never know. The pan should have the two RCA cables connected. Remove the pan, and meter the input and output to the pan itself. You have to go inside the pan, and measure right on the RCA jacks. You should get real low readings; maybe 10Ω on the input and 47Ω on the output. If this is so, the pan is good. Now leave the pan out, and put a signal into the preamp. Scope the RCA cables coming from the preamp. One will be the output of the reverb (from the transformer), but it goes to the 'input' of the tank (if that makes sense, great). Signal? Great. Now attach your generator to the 'other' RCA cable. That's the input to the reverb recovery stage, coming from the 'output' of the tank. Do you get a signal? If so, great. If not, you have a recovery stage problem. Could be a tube, could be an open Plate load resistor, capacitor, etc. Divide and conquer.

..... As for the tremolo circuit, I can see a signal, a strong one, but I don't know what it is supposed to look like on the scope?

The tremolo is just a low frequency oscillator; usually about 10Hz or so. Look right around the speed control; it goes to the tube, through a few capacitors. Now you will see three capacitors and a couple of resistors bunched together around that tube. Maybe the capacitors are .02uF or .01uF, and the resistors are 220K, 470K, or around there. If you have an analog meter, great. Put it on the Plate of the oscillator tube. The needle should swing back and forth, varying with the 'speed' setting. If not, just 'shotgun' it, and change all three capacitors.

That's about all I got. Without a schematic, it is tricky. But you can do it. Go slow, and divide and conquer.

[dragonworks]

As I stated, the reverb tank is hard wired, no rca plugs. The signal to the input of the tank, as far as I can tell, does not go through a transformer, but through a resistor cap network to get there. On the other side of the tank I have either no signal or a very weak one.