6SC7 preamp

[robbrochu]

I bought a 6SC7 tube microphone preamp off an auction site. I am trying to marry it to a Baldwin PC40 organ amplifier. The Baldwin doesn't have a preamp powerful enough to amplify a bass signal. If I hook up my Sansamp in front of it, the amplifier works perfectly. So I bought this other preamp to connect to it in the hopes that I will have an all tube bass rig fairly cheaply.

The problem is that what I thought would be a simple project, has turned out to not be so. I have a minimal understanding of how tube circuits work however, I don't have any experience with the circuits myself and really have no one local that does and can lend assistance.

I have connected a 1/4" input replacing the RCA style input that was originally attached. I have the jack hooked up correctly. the signal goes to the tip and the ground goes to ground.

There's a 6SC7 tube and a 10MFD 450WV capacitor on the top of the preamp. It has an output wire and 4 other wires. The yellow and white twisted wires are connected to pins 7 and 8 which according to the pinout are for the heaters so I am assuming I can tap into the 6.3V heater wires coming off the amplifier transformer to get heater power. The black wire is connected to the preamp chassis ground so I assume I can connect the other end to the main amplifier ground. The last preamp wire is red and it connects to a terminal strip along with a couple of resistors and 1 of those resistors is connected to the capacitor.

I have NO IDEA where on the main amplifier I should connect the other end of that wire and I'm hoping that one of you fine people will be able to help tell me what purpose that last wire serves. I'm assuming it's a mains power wire, but I don't want to hook it to power until I am sure because I don't want to blow anything up, nor do I relish the idea of getting an electrical current passed thru my body. If anyone can help I would greatly appreciate it.

 

[Frank Berry]

The red wire goes to the B+ supply ... nothing greater than about 250 volts.

 

[robbrochu]

Thank you Frank. can you explain to me how this works without any transformer at all? Does the pre depend entirely upon the main amplifier for necessary transformer use?

I THINK the B+ "fills up" the capacitor providing power to the tube and as the capacitor is depleted, B+ "tops it off"?

 

[Frank Berry]

The preamp will get all its power from the amplifier. The capacitor helps to keep the B+ noise-free by reducing the residual hum from the ac power source.

 

[robbrochu]

One more question sir. Can I scavenge the voltage from the plate voltage of the 6SL7 on the main amplifier?

 

[Printer2]

From the filter capacitor for the 6SL7. Just checking, what voltage was the preamp suppose to run off of.

 

[robbrochu]

I wish I could tell you that, but I can't. Not that it's some super secret, but that I just don't know. All I know is that it was handwritten labelled as a mic preamp and I have been playing hell trying to figure out where this one particular wire went to make the damned thing work.

 

[Printer2]

Are there any voltage ratings on the capacitors? Sorry, just re-read the op. 450V, I would not worry about the voltage.

 

[robbrochu]

attached to the 6SC7:
Solar sealedtite wax molded paper capacitor
pin2: .01MFD 400DCWV
pin 3: .005MFD 600DCWV
pin 4: .005MFD 600DCWV and other end is connected to 1/4" input jack
pin 5: .02MFD 600DCWV
and there's another .01 MFD 400DCWV with one end going to ground.

There's a big metal capacitor labelled 10MFD 450 WV marked twice. once with a triangle and the second with a square.

the triangle marked side of the big metal capacitor is connected to pin 5 and the square marked side is connected to a terminal strip that also has that unknown to me wire.

 

[Osvaldo de Banfield]

I suggest to replace all wax caps as they are or will became leaky. A photography of the device can help us to help you.

 

[robbrochu]

Osvaldo, I'll attempt to post an outside picture along with an inside picture showing the wiring. Bear with me for it may take me a little while to figure out how to do it. Thanks.

 

[Mooly]

Like this

How to attach images to your posts.

 

[robbrochu]

Here are the pictures of the preamp I'm trying to work on.

 

[Osvaldo de Banfield]

The red wire seems to be the positive supply. If the yellow wire goes to 8 or 7, then it is the heater. Black wire seems to be ground.

 

[robbrochu]

Yes the yellow and white wires are pins 7&8 and are the heaters. the black goes directly to ground. Red was the one I was indeterminate about. I didn't know whether the red went to mains power or not. Thank you.

 

[Osvaldo de Banfield]

Red wire clearly is power. It goes through two resistors to both capacitor cells and then to the plates, 6SC7 is a double triode common cathode, surely they are wired in series (to signal) to amplificate it several times. The jack seem to be the output.

 

[robbrochu]

Osvaldo, the jack is the input. It had an RCA jack in place of the 1/4" in place now. I changed the RCA to 1/4" and wired and soldered it exactly like it was when I got it. In the first picture you can see a single wire covered with a braid. That is the output wire.

 

[Osvaldo de Banfield]

And cathode lead?