Today's Mystery Guest (RCA CTC-4)

[Phil Nelson]

Miller 6195 -- thanks for the tip. PTOP is the outfit that has 4.5-MHz traps in stock. They also have the Miller 6195 coil and they suggested another that might work, so I ordered them both.

Phil Nelson

[Electronic M]

Originally Posted by Phil Nelson

Miller 6195 -- thanks for the tip. PTOP is the outfit that has 4.5-MHz traps in stock. They also have the Miller 6195 coil and they suggested another that might work, so I ordered them both.

Phil Nelson

Phil, could you please measure the L and C of the traps when you get them?

[Phil Nelson]

Originally Posted by Electronic M

Phil, could you please measure the L and C of the traps when you get them?

Sure I can measure. The parts are supposed to arrive in a few days.

Phil Nelson

[Phil Nelson]

Okay, here are the key video preamp parts. From left to right, we have three Miller TV-151 4.5-MHz sound traps, a Miller 6195, and a Miller 4566:



There are minor differences among the three TV-151s, but presumably they work the same. The last two are adjustable across the target value of 400 muh given for L3 in the schematic; I'm hoping that one or the other will work.

Tomorrow I'll gather the other components and start building.

Phil Nelson
Phil's Old Radios
http://antiqueradio.org/index.html

[Phil Nelson]

Originally Posted by Phil Nelson

Sure I can measure. The parts are supposed to arrive in a few days.

Here's the data sheet for the 4.5-MHz trap:



The sheet identifies that as a 47-mmf cap. Two of my traps have a 47-mmf cap and the third has a 45-mmf cap. They all measure 2.2 ohms across the coil:



I put one of them on a scope with a 4.5-MHz signal and was able to peak the amplitude by turning the adjuster. I don't know if that's where you want the adjustment to be in the preamp, but I was curious.



Haven't done much beyond that inspection, except to collect the needed parts. Perhaps I will create a new thread about building the preamp.

Phil Nelson
Phil's Old Radios
http://antiqueradio.org/index.html

[Phil Nelson]

Back to the video preamp plan, with a question about the heater connection. I posted the original article at:

http://antiqueradio.org/art/RCA_Broa...eo_Preamps.pdf

The plan I'd like to build is on the last page of the article, which says, "The heater connection is made to pin 4 of the adaptor socket."

The heater connection is to service the 12AT7 tube on the preamp. Here's the 12AT7 basing diagram, showing that its heater is center-tapped at pin 9.



The article tells you to connect the heater connection to pin 4 of the adaptor socket. That's clear, but I want to make sure I understand the way they drew the heater connections in the preamp schematic:



The drawing seems to show pin 4 of the adaptor connecting to pin 9 (the center tap) of the 12AT7. And pin 5 appears to be jumpered to pin 4.

Or am I reading the schematic incorrectly?

(The ground connection for the heater is made through pin 5 of the 6CL6 1st video amp tube, whose socket the adaptor plugs into.)

Phil Nelson

[jr_tech]

Originally Posted by Phil Nelson

The article tells you to connect the heater connection to pin 4 of the adaptor socket. That's clear, but I want to make sure I understand the way they drew the heater connections in the preamp schematic:



The drawing seems to show pin 4 of the adaptor connecting to pin 9 (the center tap) of the 12AT7. And pin 5 appears to be jumpered to pin 4.

Or am I reading the schematic incorrectly?

(The ground connection for the heater is made through pin 5 of the 6CL6 1st video amp tube, whose socket the adaptor plugs into.

Looks ok to me... remember that the 12AT7 actually has two 6.3V heaters (one for each triode). They are wired in series between pins 4 and 5. If you used just those two pins, you would need to apply 12.6 V to light up the heaters. Pin 9 is the center tap between the two heaters... by connecting pins 4 and 5 together and applying voltage to that connection and pin 9, only 6.3 V is required, as the two heaters are then wired in parallel.
What could be missing on the schematic is the connection from ground of the TV to the ground of the added preamp... IMHO, the wire from pin 5 of the 6CL6 socket should also be connected to ground(s) of the preamp.

jr

[old_coot88]

Yup, the whole family of 12V twin triodes (12AT7, 'AU7 'AX7, 'AY7, 'AZ7 etc) have the centertap for 6V heater operation.