[marty]

Hi All;
I am building an Oscillator Circuit based on my Philco 34 Radio, I am using as a model the Philco 39, since it is essencially the same circuit, except that the 34 is a four Band and the 39 is a two band Radio.. The Philco 39 is simplier to follow..
What I am asking is -- I want to using my scope find out what frequency the oscillator is oscillating at, So I figured that If I connect a Capacitor to the plate, I can measure the frequency from the Cap..
What size of CAP should I use ??
The tube is a 1C6 Pentagrid converter, I am only going to hook up the oscillator part of the circuit, and not the RF, that would only be hooked up thru a resistor to -3 as per schematic..

http://www.nostalgiaair.org/Resources/185/M0013185.htm Philco 34

http://www.nostalgiaair.org/Resources/265/M0013265.htm Philco 39
THANK YOU Marty

[BrendaAnnD]

Use something small, perhaps 5 pF. Otherwise, you will load the oscillator and change the operating frequency significantly.

[marty]

Hi All;
Thank You Brenda Ann, That is exactly what I needed to know.. And Why..
THANK YOU Marty

[morzh]

You can use any cap you want.
The scope impedance is at least 10M which means that whatever capacitance you use it will not load the oscillator.
You may have problems seeing a signal at units of MHz / 100s of kHz through 5pF (300kOhm at 100kHz).

Also the working frequency of an oscillator is dictated by it's resonant tank and not by its load (until you really overload it of course).

Use 10nF / 600V or so, you'll be fine.

[marty]

Hi All;
I have wired up the Oscillator circuit, according to the schematic I have for the Philco 39, It does one of the things that my Radio does.. It overheats the 5K resistor, that is between the Plate and the 2nd Grid.. The voltages are suppossed to be on the Plate +136Volts, which it has correctly.. On the 2nd Grid it is suppossed to have +120Volts and it has arround +50volts.. Grid 4 (tube cap) is --3 which is correct.. Grids 3 and 5 have +67Volts which is correct..
Am I Missing something as to "WHY" The 2nd Grid is being pulled so Low, also attached to the 2nd Grid is one side of the Coil, and the other side of the coil goes to a Capacitor, so there would be NO DC flow there, which ends up going to ground thru a 51K resistor..
THANK YOU Marty

[Mondial]

It seems like the tube is not oscillating. Grids #1 and #2 form a triode oscillator with grid #2 acting as the plate. When oscillating, grid #1 develops a negative bias by rectifying the RF signal generated there by the tuned circuit. If the circuit is not oscillating, the lack of negative bias on grid #1 will cause grid #2 to draw excessive current, which is why the voltage is dropping so low and the resistor is overheating.

Something is wrong with the coil and capacitor which comprise the oscillator tuned circuit. The feedback winding of the coil should return energy from grid #2 ( "the plate" ) to the main tuned circuit in the grid #1 circuit, causing it to oscillate.

[morzh]

marty:

See if you observed polarity of the RF transformer. The feedback should be positive.

Try to swap one of the coil's end. Only one!

[marty]

Hi All;
Thank You Both for Your replys.. I swapped one set of wires and NO change.. The coil I am using is one I wound using a blank coil form and counting the turns on the one in the radio.. I have Philco Universal Osc coil, I am going to try it and see If that makes any difference.. All of the other Oscillator coils I have are 3 wire and this is a four wire set up.. Unless the circuit is drawn wrong from Philco..
Morzh, I don't have an RF coil in the circuit at present, I do have that pin tied to --3 volts thru a 300 ohm resistor.. I only want at present to make the oscillator work, then I can add the RF coil..
THANK YOU Marty

[marty]

Hi All;
I changed coils, instead of my Philco Universal, I changed my Miller 3 lead Osc coil to a 4 lead coil, and was able to use that instead..
Basically little change, the Voltage on G2 went from 50 Volts using my old coil, to 78 Volts using the Miller Coil.. I tried it in both directions and no change..
THANK YOU Marty

[morzh]

I mean the oscillatolion coil 11. You need that to oscillate. In 39/39A sch.

[marty]

Hi All;
Sorry Morzh, about the confusion.. When you Said RF I thought you meant the RF coil, when you meant the energy (Frequency) of the Oscillating coil being RF..
Would it help to simplify things to (for now) go to a Triode type tube or something else ?? I have the following tubes -- 30, 32, 34, 2A5, 2A6 and 2A7..
So, would changing the circuit to using a 30 tube be duable ?? Just to see If i can make it oscillate..
THANK YOU Marty

[BrendaAnnD]

The 30 makes a very nice oscillator for BCB frequencies. IIRC, it's basically an old school 6C4.

[marty]

Hi All;
Thank You Brenda Ann, for your reply.. I am going to look thru my Philco radio schematics and find one with a triode Oscillator and go from there..
THANK YOU Marty

[morzh]

The classic tube oscillator uses triode.
The oscillation section of any pentagrid is in fact a triode which plate is the second grid.

So it is in fact better (simpler) using a triode unless you want to eventually convert that thing into a mixing circuit.

Also remember that the two main conditions of oscillation is 1)the gain exceeding unity at the resonant frequency and 2) the feedback being positive.

SO besides the coils you do need some capacitor to form a tank.

You also need the decoupling cap across the power supply, without which the oscillation will not be stable.

It'd be easier if you simply drew the circuit you are trying to assemble, scanned it and placed it here. Then we could comment on your circuit and not make guesses. Otherwise it is kinda blind leading blind situation.

[marty]

Hi All;
I GOT it to OSCILLATE.. And I can vary its frequency..
The Extra Cap accross the coil, for some reason was preventing it from Oscillating..
I went back to my origional Philco 39 circuit.. And when taking out that Cap which was a trimmer, I had planned to try some fixed caps, it started to oscillate..
I don't understand "why" all I know is this proves the basic circuit works..
Now to modify it to work like the Philco 34.. A slight modification.. One side of the coil and the Cap are reversed in the circuit..
THANK YOU Marty