Discrete components oven controller, modenize help

Hello, I have found this oven controller in an old magazine.
I would like to know:

1. can I use 2n2222 instead of 2n3904?
2. My thermistor is 10K instead of 1k. What should my R6 be? (estimation)
3. Can I use 12v instead of 9v in the first section?
4. Can you propose me other modern transistors than 2n5189?
5. How is the oven heated? By the collector resistors or by the 2n5189 heatsinks?

1.Yes, none of the transistors are critical.
2. 10 times whatever R6 used to be, no value is shown on. the schematic. At a guess, around 10K.
3. Yes, I'm not sure why they used two supplies originally.
4. 2N5189 went out of production in prehistoric times. I would suggest something like a BD135, BD137, BD139, BD439 or TIP31 although they are all in different packages.
5. Both will get hot but given that the 2N5189 is in a TO-39 package which would be awkward to use, I suspect the resistors were used as the heat source.

I'm guessing this is a crystal oven heater. There are easier ways to do it these days but the old methods still work.

Brian.

Thank you very much Brian,
Yes it is for an OCXO.
I like the discrete design if it works ok, rather than using an opamp for the purpose.

I wonder, if I use these BDxxx transistors, could I use a small 10W 12v incandescent or halogen lamp to do the job?
Also why two power transistors are used? Couldn't I use just one?

This circuit also works fine (any usual op-am can be used):

http://www.w6pql.com/crystal_oven_controller.htm

That's a nice web site vfone.

As the only purpose of the last three transistors is to boost the current through the resistors I can't see any reason why you couldn't replace them with a single Darlington. The resistors are needed to produce distributed heat though, a lamp would produce a hot spot and also not be as reliable. Instead of one big resistor, an array of smaller ones would work better as in the design vfone posted.

Strictly speaking, you should bond the first two transistors together so they are at the same temperature as each other but given that you have a feedback mechanism though the thermistor it isn't too important in this application.

The object of the exercise is to produce a stabilized temperature a few degrees higher than ambient so I'm not sure why such big resistors were specified in the old design. You should be able to use lower power as long as you thermally bond to the crystal case and insulate it well.

Brian.

About 50 years ago I used to used crustal ovens and the following point then applied:- The ovens were plug in with an international octal base. So they were about 40mm diam and about 50 mm high. The outer cover was shiny aluminium. Then there was an inner tube with the wire element wound around it. In the centre were the two crystal holders. Some where there was a thermostat but I can't remember where. The really cunning bit was that all the wires that ran into the plug base were silver plated eureka wire. This was used because it thermally isolates the inner works from the outside ambient temperature and substantially improved the thermal stability.
Frank