3C24 glass envelope temperature

[Kmet]

Hi there...

Currently I make one design with old tubes 3C24. In original datasheet says that is tube cooled by plate radiation and air circulating around envelope but without any clues how its that air circulation is encomplished and what is rate of ventilation. I will push tube up to maximum 20W plate dissipation (with safe margin of 5W). I will use machined aluminium HR-1 electrode cooling cap for plate and grid.

My point of cocerne is:

- what is maximum temperature on grid/plate sealing glass?
- some say that plate must be cherry red to activate getter, it's that true or myth?

Thanks in advance for helping me..

Dubravko
Croatia/EU

 

[TheGimp]

The manufacturer datasheet specifies that the plate will glow cherry red at max dissipation. It is designed to do this.

There is no mention of forced air cooling, Only "Natural". I take that to mean convection with little restriction of air flow.

The electrode cooling cap for plate and grid are specified and a good plan.

 

[Chris Hornbeck]

- what is maximum temperature on grid/plate sealing glass?
- some say that plate must be cherry red to activate getter, it's that true or myth?

Maximum temperature depends on the type of glass; Eimac used uranium glass for some larger glass valves (it looks distinctly yellow) like the 304TL, etc. If you use aluminum radiators, you'll have done all you can do.

Plates will run hotter than conventional receiving valves and show visible light. Later Zirconium coatings included.


All good fortune,
Chris

 

[316a]

It depends on how hard you run the tubes . Using 3C24 as a driver , I can get colour from the anode from around 6mA , and at such a low operating point , I won't even bother with the heat dissipator . If I was to use in an A2 amp with positive grid bias and much higher anode current , a different matter and I would use a dissipator on both grid and anode

316a

 

[Kmet]

I checked seal glass on my tubes using UV LED, my tubes have uranium glass seals. For start (testing purpose) I will use heatsink milled from aluminium round bar, and in future will be replaced with copper one. 3C24 tube will be in amplifier on end of tube chain, in function of end/power tube. My design will be conservative, only true class A mode (without A2 mode). Tell me Mr. 316a , what is your plate voltage and driver tube for 3C24 in your amp?

Thank you guys for help..

Dubravko

 

[Hearinspace]

My point of cocerne is:
- what is maximum temperature on grid/plate sealing glass?
- some say that plate must be cherry red to activate getter, it's that true or myth?

I had one Eimac doc that talked more about this than the data sheets but it's on an old hard drive and a search today didn't come up with it.

Online docs indicate that Eitel and McCullough, working for Heintz & Kaufman (before they started Eimac) developed the tantalum plate in order to get around patents on conventional gettering. The materials were treated to remove impurities prior to their incorporation into the tube.

In addition, the lack of conventional internal support structures, reduced potential sources of gas within the tube. The tantalum itself, run at the right temperature acts as a getter to take care of the rest. (See section on Tantalum on second page of linked PDF). The Net impact of this is that the plate will show some color, from (an almost invisible in daylight) very low reddish glow on up to cherry red, but at bright yellow or white it's too hot.

The HR heat dissipating connectors can allow for running the electrodes they're connected to at a higher dissipation than if you don't use them but as far as I know their chief advantage is in protecting the integrity of the seal where the metal comes through the glass envelope.

My experience with the type is limited. A breadboard stage 3C24 that I ran for something under a year and a 35T experiment that saw a lot of use for a couple of years. The 3C24s were run cool without the HR-1 and after some time they began to fail. Whether it was because of the plate not being run hot enough or because of the seal becoming defective , I don't know. The 35Ts have been run cherry red with dissipating cap and are still going strong.
That's all I can offer. HTH

 

[316a]

I checked seal glass on my tubes using UV LED, my tubes have uranium glass seals. For start (testing purpose) I will use heatsink milled from aluminium round bar, and in future will be replaced with copper one. 3C24 tube will be in amplifier on end of tube chain, in function of end/power tube. My design will be conservative, only true class A mode (without A2 mode). Tell me Mr. 316a , what is your plate voltage and driver tube for 3C24 in your amp?

Thank you guys for help..

Dubravko

3C24 was used as a driver 350V >6mA also 275V , 50mA in A2 . Driver for the A2 use was a mosfet connected as a source follower . This tube is not suitable for a class A1 output stage . I just can't see how you will get any reasonable power or distortion low enough . Damping will also be poor , unless you have a 32k output transformer or use tons of feedback

316a