Old mica transmitting caps vs modern ceramics for HF amp Input.

[Naysayer]

I need to choose between a modern ceramic cap or an old, molded mica transmitting cap for the input to a Hombrew tetrode amp. Both are .05uF although the ceramic is 1kv while the old mica is 1.2kv. Protecting the Exciter is the purpose and I presume 'bigger is better' is the rule I should follow. Is that correct ?

I'm also following W8JI's grid-driven tetrode formula to determine Rg and Rs which at 50W each, takes up a lot of space in the blower box. I can fit the old cap no problem but are there any other reasons to favor one cap over the other? HF only.

Perhaps someone can verify what I'm seeing with regard to swamping R's. With Rg & Rs in series, it still adds up to 50 ohms such that I can get away with the same drive as if I used a 100w, 50 ohm Rg without Rs. Am I seeing that correctly?

 

[nomadradio]

If this were a cathode-driven setup I'd recommend the mica. Has more metal in it, and should tolerate the higher RF current it would see.

A grid-drive amplifier should work okay with either type.

73

 

[Naysayer]

If this were a cathode-driven setup I'd recommend the mica. Has more metal in it, and should tolerate the higher RF current it would see.

A grid-drive amplifier should work okay with either type.

73

Thank you Tom. I wanted grid-driven for the low drive requirement but that changed after reading more about Tetrodes. It appears resistive matching best for HF. I have small LPF for btwn Exicter & amp as per KK5DR's "Improperly designed tetrode amp ahead" piece. The ATU he advises, I'm not sure of yet, perhaps an L in blower-box if it fits, some inductance.

Thanks for capacitor advice, neil n2eye

 

[Onelasttime]

Mica all the way! The use of ceramic caps unless we are talking a doorknob is normally about cost. You would see CB amps loaded with ceramics because they are cheap! That is even more true today.

Voltage tolerance surge tolerance, rf resistance and heat resistances favors mica. Few of us can afford to always use the best possible components though and now we have the supply chain issues as well!

 

[Onelasttime]

You said old! I do not deal with old tuning transformers that had mica cap built in but as I understand it they can get mold growing on them??? I might be misunder standing this. I have seen green poly caps grow something white that splits them open.

 

[Naysayer]

I'm about to resume work on that project. Had to stop and rebuild my bench power supply that quit right after I got the 2x 4cx300a producing RF but could not complete the tune. Bad bench supply stops everything, ya know? More to come of the amp in few weeks. Thanks again
neil NYC

 

[nomadradio]

2x 4cx300a

Two four-see-ecks three hundred?

That one takes a socket that costs more than the tube. Strictly a low-drive tetrode. Not suited to grounded grid.

73