Uncle Doug mentioned an air variable pF tool.

[Mike_J]

Don't know that I described this very well in the subject matter. Some of you are pretty familiar with old radio tube amp technology. Apparently there were parts that provided variable pF values used in some old radio tube amps controlled by a dial. Uncle Doug said the dialect for this thing was air. Think it went from zero to 330pF although the top number may be a little fuzzy.


Reason I bring this up is I want to experiment with the amp when I finally get it up and running which will be as soon as I get up the nerve to plug it in. Anyway, the 100pF cap for the bright channel is fine. The 250pF cap in the tone section is fine but the 47pF between the PI plates is still questionable. I believe it was Adkins that said if the amp had ice pick highs and fizzy overdrive the 47pF cap can be raised but once it got to 100pF it could impact the audible high frequency range. Thought one of those variable pF things would be useful in finding out at what point the ice pick highs and fizzy overdrive are eliminated. Think I remember it having those issue from a long time ago. Who knows about now but planning ahead.


Does anyone know anything about what I am talking about?


Thanks
Mike

[PRR]

Air???

https://www.amazon.com/variable-capacitor/s?k=variable+capacitor

But you are not trying to separate CFRB-AM from WINS-AM. A fist-full of 50, 100, 150, 220, 470pFd caps will usually lead to a solution very quickly. And with less junk hanging outside the chassis picking-up WCAU-AM at any setting.

[Mike_J]

Air???

https://www.amazon.com/variable-capacitor/s?k=variable+capacitor

But you are not trying to separate CFRB-AM from WINS-AM. A fist-full of 50, 100, 150, 220, 470pFd caps will usually lead to a solution very quickly. And with less junk hanging outside the chassis picking-up WCAU-AM at any setting.

Was thinking about that as an option. May have already made something with the parts in it that I can alligator clip to the place I want to test. Will need to check. Was thinking the variable capacitor trick could dial in the value then I could measure it and get a cap in somewhere near that value. Will need to check to see if I have already done something like that.


Thanks
Mike

[Mike_J]

Air???

https://www.amazon.com/variable-capacitor/s?k=variable+capacitor

But you are not trying to separate CFRB-AM from WINS-AM. A fist-full of 50, 100, 150, 220, 470pFd caps will usually lead to a solution very quickly. And with less junk hanging outside the chassis picking-up WCAU-AM at any setting.

You found it. Could get a 100pF unit which is what I am hoping would work for now. Probably need to follow your advice. If there is only a 60pF and an 80pF between 47pF and 100pF we don't have a lot to choose from. There are three between those ratings and I have two of them on hand so that is a start. Just have to plug in and see what happens.

[tubenit]

An on-off-on mini-toggle will give you three choices using the on sides to parallel caps.

I can't imagine really using more choices than that?

With respect, Tubenit

[Mike_J]

An on-off-on mini-toggle will give you three choices using the on sides to parallel caps.

I can't imagine really using more choices than that?

With respect, Tubenit

Thanks Tubenit. Have a picture of the tool I used to remove the blocking distortion from the 5E3 build. Used the bypass resistor / cap selector side and it worked great. The plate resistor / coupling capacitor side of it needs a little more help from looking at it today. Haven't used it because looking at it now it is incomplete. Need a connector to the two that would run to the plate of the preamp tube. A connector from the other side of the resistor to power and the cap to the connection at the cw pot. Was inspired at the time I made that side but hadn't quite given it the thought it needed. If I need such a thing, which I probably will, then I'll make the change. Could have variable resistors by only running that side or variable capacitors by only running that side or both.


Looks like I used a pretty big pot for the bypass resistor. Got a couple of real nice CTS 2.5KL pots by mistake. Could use the 470r resistor as the base and run the rest of them on one of those pots. Would spread them out more and I don't think I would need a bypass resistor bigger than 3K do you?


Don't have a tool for pF capacitors. Probably because there are so few size selections but your plan is a good solution. Could probably run a number of them on a board. One for the cap across the PI plates, another for the treble cap and who knows what else. Thanks again for your solution.


Thanks
Mike

[Mike_J]

An on-off-on mini-toggle will give you three choices using the on sides to parallel caps.

I can't imagine really using more choices than that?

With respect, Tubenit

I was thinking. When I looked at the coupling cap selections they looked like they were very much in need of a few more selections. Probably could dig up a 12 position switch or two around here somewhere. As I recall they were 250VAC so they should do for a tube amp. Then I could use a mini toggle switch as you suggested to include a second cap selection of pF capacitors. That way everything would be in one box. Would only give me one thing to forget I had instead of many. Don't you think that would work?

[acheld]

Only the old guys will know in intimate terms the variable capacitors of the past.

The one shown below is the type, and they work exceedingly well.   However, bear in mind that this particular unit is 7x7x4 inches.   

LOL, about as big as TR iron. The selector boxes you are looking at work well, if not terribly precise.
 

[Mike_J]

Just so happens I bought this flashing that I took a picture of. Will make a good faceplate for my second attempt at a useful amp tool. Bought the flashing because squirrels have found their way into the attic again. Yesterday I was sitting at my table where I do my amp work in the Florida Room, big screen porch in the back of the house. What do you think comes walking by on my screen but this big fat squirrel. He stopped and looked at me for about 30 seconds like he owned the place.


Then I heard a noise in the attic. The vermidon are back. My fix before was foam insulation. Just got out of the hospital from a AAA rupture repair and was feeling somewhat puny (west Texas talk for not at your best from a health standpoint). Felt even more puny when I fell off the ladder. So I used foam insulation to keep them out.


Has worked for quite a few years but they have managed to dig through the foam and are back. That is why I bought this flashing. Will create a metal barrier between those @#$%^& and my attic. Also is pretty much the same material the faceplate was made of before.


Don't need more than 18" for the squirrel problem. Will have a lot left. Have been using that metal tape to put on the cabinet to keep RF noise out of the opening of the chassis. Think I will upgrade with some of this flashing.

[Mike_J]

Only the old guys will know in intimate terms the variable capacitors of the past.

The one shown below is the type, and they work exceedingly well.   However, bear in mind that this particular unit is 7x7x4 inches.   

LOL, about as big as TR iron. The selector boxes you are looking at work well, if not terribly precise.

You have to make your own selector boxes. The boxes you buy online use cheap, 100 volt caps and cheap switches which are completely worthless for what we are doing in my opinion. Have to build your own box and use 250VAC components for the switches and 400VDC or more capacitors. Think I could find about twelve selections of pF values and place them in the box. Problem is there aren't a lot of values in the capacitors we use. For example between the 47pF and 100pF values there are 56pF, 68pF and 82pF so that is five to select from. Leaves seven more spots which could be used for something in the tone cap range. From what I understand anything 47pF or less isn't audible so there is no need to include anything under 47pF as far as I am concerned.


Thanks for showing me the variable capacitor. Those old variable capacitors go up to about 380pF as I recall. A box would use only values we can get and could go up to whatever values one would want. The variable capacitor idea made me curious but based on what I mentioned and what you mentioned about their weight would not be practical for what I would want it to do.


Thanks
Mike

[Mike_J]

Thought about it some more. If whoever said that at the 100pF range for the cap across the plates in the PI you may start to loose some high end is right it doesn't matter because if you still have an ice picky amp with fuzzy overdrive you need to increase the capacitance. That is the most important factor to deal with. Don't think it would be necessary to do much more than a 250pF, 500pF and 1KpF for the tone caps anyway. The rest should be used to cure the fuzzy overdrive and the ice pick sound.