[sdiy] tube ladder

[metasonix at earthlink.net]

>>Surplus 6136s are available--the "five-star" premium
>>version of 6SK7.
>>Lower cost desired, use 6BA6s.
>6SK7 seem popular based on what I've found.  If 6136's are both better and
>less money then it's a no brainer.  Is that what you're saying??

Well, the 6136 is a premium 6SK7, with lower noise and microphony specs.

Otherwise it's a space matter. 6SK7/6136s are large metal cased
octal tubes. If space is not extravagantly available,
I'd suggest you use either 6BA6s, which are small
7-pin miniatures; or subminiature tubes.

>>On the other hand, if you want low microphony and small
>>physical size, look into subminiatures. Types 5633 or 5907.
>>Bonus, no sockets needed, just solder them to circuit points.
>Do they have lugs or are they some sort of PCB tubes??

They have wire leads like transistors. Unlike all other tubes,
subminis can be plugged into those solderless breadboard socket things.
For a breadboard, socketed tubes won't matter anyway, you will be
soldering and unsoldering (or pluggin/unplugging) other components.


>Given that heat rises, it should stay cool enough if I vent the box.

Yes.

 
>>Now to use a tube to do this, how would that work?
>>There is DC current flowing through the tube plus a
>>little AC current from the signal. But the plate R
>>turns that into voltage.

"the plate R" does not turn anything into anything.
A pentode behaves electrically like a MOSFET or a JFET.
It is a voltage-variable transconductance device.
It can be used as a voltage-variable resistor. 
The only issue is that the cathode cannot sink electrons; 
so the tube is a single-polarity device. 
The anode MUST remain more positive than the cathode.

Using pentodes in a transistor-ladder filter should work, the 
main difference being that some current must run thru the tubes
at all time to avoid negative clipping. 

Also, some kind of resistor string will be needed, to provide 
suitable voltages to the screen grids of the tubes. Each screen
will need to see a DC voltage somewhere between the idling
voltage of the anode and the cathode of that tube, to properly
set the device up as a pentode, with current limited anode curves
(like a transistor or FET) and high gain.

Actually, you could control the tubes by modulating the screen
voltages, instead of (or in parallel with) the control grids.
Allowing an additional control input that a transistor ladder
would not possess.





E. Barbour
metasonix