Vacuum Tube saturation circuit - Page 1
Forum section: electronics / projects

author: AndreBibault
author_rank: Newbie
timestamp: February 13, 2020, 10:41:17 am
content: Hi,I am trying to build a circuit that inputs and outputs line level signals but with variable tube saturation. In university they only taught us about semiconductors and I have ZERO experience with tubes. Does anyone know of an open source design(s)/book references to check out/tips?Best regards

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 13, 2020, 07:23:10 pm
content: You mean using the low plate voltage, saturation region in the same way that the same region is used in FETs (triode region)?The traditional approach is either a variable Gm tube (which inevitably introduces some amount of signal distortion for a given SNR), or a dual-grid tube where the plate current is the product of the two grid transconductances (thus forming a one-quadrant multiplier). Examples being remote-cutoff pentodes like 6BA6, dual-control pentodes like 6HZ6 or 6HS8, beam control types like 6BN6, and pentagrid converters like 6BE6 (which I think is also remote cutoff? a number of pentagrid variants were made with both characteristics). There were also specialty tubes for more in-depth operation (like the 7360? beam deflection tube for two-quadrant multiplication).Plate characteristics tended not to be used because of the weak current flow around zero volts, the poor control of it (triodes act more like Thevenin equivalent sources with a dependent voltage, moreso than a variable resistance; the plate resistance does still vary, of course), and the ponderous screen current required in the case of pentodes (total cathode current varies little with respect to plate voltage; in plate saturation, all the current that would've reached the plate, ends up turning back to the screen instead, and toaster-grid ensues).Similarly, JFETs stink in that the triode region's width is Vgs - Vpo, and the slope is only variable over a modest, say factor of 2 or so range, for a reasonable drain voltage range (before entering linear (current saturation) mode). It's usable, it's certainly been used in practice before; but the wider range, more general mechanism is usually an OTA, or Gilbert cell or the like.Tim

author: duak
author_rank: Super Contributor
timestamp: February 14, 2020, 01:24:19 am
content: Andre, you'll have to give us more info on what you mean by saturation.You use the term "Line Level" so I'm assuming it's some sort of audio circuitry, rather than some sort of device using vacuum tube logic.Somewhere in my vast collection of obsolete crap I have some ex-Telco audio compressors from the 60s. Each has a couple of dual triode vacuum tubes and bunch of germanium diodes. I have no schematics for these things but I suspect that the diodes were used as variable resistance elements controlled by some DC signal derived from the signal amplitude.Cheers,

author: graybeard
author_rank: Frequent Contributor
timestamp: February 14, 2020, 02:51:20 am
content: Do you mean you want realtivly flat IV curves? If so you want to use a pentode.

author: graybeard
author_rank: Frequent Contributor
timestamp: February 15, 2020, 05:11:36 pm
content: My favorite book on vacuum tube theory and circuits is the4th edition of the Radiotron Designer's Handbook. It is also one of the best all around references on many electronics subjects including speakers, circuit theory, transformer design, and many other subjects.It is one of many references available fromtubebooks.org

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: February 15, 2020, 06:10:45 pm
content: http://www.john-a-harper.com/tubes201/