Just curious. I have seen all kinds of unconventional pentode connections including driving the screen instead of the control grid but what happens if you tie the screen grid and control grid together applying the input to both. It seems like g2 would act as a weak control grid working in tandem with g1 giving a triode like response but with different mu from normal triode connection.
Is this an option to triode wire pentodes whose screen grid ratings are inconsistent with normal triode operation?
Is this an option to triode wire pentodes whose screen grid ratings are inconsistent with normal triode operation?
Thanks. Will check those out.I would not have thought of that since I was thinking of normal negative bias.
In the 1960s it was common to turn almost any pentode/tetrode into a Class B zero bias triode by feeding the signal to g2, which was connected to g1 via a resistor (often 22k). The resistor stopped the control grid from overheating.
Ah, then this is not helpful since zero bias triode has pentode characteristics. So presumably the way to deal with low screen dissipation/voltage capability is level shifting such as with zener or other constant voltage between the anode and screen.
Beside the zero bias schemes.
There was Twin Drive, followed by Crazy Drive, which used both grids for drive. Both have pentode like output characteristics. These produced near constant gm1.
Getting a pentode to act like a triode has been discussed many times with all sorts of schemes. Difficult to find them all by searching however.
This one came up just today in your "pentode noise and local feedback" thread:
http://www.diyaudio.com/forums/tubes-valves/98330-people-dislike-ultralinear-8.html#post1176732
It uses active UL, with a N Fdbk scheme to the screen grids, to produce true triode like linearity rather than "Ultra" linearity, whatever that advertising moniker means.
Other schemes that have been discussed are triode emulation, where the pentode is driven to emulate an "emulant" triode example.
CFB will give you triode like low Zout curves. So will shunt "Schade" N FDbk.
There are some curves posted for things like grid2 drive with shunt "Schade" N Fdbk to grid 1. And other oddities, most were not too satisfying.
There was Twin Drive, followed by Crazy Drive, which used both grids for drive. Both have pentode like output characteristics. These produced near constant gm1.
Getting a pentode to act like a triode has been discussed many times with all sorts of schemes. Difficult to find them all by searching however.
This one came up just today in your "pentode noise and local feedback" thread:
http://www.diyaudio.com/forums/tubes-valves/98330-people-dislike-ultralinear-8.html#post1176732
It uses active UL, with a N Fdbk scheme to the screen grids, to produce true triode like linearity rather than "Ultra" linearity, whatever that advertising moniker means.
Other schemes that have been discussed are triode emulation, where the pentode is driven to emulate an "emulant" triode example.
CFB will give you triode like low Zout curves. So will shunt "Schade" N FDbk.
There are some curves posted for things like grid2 drive with shunt "Schade" N Fdbk to grid 1. And other oddities, most were not too satisfying.
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Every triode data sheet I hhave seen shows pentode like curves when grid goes positive. I would be delighted to see differently. Of course there would still be the heroics of driving the grid current.
(E.g.
Real live experiments with dual drive / crazy drive / whatever you want to call it drive, where drive signal is fed to the screen alone, or in combination with a voltage divider to G1, results in curves looking somewhere between pentode and triode, but plate resistances in the pentode range or sometimes higher. Some type of feedback will be needed to get the amp's impedance down unless you are building a current drive amp.
Why stop with 2 grids? The 6LE8 and 9KC6 tubes have 3 grids with ballpark similar gm. One can make all sorts of strange devices with them. How about triode to pentode conversion across the curves:!
1) 6LE8
2) 9KC6
3) 21LG6 Crazy drive
4) 21LG6 normal grid1 drive
1) 6LE8
2) 9KC6
3) 21LG6 Crazy drive
4) 21LG6 normal grid1 drive
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The grid negative and grid positive curves both show a set of parallel lines, although on quite different scales. Grid=zero appears on both, is therefore parallel to both, and therefore all are parallel. How can a set of parallel lines (i.e. the same anode impedance) look like a triode with negative grid and pentode with positive grid? Are the different scales causing confusion?mashaffer said:
dV/di
All amplifying tubes and even transistors have curves that are essentially parallel lines. The difference lies in the spacing, slope, and shape of the knee. In the region around zero bias the curvature is in transition as well. This obviously can be dealt with but it is a distinctly different characteristic than a negative bias condition.
All amplifying tubes and even transistors have curves that are essentially parallel lines. The difference lies in the spacing, slope, and shape of the knee. In the region around zero bias the curvature is in transition as well. This obviously can be dealt with but it is a distinctly different characteristic than a negative bias condition.
I have been chasing this concept for at least 10 years with small successes and serious setbacks. It has been discussed here for longer than that. I did lots of experimentation with pure screen drive and saw lots of power from small tubes, and quite a pile of burnt parts over a two year period about 10 years ago. Some testing buried in this thread:
Tube sale at AES
In 2010 I got the idea of driving both G1 and G2. Again not a new concept since it is shown in the "150 watt amplifier" schematic contained in the Genelex KT88 issue 3 circuit supplement data sheet. (too big to include here) One of my experiments is shown in post numbers 29 and 30 of this thread:
G1=G2/mu Scaled Drive Strawman Design
Well after 10 years and lots of setbacks, I think I'm getting close. I tried something new about 4 years ago, but abandoned it due to some stupid mistakes and lots of blown parts. A spark rekindled the old fire and I dug out the old boards and started down a different path. After literally exploding one tube and cracking another I decided that all testing should be done with $1 tubes and pressed on.
I have gone from "burnt boards and exploding parts" to "nice sounding amp" in about a year. All is not totally well yet, but I believe that there is some light at the end of the tunnel, and it doesn't sound like an oncoming train, it sounds like music.
So, is this a triode or pentode? Note that these are hand traced curves made with linear power supplies and plotted in Excel. The real spacing is uniform, the left most green line is in error caused by an unstable grid supply. Line spacing is 5 volts. Note that these curves extend out to 550 volts, and up to 300+ mA, the limits of my Fluke 407D power supply. The tube being tortured here is a smallish TV sweep tube from the $1 list.
Previous experience has taught me not to reveal the schematic until I have some boards ready, and this is still a ways off, but as with all my projects full construction details, schematics, theory and proven parts lists will be coming.
The first picture shows a pair of test boards I build several years ago. The left one is connected up to one of my Universal Driver boards with a large sweep tube in the socket. The other is not connected. These boards had a pair of mosfets, source and sink, on every element in the tube. The idea was to test EVERYTHING in every way possible. It was best used for generating smoke and dead parts, and was abandoned. I dug them out of the "box of broken dreams" for use here about a year ago. I have since moved on, so I just put them here for the picture.
Some limited success led to the 3 boards lined up across the edge of the bench. The blue perf board was used to extract lots of power from some little cheap tubes. Things like 25 watts from a pair of 50C5's happened on the blue board. Drive came from the Universal Driver Board to it's left and another twin pentode driver board to it's right. The green perf board is where the exploding tubes eventually morphed into the tubes curves shown. A careful look will reveal several burnt spots from disappearing cathode resistors caused by bad arithmetic.
The combination of the green perf board and the driver test board led to the complete test amp PCB sitting on the Sylvania box. It is capable of making 50 WPC from that Antek toroid behind it and the two OPT's to the left of the Antek. I have seen 80 WPC flow from the board without smoke or the red glow of death on a regulated power supply set to 500 volts. The Antek makes 520 volts at idle and 465 at the edge of clipping (55 WPC, both channels driven). There are still a few issues to solve, so I believe that at least two more iterations of PCB's are needed before something is ready for prime time. I plan versions of this design from smaller than this board to 500 WPC.
Note that these three very different tubes all contain identical guts. Only the plate fins are clipped off on the smallest. The biggest one is where I cracked the 100 watt barrier for the first time back in high school (1968) using an OPT from an old Stromberg Carlson PA amp. The smallest is used here.
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