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I see there is a new version of the EL509/EL519 tubes, the JJ EL509S. The main difference is no top-plate cap and an 8 pin octal base but it's supposed to have the same electrical characteristics. What's interesting to me is if it is screen driven with a high current source a push/pull pair is capable of well over 200 watts in triode.
Follow Ups:
McIntosh is using 8xEL509S in the recently announced MC3500MKII. The MKI version used 6LQ6s as finals, a genuine old time TV horizontal sweep tube. I question the wisdom of designing and marketing an amp using expendable components available from one source...but what do I know.
Edit: A pentode/beam tetrode driven at g2 is not a triode. Looking at the curves of a typical pentode where g1 is held constant (usually 0V or cathode potential) and g2 is varied, you clearly see a pentode characteristic: steep rise at lower anode voltages and then a relative leveling off.
Edits: 11/10/21
But if you drive the screen of an EL509 it becomes a triode and the curves are triode curves. McIntosh uses it as a pentode. Both Melos and David Berning used screen drive and had over 200 watts from a push/pull pair in triode. The Berning amp was the Audionics BA150. It was listed at 150 watts but that was David being conservative and testing with 100 volts AC into the power supply. Plus Melos produced triode mono blocks with over 400 watts triode from four EL519 tubes but only advertised 400 watts.
I'm not familiar with the amps you mention and couldn't find schematics but if they truly utilize g2 drive, the finals aren't being operated as triodes. That they're described as triode is probably marketing license. A couple of decades ago g2 drive was somewhat of a fad and was referred to as "enhanced triode mode" and similar. Regardless, the plate curves tell the real story. I couldn't find appropriate curves for the EL509S but I could for the 6L6GB. Although not an EL509, in a general sense the two reasonably similar. Below are three sets of curves for the 6L6GB
This is the set of curves for a triode strapped pentode where g2 is tied to the anode and g1 is driven. The family of curves are clearly typical of a triode.
This is for pentode/beam tetrode operation where g2 is held constant and g1 is driven. The curves are typical of pentode operation. Note how they rapidly flatten out at a relatively constant anode current regardless of anode voltage.
This one is for "g2 drive" where g1 is held constant (typically 0V) and g2 is driven. Note that while the values/signs of the numbers are different, the general form of the curves are nearly identical to those of true pentode operation. Clearly g2 drive (g1 held constant) is a form of pentode/beam tetrode operation.
I just heard from the Melos amp designer. He says if there are only 3 charged elements in an envelope as in screen drive it is a triode plus it performs identically to a triode.
...Did your designer comment on the curves?
A beam tetrode and a pentode are different designs but the curves are nearly identical and they're used nearly identically for audio apps. When a pentode or beam tetrode is strapped as a triode, it still has 4 or 5 "charged elements" but the curves are nearly identical to those of a true triode and designs using them this way are also nearly identical to those of a true triode design. So is a triode strapped pentode behaving as a triode or as a pentode in these designs? How should one refer to a design using a triode strapped pentode? By the logic of your designer, it's a pentode design. By the curves (and convention) it's a triode design although purists might prefer "triode strapped pentode".
His email said it's a triode and for what it's worth he has an IQ that Einstein would respect and technically is brilliant. I know high IQ doesn't guarantee truth but it's a good start and he's not the only person who told me the screen drive is a triode. So did his co-designer(years ago) and a best friend who was involved with Berning when he did the BA150 and in fact produced the 1st Berning pre amp in the late 70s.
Anyway both the Berning and Melos amps were exceptional and produced hundreds of watts like solid state amps.
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"Class B Modulator with Type 807 in Special Triode Connection" as described in RCA TT5 published sometime in the 60s. Have never seen curves published for this specific connection. Maybe resembles pentode UL?
...-With 750V on the anode, that's clearly an ICAS design. I find the need for a 10W low impedance driver kinda off putting. By contrast, RCA suggests a conventional pentode ICAS AB2 operating point @ 750V/300V that's capable of 120W with only 200mW drive. It seems the primary advantage of the "special triode" connection is that it does away with a decently regulated g2 supply but adds the expense of a relatively powerful driver.Curves for this connection are probably "interesting".
Edits: 11/13/21
I recall many years ago that David Berning published curves for his screen driven output tube configuration. The curves were VERY linear and triode-like. This does require a hefty voltage swing, but that is not an insurmountable problem by any means.
...g2 drive does appear to be more linear (curves are more evenly spaced) than conventional pentode g1 drive but based on published data and limited personal design experience, I find it difficult to believe that a true triode characteristic is achieved. A healthy dose of corrective feedback could change effective curves significantly.
Unfortunately I don't remember where they were published. It was probably 35-40 years ago. Yeah, I'm old.
...Berning's website looking for the curves for g2 drive. Didn't find any but I did find Berning's patent for g2 drive #3,995,226 dated Nov. 30, 1976. Primary benefits/characteristics cited are increased reliability at high anode voltage, cooler operation and "..linear, highly efficient operation resulting in relatively high power output.." . Nowhere is triode or triode-like operating characteristics mentioned. The circuit is as I described earlier where g1 is strapped to the cathode and drive applied to g2. Read it for yourself at the link above. Seems to be an interesting error in the second paragraph of "BACKGROUND OF THE INVENTION".
Diodes?
I assume the patent writer intended to use "tetrode" or "beam tetrode" where diode shows up. OTOH, Berning is innovative and maybe he envisioned an output config. somehow using diodes instead of traditional control grid devices. Who knows?...David Berning knows!
Thanks Gary. Both David and Murray Zeligman told me the curves were triode as did the designers of the Melos amps. The biggest problem was getting full power from the output tubes since screen drive needed high current to drive to full output. David used a bipolar transistor. Melos used MOSFET followers on the driver tube. I believe David later built a 100 full tube version with tube driver.
The G1 connection in that schematic is made through a 20K ohm resistor. That means once the driving signal goes positive, the control grid draws current and quickly saturates. That leaves only the screen as a variable control element over the remainder of the cycle. Also, whenever the input signal approaches the zero line, the tubes cut off hard due to lack of screen voltage. I doubt that the curves or performance this produces even remotely resemble a triode.One other thing - Modulators are typically used to amplify only voice frequencies between 300 Hz and 3 kHz. That narrow range allows the use of filtering that greatly reduces the harmonic content and reduces the audible effects of distortion for purposes of communication. This is why so many modulators can operate in class B and take advantage of the additional output power.
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Buy Chinese. Bury freedom.
Edits: 11/13/21
It would be interesting to see what the new EL509s would do with that.
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