These tubes have low-ish internal resistance , that's why can produce almost 50W at just 300V plate and 150V G2
More EL36 ideas, this time screen grid drive and SE ...
The 6CM5/EL36 for Single Ended Audio Applications
"The results were looking promising but still less than the requisite 4W was forthcoming. At this
point the 6CM5 had the screen grid at about 150V and the grid bias set to draw about 45mA
anode current. What did become apparent was the less bias the 6CM5 had, the more power it
would produce without distortion. So, why not get rid of the grid bias and just control the anode
current with the screen grid voltage?
That's where the breakthrough came! With no bias and 25V on the screen, the anode was drawing
45mA at 240V. I couldn't believe it when I was getting 3.8W before clipping into the 15 Ohm
load resistor, (and at least 4W into the transformer primary)."
The 6CM5/EL36 for Single Ended Audio Applications
"The results were looking promising but still less than the requisite 4W was forthcoming. At this
point the 6CM5 had the screen grid at about 150V and the grid bias set to draw about 45mA
anode current. What did become apparent was the less bias the 6CM5 had, the more power it
would produce without distortion. So, why not get rid of the grid bias and just control the anode
current with the screen grid voltage?
That's where the breakthrough came! With no bias and 25V on the screen, the anode was drawing
45mA at 240V. I couldn't believe it when I was getting 3.8W before clipping into the 15 Ohm
load resistor, (and at least 4W into the transformer primary)."
For safe operation in pp pentode i suggest 2.5k aa load, Ug2 150V, Ua 300V, Ia0 50mA, rated output at least 35W with any half decent opt. It is possible to get higher output power by increasing Ua and ZL. Price to pay is increased distortion and decreased life/reliabilitie (if max power and efficiacy is the goal than biasing must be lean, as above in the example provided by Fuling, in wich case the distortion peak will be high allready at lowish output powers, so pick your poison).
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If high output power is not a priority I can recommend PL36 in triode connection. Not too far from a 2A3, plate curves are available via Google.
Now some further questions are wandering in my mind.
-What bias point would you use for a CFB PL36 PP stage, considering HiFi? By playing with LTSpice, the more I increase bias into class A, the more reduction of 3rd and further odd harmonics. Of course it makes sense, but what would you consider an optimum bias?
-So far, a 20-30% of CFB could not replace the benefits of a high quiescent current in terms of THD reduction. What about combining CFB with other types of N.feedback?
-Member @smoking-amp gives some precious guidelines on intertwining the S-shaped gm curves for optimal biasing. I need to read a bit further on this, however I was hoping for a cheat to achieve low distortion with low biasing, closer to class B.
-What bias point would you use for a CFB PL36 PP stage, considering HiFi? By playing with LTSpice, the more I increase bias into class A, the more reduction of 3rd and further odd harmonics. Of course it makes sense, but what would you consider an optimum bias?
-So far, a 20-30% of CFB could not replace the benefits of a high quiescent current in terms of THD reduction. What about combining CFB with other types of N.feedback?
-Member @smoking-amp gives some precious guidelines on intertwining the S-shaped gm curves for optimal biasing. I need to read a bit further on this, however I was hoping for a cheat to achieve low distortion with low biasing, closer to class B.
Arf! We all want to circumvent the Second Law.
For audio reproduction, folk seem to get away with Class A, monotonic, operation up to a 0VU level that still includes a larger peak available output without clipping (too much). So, for example, for a very loud 85dB SPL at listener's position measured in the usual averaged way, if the amplifier remained in Class A and monotonic to 85 dB SPL, it could transition into AB for peaks up to the professional standard allowance of +20dB, for peaks of 105dB SPL, without most folk hearing the transition.
That's the classic answer, but it requires the amplifier to make the A to AB transition, and back, seamlessly, without artifacts or di/dt blips. Modern, non-commercial design is headed in that direction, with better damping of OPT primaries. Lots of good stuff happening right here on diyAudio, whether folks recognize it at the time or not.
All good fortune,
Chris
For audio reproduction, folk seem to get away with Class A, monotonic, operation up to a 0VU level that still includes a larger peak available output without clipping (too much). So, for example, for a very loud 85dB SPL at listener's position measured in the usual averaged way, if the amplifier remained in Class A and monotonic to 85 dB SPL, it could transition into AB for peaks up to the professional standard allowance of +20dB, for peaks of 105dB SPL, without most folk hearing the transition.
That's the classic answer, but it requires the amplifier to make the A to AB transition, and back, seamlessly, without artifacts or di/dt blips. Modern, non-commercial design is headed in that direction, with better damping of OPT primaries. Lots of good stuff happening right here on diyAudio, whether folks recognize it at the time or not.
All good fortune,
Chris
These pulse power tubes were not designed for linear action in their pentode mode. But yes, triode mode, they are basically similar to those and the indirect heater family of the 'A3' style plate (6n7, 6a6, 6as7).
The circuit you have posted with the screens connected inverse to UL was refereed to several years ago by Menno Vanderveen of Plitron.
The link I had to his papers is quite old, tried it just now & I got the 404 error. The connexion is Positive FB, so damping factor is reduced.
But could be brought back to something normal as you have shown in your simulation.
I think Menno had some transformers wound so that people could experiment with this circuit. Check Plitron Transformers.🙂
I know OP @50AE is a master winder who will do his own transformers, but if you, as a non-winder, are looking for Menno’s, I believe Menno van der Veen is no longer associated with Plitron. His transformer designs can still be purchased from him directly. Look here for his “Specialist” series of transformers.
https://www.mennovanderveen.nl/index.php/nl/producten/specialist
https://www.mennovanderveen.nl/index.php/nl/producten/specialist
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Bumping up the thread a bit, the link content is no longer active. By chance, has someone saved its contents and willing to share? Many thanks!
