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Old 21st September 2013, 03:33 PM   #11
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Looks like +6V on the 6JD5 g1 only gets one up to 100 to 120 mA plate current with a 100 V knee. Unless one can get the g1 voltage up a bunch more, these look to be only usefull with exorbitantly high B+. Cost around $5 is not too bad, but...

For a reality comparison:
21HB5A were available for the last few years at $1 each. These can do 550 mA at just 0V on g1, and with just a 65 V knee. (or 300 mA with just a 40V knee, or 120 mA at a mere 20V knee) Max B+ rating is 770 V, and peak plate voltage rating 6000V. With 800 mA peak current rating. These work with quite practical OTs. They also have the same size plate as the 6HS5 (which is rated at 30 W). And they're fairly linear..... especially (in fact outstandingly) in a still practical screen drive mode.

Beam triodes = eye candy only
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Last edited by smoking-amp; 21st September 2013 at 03:45 PM.
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Old 21st September 2013, 09:50 PM   #12
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I have to admit that my initial enthusiasm for beam triodes has faded. At first glance, they looked very promising as a tube that could produce reasonable power at low plate voltage, but after plotting a few loadlines and encountering a few tubes where the knee voltage was higher than expected, they don't seem quite as appealing. That's part of the reason why I put the project on the shelf for a couple of years. Even so, I'll probably still build some sort of amp with them, just to see what they are capable of in practise.
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Old 21st September 2013, 11:05 PM   #13
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The beam triodes (BTs) seem to be horizontal outputs (HOs) that have had g2 deleted. Now if they would have made some with g1 deleted instead, we would be sailing into high linearity screen drive mode for cheap. But the existing $5 dollar BT tubes requiring 2X to 3X more costly magnetics just doesn't cut it for me.

An interesting scheme came up recently in the latest screen drive thread. Instead of driving the g2 (or even the latest dual drive of g2 and g1/Mu), fix the g2 DC voltage at say 1/4 of it's usual level and drive the cathode negative with a P-channel MosFet follower (g1 connected to the cathode, Mosfet drain to B-). The large voltage swing at the cathode injects the driver power into the output effectively. 50% or so more power output could result from the same tube, compared to the usual grounded cathode mode. (since the increased effective B+ occurs when minimum voltage drop is across the tube)

So one can now use a modest B+ and a modest B- and greatly improve power handling. Uses extremely linear, effective, g2 drive besides. Not exactly a triode, but the Rp is similar to the BTs for a decent size HO like 6HJ5 say (also $5, 5K Ohm Rp). I think George (Tubelab) got around 150 Watts in normal grounded cathode with a pair of these in class B, P-P (6HJ5). I would expect more like 200 to 250 Watts available from a pair in this cathode/g2 drive mode. All this from a 24 Watt P-P tube set!!

Some little mini HOs, the 6GF5, are available for super screen driver (or cathode/Mosfet gate driver here) stages at $1 still (fully max rated at 770 V B+ like the big ones still).
13GB5, 16/21GY5 (like 21HB5 but with a plate cap), 21/29KQ5 and 17GJ5 HOs are still available at $1 each.
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Last edited by smoking-amp; 21st September 2013 at 11:29 PM.
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