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    Building, troubleshooting and testing of these amplifiers should only be
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Just for fun - Comparison of 6C45P, 6Z52P-Triode, and 7788-Triode curves

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Curves gathered from various places around the innerwebs
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And for $0.33 each, I got a big box of these GE MIL 6197 tubes. (similar to 6CL6) (price has increased considerably since, $1 now)

Rather linear in pentode mode, not bad in triode mode, and amazingly consistent.
Can almost pick two tubes out randomly from the box and they are very near matched. Never seen that before.

a) Pentode 50V/div Horiz., 5 mA/div Vert., 0.25 V steps, 110V on screen grid

b) Triode mode 50V/div Horiz., 5 mA/div Vert. 1.125V steps Mu 22
 

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12HL7 50V/div Horiz., 5 mA/div Vert. mu 25 (curve tracer selected)
$1 at the time purchased

Oh yeah, I got a couple of sleeves of those after I saw your post those curves a few years ago. I have a spud headphone amp I built with two of these running in triode, one per channel into Edcor 8k50 SE OPTs (no gNFB). It sounds very nice. I use it every night with either Sennheiser HD650 (300 ohms) or Fostex T50RP (60 ohms) cans. It drives both just fine. I had to measure a bunch of 12HL7s before I found a pair that matched close enough for use as a stereo pair.

Here is a 6JC6 Magnavox (Japan) in triode mode.
50V/div Horiz., 2 mA/div Vert., 0.5 V steps, Mu 60
Got a box of these for $0.50 each a while back at Vacuumtubes.net ( quantity sales)

Mu of 60? How's the input capacitance in triode? I have a bunch of these too.

And for $0.33 each, I got a big box of these GE MIL 6197 tubes. (similar to 6CL6) (price has increased considerably since, $1 now)

Rather linear in pentode mode, not bad in triode mode, and amazingly consistent.
Can almost pick two tubes out randomly from the box and they are very near matched. Never seen that before.

a) Pentode 50V/div Horiz., 5 mA/div Vert., 0.25 V steps, 110V on screen grid

b) Triode mode 50V/div Horiz., 5 mA/div Vert. 1.125V steps Mu 22

That's a new one on me. Looks like another 9-pin mini in the EL84-ish camp. Kinda sorta, but lower max Pdiss (10W plate+screen). Maybe it's capable of more in real-world use? rp of 2k in triode is pretty usable. Great for high current diff-driver pairs if they're that easy to match up.
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