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Old 21st April 2008, 03:25 PM   #1
GordonW is offline GordonW  United States
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Default Anyone used the 7C7 loktal pentode?

Was thinking of cobbling together either a Hafler-circuit or Mullard-circuit PP amp from some junk here. Among other things, I got a gaggle of loktal tubes... a set of 4 7C5 (same as 6V6, outputs), a few double triodes (6F8, 6N7, 6V7, etc), and three 7C7 pentodes...

Well, I looked up the available data... and darn, if the 7C7 isn't about the closest thing I've seen, to matching the parameters of an EF86 or EF804. The transconductance is somewhat lower (about 2/3 that of the EF86), and it can't handle as much g2 voltage, but ALMOST ALL the other parameters are a very close match!

Anyone used these guys? They're supposed to be similar to 6J7 (they are listed as the same application data for both in many Sylvania tube manuals), but they look to be a bit more "friendly" for the Mullard application. I would love to get hold of a set of plate curves!

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Gordon.
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Old 21st April 2008, 05:28 PM   #2
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There are curves in the 1946 Sylvania manual - I scanned the 7C7 data and sent it to Frank Philipse - http://www.mif.pg.gda.pl/homepages/f.../084/7/7C7.pdf
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Old 22nd April 2008, 12:09 AM   #3
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Gordon,

6V6 family tubes are easy to drive. Do you have any 12AT7s in your pile? If you do, the 7C5s and 'T7s will provide the basis of a fine "El Cheapo".

Maybe you could get some milage out of the 7C7s in the 2nd gain block of a phono preamp. Hmmm, 6GK5 1st gain blocks, 7C7 2nd gain blocks, and 6F8 O/P buffers just might work out.
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Old 22nd April 2008, 12:26 AM   #4
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I'd be inclined to use a 7F7 section per channel for gain, with 7N7 cathodynes, driving 7C5 outputs run as pentodes. Add a bit of global NFB and it should sound sweet.
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Old 7th May 2008, 04:13 PM   #5
GordonW is offline GordonW  United States
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On the subject of similarity of 7C7 to EF86- I found "typical" plate curves for both. From my view, they look EXTREMELY similar. I would be interested in any comments anyone might have, if they notice anything that should make them work significantly differently from each other...

Here, the 7C7:
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File Type: jpg 7c7 plate curve.jpg (97.4 KB, 227 views)
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Old 7th May 2008, 04:15 PM   #6
GordonW is offline GordonW  United States
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Here's the EF86. Yes, the scales are a little different... but note that the current vs. voltage is about the same everywhere. And note how, despite a bit of "gain" difference, the shape of the curve spacing near the bottom of the graph, is VERY similar. In fact, the 7C7 may actually have BETTER spacing "evenness"!
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Old 7th May 2008, 04:21 PM   #7
GordonW is offline GordonW  United States
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If this tube is as good as it looks, it may actually supplant the EF86 in my screen-drive amp project... use the 7C7 to drive a 6C4 split-load phase splitter, in turn driving a 6SN7 diff amp for additional voltage gain, then into a 6BL7 cathode follower, driving the g2 of a pair of 6JB6s in PP...

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Gordon.
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Old 8th May 2008, 12:46 PM   #8
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Yes I am using them in an upcoming project (just need to finish the chassis).

Reliable 7C7 curves for Eg2 = 100V and also 7C7 triode strapped curves can be found on my homepage.

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Tom Schlangen
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