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Old 27th January 2007, 05:25 AM   #1
hacknet is offline hacknet  Singapore
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Default Tube choice for Class A2

hey guys,

woke up this morning with this itch to try a class A2 output stage.

after doing some reading, i realised that there are only a couple of tubes that are recommended for class A2, namely the 845, sv572 and the 811.

the 811 and 572 are pretty expensive and the 845 needs pretty lethal voltages.

i was wondering if i could get a taste of class A2 with more common tubes like the chinese 807 which is in abundance in this part of asia.

what do you look for in a tube for it to operate well in class A2?

thanks man!
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Old 27th January 2007, 05:57 AM   #2
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Default Class A2 triodes

I've been thinking about class A2, too. The 811A is my first choice, and the Russian and Chinese 811As are not too expensive and seem to work well.

There is a connection of the 807 I have seen that makes them a very high mu triode - connect grid #1 and #2 together and use this as the control grid. The mu is several hundred. This should work well in class A2.

I've played around with 12AX7s in class A2 - they actually work OK, although it is easy to get the plates to run red-hot. If you want to fool your solid-state engineer friends, put a 12AX7 onto a Tek 575 transistor curve tracer with the plate going to the collector, the grid going to the base and the cathode to the emitter. You need to supply heater power. When you trace the 12AX7 as if it was a transistor, it looks like a lousy power transistor with a low hfe! This is with the grid being driven by current.

- John Atwood
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Old 27th January 2007, 06:03 AM   #3
hacknet is offline hacknet  Singapore
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great to hear a reply so soon!

i dont know that the chinese had 811s. in that case, maybe i will keep an eye out of them in that case.

the 811s have pretty hot filaments..! 6.3v @ 4A!

but what exactly makes a tube a good for class A2? high gain? a tough plate?

thanks guys!
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Old 27th January 2007, 08:33 AM   #4
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My choice for this is the 812, rather than the 811. The 812 has somewhat better linearity and a lower r(p).

Click the image to open in full size.

Pretty good, I'd say.
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Old 27th January 2007, 09:59 AM   #5
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Quote:
but what exactly makes a tube a good for class A2? high gain? a tough plate?
thanks guys! [/B]
A tough control grid, more likely. It's got to dissipate power equal to grid voltage times grid current, when the drive takes you out of class A1.

So what tube? I don't know, but 6L6GC data sheets always show specs for a pair in class AB2. So they ought to be fine in class A2 SE with the right parameters.

Available from China too!

--Paul
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Old 27th January 2007, 04:32 PM   #6
wfmali is offline wfmali  Europe
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Hello,
absolutly worth a try are VT76 (=CV1076=DA41=TZ40=...) and SV811-3... Currently build a SE A2 with the latter one, driven by 2A3 & interstage. Works good at the bench already.
But: none of those tubes are very common, at least not anymore.
Marcus
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Old 27th January 2007, 04:40 PM   #7
hacknet is offline hacknet  Singapore
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yeah.. been searching for chinese 811 but i havent found any online. the sv572 looks like its going to cost the moon. i'd prefer to stay on the cheaper side of things.

i've been thinking if i can drive these buggas with something like an akido with a tube choice like 5687-5687, might be able to get low enough output impedance to do some work. i dont know if i can get enough voltage swing thou..

hows a low voltage 845 in A2? defeats the purpose?

i've seen the 6l6 in class a2 spec but it has never been recommended. i was thinking of the russian ones instead because they come in abundance. 807 might look alittle more snazzy but i dont know how they would measure out..
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Old 27th January 2007, 05:16 PM   #8
rdf is offline rdf  Canada
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Quote:
Originally posted by hacknet
yeah.. been searching for chinese 811 but i havent found any online.
E-bay, a matched Shuguang pair is under $12 from china_best2006.
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Old 27th January 2007, 05:18 PM   #9
hacknet is offline hacknet  Singapore
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will try that man! thanks!
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Old 27th January 2007, 05:29 PM   #10
hacknet is offline hacknet  Singapore
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is there a difference between the 811 and the 811a?

is the 812 better suited for class a2 operation compared the the 811?
they dont cost very much more and it might be interesting.
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