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Old 11th April 2006, 05:24 PM   #1
Giaime is offline Giaime  Italy
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Default 802 as an audio output tube

Hello everyone! I found several new RCA 802 for 10 euro each.

They look so good, do you think we can make a tube amp with that?

I found RCA sheets, but not much info. What do you think about it? I would like to go PP, since I need 7-10W at least. I could try pentode SE, but I think it will require feedback to get reasonable damping factor. So, maybe, triode PP?

Any thoughts are welcome - if it's not suitable please tell me, the same place has RFT EL36 new in box for half the price... but 802s look soooooo good!
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Old 11th April 2006, 10:45 PM   #2
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hi!

don't know what they mean with d.c. supresoor voltage (+40v), but regarding it as a normal pentode, it should work.....

quick draw PP at:

B+300v, ia=20ma, vg=-23, vg2=250v, zaa=20k
notice the pd curve(plot it with PD,ccs=6,7W I=u/p)
i don't know if one can exceed it too much with this one...

this is what the datasheet says:

err...Pd,ccs=maximum plate watts, at 'continous, commercial service' not a current source....
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Old 11th April 2006, 10:48 PM   #3
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Old 11th April 2006, 11:31 PM   #4
cerrem is offline cerrem  United States
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I did some quick number crunching... for a pair of 802 tubes in Push-Pull...
If you operate the screens at thier max of 250V regulated of course... Then with a plate voltage of 400V and a optimum plate load of 1K plate load you get 12Watts RMS output....
450V plate is roughly 14 watts and 500V plate is roughly 16 watts.....
This is with the suppressor grid at +40V......
The suppressor + voltage can help with increasing the gm of the valve, provided the plate voltage does not dip below or close to 40v durring AC dynamic swing...then you will get arcing or discontinuity in current .... If you examine the plate curves you will see that the plate load swings down to roughly 100V in most normal designed circuits...
If one plans to drive into grid current ClassA2 or AB2 ...you can get some pretty respectable amount of power from these valves...

Chris
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Old 12th April 2006, 12:00 AM   #5
Rocky is offline Rocky  Norway
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What if you chose some cracker like the russian 6c45p to drive those things? then perhaps you could do this 2-stage and really cheap!

EDIT: You might think those small russians would look dwarfed next to the 802s, but you can drive them almost to meltdown way above specs without worrying too much, an by then they give off *alot* of light. I've burned some of them at voltages and currents you wouldn't believe
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Old 12th April 2006, 08:09 AM   #6
Giaime is offline Giaime  Italy
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Thank you all!!!!

I was also thinking A2... it seems the way to go for me.

So... options:

1) triode PP A2, cathode bias, G3 tied to a voltage divider from B+. Driven maybe... from tubelab powerdrive?

2) pentode SE, A2, cathode bias, G2 regulated and G3 tied to a voltage divider off from G2 line, I think I would still drive it from a powerdrive.

3) pentode, PP, A2, some SERIOUS power out there

I've got to buy only one of those and experiment a bit with A2 conditions... I have here a PT that can give 400V B+, and would like to use this (it seems big enought), but no OTs, I would also like to try mains toroidals for output.

Some suggestion on possible loads for the various options? I would like to go triode PP for the sound and the high damping factor.
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Old 12th April 2006, 08:12 PM   #7
Giaime is offline Giaime  Italy
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I've tried something with the plate curves, SE for the moment, I will do PP calculations soon.

1) A1: Z = 8k, Iq = 40mA, Vq = 320V, Pout = 3.9W, Second harmonic = 0% (!!!), third = -8%.

2) A2: I don't know how to calculate this. I know that I must take 2 points: the point where the grid voltage is zero, and the point where the grid voltage is the double of the quiescent one. So, if I allow the grid to go 10V over zero, I should allow the grid to go 10V below the other point. So I get those results:
Z = 4.4k, Iq = 52mA, Vq = 250V, Pout = 4.3W. So little for 10V drive! I shall try some very high Z loads and very high plate voltages...

edit: Iq = 30mA, Vq = 400V, Pout = 5.7W for 16k. But 16k trannies hare hard to find...
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Old 12th April 2006, 08:45 PM   #8
Rocky is offline Rocky  Norway
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Quote:
Originally posted by Giaime
third = -8%.

You'll have to 'splain that one for me... minus 8% 3rd harmonics?
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Old 12th April 2006, 08:51 PM   #9
Giaime is offline Giaime  Italy
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It may look a bit funny, but...

http://members.aol.com/sbench102/pent.html

Quote:
Notice the minus sign. This indicates that the harmonic content subtracts from the fundamental (flattening it) when the fundamental is at its crest. This *usually* happens on third harmonic distortion in tubes.
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Old 12th April 2006, 09:25 PM   #10
Rocky is offline Rocky  Norway
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ah, well OK, I see... thanks for splaining it, it just sounded weird at first glance.
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