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Old 22nd December 2010, 01:03 AM   #1
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Default 807 SE Schematic Critiques

Hey guys,

I drafted this schematic a week or so ago. Please tell me what you think and if any improvements could be made. I will not be implementing any global feedback just yet. Here's my schematic:
Click the image to open in full size.
(larger picture: http://imgur.com/1kfQC.jpg)

One thing to note: the maximum plate voltage is 400V for an 807. I have seen several circuits that push this rating. The others I've seen also use lower resistance cathode resistors, but I am hoping by using a larger value, I will prolong the life of the tubes. Here's the curve:
Click the image to open in full size.
(and a larger picture here: http://imgur.com/MBDan.jpg)

Thanks!

Kyle
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Old 22nd December 2010, 01:09 AM   #2
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I haven't looked at any of the numbers, but thoughts straight aways include:

triode may be fine, but you are likely to want more than just u/l to tame the pentoady-ness of the 807 in u/l mode.

Will the SL7 have enough oomph to drive the 807 in triode mode?

Why not stick a ccs under the 807?
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Old 22nd December 2010, 01:17 AM   #3
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try triode mode for the 807
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Old 22nd December 2010, 01:48 AM   #4
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just had a quick look - pick this to bits.

triode mode, set the current using a ccs at 40ma through the 807. 420v B+

set up your driverto provide 37V pp to the 807 g1

you get pretty close to symetrical 300v swing at the 807 anode

300v pp @ 40ma means about 8w output (math check please brighter ones)

STC 807 datasheet - its a goldmine
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Old 22nd December 2010, 01:58 AM   #5
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Haven't tried it myself, but I've found a number of reports that basically say 807 triode strapped doesn't sound good at all. Don't mean to stop you from your exploration though, it's best to make up your own mind.
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Old 22nd December 2010, 03:56 AM   #6
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I have a SPDT switch to switch between UL and triode. If I don't like one, I'll just keep the switch in the other position. I think UL is a good compromise between triode and pentode modes.

The *SL7 should be powerful enough. I've seen other circuits use similar tubes, all with fairly high gain. I need 64V P-P to drive the 807 to full power. I really like the idea of a CCS. I'll be able to adjust it much more easily.

From what I understand, the 807 is a 6L6 in a different package. I'm not sure if people like 6L6 SE amps or not, but my guess is that they'd sound similar. This whole build is going to be an experiment, so we'll see how it goes.

Thanks guys!

Kyle
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Old 22nd December 2010, 05:21 AM   #7
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the -32v bias is going to give LOTS of distortion... Anode swings down to around 185, up to around 560. Thats 215v down, 185v up...

Here's an 807 SE that works. Note the bias point.

Having said that, I'm all or experimentation! Go for it...
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Last edited by aardvarkash10; 22nd December 2010 at 05:33 AM.
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Old 22nd December 2010, 08:01 AM   #8
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Quote:
the -32v bias is going to give LOTS of distortion... Anode swings down to around 185, up to around 560. Thats 215v down, 185v up...
Can not find those figures in the diagram. If we go for 6300ohm load we get 150-375-560 and asymmetry is 22% which correpond to 5% 2H just before clipping. Where is the "LOTS of distortion"? Think it should be called average. Unfortunately output power will be very low. 160-375-550V, 5000ohm, 23% asymmetry, 2H 5%. But when going for 3300ohm it gets worse. We get 175-375-520V, asymmetry 38%, 8% 2H.

But doublecheck my course figures, made this at a short break at work.

Also remember you get distortion-cancellation in a 2-stage amp, so figures will look a lot, lot better IRL.

About the RH807 its better to check this site about Schade to get it right.

Last edited by revintage; 22nd December 2010 at 08:06 AM.
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Old 22nd December 2010, 08:15 AM   #9
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Hi!

The 6SL7 is a great tube, but not that well suited as a driver IMHO, I'd prefer something beefier there.

If you want to stick to the 6SL7, don't load it so heavily. It has a rp of about 45k. You load it with a 100k plate resistor and 100k grid resistor in parallel, that is a 50k load! This will leave about half the mu of the 6SL7 as gain!

Increase the plate load resistor to maybe 150k (you can use higher B+). And increase the grid resistor of the 807 to the maximum allowable value. Or use a different driver.

Best regards

Thomas
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Old 22nd December 2010, 09:40 AM   #10
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The only "different" dual-triode driver that comes to mind is E88CC albeit with mu on the low side. But maybe Thomas has an ace up his sleeve . About single triodes there are plenty to choose from.

If you run 6SL7, do some loadlines there to.

Last edited by revintage; 22nd December 2010 at 09:56 AM.
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