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Old 7th May 2006, 04:17 AM   #11
lndm is offline lndm  Australia
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Well, problem seems fixed, and I have all the headroom I had with plain cathode bias, plus I have the damping factor. The 1812 can't break it .

I don't think it is very elegant, much tweaking to go, but thanks to all for helping me come to a solution.
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Old 8th May 2006, 09:47 PM   #12
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I have on occasion used a s.s regulator type of topology to provide a fixed cathode voltage (mine about 44V), with the advantage over many zeners that you can adjust it and it can sink a rather large current. Regulation can be anything you want. The Zeq of mine is below 150 milli-ohm over the whole audio range. Topology can also include an overload sensor (to trigger an h.t. cut-out circuit) and other handy features.

Regards.
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Old 9th May 2006, 05:30 AM   #13
lndm is offline lndm  Australia
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I have a couple of mje340's. I might be tempted to wrap them around the zeners (ala simple Vreg). I hope they may at least shield the zeners from whatever makes them groan and oscillate, and reduce the impedance.
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Old 10th May 2006, 01:27 AM   #14
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Careful here. Indm - they must be PNPs. The tube is the load, with + at anode and - at cathode, The transistor is between common and cathode, sinking the cathode current. Convenient to screw the collector directly onto the chassis without insulation.

My circuit was somewhat more involved, with a LTP (PNPs) sensing on one side and having a 6,2V zener on the other, then fed through a NPN to the power transistor (in my case TIP 42 - simply readily available; I needed to sink 1A). As said one can then sense over-current somewhere inside, and a compensating cap in the right place will extend the frequency response, as in giving a low Z over the audio range.
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Old 10th May 2006, 03:24 AM   #15
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One more take on blocking distortion - I'm going out on a limb here and suggesting that the current source in the cathode (suitably bypassed) as I used in the Baby Huey circuit here
EL84 Amp - Baby Huey
addresses both forms of blocking distortion - it eliminates that due to cathode bypass charging up with grid current and ingnores that cause by a grid voltage shift.

Am I deluding myself???

Cheers,
Ian
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Old 10th May 2006, 03:50 AM   #16
SY is offline SY  United States
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I can't convince myself that this is immune to blocking. If, under overload, the bypass cap charges up to follow the CS voltage, when the CS voltage tries go the other way, it still has to discharge.

Or maybe I'm not seeing it right.
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Old 10th May 2006, 04:39 AM   #17
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SY - you're right as usual.
If the bypass cap charges up as a result of overload then tube current will reduce by the same amount as the cap discharge current supplies part (or even all) of the constant CCS current.

I really need to think these things through a bit better before dashing of a post during my lunch hour.

Cheers,
Ian
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Old 10th May 2006, 04:48 AM   #18
arnoldc is offline arnoldc  Philippines
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Indm, can I use just a scope and signal generator to see my amplifier's overload recovery capability or lack of? If so, may I ask how?
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Old 10th May 2006, 05:15 AM   #19
lndm is offline lndm  Australia
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Thanks for the heads up there Johan, I haven't thought through to the finer details yet. Your idea will take some digesting but it does appear more refined than what I have now. I haven't seen much like it on the www .

Arnoldc, with an output stage cathode resistor and bypass cap, I had one channel watching the cathode and one watching the dummy load. There was appreciable signal at the cathode despite the cap. I would wind the volume, and when the signal on the load revealed I was drawing grid current, the cathode trace would shoot off (with DC) and return shortly after (though I was AC coupling the channel come to think of it).
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Old 10th May 2006, 05:34 AM   #20
arnoldc is offline arnoldc  Philippines
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Hmmm, I'll try that but my amp right now shares the same cathode R. Hope it's close or similar in result.
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