Hey boys, don't fight! 
I'm no expert, but I've built a circuit like artosalo proposed, except I used a 5687 (very easy to find, inexpensive, great tube). It does sound really fine. It's not hard to get working correctly, either. Snaps right into place.
I used an LM317 in the "tail" instead of the DN2540, because the LM317 requires only a single resistor to set the current using its built-in 1.25V reference, and it works every time. I know the DN2540 performs better in the upper frequencies, but I went for the convenience of knowing exactly what current I'd get with a particular resistor value (for Vref = 1.25V, Vref/R = I in amps).
artosalo, is there enough voltage between the 6N6P cathodes and ground to keep the DN2540 from switching off? Do you have like +5V at the 6N6P cathodes? Is that enough for the DN2540?
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I'm no expert, but I've built a circuit like artosalo proposed, except I used a 5687 (very easy to find, inexpensive, great tube). It does sound really fine. It's not hard to get working correctly, either. Snaps right into place.
I used an LM317 in the "tail" instead of the DN2540, because the LM317 requires only a single resistor to set the current using its built-in 1.25V reference, and it works every time. I know the DN2540 performs better in the upper frequencies, but I went for the convenience of knowing exactly what current I'd get with a particular resistor value (for Vref = 1.25V, Vref/R = I in amps).
artosalo, is there enough voltage between the 6N6P cathodes and ground to keep the DN2540 from switching off? Do you have like +5V at the 6N6P cathodes? Is that enough for the DN2540?
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You can use a constant current diode instead of the LM317. They are also called current regulating diodes:
Mouser Electronics - Electronic Component Distributor Current Regulator Diodes
They are just 2 terminal devices, just like a normal diode, except that there's an FET inside, arranged as a constant current source.
Mouser Electronics - Electronic Component Distributor Current Regulator Diodes
I used to use those a lot, but I found they were too expensive, and didn't come in high enough current ratings. But I see that's all changed. Thanks!
The DN2540 and 10M25S depletion mode MOSFETs are the preferred choices these days (SY here turned a lot of people on to those). The LM317 is a poorer performing choice, but it's cheap, ubiquitous, and really easy to work with. And since the OP wants a simple project, I thought I'd throw that in.
But the CRD's you pointed out are even easier. You want 30mA? Put two in parallel and there you are. And they're less than $3 each now.
I wonder how they perform... Anyone?
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I used to use those a lot, but I found they were too expensive, and didn't come in high enough current ratings. But I see that's all changed. Thanks!
The DN2540 and 10M25S depletion mode MOSFETs are the preferred choices these days (SY here turned a lot of people on to those). The LM317 is a poorer performing choice, but it's cheap, ubiquitous, and really easy to work with. And since the OP wants a simple project, I thought I'd throw that in.
But the CRD's you pointed out are even easier. You want 30mA? Put two in parallel and there you are. And they're less than $3 each now.
I wonder how they perform... Anyone?
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The cathodes are at +58V? I can see why you need the input blocking caps.
For some reason that wasn't obvious from the schematic. How much current do you have the 6N6P triodes drawing? About 10mA each?
The 220R resistor sets the 'bias'? Ah, I see. The grid leak resistors define a floating ground, and the bias resistor goes there too.
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For some reason that wasn't obvious from the schematic. How much current do you have the 6N6P triodes drawing? About 10mA each?
The 220R resistor sets the 'bias'? Ah, I see. The grid leak resistors define a floating ground, and the bias resistor goes there too.
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Performance depends on the current rating, with lower current values maintaining constant current over a wider voltage range than the higher current values. I am not sure about the conditions used for this test shown in the charts though. Also I think it's important to understand that when used in the tail of a long tailed pair, the voltage doesn't change a lot anyway. Here's a page about them:
http://www.centralsemi.com/product/cld/index.aspx
These curves look a lot better:
http://www.centralsemi.com/PDFs/products/ALL_SMD_CLD_curves.pdf
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That's a good spot. I've used them there and they work well. I like that tube, too. It's a smooth sounding one.
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Yeah, what's confusing to me is that other manufacturers, including Central Semiconductor, show almost a ruler flat curve for their CLD's (CRD's) over the entire voltage range, and Semitec does not. Vishay stopped making them apparently, but someone else is making their TO-92 style device.
For the two load resistors of the long tailed pair, I'd use a current mirror instead. There are current mirrors made as one device with 4 legs, so it's easy to implement.
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