These two buffers are very similar. In fact, the XL came about as a simple mutation of the regular version when the cost/size of an extra tube was not an issue and there was the possibility of driving a lower impedance load (long cables, solid state amps, etc). In theory, the power supply requirements are identical down to the B+ current draw. “In theory.” Although if you study the schematics carefully and read the write ups, you’ll see that my steel-trap mind is missing a spring or two.
The original Muchedumbre called for a Triad C-3X (500 ohm DCR). On the schematic for the XL, I seem to have spec’d the Traid C-7X (270 ohm DCR). The inductance rating for these two chokes is identical, but the C-7X has about half the DCR. Is that going to be a problem? In short: no. I love tubes; they make up for all my shortcomings.
Current draw between the two configurations (XL vs vanilla) is the same on paper, despite the extra triodes in the XL (the added triodes are in series, so the same current flows through both). If using the same choke, the B+ should end up the same. With the modest current in the pre, however, you have plenty of leeway in DCR before you have to worry about big B+ changes.
ΔVdchoke = (500 ohms – 270 ohms) * (10mA x 2) = 4.6V
No problemo.
Say you have a big old Hammond 193M at 10H and 63 ohms DCR. You could very safely use that, too. The lower DCR gives you a little more room in the B+ to increase the series/sense resistor if you’re amps’s input impedance suggests it. You gain some B+ in the choke DCR and drop it back down across the sense resistor. Current across the DCR is twice that of each channel, so assuming 10mA bias per channel:
ΔVdchoke = (500 ohm spec – 63 ohm part) * (10mA bias x 2 channels) = +8.75V
ΔVdsense = (1k5 ohm spec – 3k ohm part) * 10mA bias = -15V
Net result is a 6.25V deviation from the ‘on paper’ spec’d parts. That’s less than 5% of the B+, so better than parts tolerance in a lot of cases (e.g. low frequency chokes). The x-axis divisions on most tube datasheets are 20-25V, so they’re only so precise. Generally, 10% changes in B+ based on available part specs is probably about where I would return to my load lines to recalculate. In practice, I sometimes build anyways and then measure actual B+ before worrying about anything.
Solder and wire beats paper and pen.
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