Just purchases the Bottlehead's new OTL headphone amplifier, the Crack . Seems like an interesting addition to my headphone system. It is relatively cheap, and looks to have plenty of room for upgrades/tweaks and expansion. Never used an OTL amp before, so it'd be interesting to build and compare this to the other headphone amps I have running.
The tube complement is very standard and readily available. The NOS prices on the two tubes, 12AU7 and 6AS7 are quite reasonable, moving up to the very expensive. But since it only uses one tube each, tube rolling may be an easy reality. Upgrades such as capacitor replacement (it looks 5 electrolytic can be replaced), CCS to the driver tube, resistor upgrades, sockets upgrades, etc. are possible.
Bottlehead seems to imply up to 4 weeks wait time, which is fine, as I am in no hurry to build. Over the last few months, I have come to the conclusion that headphone audiophile will be the main method of music enjoyment for myself. Mainly due to work and family arrangements. So, in the meantime, using my HD650, I will endeavour to build and listen to a wide a range of headphone amps as possible!
For reference, the following tubes may be suitable as substitutes.
12AU7 - ECC82, CV491, 5814, 6189, 5963, 6680, ECC802, E80CC and B749
6AS7/6080 - 7236, ECC230, 5998, WE421A, CV2523, 6336, 6520, 6h5c/6n5s, 6h13c/6n13s