This is a don't-try-this-at-home amplifier; the tubes are hard to get and very expensive. 5998A is a close relative to 6AS7G/6080. Many of its parameters are similar but gain is higer. Below, a list of similar tubes, picked up at Head-Fi.org
6080 - low gain @ 2- great vocals, wide soundstaging
7236 - medium gain @ 4.8 - great bass, fast and dynamic.
5998A - medium gain @ 5.4 in straight bottle. Plate construction like 6AS7.
5998 - medium gain at 5, same current load as 6AS7
421A - same as 5998 but with matched plates/sections
6AS7G - low gain @ 2, the standard tube for most, plentiful
6AS7GA - low gain @ 2, like the standard 6AS7G in striaght bottle
6520 - premium 6SA7G gain @ 2, sometimes with 5998 plates
6N13S - russian 6AS7G equivalent.
ECC230 - european 6AS7G equivalent.
6528 - equiv to 5998 but very high gain at 9, twice the current load.t
6336 - like 6528 but low gain at 2, still twice current load.
There are a few very similar tubes that can be used to make up a minimalistic amp. Picking a tube like 5998A that has a higher gain than 6AS7G and it is easier to find a suitable driver. Here the driver is a 12AT7. Two tubes - one driver and one power that will make a rather stripped down amp.
The transformers are Banbury, now discontinued and mains transformer is German.
A few notes:
Input connectors are placed on left hand side, close to the front to avoid picking up dirt if placed at the back of the amp. There's an iPod docking station added just for fun.
It may appear that the transformers are situated rather strangely, but The chassis was used for another project and holes were left here and there. The transformers are placed like they are to cover "old sins".