I do own a pair of Hartung 50W OTL's.
Construction:
Good double layer case - it means there is an internal sub-chassis where everything is assembled and then there is the outer black decorative metal case that sits about 5mm above the inner (to insulate against heat). The very large toroidal mains transformer is fixed to the inner case by a strong arm and bolted. the speaker posts, input posts are quality but not the top of range. Over all the case seems sturdy and well built. All Tubes are on top and heat insulated from the components inside.
Circuit and components:
Reasonable quality, room for a fair bit of upgrading. resistors are mainly 0.25W metal film, signal capacitors are run-of-the- mill plastic caps, there is a volume control which is a stepped resistor bank (not really necessary - I bypassed mine) - there is also balanced input and a cheap slide switch to activate it, again not required by me, so I bypassed them too. The interconnect from the phono socket to the main board is a cheap one. The toroidal mains transformer is huge! at least 600W - the smoothing capacitors are quality Nichicon 5000Hr gold. In short, the expensive main components are quality, but the rest can be upgraded.
Sound and Operation:
The amp uses one ECC81 as input stage, another two as driver/splitter - these driver four Russian Military 6C33C in a push-pull configuration biased carefully for low DC offset at speaker terminals which has a protection fuse. The biasing is done through a small hole at the top. there is a fine multiturn pot under there which is high quality. After a bit of perseverance, I managed to fine tune the bias for less than 10mV DC at speaker terminals, and it stays there (when warmed up) and does not drift! which is a good thing.
I love the sound! it drives my Sonus Faber Grand Piano Home speakers (min imp. 6 Ohm) with ease. The bass is powerfull and well defined - the mid range is fluid and open, and highs are high! because usually with tube amps that do have an output transformer, the treble rolls off - but not on these - they go supersonic (which is apt, since the 6C33C's are out of a Mig fighter jet). Bear in mind that I have done the following upgrade/tweaks to the amp, but even as it came of the box, they were very good.
Upgrades done;
1- Bypassed the volume control, the balanced input and slider switch.
2- Changed the interconnect internally for a quality cable, direct to the board.
3- did a star earth, to minimize the slight hum that there was.
4- changed all signal capacitors for cheap Russian K40 paper/oil capacitors.
5- bypassed all smoothing capacitors with 4.7uF plastic caps.
6- doubled the smoothing caps with identical caps (cheap enough since they are 200V versions)
Total cost of upgrades - not much! - but they did improve residual noise and clarity by a great deal.
Final word - they are genuine amps, well made and easy and cheap to upgrade . Also the seller is very helpful, and honour the guarantee without fuss.
For the price, you would be hard pushed to find better.
