I'd always been a solid state amp guy. I played around with a XDuoo TA-02 a while back, and while it had 2 EF95 family tubes, it still relied on solid state circuitry for power amplification. It sounded nice, but now resides on my desk at work. I also built a nice buffer based on the X10-D and enjoy it quite a bit with my Krell KSA5 Klone. I finally got a full tube amp when I joined the Little Dot MkII drop from Massdrop last month. I had ordered a selection of different tubes off of eBay while waiting for shipment, and got the amp and tubes all at once. The first listen with the stock Chinese tubes, was ok. The bottom end was somewhat muddy, the mids were acceptable, and the highs were somewhat harsh. I didn't give it a chance to burn in. I swapped in a pair of Novosibirsk 6N6P marked 8th week of 1973 and played around with some EF95 tubes. The sound was nice, especially with the Soviet tubes (more below). I opened up the amp with a plan to make a switch for the EF92/95 jumpers and lo and behold, the horrors!
The board was marked version 3. The build quality was dodgy at best. There was flux residue all over the place and while the soldering was solid, it was messy. The WIMA input coupling caps were melted at the bases. They were marked 3.3uF, and being tiny, were likely polyester. The other film caps were nasty generic globs whacked in willy-nilly. The film resistors were marked 1%, and when I measured them later after I cut them out, they were 1%, but barely. The big 5W resistors were 10% jobs. The LED was mounted too low and did not fit into the hole in the front panel. Finally, the bridge rectifier was some yucky no-name job. On the plus side, the electrolytic caps (UCC's, Nichicon's, and Rubycon's) were quite nice. The pot looked chintzy, but it's an Alps and actually measured well and sounded smooth.
An order was placed with Mouser, and after the package arrived, I got busy with the snippers and desoldering iron.
That's some Shenzhen market special!
After some work, much better!
The film resistors are Vishay RN55 (matched to 0.1% between left and right channels for the signal resistors) and CFP series. Yes, I even replaced the resistor to the LED. It was $0.10 so why not? The large 5W resistors are 1% and also Vishay's, but wirewound (don't care about inductance there). The film caps are WIMA 5% polypropylene. The large coupling caps are Kemet 3.3uF polypropylene. I have a pair of them in my Lehmann BCL clone and they sound very nice. There's a proper Vishay bridge rectifier, a Fairchild diode (again, also $0.10), a properly aligned LED, and a DPDT switch for the jumpers. The connecting wires were also switched out. I did not modify the circuit because I wanted to know what this little guy is truly capable of with good components.
The first set of driver tubes were a pair of Voshkod 6ZH1P-EV marked 11th week of 1981. Before the innards swap, they sounded nice. Good bass response and punch. Nice midrange, especially with female vocals, and clear highs. After the new guts, everything holds, but better. The bass extends down deep, but is controlled, tight and well defined. I wonder if the original WIMA's were actually 3.3uF. They were probably of a lower value so that the bass roll-off frequency was higher. The midrange is now front and center. Male and female vocals are lush and full. Mark Knopfler's Strat sounds so sweet. The highs are bright without being fatiguing. High-hats and cymbals are crystal clear with a smooth natural decay. The soundstage is astounding, giving my solid state amps a run for their money. There was a thin veil before, and now it's gone. Needless to say, I haven't got around to the other tubes.
The Little Dot will not replace my solid state amps, as when one gets down to the nitty-gritty, the solid states are technically superior. But boy, is the Little Dot fun, and I am looking forward to playing with the other tubes in my collection.
