If you browse the internet you will find many positive comments on this kit. I don’t want to repeat what has been said already. Perhaps to mention that the whole build process is such a pleasure which is due to the well-thought layout of every small detail including packaging. The same applies to the assembly manual. It is like a time travel working on one of these ancient Heathkit kits. Placing the decals reminded me of building Revell aircraft kits 50 years ago.
The TU-8800 is a smart design supporting 3 different operating modes for the most common octal pentode/tetrode power tubes flipped by a single switch. The anode/G2 currents are kept stable using a servo regulating negativ bias. No need to worry abou bias point drifting. The amplifier is protected against tube failure by monitoring current through a pair of gyrators. Switching of anode voltage is done by switching in or out an additional transformer winding in the HV-section.
The bias point details per operating mode are:
|Ia / mA||41,8||78,2||78,2|
|Ua / V||262,3||245||324,9|
|Pag2 / W||11,0||19,2||25,4|
The flexibility of this design can also be exploited to use output tubes not originally supported. Of course you have to use adaptors readily available on the market. I have successfully tried Siemens F2A11, Telefunken EL156, Valvo EL12 and Telefunken EL84. The power modes to use are Low for EL84 and EL12, Mid for F2A11 and High for EL156. The F2A11 can also be run in High power mode but I don’t need the increased output power for my Lowther TP-1 speakers. Also, I prefer the F2A11 in Mid power mode from the sonic point of view.
I bought the kit including the Lundahl transformers. I have upgraded the following parts:
Replaced stock coupling caps with Duelund JDM Silver Capacitors (0,01µF) and V-Cap ODAM (0,1µf).
Changed volume pot with a TKD 2CP-601 which I prefer to the 2CP-2500 series from the sonic point of view.
Resistors in position R101 .. 112 and R201 .. 212 have been replaced with Takman Metal Films.
Also PTC101 and 201 (in series to the screen grids) have been replaced with Takman Metal Films. I don’t like non-linear resistors in that position!
C105, C106, C108, C208 have been bypassed with Soshin 0,01µF Silver Mica capacitors.
C112, C212 have been bypassed with V-Cap ODAM 1,0µF capacitors.
… and how does it sound? I can tell you that my TU-8800 gets quite some listening time. When this blog grows you will know what that means. Descriptions of other kits I have built in the past years will follow.