This is a gem. This is my second SET 45. This time I built it in the same enclosure and the results are amazing. If one has the appropriate speakers for SE tube amps and low power ( the 1st watt is all that matters), than this amp is a must to experience. I've spent many hours to experiment with different driver tubes. My goal was to make a 2 stage amplifier where the driver has minimum distortions with a dominant 2nd harmonic. This way the amplifier as a whole will get a predominant 2nd harmonic and will reveal the gorgeous sound of the 45.
The result of this brainstorming was an amp with lower input sensitivity. I used type 76 triodes to drive the 45. The 45 has a Miller capacitance of about 40pF ( including some stray) and it is easy to drive in class A by using a low current driver such as the 76 at 5mA, without introducing slewing distortions. The contribution of the 76 driver of the overall distortions is less than 10%.
The power supply is using a over built Plitron transformer to supply the high voltage. The same transformer is used to provide 6.3V to the 76 filaments . The rectifier are SiC Schottky and quais choke input with first cap Clarity ESA followed by LCLC with two Hammond 193D 8H 150mA and two combination 100uF Mundorf plus 33uF Obbligato oil.
The low voltage raw supply uses all Triad transformers used to provide raw DC supply to the Coleman filament regulators for the 45s. The high voltage comes delayed 25-30 seconds to allow the 45 filament to reach a quasi-steady state and extend their lifetime. The delay timer is based on the 555 IC.
The main amplifier case is built out of Maple solid hardwood stained with light oil urethane, 4 coats. It is lined with 5 mil. copper sheet for better screening. The top plate is gold anodized aluminum made by Front Panel Express. The main amplifier is provided with Neutrik RCAs and Cardas binding posts. A nice Shurite 5V panel meter can be switched to measure each of the 45 filament and allow precise adjustment to 2.5V. There are separate 10 turns 2W pots top panel accessible for this adjustment. The output transformers are James 6113HS.
I always measure my amps and in majority of cases the measurements tell the truth about the sound of the device. In this case, e from the distribution of the harmonics, there is a 2nd harmonic dominant SE amp that sound as good as it measures.
Input sensitivity:
1 Vrms for 0.25W output, 2.2 Vrms for 1W output, 3 Vrms for 1.75W output max
Frequency response: 20Hz-30kHz @ -1dB
THD : 3.5% @ 1.5W
Dimensions are 19"x13"x9" (WXDXH)
Weight 40 lbs
I’ve needed a bit of gain to drive this amp to max power. I found that my 18dB 01A preamp is a great fit. I’ve also tried 26 pre and 10Y pre and they all sound excellent combined with this SE 45.
