Here is another 4P1L design. All started when I tried to figure out a gain stage for a First Watt F4. The F4 is a unity gain 25W class A power amp that needs about 14Vrms input in order to deliver max power.

4P1L offers a x9 gain. Therefore, using a 4P1L gain stage is appropriate and the input sensitivity of the hybrid amp will be about 1.55Vrms.

I built my first F4 in a 4U standard ModuShop Dissipante. seen at http://www.modushop.biz/site/index.php?route=product/category&path=66_97_107.

I had to built the 4P1L gain stage so it fits inside this case. The max dimensions were 12″x12″. I built the first one on a garolite FR4 board and attach all components and small PCB to it (see previous built https://simplepleasuretubeamps.files.wordpress.com/2014/12/photo-1-1.jpg). It was a hassle to position and attach each board to the main FR4. This led to the design of a single PCB board. I also wanted to integrate the Coleman regulators and use standard heat sinks such as the Wakefield 657-20ABP. Rod helped me figure out the size of the heatsink.

The schematic  is simple: The power supply uses 3 transformers ( one of the 187D20 for each filament, and one Antek 50VA 160V for the HT.). Ht supply is filtered by a CLC (470uF-15H-470uF). The raw supply for the filaments uses 1N5822 and CRC 10mF-0.47R-10mF. The Coleman regulators need to be preassembled except the power transistors who are mounted on 2″ Wakefield heatsinks. The 4P1L is biased at about 115V Uak, 35mA and 560mA starved filament. Th headroom in this case is about 7dB. The load for the 4P1L is a cascoded DN2540 CCS. The PCB measures 266mmx274mm. It was much cheaper to order it on 1.6mm thickness and I provided many holes for support. One can use Aluminum stand-offs to make sure that the board stays rigid.

The only concern is how the 4P1L’s socket is attached to the board. I would use neoprene damper sandwich mount, rubber rings and even a dead weight ( in the pictures I just soldered the sockets to the board but I have to remove them and use the dampers..there is a hard to notice noise that comes from the very small vibration of the power transformer and the 4P1L is very sensitive to it).

Distortion levels are very low in the 0.0x % with a predominant 2nd harmonic. The frequency response is as flat as possible in the audio band.

For anyone who wants less gain there are few options. One can use  a step down direct feed line output transformer, such as LL1671/30mA. Another choice would be to use a parafeed line output transformer , such as Jensen JT-10k61-1M.

The sound is very rewarding. The DHT is complementing the solid state push-pull, and the resulting music experience is balanced, with great dynamics and nice sounding harmonics.

A possible BOM is listed below: