I am trying to imagine the best scenario for the OT of a PSE 4P1L amplifier. From different plate characteristics posted on the web (old and new), and an optimal operating point of 250V, 35mA, -21V one can calculate the Rp(Ra) of 4P1L. IT varies between 1200 and 1400ohm. I would say that an optimal primary OT for PSE 4P1L would be 2.5-2.8Kohm. I opted to look rather at a 3K trafo than a 2.5k. After measuring my tubes most of their Rp is somewhere between 1400-1500ohm (at the operating point shown above). Here are my findings:
Make Model Pmax SE (W) Gapped (mA) Iprmax (mA) Primary Secondary Frequency range Weight (lB) Price$/pair
Lundahl LL1620 13W @ 30hZ 80 3.3k, DCR 308 ohm, 45H, 11mH leakage 4/8/16, DCR 0.8ohm for 8ohm 7hz-25khz +/- 0.5dB 5.5 520
Lundahl LL1664 10W @ 30hZ 100 ( not sure if they can gap at 70) 200 3k, DCR 72ohm, 17H, 8mH leakage 8, DRC 0.5 ohm Not available 3 320
One Electron UBT-3 15W @ 50Hz 110 110 3k, DCR 286 ohm, 17H 4/8/16 20hZ-40khZ, -1dB 5 220
Electra Print Custom order 5W 70 3k 8 Min 25Hz-20khZ, -1dB ? 350
Audio Note 144-V2 15W 90 90 2.5k 4/8 20hZ-20khZ, -1 to -1.5 dB ? 230
Edcor GXSE15-6-2.5K 15W ? 120 2.5k, DCR 95ohm, 6H 6 40hZ-18khZ, <1dBu ? 75
Edcor CXSE25-8-3.2K 25W 200 3.2K, DCR ?, Lpr ? 8 20hZ-20kHz, <1dBu 10 166
James JS-6112HS 10W @ ? 70 ? 2.5K/3.5K 8 29Hz-50kHz, -2dB 3 300
Tango XE-20S 20W 80 160 2.5/3.5K 8/16 25Hz-90kHz, -2dB 4.66 660
As always compromises have to be considered. In order to balance price and sound quality balanced with all the components, schematic (so we are not creating a weak link), I would consider LL1664 (if they would gap for 70mA), Electra Print and One Electron UBT-3. Also Audio Note can be considered as it has good reviews.
I am experimenting now with a UBT-1 that I had available. I started with a quad 4P1L that is appropriate for the UBT-1 (1.6K/4, 8, 16). To implement filament bias at 21V and 2.5A is too challenging from a heat management point of view. Despite that I would like to see a quad 4P1L, I had to go for a PSE 4P1L where the filament bias will dissipate acceptable levels of power on Rf (21V*1.37A=28.77W) to achieve the desired sound.
If you are interested in building this amp, I am sure that the challenge of choosing the best fit OT came to your attention.
What is your best fit and experience?
Best Regards,
Radu