801a Parallel Single Ended 12 Watts

[Paul Barker]

This use triode connected D3a at 150v 15mA coupled to parallel 801a’s with Hammond 10k//10k Bifilar 126C, which is limited to 15mA DC. D3a no cathode bypass cap, both channels shared cathode resistor. Facilitating some feedback. Reduction in gain not so great to matter, raised internal impedance r*mu+1 doesn't take the relationship with interstage outside design parameters. It is because of all this I decided on transformer coupling and triode mode. Many people like to use d3a pentode to blend the harmonic distortion at the front of a SET design. I chose this path instead to eliminate coupling caps. The output stage each valve ac heated out of phase with its partner to cancel 100hz hum naturally (simply too by the way). each channel shares cathode resistor bypassed with Russian oil caps -3db at 5hz from memory. OPT’s are 5k so equivalent to 10k with single ended. Power output 12 watts with 2nd and 3rd harmonic combined just under 1% at 12 watts. There is no facility to drive up to A2, a deliberate choice because the harmonic distortion in class A2 sky rockets lead by the odd order harmonics. The b+ supply choke input Black Gate Capacitors. b+630v max capacity 250mA. But the choke is beyond with a capacity of 400mA and 32 henry. The 4 801a’s are pulling around 35mA each. I would rather have more excess capacity in the power transformer, but such transformers are short supply these days, and to get said bespoke built out of my range.