Falstaff Hybrid Circlotron Power Amplifier is an excellent design but in truth too heavy and also very expensive so I thought of a lighter version …a kind of small Falstaff…………. R O B I N !!!!
I tried to simplify the schema and reduce the number of components but without penalizing the audio quality too much. I found a little magician: the E180F tube ( or russian equivalent 6j9p ).
This gold pin, long life little pentode in triode connected perform state of art features: high amplification factor ( mu=50 circa ), low plate resistance (rp around 3 Kohm ) and very linear anodic characteristics.
A perfect partner for this tube is the interstage transformer Lundahl LL1660 SE 10mA, connected in this case in mode ALT-V but with the two primary section in series ( 4.5 : 2+2 ) to ensure high inductance ( 130 Henry ) and consequently great bass and an overall flat 25Hz-40KHz bandwith. In the input voltage amplifier stage I use a LED cathode bias.
No more 10mA of current must flow In the E180f ( and primary interstage transformer ). You can choose another type of LED ( red, orange, green…) to achieve this. In this stage the anode voltage is well filtered by a mosfet ( STP2NK90z, zener protected ) capacitor multiplier. The output capacitor, on Mosfet source, is an MKP type ( 10uF, 400V ).
HV power supply consist in a 160 V CT at 100mA, a 6X4 full wave 7 pin tube rectifier and an CLC ripple filter ( first 1,5uF-400V MKP type capacitor ). The output voltage is 160 V shared to both R-L channel. For E180f filament and circlotron mosfet bias I use a classic, slow rising, LM317 power supply with four 3 A schottky diode as bridge rectifier. The 1 kohm trimmer set the output voltage to 6,3 V.
For the power stage circlotron are needed two 225 VA toroidal power transformers with two separate secondary each ( 25 V at 4,5 A ). The electrolytic capacitors in the following schema are audio grade ( Kendeil, Elna, Nichicon… ) 10.000 uF- 40 V and the resistors 0,22 ohm – 5W. For best audio performance use schottky diode otherwise normal 25 A bridges are OK.
Caution is never too much: I add one per channel uPC1237 board DC protection: a relay disconnects the speaker in the event of a failure in the final stage. Remember Circlotron is a floating amplifier ( no ground common output connection ) therefore we need two protection board powered by two 12 V transformer ( the two green transformer in photo ).
For setup the amplifier refer Falstaff: is the same but here the current must be set at 1,2 A.