The oldest amp from Sepp, a Harvard 5F10. The "successor" of the Deluxe 5E3. For harp players, this circuit is (disregarding the Champ) the second most important foundation. Better than a 5E3 for various reasons.
The most obvious difference are the three inputs with good sensitivity decreasing from left to right - but only one triode, one 6AT6 for all three.
The power amp is now "fixed bias" and no longer "cathode bias". You can tell by the selenium cube under the lamp. Left at the fuse there's still the cap. And the red capacitor in the middle ... the attempt of a modification ... a "dull cap" how the americans love it. That was the reason for the non-functioning tone potentiometer. All components still have the correct values and functions, the electrolytic capacitors have been partially exchanged already.
Well ... a 230V transformer is needed. The ones from the Champ and from the 5E3 do not fit. The 290BEX neither. There's only one left, a universal transformer.
After a short panic, I soldered the HV output of the transformer with two diodes and one resistor in between to the original bias resistor and turned it on .... 29V to the grid. 27V are in the diagram, the tubes run at 25mA - that was easy, the estimated value of the resistor fitted perfectly. A little cleaning and a new switch .... there you go.
This amp is too valuable to modify, that's why it remains a guitar amp.