Strange circuit Echolette B30 amp

[cfortner]

Hi,

a friend of mine brought me a not working Echolette B 30 (built around 1960). Hum, 1,5W output, oszillation, it don't work really.

I'm wondering about this schematic. Grid 2 of the valves EL500 is connected (connection marked with 1 ) to the center tap of the power transformer hivolt secundary winding.

The point marked with 1 isn't at ground level, isn't it? I don't understand the funktion of this wiring, because the center tap for me is AC, not DC.

I attached also the schematic of the B 35 from 1966, there grid 2 is attached at anode voltage-level over 1k and fixed BIAS is applied to grid 1. Seams to be a better solution.

But how does the circuit of the B 30 work?

Thanks

Christian

[sluckey]

The voltage at point 1 is DC, not AC. It is half the dcv at point 2. The circuit you understand is not always a better solution.

[PRR]

Bogen did the same half-voltage tap on several very excellent PA and motor amps.

That's not your hum and oscillation.

[cfortner]

Thanks! The amp was „repaired“ earlier, now I have to check all these issues.

May be the taps of the pt are attached in a wrong way.

[PRR]

Nevermind the PT. What DC voltages do you have at plate and screen?

[cfortner]

320V plate, I have to measure the other voltages