Oahu Tonemaster 230K amp-Change field coil to permanent magnet

[kagliostro]

A better version of the schematic



Franco

[DummyLoad]

Dummyload, what/where would you have used the choke and resistor?  Sorry but I'm confused.

Field COIL acts like a choke and a choke is a COIL - Field coil adds PS filtering, 230K field coil schema shows 612Ω DC resistance - Hammond is 157L 430Ω make up the difference with 180Ω puts PS B+ to 6V6 anodes at near same as the field coil. Won't have the same inductance as the field coil, likely lower, but I'd want the some of the PS LC filtering character and ripple reduction of the original.

How I selected: The schema shows 15V on 6V6 common cathode R of 200Ω so about 75mA at idle - add another 3-4mA for the preamps, and 8-10mA screen/anode when pushed. the Hammond is rated at 75mA conservatively, has a DCR of 430Ω, closest with similar current need and DCR, still short about 180Ω of the 612Ω of the field coil, that's the series resistor. If you're not comfortable pushing 75mA part, find a 100mA part, take that DCR and add what you need to make 612Ω. 

--Pete

[wsscott]

The P-TGC-001 choke I wound up using is a 4H, 50mA choke with 170 ohms of resistance.  So I added a 500 ohm, 5 watt resistor in series.

DummyLoad, What would the difference have been if I had used your selection?