45s in steromour II ?

[Paul Birkeland]

The power supply voltage available needs to go from 380 to 350.  The Stereomour I used a total of 1200 Ohms of series resistance in the power supply to accomplish this, you could start by replacing the four 130 Ohm/2W resistors with 600 Ohm 5W resistors.  There's plenty of space in the layout for this.  If you find that you don't quite have room for four 5W resistors, you can leave one 130 Ohm/2W resistor in each channel and use a 1.1K dropping resistor in place of the other 130 Ohm resistor to get your voltage drop, but it should be a 10W part.

The filament voltage is dropped from 3.15V to 2.5V through two 0.15 Ohm resistors. I would try switching these to 0.25 Ohm resistors.  Be prepared to check your AC voltage after you do this and adjust as necessary.  If I was assigned this task, I would buy 0.22, 0.25, and 0.27 Ohm 3W resistors, but I still wouldn't be surprised if I needed a different value.

The cathode bias resistors need to be changed to 1.6K/10W resistors.

The higher inductance tap on the plate choke means disconnecting the black wire and connecting the red wire in its place for each plate choke.

The parallel feed cap should go somewhere around 8-10uF.

If you end up doing this, please post the voltages that you end up with after the changes (including your value for the filament voltage resistors and resulting AC voltage), and PJ can look it over and see what might need to be tweaked.

 

[rockdoc]

Thanks PB!

Steve T

 

[Paul Joppa]

Parafeed cap should be around 5uF.

The stock power tube cathode resistance is 2000 ohms in parallel with 3000 ohms, giving a net 1200 ohms. For 1600 ohms, you can replace the 3000 ohm resistance with 8000 ohms, keeping the 2000 ohm resistor. In the practical real world, 8200 ohms is close enough, and a 2 watt or greater rating is adequate. I chose this design specifically for the possible 45 conversion. Later we decided that there were likely to be very few conversions actually done, so in the interest of keeping the manual simple we did not describe the change.

A third possibility at the initial design stage was to get 1000 ohms (by replacing the 3000 ohm resistor with a 2000 ohm resistor) for use with a 3000-ohm output transformer such as the one used in Kaiju, or a Magnequest TFA-2004. This has some other complications, serious enough that we are not planning to pursue this option at this time.

 

[Paul Birkeland]

Parafeed cap should be around 5uF.
[...]

A third possibility at the initial design stage was to get 1000 ohms (by replacing the 3000 ohm resistor with a 2000 ohm resistor) for use with a 3000-ohm output transformer such as the one used in Kaiju, or a Magnequest TFA-2004.

Oops, I used 3K when calculating that parafeed cap value.