45s in steromour II ?

[Paul Joppa]

The output transformers in Stereomour II are 4K ohms, and will run with 45s very well.

Conversion to the 45 is moderately simple, but there are a few non-obvious things involved. Some of the resistor values must change, and to confirm the right value we'll have to make the changes ourselves and measure the results. I THINK this is the list of changes:

Reduce power supply voltage (increase the 130-ohm 2-watt resistors)
Adjust the filament voltage (increase the 0.15 ohm resistors)
Change the cathode bias resistors
Use the higher-inductance tap on the plate choke
[Replace the parafeed output cap - optional]

Incidentally, I originally designed it to have some other output stage options, not just the 45. It's not clear which of them we might support, at what level of support. So I won't say more just yet.

[rockdoc]

sounds good, thanks Paul!

[lordvader]

What operating points are the 2a3's running at (so I can math the rest) ?

[Paul Joppa]

The target operating point for the 2A3 in Stereomour is 300v plate to cathode, 50mA plate current, and 60v bias.

The 45 should run 275v, 35mA, 56v bias. Be aware that at the reduced current, the PSU voltage will increase slightly.

[rockdoc]

Conversion to the 45 is moderately simple, but there are a few non-obvious things involved. Some of the resistor values must change, and to confirm the right value we'll have to make the changes ourselves and measure the results. I THINK this is the list of changes:

Reduce power supply voltage (increase the 130-ohm 2-watt resistors)
Adjust the filament voltage (increase the 0.15 ohm resistors)
Change the cathode bias resistors
Use the higher-inductance tap on the plate choke
[Replace the parafeed output cap - optional]

Incidentally, I originally designed it to have some other output stage options, not just the 45. It's not clear which of them we might support, at what level of support. So I won't say more just yet.

Paul, any updates on the 45 conversion? This thread from a year ago is the most recent I've seen on it. Thanks!

Steve T

[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.