Monamour 45???

[kevner21]

Can a monamour be changed to a 45 tube output?

[Paul Birkeland]

It could, but the output transformer impedance isn't ideal for the normal range of #45 operating points.  If you want to run the #45, the Stereomour II has some documented modifications to allow this and uses different iron that better supports the #45.

[Brissie]

Hi Paul,
I am thinking of converting my MonAmour amps to #45 tubes as I have quite a few good pairs laying around.
I have a spare set of OT2 transformers (I think they are 4K?) which are probably a good match for the 45?
What other changes would need to be made to be able to run a #45 tube in the MonAmour amplifier?
I assume the changes are similar to what you already recommended in the Stereomour II 45 Conversion thread but I suppose the MonAmour is a bit different circuit.
Thanks, Steve

[Paul Joppa]

Sounds possible. Let me give some thought to the operating point. A big resistor will make more heat under the chassis; PB may have some ideas about that.

[Paul Birkeland]

You'll need to change a bunch of resistors and figure out how to mount the OT-2s to the chassis. Don't forget to increase the filament dropping resistor to bring the filament voltage down and to replace the parallel feed capacitor to either a 6.8uF or 8.2uF part (you could probably leave the 12 for testing, but it would make sense to get an 8.2uF Solen to see if that makes much of a difference).

I don't know what you would do with the switch, as it could no longer serve any purpose that I could think of.

[Brissie]

Thanks Paul, Paul,
I got side-tracked for a bit but like to pick this up again.
Previously you gave the following guide for upgrading a Stereomour II to run with 45 tubes:

1.  The 3K resistor in parallel with each 2K resistor should be removed and replaced by an 8.2K resistor (a Vishay PR-03 will probably fit OK).  This adjusts the cathode bias voltage.
2.  The 130R/3W resistors in the power supply need to be removed and replaced with 600 ohm 5W resistors.  It may be easier to just replace the two resistors that go from the 5 lug strip by the power transformer to the other two 5 lug strips with 1.1K/10W resistors and leave the other two 130 ohm resistors where they are.  Your mileage may vary, let us know which way works best.  This change lowers the power supply voltage for the 45.
3.  The 0.15 ohm resistors need to be removed and replaced with 0.22 ohm 2W resistors (3W is fine too).  This will reduce the filament voltage to 2.5V when the 45 is plugged in (the 2A3 draws more current, so if you don't change the resistors the filament voltage will be too high).
4.  The black wire from each plate choke needs to be disconnected from the power supply and the red wire connected in its place.  This sets the plate choke to the higher inductance/lower current setting for the 45.
5.  The parallel feed capacitor should be increased to a 5uF/630V part (4.7uF is totally OK).  This works in conjunction with step 4.
The presence of the DC filament upgrade will change the value of those 0.22 ohm resistors.  If someone can report what filament voltage they are getting (DC voltage between pins 1/4) with the 45 and the DC filament supply, we can suggest  tweaks to those values.
I think this is everything.  If anybody buys the parts and performs the steps, let us know and we can direct you to some voltage measurements to check that things are operating properly.
I would also note that the shunt regulated driver stage might call for tweaking the power supply dropping resistors a little bit as well.  It may be that a pair of 1K/10W resistors and 2 of the 4 original 130R resistors might be a better combo in the power supply.

Obviously the MonAmour is a different circuit and uses different value components?
Would you be able to recommend the component changes?

Also would anything need to be adjusted in the driver (any changes to components on the CCS boards?).

Thanks, Steve.

[Paul Birkeland]

The OT-5 needs to be swapped out with an OT-2.  In my opinion this isn't all that bad since there's a shroud over the PC-5, so adding holes to the top plates doesn't have such dire consequences if you make mistakes.

Going off the Stereomour I conversion changes, the 130R 5W resistors in the power supply should be changed to 600R/5W resistors.  The 910R cathode bias resistor could be replaced by a 1.6K cathode bias resistor.  You'd need to not install the switch and its associated wiring.

The 0.1/3W resistor needs to go up in value.  Try 0.2/3W and measure the filament voltage to see what it lands on.  You also need to factor in that the AC ripple will add to what you see as DC, so getting to exactly 2.5 isn't exactly necessary.

The parallel feed cap could go down to 6.8-8.2uF.

PJ may have more ideas about properly tweaking the circuit.  The PT-10 and the PT-6 certainly aren't the same transformer, and he would likely at minimum want to see a complete set of voltages with the recommended resistor changes, then there would probably be other tweaks to get everything where it's supposed to be.