Stereomour II 45 Conversion - Anyone Do It Yet?

[Sugar Man]

Has anyone tried this yet? If so, how'd did it go? If not, is this still the latest thinking?

I'll have another crack at this, the list is kind of long:

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.

[Paul Birkeland]

I think there's still time for someone to be the first.

[Jamier]

I remember that Greg (gstitt) was working on it last year. You might ask him. Check his posts on page 3.

Jamie

[Sugar Man]

I think there's still time for someone to be the first.

I think I'll give it a shot, but I will definitely need some help from the experts. 

I will even try to resist the urge to start with boutique tubes and capacitors. Solens for the capacitors? Any recommendations for reasonably priced 45's?

[Paul Birkeland]

Yeah, keep it simple for now.  A used pair of 45s on eBay isn't going to be particularly expensive (once everything is working as it should, then consider the fancy new production stuff).

[2wo]

If you have any kind of tube friendly speakers, the 45 can be pure magic.

 My SR-45 is still my number one amp by a long margin...John

[Alby0521]

Hi all.

Is the quote of the 1st post the most recent ‘’guide’’ To convert Stereomour II into 45?
If I follow those instruction I’ll be able to run 45 on it?

Does anybody know what the correct reading of resistance and V I should have once I do the change?

I’ve been looking to do this change since a while but never got to it. Maybe this is the right time.

Thanks in advance.

[Paul Joppa]

We have not actually done it ourselves, so we can't say with confidence what the voltages will be. I can calculate expected voltages at the tube pins, but detailed resistances and voltages really must be measured on a working sample if they are to be believable.

[Jamier]

If you have any kind of tube friendly speakers, the 45 can be pure magic.

 My SR-45 is still my number one amp by a long margin...John

John, I remember that you said you listen to speakers with sensitivities in the mid 80's in conjunction with your 45 amp(s). Do you listen near field or in a larger area? The 45 version of the SII is something I have wanted to try for a while. I'm trying to get a handle on what to expect from 1.75 watts in a nearfield environment.I have 92db sens MTMS that I use with the 2A3 version and they play plenty loud.

Jamie

[Deke609]

I haven't found a post on it, so I'll add this to the mix: EML has a new "45B" tube that can deliver twice the output power of a regular 45. It can be found at jacmusic.com under Tubes/Products/ (the website architecture isn't entirely intuitive).  I don't have the technical knowledge to determine whether this tube would work with the SII, but the promise of a Stereomour 45 with no loss of power sure is appealing!

Here are the basic specs from the website:

EML 45B  Filament Ratings
Filament Voltage    
= 2.5Volt (AC or DC)
Tolerance on filament voltage   
5%
Filament Current    
~ 1.5Ampere
.
EML 45B Maximum Conditions
Plate Voltage    
420Volt
Plate Current    
53mA
Minimum Plate Dissipation   
8.5Watt
Continuous Plate Dissipation   
22Watt
Power Output in Class A    
5.2Watt
Grid resistor   
500k Ohm

 

[Paul Birkeland]

I haven't found a post on it, so I'll add this to the mix: EML has a new "45B" tube that can deliver twice the output power of a regular 45.
Continuous Plate Dissipation   
22Watt
Power Output in Class A    
5.2Watt

This will potentially put out more power than a 2A3, but you'll need a different power transformer with a much higher voltage winding. (Think the Kaiju HV supply and Stereomour LV supply)

You'll end up with an amp that will only work for the EML 45B.

It does look like the 45B would plug into a 45 modded Stereomour I/II and work just like a 45, but you aren't going to get any extra power.

[Deke609]

Bummer. So is the drop in power output going from a 2A3 (3.5W) to a 45 (1.75W) because the 45 is half as efficient as the 2A3? And therefore, in an SII, you'll never get more than 1.75 Wpc from a 45, regardless of how much power the 45 can handle?

[Paul Birkeland]

The 2A3's plate can dissipate 15W, while the 45 can dissipate about 10W.  The filament of the 2A3 is also substantially beefier than the 45.  It isn't a matter of efficiency so much as physical size and capabilities of each device. 

[Deke609]

Thanks PB

[Jamier]

PB, The EML website says that their 45s can have center tapped filaments(?) Can the SII be modified to use this version of the tube?

Jamie