Stereomour II 45 Conversion - Anyone Do It Yet?

[Deke609]

Electraprint will build custom plate chokes to whatever specs you want. I have purchased several. Jack is very knowledgeable and can advise you about what will work well with different tubes.
My favorite amp is a low power parafeed 845 with Electraprint iron throughout.

Michael

Many thanks Michael.  Some custom Electraprint plate chokes for twin Stereomour 45s could be a fun and manageable project. It's good to know that you've been happy with their products.

cheers,

Derek

 

[Deke609]

The second SiI arrived today and I've started the assembly. I want to give the new amp a minimum of a month of burn-in before bridging the two amps.

In the meantime, I'd like to convert my original SII to 45 and listen to the EML 45Bs. Do PJ's instructions set out on the 1st page of this thread capture the first steps?

Since I will be placing a parts order with Digi-Key, are there any other values of the quoted components that I should pick up in anticipation of having to make some tweaks? Factoring in shipping costs, it will likely be cheaper for me to order a scattershot range of component values in one go than to make make a series of more targeted orders for fewer components.

Many thanks in advance,

Derek

 

[Deke609]

...

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).

...

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 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.

@PB - I have both the DCF and SR upgrades. So I'm thinking my best approach is:

(1) leave in place the two 130Rs that connect 34L/31L and 30L/27L, remove the other two 130Rs and add wire leads to their termination points to make it easier to insert different replacement values (1.1K or 1.0K 10W - any other foreseeably likely values?  E.g., 950 or 900?)

(2) order a range of values for replacing the 0.15 ohm resistors - would 0.18 - 0.25 in 0.1 increments be a safe bet? Or is their some other range you would guesstimate?

Many thanks in advance,

Derek

 

[Paul Birkeland]

There's no reason to expect the 1.1K value to be a problem, you shouldn't need to try different values.

Since you are going to start with AC heating on this build, the 0.15 ohm resistors in the stock build can go to about 0.25 ohms instead (2W is OK). 

When you get to the DC filament installation (which you should do dead last), you can start by installing the resistors provided, then let us know what DC voltage you see across the filament pins.  When I dialed in the kit design, I was ordering resistors in 0.02 ohm increments. 

-PB

 

[Deke609]

Many thanks PB - and just to clarify: I am converting my existing SII that already has the SR and DCF upgrades installed. Is that problem, or would you suggest building the new kit as a 45 from the start?

My thinking is that I'll get a quicker sense of the 45 sound with my original SII b/c the caps and other components are already burned in. And I believe the EML 45Bs come with a partial factory burn-in.

 

[Paul Birkeland]

Ah, that changes things a little then.  I would probably just remove the resistors added on the 4 pin socket during the DCF upgrade and then let us know what DC voltage you have after you've done that. 

 

[Deke609]

Many thanks PB - So just so I have this, I am to:

(1) remove the four 0.13 ohm resistors to cathode pins 1 and 4 added during the DCF upgrade;

(2) leave the 0.15 ohm resistors (for now) on cathode pins 1 and 4;

(3) take DC voltage measurements to determine what R value replaces the 0.15s.

Is that right?

Many thanks,

Derek

 

[Paul Birkeland]

Yes, and don't be too annoyed if I come back and suggest 3 different resistor values to buy, even with your measurements in hand. 

 

[2wo]

Isn't the point of the 45b  is that it can be run at a significantly higher operating point than a standard 45 looks 420v at 53ma max.  This is close to 2A3  maybe you could try adjusting the cathode resistor might be all you need to start ...John

 

[Doc B]

We have been developing a 3.5 watt 45 tube too.

 

[Paul Birkeland]

This is close to 2A3  maybe you could try adjusting the cathode resistor might be all you need to start ...John

He'd also have to adjust the filaments a bit too because of the lower current consumption. 

 

[Deke609]

We have been developing a 3.5 watt 45 tube too.

Yeah, that was a common response to the 45B on a lot of threads: the reinvention of the 2A3!  Having not even heard a 45 tube, I am taking a flyer with the 45B. We'll see. Hoping for some of the 45 magic I've read so much about.

 

[Doc B]

My take on this is that if you want to hear a 45, buy an old school RCA 45. If you want to hear a 45B, buy a 45B. And understand that they have been made for different conditions and will most likely sound different because of that. We went through this with all the new production tubes that came out with 300B in their designation back in the late 90s and early 2000s. Most sounded very good but all sounded different, particularly the superhero versions.

 

[Deke609]

Isn't the point of the 45b  is that it can be run at a significantly higher operating point than a standard 45 looks 420v at 53ma max.  This is close to 2A3  maybe you could try adjusting the cathode resistor might be all you need to start ...John

He'd also have to adjust the filaments a bit too because of the lower current consumption. 

Are you guys suggesting that there might be a way of operating the 45B for more than 2Wpc in the Stereomour?  I had assumed from PB's posts (quite possibly erroneously) that getting more than 2W from the 45B requires a higher voltage/current combo than the SII iron can provide.

EML provides a number of suggested operating points for the 45B. For 2W output: 275V and 36mA. The next higher suggested operating point is 3.1W via 384V and 41mA, which according to the website is one of the better performing operating points.  I've not found a published curve for the 45B, so I have no idea if there are good operating points between 2 and 3.1 watts.

 

[Paul Birkeland]

Are you guys suggesting that there might be a way of operating the 45B for more than 2Wpc in the Stereomour?

Yes, but you were also very interested in using the full 40H from the plate choke, which is not possible when operating under the 2A3 conditions.

A matched pair of old stock #45 tubes can generally be bought on eBay for well under $100.

 

[Deke609]

Yes, but you were also very interested in using the full 40H from the plate choke, which is not possible when operating under the 2A3 conditions.

A matched pair of old stock #45 tubes can generally be bought on eBay for well under $100.

Thanks PB. Yes, that's true - I am interested in hearing more low-end, which I take it requires switching from 20H to 40H to get the added bandwith. But I would also be interested in hearing the 45B's with more than 2Wpc just to keep some headroom. If the 3.1W operating point is an easier modification, could you help me with that to start?  And then after a month or more, I could move on the mod outlined in this thread?

many thanks,

Derek

 

[Paul Birkeland]

PJ would have to comment on how much plate current the choke will take before saturating.  Even the 3.1W operating point may push the entire winding a little too far.  Also, going from 2W to 3.1W isn't quite a 2dB increase in level, so not exactly earth shattering.

 

[Deke609]

... going from 2W to 3.1W isn't quite a 2dB increase in level, so not exactly earth shattering.

Thanks PB. Yeah, not a ton of extra headroom, but every little bit helps.  I think a 45B amp at 3.1W would make for a fair comparison with a 2A3 amp at 3.5 - I won't get the bass extension, but the 45B is supposed to have a very nice midrange, and I can listen for that and not need to factor in differences in headroom between the 2A3 and the 45B.  I have in mind 3 different comparisons:

(1) 45B/3.1W vs. 2A3/3.5W - roughly equal headroom

(2) 45B/2W vs. 2A3/3.5W - for whether I like/want the added bandwidth - I may pick up some regular 45s for this as well.

(3) one 2A3/3.5W amp vs. bridged 2A3/7W two-amp combo - for benefits of 3dB of added headroom over my existing single 2A3 amp

Based on the above experiments, I can decide on which configuration/compromise suits me best: bridged pair of 45Bs @ 6.2W (more headroom) or @ 4W (greater bandwidth), or a bridged pair of 2A3s @ 7W.  If greater bandwidth wins the day, I may even consider parting with my LCD4s and looking for a used pair of the LCD4z.  Man, the LCD4s like a lot of juice.  Example: vol. pot settings listening to Rush's Tom Sawyer at the perceived same loudish volume level: LCD4 @ -6dB vs. Focal Elex @ -24dB.  And that's with the BeePree fully open. at -9db. With the BeePre fully open, I turn the SII down to -18dB on the LCD4s, but I hear what sounds to me like a bit of distortion or auditory halo around notes (which works quite well with the bloozy rock I often listen to)

I should clarify again, for others that who might read this, that I am not trying to solve a perceived performance problem. My BeePre/Stereomour II combo is absolutely killer with my LCD4s. IMHO, anyone with headphones that are hard or moderately difficult to drive should consider the SII w/ the DC Filament upgrade (but I recommend getting the other 2 as well - the Shunt Regulation upgrade is particularly amazing).

cheers,

Derek

 

[Paul Joppa]

PJ would have to comment on how much plate current the choke will take before saturating.  Even the 3.1W operating point may push the entire winding a little too far.  Also, going from 2W to 3.1W isn't quite a 2dB increase in level, so not exactly earth shattering.

My measurements indicate 45mA as the limit for the two chokes I measured. I have been rating the PC-3 as 40mADC (full winding), which is a little conservative and allows for some variation in manfacturing.

Using the 45B, you could probably not change the high-voltage power supply - just make the other changes. You might see 310v or 320v plate to cathode, rather than 275v. With the 1600 ohm cathode resistance, I estimate 41mA would be drawn, and you might get 2.6 watts output. Plate dissipation about 13 watts - don't try this with a standard 45!

For a little perspective, I note that the EML operating points are higher voltage without much higher current; consequently they are optimized for much higher impedance output transformers (6500 or 9200 ohms, vs. 4000 for Stereomour). While I agree this is a good idea, it is not usually done with the standard 45, which is limited to 275v maximum plate to cathode.

 

[Deke609]

Many thanks PJ.

So initial steps for the 2W+ conversion for EML 45B tubes are (please correct me if I'm wrong):

(1) Replace the 3K cathode bias resistors in parallel with the 2Ks with 8.2Ks to get 1.6K combined;

(2) Leave the 130R resistors in the power supply alone.

(3) Remove the 0.13 ohm resistors that are in parallel with 0.15 ohm heater resistors.

(4) Connect red wire of plate chokes in place of black wire - do I simply insulate the exposed end of the black wire and leave it disconnected?

(5) Swap in 5uF or so 630V parafeed caps.

(6) Measure DC heater voltages and report back for tweaking the heater resistor values.