Stereomour - Stereomour II

[M42]

I was wondering if anyone who has owned or heard both the Steromour I and II might give me their impressions of the two. Is the SII a "significant" sonic step up from the SI?

[Paul Birkeland]

The Stock circuits are very similar.

The Stereomour II will have a dramatically lower noise floor compared to the Stereomour I when the DC heater supply is added (this is not an upgrade that's compatible with the S1)

The  Stereomour II driver will be as refined as the Paramount 1.0 driver with the shunt regulated driver upgrade.  This is also not compatible with the S1.

I also think the S2 is a bit easier to build and has more room for swapping out parts down the road.

[Alonzo]

Okay, here's a completely non technical review. YMMV/IYBITIT,etc.  The stock SI is more "tubey" and clips more softly than the SII.  The SII (stock) is detailed and more direct (better response on vocals, especially high notes) and has crisper bass (bass notes aren't muddled, you can hear individual strings strummed on a standup bass, they don't muddle together for example).  These are the notes I have after building both (before I ripped them apart to change items).

Yes, the SII is a significant upgrade to the SI.  But it depends on your ears. (But, and a very big one at that, the SI can be set up as a 45 amp and it's just glorious.  Beats my SII every day of the week (planning a 45 SII if I can get Mikey to make some iron).

[M42]

Paul, Alonzo, thanks for your replies. The 45 conversion sounds intriguing. I haven't had much time to research what's involved...is it a difficult conversion, would detailed instructions be available? The only reason I get these kits together is due to the superb instruction manuals and the help on this forum.

If memory serves I used the 4 ohm tap when I wired the amp. I think this was to lower the noise floor somewhat? My listening room is very small, my Frugel Horn's are fairly inefficient (87db?). My Reduction has been replaced by an Eros.

Given these circumstances, in your opinion, would the 45 conversion be a viable option?

[Alonzo]

So, I like my SII better on Frugal Horns, I have the doctored FE126en's, but I hear they are even better with his other drivers.  The conversion from 2A3 to 45 is in the SI manual.  No instructions for the SII.
You should try the 8 ohm tap, see if you hear a difference before you consider the 45.  You lose maybe a half watt or so and some of the depth in the lower registers with a 45 amp.  The FH3's I have would sometimes need a sub using the SI 45 but not with the SII 2A3.
But there's a caviat to this, none of my amps are stock, I have different outputs, chokes, added chokes, etcetera so my system (and hearing) will be different.
There's lots to do to a stock SI to change the sound before you step up to a SII.

[Paul Joppa]

Several different subjects in this thread - here are some random notes about the design ...

As PB pointed out, the Stereomour II is basically the same circuit as the original Stereomour; the main design idea was to allow more upgrades, so it can reach the same level of performance as the Kaiju (but at half the power).

However, the one audibly significant improvement in the stock version was to the 12AT7 driver stage, which now has more current - 5.5mA compared with 3.8mA originally. This gives it a bit more drive capability. More important in my opinion, it allows a smaller cathode bias resistance, which in turn allows that resistor to be unbypassed.

The Stereomour II was designed to accommodate a conversion to 45s, but we have not put together instructions for that - mostly because few have asked, and because we have not yet done the conversion ourselves. The conversion would involve changing the filament resistors, changing the cathode bias resistors, changing the power supply resistors, changing the tap on the plate choke, and changing the parafeed capacitor. All these parts changes are readily accessible. On the plus side, the bass goes a half-octave deeper; on the minus side the power goes down to 2 watts from 3.5 watts.

If one is happy with a Stereomour I set to 4 ohms and driving an 8 ohm speaker, the power would be about the same with a 45 using the 8 ohm tap driving the same 8 ohm speaker. The noise floor issue for very efficient speakers can be resolved with the DC heater board, if it is a problem in practice.

[Sugar Man]

PJ, It's time for the SII 45 !!! Like M42 above, I too need detailed instructions to avoid releasing the dreaded blue smoke.

[Paul Birkeland]

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.

[Natural Sound]

PJ, It's time for the SII 45 !!! Like M42 above, I too need detailed instructions to avoid releasing the dreaded blue smoke.

Hopefully you aren't in a hurry. I've been waiting since 2016. I'd even be OK with a description without the detailed instructions. I've been dreaming of a Stereomour II DHT 45 desktop system for a long time. In the meantime my SEX amp is filling the void adequately.

EDIT: looks like PB was busy while I was typing. Thanks PB!

[Alonzo]

Look like it's time to get another SII, time to go meet with the CFO!