Stereomour II Transformers

[faskenite]

I am interested in the Stereomour but saw that someone on the board said the other day that the Kaiju transformers are superior to those in the Stereomour.  I am fully stocked on the 300B amp front, but I know transformers matter.  Is there an upgrade for the standard Stereomour transformers?

[Paul Joppa]

It's a possibility we have not (yet, at least) made into a reality. Both sets of iron are designed to the same philosophy and in general sound similar at their respective design points.

Most SE transformers are rated for power down to 30-40Hz. The current Stereomour output transformer is rated 4 watts at 25Hz. The plate choke is a bit over 20 henries. This iron complement is a large improvement over the now-obsolete Paramour from which Stereomour was derived.

The Kaiju transformer is rated 8 watts at 22Hz - so it would handle 4 watts down to about 15Hz. The plate choke is 40 henries at 75mA when used with a 300B, but has a tap to give 55 henries at 65mA.

From these numbers, you can see that the Kaiju iron, when used with a 2A3, would comfortably go deeper in the bass - but there is little musical information that low. I still want to try it and see (hear) what the real-life difference might be, though.

The conversion would require a decrease in plate voltage and an increase in 2A3 plate current, because the Kaiju transformer is 3000 ohms, vs. 4000 for Stereomour. The damping factor would also be lower because of this difference. Preliminary estimates have been made of what component changes would be required, but as I said, we have not (yet) built one to confirm the values, and determine if the differences are enough to justify the investment. For what it's worth, the chassis plate does have suitable mounting holes.

[faskenite]

That's really interesting, thanks very much for this Paul.  My initial sense is that I don't need an SET that goes to 15 Hz.  I wonder if my subwoofer does, or it does whether that could matter musically, as you indicate.  I recall that when I first got my sub (a pretty good REL) I was primarily struck by the fuller lower frequencies it provided to instruments generally thought of as being in the 'mid-range' - John Lennon's voice is an example I recall pretty distinctly.  But could we possibly be talking 15-20 Hz as far as that is concerned?  In John's dreams!  (RIP!)

[Paul Joppa]

For what it's worth, the old Paramount would handle full power down to about 30Hz, and the Kaiju (same basic circuit) as I said went to 22Hz. Plate choke went from 24 henries to 40 henries. The difference was quite audible. Of course there were a few other changes as well, so it's hard to be sure, but it's my opinion at present that the iron change is the main thing in the deep bass.

[Deke609]

I still want to try it and see (hear) what the real-life difference might be, though. ... we have not (yet) built one to confirm the values, and determine if the differences are enough to justify the investment.

@PJ: Happy to be the guinea pig on this. I'll happily purchase the Kaiju iron and if you provide some initial install instructions, I will take any and all measurement values you ask for to tweak the install.  I'd be willing to spend some serious time on this.

For listening A/B comparisons between the SII and Kaiju OTs, it would be interesting to have the Kaiju OT on one channel and the SII OT on the other. I use headphones with my SII and could quickly swap channels to hear any differences.  My HPs (Audeze LCD4s) have great bass extension, so I think they would highlight any sonic differences between the two irons.

[Paul Birkeland]

Another issue with putting Kaiju iron on the S2 is that the Kaiju output transformer is 3K and the S2 output transformer is 4K.  This will require some tweaking of the operating points of the 2A3 to nudge the plate voltage down a little and the current up a bit as well.

Another option is to only use the Kaiju plate choke in conjunction with the S2 output transformer.  In my experiments with this combo, I was -2dB at 8Hz.  This, IMO, would be the better allocation of funds without going too overboard (Kaiju iron is EXPENSIVE!!!).

[Jamier]

[quote author=Paul Birkeland
Another option is to only use the Kaiju plate choke in conjunction with the S2 output transformer.  In my experiments with this combo, I was -2dB at 8Hz.  This, IMO, would be the better allocation of funds without going too overboard (Kaiju iron is EXPENSIVE!!!).
[/quote]

Buying the iron alone would be less expensive than buying the whole kit just to pull the iron out, which you know some might be tempted to do if the iron isn't sold seperately. Not me, of course. At least I don't think I would.....
.....well maybe. Let's face it, no one makes money selling self-control.

Jamie

[Deke609]

Another option is to only use the Kaiju plate choke in conjunction with the S2 output transformer.  In my experiments with this combo, I was -2dB at 8Hz. 

That's a cool idea!  PB - what's the comparator dB value at 8Hz for the SII (stock), if you have it?

... without going too overboard ...

Yeah, true ... but to quote Springsteen,  Blinded by the Light, "But Mama, that's where the fun is!" 

[Paul Birkeland]

The stock iron is about 1dB down at 20Hz. 

[Deke609]

PB - I just discovered this post you from about 2 weeks ago on another thread, stating that the Kaiju PC/SII OT pairing is only down 2 dB at 6 Hz!

Yes, parallel feed output splits the duties handled by a typical series feed capacitor into two individual components (the plate choke and the parallel feed output transformer).  In testing the Kaiju plate choke and the Stereomour output transformer together, I am down about 2dB at 6Hz.

So -2dB at 8 Hz or 6 Hz, either way that's pretty amazing. 

I would love to swap in the Kaiju PCs - is there any way of making this happen? 

Many thanks,

Derek

[Paul Birkeland]

I had to go back to my notes and double check, but it's 8Hz, not 6Hz.  There are no plans currently to offer the PC-5 for the Stereomour, though we have discussed it in the past. 

[Paul Joppa]

There are several options for putting Kaiju iron into a Stereomour. One of the reasons I have wanted to try it out is to see which options might be worth documenting...

Here are three simple ones:

1. Just swap the iron, and increase the parafeed capacitor - no other changes. The power will increase  slightly, to around 4.5 watts, but the tube distortion will also increase slightly.

2. In addition to swapping the iron and cap, change the 3K cathode resistors to 2K (leaving the existing 2K in place). This changes the total bias resistance to 1000 ohms, from 1200 ohms stock. The plate dissipation will increase to 18 watts which is OK for the Sovtek 2A3 but exceeds spec on original-style 2A3s. You'll get the same ~4.5 watts without the increased distortion.

3. The full change also reduces the high voltage power supply, to restore the 15-watt plate dissipation and 3.5 watts output. This is similar to what is done for the 45.

With respect to just changing the plate choke: the Kaiju choke has a 2A3 tap providing 60 henries (Kaiju gets 40 henries, stock Stereomour gets 20 henries). The choke, in conjunction with the parafeed capacitor, sets the small signal bass extension.

The output transformer sets the overload point for magnetic saturation - in other words, the large-signal bass extension, also called "power bandwidth". The stock OT-2 (Stereomour) saturates at about 22Hz with 3.5w output. The OT-5 (Kaiju) at 3.5 watts saturates at 15Hz. This magnetic hysteresis distortion falls as frequency increases in the octave above the saturation point so there are measurable effects up to 44Hz and 30Hz respectively.

[Jamier]

PJ, option 3 seems to make the most sense as it allows the use of any 2A3 and it isn't much more work.The extra watt in the other options just doesn't seem to be worth the tradeoffs. In all the options mentioned you  are implying that both the PC and the OTs  are being changed, right?

Jamie

[Deke609]

I'm game for trying out any or all of those options.

[Jamier]

Doc, would you consider selling Kaiju iron on a presale basis?

Jamie