What is the plate current through the 2A3 in stock Stereomour II?

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Deke609

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Both my SII's are partially dismantled so I can't measure and calculate this myself.

I just want to know what my starting point is with the stock circuit so I can maker a better guess about how to get 45-48mA with an EML45B.

cheers and MTIA, Derek



Offline Raymond P.

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Based on notes from my initial build, my SII 2A3 plate current is ~50mA. But based on voltage check specs, it should be ~48mA.

Raymond P.


Deke609

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Awesome. Thanks Raymond.

Edit: Given that the EML 45B can take a max of 53mA and has higher Rp than a 2A3, I think I'm safe to briefly try the tube with the stock/original power supply and cathode resistor, then take some measurements.  Very cool.  I'll test the reversion to stock power supply with Sovtek 2A3s before trying the EML 45Bs - just in case I make a wiring mistake.
« Last Edit: April 19, 2020, 12:55:12 PM by Deke609 »



Offline Paul Joppa

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I am looking at the 45B plate curves, and it appears that at the design operating point of Stereomour (300v at 50mA, -60v bias) the plate resistance of the 45B is 1100 ohms. That's pretty well suited to an OPT of 3000-4000 ohms.

Paul Joppa


Deke609

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Excellent! Many thanks PJ for figuring this out.  Much appreciated. I will inevitably play around with different loadings on the secondaries via different value resistors in parallel with the 200 Ohm headphones, but it is good to know that I am starting from a decent initial setup.

cheers and thanks, Derek






Deke609

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Got the amp reverted to stock power supply. With 120VAC incoming and the Sovtek 2A3's, I got 57.5V dropped across the 1.2K cathode resistor, making 48mA.  But with the EML 45B I got 61.3V, making 51mA.

I clearly don't understand Rp.  I would have thought that higher Rp (the EML 45B) would decrease current, not increase it. Perhaps the fact that the Sovteks were never burned in has something to do with it?

Anyway, time for a listen. My right ear is still a bit wonky but the Benadryl improved things a lot (thanks for the idea Doc!). I'm going to use a variac to reduce incoming AC to 118V - just to be on the safe side.

cheers, Derek




Offline Paul Joppa

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Have you adjusted the filament voltage? 2A3 needs 2.5 amps, 45B only 1.5 amps, so the series resistors would need to change between them.

Plate resistance is a dynamic resistance, usually written lower case rp to indicate small signal. The tube voltage divided by current is called beam resistance and is a static measurement RB.

Paul Joppa


Deke609

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Thanks PJ.

The filaments were previously tweaked for the EML 45Bs after doing the 45-conversion. So I left them as is (setup for the 45Bs), and simply reverted to stock B+. At 120VAc incoming, I get B+ of 380VDC. 

cheers, Derek



Deke609

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Yet another lesson to always read the manual/schematic: I messed up the plate choke wiring. Instead of red and blue for the full winding, I connected red and black, giving me what I guess was 15H instead of 55H.  That explains why it sounded off.  Back to the work table. 



Deke609

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@PJ or @PB: does it matter if I reverse the current direction through the chokes? I.e., if I connect red to anode and blue to B+?

Edit: and just be clear, I am wiring the Kaiju choke into the SII and so using the full winding.

many thanks, Derek
« Last Edit: April 20, 2020, 10:41:51 AM by Deke609 »



Offline Paul Joppa

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It matters a little. The choke is wound so that the blue wire is deep inside, where its capacitance to ground is small and controlled, and it is shielded by the rest of the winding.

Paul Joppa


Deke609

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Thanks PJ. I suspected something like that, so I rewired the chokes as per the schematic, with red to B+ and blue to anode.

Sounds superb. Soundstage is still smaller than the Kaiju (which has me thinking the 5670 circuit is the missing piece), but the bass and treble are fantastic.  I think I'll keep this setup until I'm ready to build the 45B amp with the other iron.  That other iron is now in the Kaiju and sounding quite fine. A different and equally enjoyable flavor. But the lower B+ needed to bring the plate current down in the Kaiju makes the amp play softer: had to go up a full click on the coarse attentuator on the Beepre.  I might get a pair of Lundahl 53H/80mA chokes just to see what the Kaiju with the amorphous core OPTs sounds like at the intended operating point.

In case it is of interest: the SII-45B amp with the Kaiju iron has the equivalent of 9uF parafeed caps. So about 3uF shy of the midpoint of PJ's 2*L/(Rprime*Rprime), but within the recommended range.  Sounds great to me.

cheers, Derek
« Last Edit: April 20, 2020, 03:54:34 PM by Deke609 »



Deke609

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On further listening to the SII-45B with Kaiju iron, 45 filament and stock B+ power supply, I'm a little less enthusiastic.

Likes: the revised amp is really well balanced from low to high, to my ears. Compared to the SII-45 with either stock iron or the Lundahl 70H/60mA and regular core headphone OPT, the Kaiju was so much better in the treble. With the Kaiju iron installed, the SII-45B amp comes very close to the Kaiju in the treble. And what it still lacks may just be a consequence of the Kaiju's bigger soundstage and instrument separation. At the same time, the Kaiju to my ears is more weighted towards the highs. Bass has tons of slam, but something about the presentation of the music always leads my ears/mind to focus on the treble. I'm more of a mids/low-end guy. With Kaiju iron, the SII-45B strikes a balance that I find really pleasing. So that's a big plus for me.

Dislikes: (1) it's a bit slow and, to use a expression of Doc B's, it sounds a bit "fat". It's as if there's a halo around all the notes. Maybe too much 2nd harmonic? (I could try to measure it, but my past attempts to measure THD turned out wonky. My scope seems to be really sensitive to the emi/rfi environment. And my place is filled with electronic stuff. LED lights seem to really throw the scope out of whack.)

(2) Narrow soundstage and lack of separation. Using headphones with the Kaiju, I can almost forget that I'm listening through headphones. Instruments and notes are placed in a 3D space around and outside my head. With the SII-45B, the soundstage is pretty much limited to the span between ear cups, and notes/instruments are often superimposed, making detailed listening more difficult. This doesn't appear to change with different secondary loadings. I've tried putting 17R in parallel with the 200R phones to get an effectivw 16R load to match my 16R OPT secondary wiring, as well simply plugging the 200R phones into the speaker outs. No appreciable change re soundstage and separation. Although the higher reflected load does seem to make the music a bit crisper, which I like.

Now, don't get me wrong, had I never heard the Kaiju, I'd be thrilled with the SII-45B with Kaiju iron. It's a significant step up IMO from the stock iron. But it's still no Kaiju. And notwithstanding that I find it a bit more treble-weighted than I'd prefer, I still much prefer to listen through the Kaiju.

I will eventually rebuild the SII-45B with different Lundahl iron and try to hit one of Jac's recommended operating points that reportedly produces -30dB second harmonic and 4+ wpc output.  And I'll use 5670s as drivers and for shunt regulation (any maybe even try to figure out how to use an EML 30A as driver, with some suitable single triode filling in the for 5670 for shunt reg).  Maybe that will give me the sound that I'm chasing: a lower centered Kaiju.

If that doesn't get me where I want to go, perhaps the 45 and 45B tube just doesn't fit the bill. And unfortunately I'll never know whether my earlier mishap of mistaking an 8R2 for an 8K2 when converting one of the SIIs to 45 damaged the tubes. I *think* not b/c the EML 45B's sound more detailed and better (to my ears) than my other pairs of regular 45s - including two pairs of NOS RCAs.  But who knows? I don't think I want to shell out for another pair of EML 45Bs just to test the question.

And maybe I'll just have to accept that the "perfect amp" doesn't exist and that there's always trade-offs. ... Nah  ;D

cheers, Derek
« Last Edit: April 21, 2020, 05:21:50 PM by Deke609 »



Offline Paul Birkeland

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If you want to use the 30A as a driver, you'd get rid of the shunt regulator entirely and load the 30A with a plate choke.

Paul "PB" Birkeland

Bottlehead Grunt & The Repro Man


Deke609

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Huh. Thanks PB. I hadn't really thought that through (still haven't). I figured that the high plate voltage for the EML 30A might rule out C4S loading, but hadn't considered its impact on shunt regulation.  Why no shunt reg? 

many thanks, Derek