New Here: Just Ordered Stereomour!

[Paul Joppa]

The 12.6 volt winding should be good for 0.5 amps, so a DC heater would be practical. Note that the winding also serves as a shield and should be grounded. In the stock design, the 12AT7 filament center tap is used to ground that winding.

[InfernoSTi]

Well, upon lots of consideration, I intend to stick with the 2A3 design. 

That being the case, I was able to pick up a set of four RCA 2A3 tubes dating from the early 1940s to compare to the Sovteks that come with the amp.  I also picked up a Mullard CV4024 (military type 12AT7) that should match well with the RCAs.  Since we all love the A/B comparisons, I figure this will be a good place to start!

In the meantime, I am building some DIY room treatments and have a 3 pack of Owens Corning 703 4" rigid insulation and some nice natural color burlap coming in. Gotta keep busy with a project!

[Paully]

Very good choices, almost too good.  To get any better you are going to be searching for Telefunken 12at7 and single plate 2a3...  But seriously, you have some good stuff there and you are definitely going to enjoy it.

[InfernoSTi]

Thanks, Paully, I appreciate the vote of confidence.  Now I know what to keep my eye out for, too!  I appreciate the tips!  Just got lucky getting these from a guy who was selling his 2A3 amp and had his "stash" to get rid of.  Funny how that works out just right sometimes.

John

[Thoburn]

Calculated uptimum values for the parafeed cap are 2.5uF for 2A3, 5uF for 45 or 2A3 in 45 mode. The range from half to twice the optimum is considered suitable, which is why the stock 3.3uF is used for either tube. A further consideration is that the smaller capacitor protects the tube from distorting due to low load impedance at infrasonic frequencies, which is of course more likely to be triggered with a lower-powered amp - hence a slightly small parafeed cap with the 45 is part of the plan. Experimentation with these values is quite reasonable, because of the interactions possible with real-world speakers.

(Parenthetically, I have posted a different but similarly exotic reason for the 3.3uF in Paramour I and II where that value is larger than the theoretical optimum. The cynical will have noted that all these ideas lead in the end to the same cap value of 3.3uF  :^)  I maintain nevertheless that this is just a fortunate coincidence. Really and honestly, I do.)

Hi Paul,
From the picture on the bottlehead site it appears these caps are Solen. Given what you say above and given that I am building the 2A3 circuit would it be advantagous to use a 2.5 or 2.7 uF value in an upgraded cap such as the Mundorf silver/oil? Pardon me for being a nub. FWIW, I will be driing a Lowther DX4 in a horn cab.

[Paul Joppa]

Within the range I talked of (half to twice the "theoretical" value) it's hard to predict what value will work best in a given system. The reason is the interaction of the cap and plate choke with the speaker's impedance, which in most cases fluctuates quite a lot in the deep bass. I can generalize - tighter, cleaner, but not as deep bass with the smaller cap - but I can't be confident that's what you'll hear in your room with your speakers.

As usual, I recommend you build it stock first, then experiment with different caps if you are interested.  The difference between 2.5uF and 3.3uF will be small; the difference between a Solen and a Mundorf may well be larger, subjectively at least.

Here's what I'd do if I wanted to get a lot of information: I'd build it stock, and give it at least 100 hours of music to be sure the cap is broken in and my ears have become accustomed to the sound of this amp. Then I'd swap in the Mundorf, again for at least 100 hours. Then I'd add the Solens back, in parallel with the Mundorf, and try to hear the effect of the larger capacitance. The capacitance effects will be mostly in the deep bass, while the capacitor sonic differences would likely be throughout the audio band and especially in the treble. Based on the results of those trials, I'd choose the one I liked the best, or decide what other things to try.

[Grainger49]

I would make the suggestion that you experiment with cheaper caps of different values first.  Then when you zero in on the value that reaponds best invest in the high dollar caps.  I'm a fan of Obbligato Film and Oil caps, their cheapest.  You could use a hand full of those to decide on the value then go to the Mundorfs.

[Thoburn]

Thanks Paul and Grainger49 for the excellent advice on the caps.

[InfernoSTi]

Well, I want to give a big "THANK YOU" to Bottlehead for their Stereomour kit!

Excellent instructions, an easy build, and 30 feet of wire!  I put mine together after work Wednesday, Thursday, Friday, and quite a bit on Saturday and Sunday. 

My resistance checks were all spot on and my voltage checks were tolerable (but not as close as I would have liked...more on this in a bit).  Since everything was within 15% and there wasn't any "magic smoke" released, so I plugged in my OB Hawthorne Duets and oh WOW!!! 

Rich, smooth, music with amazing detail, special placement/soundstage, silky highs, and buttery mids.   Everything I want in a tube amp!!!  And I can't wait until it breaks in a bit (I'm typing after about 30 minutes of operation on the stock tubes).  My hum is only 0.28 mA on the right channel and 0.33 mA on the left channel.

So my only worry is that a four of my readings are right on the edge of acceptable (the vast majority of reading are either spot on or within 6-7%).  However, I have a handful that are 10-15%, so I figure I'd ask folk's opinions to see if I should worry (or just grab an adult beverage and listen to some music and stop nit picking!).   I need to mention that my mains VAC measures 123 VAC (not the test 119VAC).

Here are the handful of high readings (just the big ones, not the 6-7% readings):

Terminal     Expected     Actual   Variance %

T2             230            207        -10%
T14           230            194        -15%

OA            230            207        -10%
OB            230            194        -15%

Should I worry?  It is on the ragged edge but everything sounds WONDERFUL!!!!

Thanks a bunch!
John

[Paul Joppa]

You got NO PROBLEM.

It's a good illustration of the complexity of making good tests. The points you identify are the driver plate voltage. Because the driver is running under fixed bias, this point will vary more like 20% or even 30% - 2 or 3 times the variation expected at more stable points. This is because of variations in individual tubes. It's just too confusing to try to say this in the manual.

If you look at the schematic, you'll see that my theoretical values for these points were 200 volts. The actual first production unit measurements were 230 volts, so that's what is in the manual - we use real measurements rather than theory. Your measurements are much closer to the theoretical value, which is based on the published tube curves, so I think you have just validated my theoretical calculations - thanks!

:^)

[InfernoSTi]

You got NO PROBLEM.

Thank you for the quick reply, Paul.  And your answer is music to my ears!  I didn't think the Stereomour could sound this good and have a problem!  

It's a good illustration of the complexity of making good tests. The points you identify are the driver plate voltage. Because the driver is running under fixed bias, this point will vary more like 20% or even 30% - 2 or 3 times the variation expected at more stable points. This is because of variations in individual tubes. It's just too confusing to try to say this in the manual.

If you look at the schematic, you'll see that my theoretical values for these points were 200 volts. The actual first production unit measurements were 230 volts, so that's what is in the manual - we use real measurements rather than theory.

I appreciate the detailed background, and I'm certain others will too.  This starts to help me understand the what and the why (and gives me a place to start looking for more info on the topic).

Your measurements are much closer to the theoretical value, which is based on the published tube curves, so I think you have just validated my theoretical calculations - thanks!

That's what we are here for, Paul. You just keep dreaming up great designs and we will keep proving them up in practice!  

Cheers!  John

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[InfernoSTi]

I have about 60+ hours on the Stereomour now and I wanted to post what I believe will become a very familiar posting as more folks build the Stereomour: the first few hours were really sweet but then I hit about 40 hours of "tightness" in the music.  At about 50+ hours, the sweetness started coming back and in a serious way!

The bloom on this amp is distinctively noticeable.  However, I don't feel there was only one area that was more notable; it is generally across the board.  That being said, what caught my attention first were the cymbals; I had one of those "hey wow, that sounds really nice, I need to pay attention" moments. 

I assume the burn-in can be noticeable for a few more hours, say 100+, but after that I assume it will settle into a nice, steady state where I can roll a few tubes and such and not worry that changes in sound are burn-in, rather than due to the changed tubes?

John

[Grainger49]

John,

The labeling you did under the chassis will be good when you went to tweak.  Not to mention it can help you find your way through the circuit.  I'm experienced but label everything.

The Solen Parafeed caps will be 98% settled out in 100 hours.  More expensive, and often better sounding, caps take longer.  I am burning in Teflon caps for 3 weeks now to get rid of the hard and congested days.

I have the first generation Paramours, two little "mono blocks."  And I am very happy with the sound.  I am as sure as you that anyone with appropriate speakers will love the Stereomour.

[InfernoSTi]

Grainger,

Thank you!  It is great to hear that you still enjoy your Paramours...that is the real test of sound quality if you still have them!

The labeling felt a bit odd writing on a beautiful blank chassis but it was a great help while building.  I hadn't thought about it, but it will help down the road, too.

Thanks for the info on the Solen's.  I think I will be rolling different tubes over the weekend just because I can't wait any longer!  I have to confess I have already swapped in a JJ 12AT7 and it was a really nice tube.  It has a signature that is pleasing to my ear. I will be trying some old RCA 2A3s and JJ 2A3-40s, as well as some NOS 12AT7s (a few of the classics). 

I have also ordered some Auricap 3.3uF caps to swap out with the Solen's at some point.  Based on the postings, that seems like a popular swap for a (relatively) small price.

I can attest that Hawthorne Audio + Stereomour = Heavenly sounds!!!  I am really loving the Stereomour's signature coming through the HA's!

John

[Grainger49]

There is a thing I say, and often, about expensive caps.  They are like seasoning.  I might like cinnamon you might like nutmeg.  At the cost of some caps, over $100 each, you can be horribly disappointed with the wrong ones. 

To heap insult upon injury the more expensive the capacitor the longer it takes to get to the final sound it will produce.  The more expensive Black Gates, Mundorfs, V-Caps and any Teflon cap can take weeks to come to rest as far as the sound goes.  Less patient men may not get there.

Tell us more about the Hawthorne Audio Duets, I'm not familiar with them.