New Here: Just Ordered Stereomour!

[InfernoSTi]

Hi guys! 

I'm new to the forum and thought I ought to make a first post. 

A little background, I have a small number of little tube amps: an original MiniWatt, a Jolida FX-10, and a 6CL6 SPUD I built from Jerry's kit over at Hawthorne Audio.  I wanted to try out a Bottlehead kit and felt the next obvious step would be an 2A3/45 SE amp.  Since I enjoyed the process of building my SPUD very much, I figured the Stereomour would fit the bill perfectly!

I'm used to small watt amps, so power isn't really a concern to me (I listen in a small room with 96 db speakers).  My question is are there any recommendations on going with the 2A3 verses the 45 tubes?  I'm fairly novice to the tube world but have read good things about the 45's, so my uneducated leaning is that direction at the moment. Anyone care to reinforce that idea or steer me in the 2A3 direction instead?  The WHY you recommend one over the other is of interest.

Great forum, by the way!

Thanks, John

 

[Paully]

This isn't meant to sound snarky, try it yourself and tell us!  I read tons and tons when trying to decide whether I should order the Paramount 2a3 or 300B version.  Agonized over it for some unknown reason.  I went 300B.  But I picked up some old 2a3 and had to try them out along with the Shuguang 2a3c.  I now believe for me that the difference between the two isn't that big of a deal so I stick with the 300B, might as well have the extra power.  But the smart bet would be on me switching back sometime just because I can.  You may decide otherwise as it regards 2a3 or 45 but you will never know for sure until you hear it for yourself.  And since you are getting an amplifier that does both and are officially a Bottlehead, you are required by the bylaws to experiment and report back.    Have fun whatever you decide.  Sorry I can't answer your question directly but if I was in your shoes I would build the 2a3 first and listen.  Buy a pair of old stock 2a3 (or the Shuguang 2a3c) and then listen to those.  Pick up a pair of 45s when you can and then give them a listen and just see what you think and also pay attention to how well the 45s drive your 96db speakers given the music you prefer.  Then you will know.

 

[Paully]

By the way, is ordering it with 45 tubes even an option?  I don't think it is.  So from my perspective all the more incentive to just go ahead and build it for 2a3 first since you will have those tube anyway and then start your experimentation.

 

[2wo]

I think you get a set of 2A3

 

[Grainger49]

I have only heard a few 45 amps.  And I heard them on speakers that were happy with that power.  They are addictive.  If you think you will do well with a 45 output tube go for it ! ! ! ! !  

Upgrading the iron with the 45 in mind will make a substantial difference too.

 

[InfernoSTi]

Guys, you make excellent points...and the logic of "it comes with 2A3s so try that first" is self-evident.  I'm glad to hear of success using the HAs with a 45 amp.  Thank you for all the quick and thoughtful responses! I feel welcomed to the Bottlehead community indeed. John

 

[Paul Joppa]

When you get the Stereomour, read over the instructions for switching tubes. There are four large power resistors that are disconnected for use with 45s - you might want to install them in a way that allows disconnecting and re-connecting easily. If you settle on the 45 permanently, you can just clip the leads and remove those resistors.

While you are experimenting, remember you can plug a 2A3 into a Stereomour that is set up for 45s. Performance (power, mostly) is in between but closer to the 45. It's a third option. The reduced plate dissipation will extend the life of a 2A3 somewhat.

 

[InfernoSTi]

Paul,

That is great advice on doing some selective "pre-switching" planning.  And I am interested in the idea of running the 2A3s in the 45 configuration.  You say the power is the main issue?  What sonic compromises would you anticipate?  If they are truly minimal, that might be a great alternative for those of us who aren't needed the extra (I'm laughing as I write "extra" meaning 3.5 watts instead of 1.75 watts) power.  The ease of swapping from one to the other would be a great convenience (pending no significant sonic issues)!   

John

 

[jrebman]

And PJ, to add to that last question, what changes may be recommended to the PF cap?  Leave it in the middle at 3.3, or shoot for something closer to 5 uF?  Would you also change the plate choke?

Thanks,

Jim

 

[InfernoSTi]

And another question (this may be answered somewhere on the boards already, so I apologize for a double post if that is the case):

Is there any worry about using the EML 2A3 tubes in the Stereomour?  I assume not, but wanted to ask to be sure.  From what I'm reading, there are some very nice 2A3 tubes that folks are comparing to 300B and Type 45 tubes (generally speaking), and one of those is the EML 2A3-S solid plate.  However, some of the other amps are requiring a soft start board for those particular tubes.

Thanks, John

 

[jrebman]

John,

The EMLs should be fine in this design as it is cap coupled, and not direct couped like the Paramounts.

I've not heard the EMLs yet myself, but I will say that the Shuguang Treasure 2A3zs in my Fi monos are outstanding.  They do take forever and a day to reach their best sound, but when they get there...

Don't poo-poo the Sovteks either, they are actually quite decent, and the Sophia Premium 2a3 balloons and JJ 2a3-40s can also be really nice in the right system.

-- Jim

 

[InfernoSTi]

Thank you, Jim.  I will look up these tubes...that will give me something to do while we wait!  I am still at the deciding to go 2A3 or 45 route thought process...after reading up on some of the 2A3s, I may be sticking with that tube for a while!  I will take a look at the Shuguang Treasures.  John

 

[jrebman]

John,

On the Treasures make sure you lookfor and get the Premium Grade A varety, and don't buy them on ebay or overseas.

Should have specified the mesh plates for the Sophias.

I've heard both 45s and 2a3s, and I think the 2a3s just give a slightly more harmonically rich tone -- to me, instruments just sound more like real wood, brass, etc.  There are others who will say the same of the 45 too, so you're just going to have to experiment.  Those Hawthornes are tone monsters, so the 45 may indeed end up being the better choice.

-- Jim

 

[Paul Joppa]

I've been out of town for the Fourth (yes, the cabin is still standing!) so I'll try to answer several questions.

Running a 2A3 in a Stereomour set up for the 45 will run the tubes at very close to the same operating point - 35mA and 275v plate to cathode. The lower plate resistance of the 2A3  would normally make me want a slightly higher load impedance; the 4K standard is 0.8 times my target. But my choice of load impedance is pretty conservative; many commercially-available amps use an even lower impedance. In fact, the Paramour II upgrade runs at 0.75 of my target. In both cases, the large plate choke inductance minimizes tube distortion at the lowest frequencies, compensating for the slight distortion increase of the low impedance. Then lower impedance will also raise the efficiency. I have not done a full set of measurements yet, so I will only guess at 2 to 2.5 watts for the 2A3 in 45 mode. At a guess, the power and distortion differences will be no more and possibly less audible than the intrinsic differences between the tubes.

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.)

I still do not know what to say about the EML tubes. They do not like certain kinds of startup transients - EML is quite straightforward about this on their web site, and they have been helpful in identifying and diagnosing problems. In most cases, a cap-coupled circuit such as Stereomour is considered safe with those tubes, as long as the time constant is short enough. Many people with many different cap-coupled amps are very happy with these tubes. Our time constant is 25mSec. I would say that is "normal" while a few more exotic amps have longer time constants, up to 5 times as long. I do not have enough detailed information to do a design that is "known to be safe" - I kind of suspect nobody does. That critical time constant probably depends on several other variables, which is probably why nobody will specify a specific universal safe value. I am not going to purchase ten pairs of EML tubes in order to do destructive testing to discover those limits! So, while I think it very unlikely there will be any problems, I won't make any promises and Bottlehead won't cover your expenses.

The PC board in Stereomour will be the same, and can be configured for, a startup time delay - but the parts are not included and I still have some cautions about the EML tubes with those boards. Even with the startup delay, the amp needs to stay off for a long time once turned off - I am saying 30 minutes at this point - in order for the slow start to re-arm itself.

Hope all that is helpful.

 

[jrebman]

Thanks, Paul.  I've got some decent but cheaper caps to play woith for the PF, but when the right value is found, if it happens to be any different than 3.3 uF, then I'll probably want to put some jupiters in there.

I've also been toying with the idea of a 12 sn or sl7 as a driver, but of course I want to hear the stock design first.  One thing that is potentially interesting about the 12sl7 is it's low heater current -- 150 mA -- and the possibility for a DC filament.  Looks like it will be close, butmay work.

-- Jim

 

[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.