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Moondog 2A3 Monoblocks


Blackbird

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Bruce,

 

I found the same thing -- activity definitely seems to be less; and what I do find seems to focus a bit more on what was rather than what is.  My wife and I were just talking about how the years have gone by so quickly.

 

But, as we acknowledged, both EP and MQ are outstanding.  Electra Print was also responsible for the DRD (Direct Reactance Drive) designs that Welborne sold towards the end.  Those amps were also unbelievably quiet, and the quality of the kits even better, IMO, than the earlier Laurels and Moondogs.  Electra-Print first published their DRD 300B circuit in Vacuum Tube Valley Magazine.  My point is that knowledge of single-ended design using these historic triodes was/is significant, and that fact could only be an advantage to their wide range of products -- from complete amplifiers and circuit designs, to an extensive line of both push-pull and single-ended transformers, power transformers, plate chokes, grid chokes, etc., etc.

 

Welborne's change from MagneQuest to Electra-Print was most definitely NOT a compromise in terms of either build or sound quality.  And just for the record, when Vacuum Tube Valley did a comparison of some of the more well-known single-ended amps from that time period, it was Moth Audio's direct-coupled stereo 2A3 amplifier that came out on top.  Moth Audio sold me their last chassis for the design, as well as all the parts needed to build that amplifier from the schematic.  While the amp does not use an Electra-Print power transformer, it does use EP output transformers.

 

 

 

 

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Hi Jeff,

 

You observed the following with regard to the (purely subjective) benefits of two stage amps and direct coupling:

 
"Once this is understood, why would anyone waste their time, money, and life ..........using approaches that are second best in  topology"
 
It's not a waste of time, money, and life if someone  prefers something else, such as a three-stage amp with a parallel-feed final (or muesli mixed with unfiltered extra virgin olive oil instead of milk or yogurt!).
 
 And what about well designed OTLs?  The school of thought that the otherwise essential lump of metal hanging on a valve amp's output stage is evil and must be banished forever!  Or other audiophiles who think I'm an absolute fool for listening to horns with outrageous efficiency, driven by amplifiers weighing nearly 50 pounds that groan (or hum) and struggle to pump out one single watt!
 
I guess my belief is that there are no absolutes.  The Moth amp might have been judged best overall in that case, but that doesn't mean that any of the others might not have come out on top under different circumstances, or with other listeners.  And you and I and thousands of others know that interstage and direct coupling were around long before Moth Audio, as are numerous other topologies.  And that's part of the journey, isn't it?  
 
And hey, how did that new amp turn out?  Are you listening to it now?  Why are your impressions?
 
 
 
 
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Hi Jeff,

 

I absolutely built the Moth Si2A3.  I didn't have all the parts for it.  It was actually my (absolutely incredible!) wife who contacted Craig U., and talked to him about the possibility of our buying the chassis, OPTs, power transformer, and other passive parts in order to build it myself.  Moth never sold their products like this, but it's how we were able to acquire what at the time would have been difficult financially.  And he agreed, sending everything in the beautiful wooden flight case with the Moth Audio logo branded right on the top of the box.

 

I was in the process of doing some rewiring of the regulated DC filament supply for the 2A3s in order to use AC current and hum null pot, when I had to have major back surgery.  Shortly after that, after my mom passed away in Tucson, my wife and I decided to move to Arizona to be closer to my dad.

 

The Moth was a very quiet amp with DC heaters, and I also used it as my main headphone amplifier.  And yes, we love the sound of it as much as its looks.  I've used with both our 15ohm Lowther PM2A drivers in a large, back-loaded horn, as well as the Klipschorns we had at the time.   I will say that direct coupling between its paralleled input/driver stage (twin triode) and 2A3 sounded very detailed, with sharp leading edges; and that is a presentation I really like.

 

You wrote above that " Adding resistance in series to a power supply, Eric,  is the exact opposite of what I strive for.  ( See Dr. Halijak 's letter, page 8,  of my KT88 build thread )"  I totally agree with that.  As you know, series resistance like that is often found between filter cap sections; and it was through the building of Dr. J. Lessard's parallel-feed Horus 2A3 mono blocks over ten years ago that I also gained an appreciation for a choke in the position instead of a resistor.  In fact, that's one of the modifications I'm doing to the pair of Welborne Laurel IIs I have here right now.

 

Simple can most definitely be a good thing; and direct coupling in a 2-stage amp such as this is surely simple.  The Moth is not a straightforward amp to wire, however, since there are three levels of wiring that are all dependent on one another.  The finished product was well worth the work though!  I'm sure I posted images of it here quite a few years ago.

 

 

Very glad to hear your amp is up and running and passed the smoke test!

 

erik

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"and it was through the building of Dr. J. Lessard's parallel-feed Horus 2A3 mono blocks over ten years ago that I also gained an appreciation for a choke in the position instead of a resistor."  I wonder Eric what your thoughts are now on Jeff L´s amps?  I still use the 2A3PP he built for me in my main listening system and have to admit I still love it!  my only source of displeasure to this day is a constantly humming hammond trannie underneath, mounted on the front.  let me know your thoughts.  warm regards, Tony

JFL2A3 bottom.jpg

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I do hope that Dr. Lessard is off enjoying his work somewhere in the world. I've searched off and on for him for years now.

 

Plus, I still thought of building the Seth monoblocks (not likely at this stage of my life), but I can't find the schematics and drawings that Magnequest used to host.

 

Bruce

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25 minutes ago, Marvel said:

I do hope that Dr. Lessard is off enjoying his work somewhere in the world. I've searched off and on for him for years now.

 

Plus, I still thought of building the Seth monoblocks (not likely at this stage of my life), but I can't find the schematics and drawings that Magnequest used to host.

 

Bruce

http://bjl.audioconcept.free.fr/Archives/01_Tubes&Transistors/01_Amplis_Tubes/2A3/P.P/2A3_s240_Manual_v1_Seth.pdf

 

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3 hours ago, Marvel said:

I do hope that Dr. Lessard is off enjoying his work somewhere in the world. I've searched off and on for him for years now.

 

Plus, I still thought of building the Seth monoblocks (not likely at this stage of my life), but I can't find the schematics and drawings that Magnequest used to host.

 

Bruce

 

I believe I downloaded alot of the SETH stuff. I also have the PDF if you need it.

 

 

seth_2a3_6N6Pi_v2.jpg

seth_2a3_schematic_v3_650.gif

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Jeff,

 

I regrettably sold those amps a long time ago.  I'm fairly sure you are trying to obtain some. mechanical decoupling with pyramid washers?  

 

In turn, I would like to suggest you try splitting. power supply duty and AC output signal between a large plate choke and small, parallel-feed output transformer -- connected in parallel (aka "shunt") feed fashion; and, instead of the usual grid leak resistor. in the finals stage, use a very high impedance grid choke instead.

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18 hours ago, Marvel said:

Thanks Ian... I'll get everything downloaded tonight. Thanks also to CWF, for the link to the French site. I've downloaded other stuff from there over the years.

 

Notice there is a version 1 and 2 of the SETH!  v2 is the correct version.

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  • 2 weeks later...
On 7/11/2019 at 5:41 PM, Blackbird said:

e technician told me that according to the schematic, two resistors were wrong.  R10 was suppose to be 2K not 1K and R13 was suppose to be 1.8K not 1K.  Other then that they sound incredible! 

Hi Blackbird,

 

Apologies to begin with here, but the person who worked on your Moondogs (and from the looks of it did a good job), is NOT correct about resistor values.  R13 is actually composed of a parallel pair, which on the parts list will be shown as resistor A and resistor B.  However, you will also see that they have the same resistor reference number as related to the schematic.  Parallel resistors will double the power-handling ability of that particular part of the circuit, but the value of resistance will be reduced -- just as two 8ohm speakers connected in parallel will reflect a new nominal impedance of 4 ohms.  Connected in series, those same speakers would double the impedance load to 16 ohms.  Resistors are the same.

 

So.  Had R13 been in another position in the circuit, such as the heater supply, the value would have been much more critical.  As you mentioned above, the amps sound good to you.  However, it would be wise, in my view, to correct this situation - and it's an easy fix: the present 1.8kohm power resistor can stay exactly where it is (no need to remove it).  Get an additional 1.8k ohm resistor of the same kind and power rating, place it on top of or along side the one that's already there, join corresponding leads, crimp, and solder.  Clip excess lead, and that's it.

 

So glad you were able to get a pair of these fine amplifiers! :)

 

Oh, I meant to say it would be very easy to conclude the resistor values were incorrect if one were not familiar with the particular amp circuit in question.  So, this is absolutely NOT saying anything against the tech!

 

I poked around a bit and found the following regarding Moondog parts layout.  I was going to dig out my old Moondog manual, so this link saved lots of time.  R14 was shown twice, so is a typo.

 

http://i-vol.com/welbourne_labs_moondog_2A3/moondog_info.htm

 

IMPORTANT:  I see I completely forgot to mention that the same is true for R10!  This is much more critical to the circuit since R10 forms the cathode/filament bias resistor.  There is heavy current going through this area, and it's important to make sure, Blackbird, that the resistors used have BOTH the correct value (2K) and power rating.  I am nearly finished repairing a pair of Welborne Laurel mono blocks for a friend, and used a 50 watt chassis-mount resistor in this place for better heat dissipation.  So once more for R10:  IF THE TECH USED A SINGLE 2K POWER RESISTOR FOR R10, HAVE HIM ADD ANOTHER ONE IN PARALLEL WITH IT FOR A FINAL VALUE OF Ik.

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5 hours ago, erik2A3 said:

Hi Blackbird,

 

Apologies to begin with here, but the person who worked on your Moondogs (and from the looks of it did a good job), is NOT correct about resistor values.  R13 is actually composed of a parallel pair, which on the parts list will be shown as resistor A and resistor B.  However, you will also see that they have the same resistor reference number as related to the schematic.  Parallel resistors will double the power-handling ability of that particular part of the circuit, but the value of resistance will be reduced -- just as two 8ohm speakers connected in parallel will reflect a new nominal impedance of 4 ohms.  Connected in series, those same speakers would double the impedance load to 16 ohms.  Resistors are the same.

 

So.  Had R13 been in another position in the circuit, such as the heater supply, the value would have been much more critical.  As you mentioned above, the amps sound good to you.  However, it would be wise, in my view, to correct this situation - and it's an easy fix: the present 1.8kohm power resistor can stay exactly where it is (no need to remove it).  Get an additional 1.8k ohm resistor of the same kind and power rating, place it on top of or along side the one that's already there, join corresponding leads, crimp, and solder.  Clip excess lead, and that's it.

 

So glad you were able to get a pair of these fine amplifiers! :)

 

Oh, I meant to say it would be very easy to conclude the resistor values were incorrect if one were not familiar with the particular amp circuit in question.  So, this is absolutely NOT saying anything against the tech!

 

I poked around a bit and found the following regarding Moondog parts layout.  I was going to dig out my old Moondog manual, so this link saved lots of time.  R14 was shown twice, so is a typo.

 

http://i-vol.com/welbourne_labs_moondog_2A3/moondog_info.htm

 

IMPORTANT:  I see I completely forgot to mention that the same is true for R10!  This is much more critical to the circuit since R10 forms the cathode/filament bias resistor.  There is heavy current going through this area, and it's important to make sure, Blackbird, that the resistors used have BOTH the correct value (2K) and power rating.  I am nearly finished repairing a pair of Welborne Laurel mono blocks for a friend, and used a 50 watt chassis-mount resistor in this place for better heat dissipation.  So once more for R10:  IF THE TECH USED A SINGLE 2K POWER RESISTOR FOR R10, HAVE HIM ADD ANOTHER ONE IN PARALLEL WITH IT FOR A FINAL VALUE OF Ik.

 

Thanks, erik2A3.

 

As you know I am not good with a solder, I will talk to my technician to see when I can send them back for him to make the modification.

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Hi Blackbird,

As I mentioned, the fact that I pointed this out is, again, absolutely not a reflection of the skill and or knowledge or competence of the person who refreshed some of the caps in your amps.  It’s important for me to emphasize that.  If there is one design I know inside and out, it would be these Moondogs, and I just had to point out this easily-made and understandable misinterpretation.  Making the corrections will be good for the health of the amps in the long run.  You have a great example of the species to enjoy - hold on to them! ;)

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erik2A3, there is absolutely no misunderstanding on your part and I understand it perfectly.

 

I have to say that after having read many of your posts, I would say that you are an extremely knowledgable person when deal with with these type of amps.  I have a lot of respect for you and when you say something, I listen carefully.  Your explanation of these resistors in my Moondogs and what need to be done are very clear and I understand it well.  Thanks again for taking your time to explain the issue to me.

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