Laurel twins repaired/refurbished

[erik2A3]

Helping rebuild/refurbish a forum friend's Welborne Labs Laurel II mono blocks.  A tremendous amount of cleanup; many feet of solder wick; complete recapping of all electrolytic, metallized film bypass, and film coupling caps.  There was terrible hum and noise in one channel, which turned out to be a heater to cathode short in one of the 6AC7 as well as a bad rectifier socket.  Many cracked wires, including capacitor leads, etc.  A main issue was over-heating in the 300B filament supply, which was replaced by a relocated full-wave bridge rectifier and CRC filter section (all rebuilt with paralleled resistors for DCR of 1ohm).  300B 5V filaments had been running 4.3VDC, and are now corrected.

 

Once wiring and parts replacement where needed was finished, there was still an inexplicable 60Hz hum in one channel.  I worked literally hours on this problem, and finally found the source:  hiding under a 3.3uf Solen cap was a wire from the 6.3 VAC heater supply (for the paralleled 6SL7 and 6AC7) leading directly into the grid.  That corrected, I have to say these Laurels are now probably the quietest single-ended amps I have.  I'm also going to take a picture of the MagneQuest 300B output transformer. They are absolutely huge, the largest output transformers of this kind I've seen.

 

And so, with one more step (new 4 pin finals sockets), I will be very sorry to see these shipped back home this coming week.  They are lovely sounding amplifiers; and if you have heard that the 300B is euphonic, syrupy, overly-warm, etc., this circuit thoroughly and comprehensively dispels that myth.  With the schematic in (basically) in my head, I'm going to build a pair after I finish up a couple of other project.

 

Forgot to mention: A large choke was used to replace the common resistor in the power supply between the main filter sections for CLCLC. Wonderful amps!

[Marvel]

Absolutely beautiful, Erik! Even with your own amps in the house, I am sure it will be difficult to get these boxed up and shipped.

 

Bruce

[erik2A3]

Thanks for your kind comments Bruce.  I have been extremely happy with the results of this work, which I did when I had time and my wife was able to take a turn with watching our 11 month old black lab puppy - who at one point suddenly jumped up, and grabbed (with her mouth, of course) one of the input stage tubes.  She ran all over the house with it, keeping it just out of my reach while I chased after her.  There was some minor damage around the base, but I am sending back with the amp a couple of 6SL7 replacements.

 

I have a sort of special place in my heart for Welborne Labs amplifiers, and over the years have had the the chance to build kits for others, as well as repair work.  This pair of Laurels were a significant involvement.  You can see the signs of severe heat stress on the solder board terminals in both amps, and the parts (voltage dropping resistors and caps) associated with that problem were in really bad shape.  All was removed and replaced with more robust power handling ability and greater separation between passive and active components.  All AC current, such as the AC heater supply for the driver stage and the full-wave-bridge rectifier for the 300B, were moved to the opposite side of the amp from the signal input.  Where the amplifiers had been terribly noisy at first, they are now much, much better -- the only audible sound being a clean and quiet hiss from the tweeter (with ear right up the the horn).

 

These amps have changed hands a number of times, and in all cases had the 60Hz heater supply hum issue I mentioned above - the result of an incorrectly connected wire.  Music has an organic quality that I find somewhat lacking in my other 300B amplifiers.  I have always found the topology of the input/driver stage to have a definite influence on the voicing of an amplifier, and, with the generally conventional implementation of the 300B final stage, I think the paralleled sections of the 6SL7 and 6AC7 metal envelope valve are key components of the sound of this amp.  Unlike the Welborne Labs Moondogs we're so familiar with, the heater supply that's shared by both front-end tubes are floated approximately 120VDC above ground (with center tap of the 6.3V supply connected to cap-filtered approx. 120VDC source directly off the 5AR4 rectifier.  I encountered this one other time in a Bruce Rozenblit (now Transcendent Sound) line stage preamp design I built in.....let's see....1993, I think it was, from a schematic published in Glass Audio magazine (now sorely missed!).  It's amazing to me that that was nine or ten years before I first logged onto this forum!

 

There was also the problem associated with the epoxy that holds the solder terminal boards to the chassis.  I had the same issue with my Moondogs, where the epoxy used had either been old or not entirely well/correctly mixed.  Each Laurel Monoblock had a few boards just sort of hanging in space with wires connected to them.  Anyway,  a well-worth-while project that took more time than I anticipated.  I thank the owner for his exceptional patience -- especially with my possibly a bit overly-meticulous approach.  I just have a high standard for BOTH safety and performance, and want to make sure what I do is the very best I can do.

 

So, to the Papa of these awesome Welborne Labs Laurel II mono blocks:  Thanks for your patience and allowing me to work on these classic SET amplifiers!  

[Delicious2]

5 hours ago, erik2A3 said:

... I thank the owner for his exceptional patience -- especially with my possibly a bit overly-meticulous approach.  I just have a high standard for BOTH safety and performance, and want to make sure what I do is the very best I can do.

 

So, to the Papa of these awesome Welborne Labs Laurel II mono blocks:  Thanks for your patience and allowing me to work on these classic SET amplifiers!  

 

Words fail to describe the amazingly generous journey Erik has taken me on through my amps!  He is unparalleled in his constancy of communication about his progress, in his knowledge about every component of the circuit and their interaction... and yes Meticulous! - an artist really!  I'm excited to hear what you've wrought Erik!

 

thanks a million!😀

[erik2A3]

You are welcome!  Glad to help get these great amplifiers up and running well again!

 

Jeff:  There is indeed a pecking order in a queue of several such projects, and I just told my wife yesterday that the Moth Si2A3 rebuild was next!  After that I decided it would be fun to build an amplifier consisting, ONLY, of parts I happen to have on hand right now, and using our heavy-duty cheesecake spring-form pan for the chassis.  Thanks for remembering the Moth!  Visually, it's definitely one of my favorites.  I'm primarily interested in re-working the power supply.

 

 

 

[Blackbird]

On 8/18/2019 at 8:54 AM, erik2A3 said:

I thank the owner for his exceptional patience -- especially with my possibly a bit overly-meticulous approach.  I just have a high standard for BOTH safety and performance, and want to make sure what I do is the very best I can do.

 

I can’t even begin to describe the level of dedication that erik2A3 had.  Such a remarkable feat!

[erik2A3]

I really enjoy this kind of work, and the reward for me is in the process - as well as of course the outcome!  The amplifiers are now on their way back home, and this is the part - shipping - where I sometimes get a little nervous....

 

Tubes aglow!

 

 

[erik2A3]

...and so it came to be that the Welborne Labs Moondogs inherited the same platform as the Laurels above, and thereby making use of half sections, only, of two 6SN7 input and driver tubes -- whereas the tube compliment of the Laurel II consisted of either a GZ37 (one of my favorites) or a 5AR4 rectifier, paralleled sections of a 6SL7 (just like my Moth Si2A3) and the metal envelope 6AC7.

 

My Moondogs are no longer really Moondogs.  They had undergone a couple of changes over the years, and so how ended up as the 300B circuit shown here -- with direct coupling in the input stage. I'm seriously contemplating reworking this circuit into that of the Laurel II.  While I would describe this amp as very fast (basically meaning good HF response) and 'clean' sounding, I found the Laurel II to have an organic sort of richness that I don't detect in the couple of other 300B designs I've tried.

 

 

[Blackbird]

Your Moondogs is really clean, erik2A3!  I am really envious of your skills.

[erik2A3]

Thanks for your kind comments Blackbird.  In reality, these are really only the shell of my former Moondogs.  Well, there are aspects of the power supply that are similar, but that's all.  I am very fortunate, though, to have a second pair of Moondogs not yet fully built; and that pair of monoblocks will be left totally as they were designed by Welborne Labs.  The Moondogs and my Lowther horns were purchased as 

a sort of promotional venture between Lowther America and Welborne Labs in 1990s.  I was already really interested in single ended tube designs at that time, but the Moondogs just really define all of this for me.  There is another forum member who owns a pair of the amps that were bought a bit earlier, when W. Labs was using transformers from  MagneQuest.  I guess I honestly look at the work I did above as something I wish I hadn't done, but now have the opportunity of remaking them into a pair of Laurel IIs - and because we were lucky enough to find a second pair of Moondogs.

 

I'm going to be rebuilding another amp I have from that time period - the Moth Audio Si2a3; and want to consult with Jeffrey Medwin regarding the use of a low-stored-energy power supply for the Moth.  In fact, he has already offered some very interesting insights.

 

As usual, I've gotten away from myself again!  I really just wanted to thank you for your nice compliment.  I hope your newly re-capped Moondogs are giving you lots of listening enjoyment!

 

.

 

 

[erik2A3]

Blackbird:  Are the hum-null potentiometers on your monoblocks working ok for you?

[Blackbird]

They are working just fine and are almost completely silent.

 

erik2A3, I know the Moondogs had been around for over two decades.  Is there any way to make a brand new pair and perhaps improve on the circuitry?

[erik2A3]

 

4 hours ago, Blackbird said:

 

erik2A3, I know the Moondogs had been around for over two decades.  Is there any way to make a brand new pair and perhaps improve on the circuitry?

Yes, you are correct.  I often include extra bits of information in my posts for those who may not be familiar with them.  And your question:  A brand new pair could definitely be made, and all parts are readily available.  I'm of course referring to the Moondog as circuit, not the packaging.  As for improving the circuitry, that is a much more complicated issue.  What aspect of the circuitry?  I think I do understand what you're asking me though - what can be done to make it a better Moondog, correct?  We can use my makeover as an extreme example, and what's left, while quite good sounding to my ears, is most definitely NOT a Moondog.  That's why I'm glad I have another pair that will be left 100% Moondog, even with the bunches of modifications that I know can be done in the name of improvement (which, at the end of the day, might not be as good as the original version).

 

Ron Welborne designed all of these kits so they could be more straightforward to build for those who may not be familiar with schematics, etc, and it's already a very, very good design.  Lead lengths could be shortened, and more direct connections to grids, plates, and cathodes could be made, etc. but the circuit would remain the same.

 

But sure.  You could use the same chassis you have, which is THE Moondog, and, if you were so inclined, completely redo the wiring.  This is essentially what I did with the Laurel IIs in this thread.  There are a couple of things I decided would be best left alone since they were doing their jobs well and the associated craftsmanship was good.  But the rest was re-done, while still maintaining the essential Laurel II design.  In other words, I absolutely did not redesign it.   One of the main things I addressed was the proximity of certain parts of the circuit.  I relocated a number of large components, as well as solder terminal strips, in order to 'open things up a bit' for better cooling as well as noise reduction.  Those same sorts of things could be done with the stock Moondog.

 

I hope this help!

 

 

 

[Blackbird]

1 hour ago, erik2A3 said:

Yes, you are correct.  I often include extra bits of information in my posts for those who may not be familiar with them.  And your question:  A brand new pair could definitely be made, and all parts are readily available.  I'm of course referring to the Moondog as circuit, not the packaging.  As for improving the circuitry, that is a much more complicated issue.  What aspect of the circuitry?  I think I do understand what you're asking me though - what can be done to make it a better Moondog, correct?  We can use my makeover as an extreme example, and what's left, while quite good sounding to my ears, is most definitely NOT a Moondog.  That's why I'm glad I have another pair that will be left 100% Moondog, even with the bunches of modifications that I know can be done in the name of improvement (which, at the end of the day, might not be as good as the original version).

 

Ron Welborne designed all of these kits so they could be more straightforward to build for those who may not be familiar with schematics, etc, and it's already a very, very good design.  Lead lengths could be shortened, and more direct connections to grids, plates, and cathodes could be made, etc. but the circuit would remain the same.

 

But sure.  You could use the same chassis you have, which is THE Moondog, and, if you were so inclined, completely redo the wiring.  This is essentially what I did with the Laurel IIs in this thread.  There are a couple of things I decided would be best left alone since they were doing their jobs well and the associated craftsmanship was good.  But the rest was re-done, while still maintaining the essential Laurel II design.  In other words, I absolutely did not redesign it.   One of the main things I addressed was the proximity of certain parts of the circuit.  I relocated a number of large components, as well as solder terminal strips, in order to 'open things up a bit' for better cooling as well as noise reduction.  Those same sorts of things could be done with the stock Moondog.

 

I would prefer to keep my Moondogs.

 

I am not an Electrical Engineer and not even a technician.  I don't know how do handle a solder.  But I know good tube sound when I hear one.  Now what if I can find the Moondog cabinets and have you build me a pair with what highlighted (in bold) in your quote plus MagneQuest OPTs?  The completed Moondogs will be paid for however long it take and no question asked.  I would rather have you build them for me or no one else.  In my opinion, I don't think anyone can build them like you can!

 

[erik2A3]

I'm generally in agreement with all of the above Jeff.  A note about lead length (with which I also agree!) is that one needs to remember that line-level inputs and speaker level outputs are unavoidably influenced by the users lengths of IC and speaker cable.  I totally agree that best practice here would see the shortest possible lengths of wire both inside and outside the amplifier.  Dynaco, Leak, and others always put their RCA inputs as close to the input grid as possible, up at the front of the amp.  And that's good.  As convenient as are input and output jacks on the back of the amp, that location is also usually in the same vicinity as mains AC current and other power supply components, which, as you observed, can potentially pollute delicate input signals in areas of high input impedance - which seems to make the possible influx of noise all the more likely. (this last sentence seems awkward and ridiculously long to me...)

 

With the revised Laurels here, the output was for all intents and purposes free of hum (both 60 and 120Hz), as well as buzz and/or any other annoying irritants. Our electrical environment is very good in this new (well, new to us) AZ house; the one in Texas was really just terrible.  Equipment was also often plagued by pesky ground-loop hum and EMI/RFI that was not present in components individually.  There are ways to deal with that, some of which being safer than others, and I was able to address it effectively (ferrite, ground-shunt input caps for RFI, etc.). And the dreaded earth-ground lift via 3-to-2 prong 'cheater plugs.'  Years ago we had some fun 'discussions' here about this!   

 

The 3-tier Moth Si2A3 is now upside down on my workbench awaiting surgery... (it also has input and output connections on the back, which I admit is convenient).  At leas they are on the side opposite AC power cord connections.

[Travis In Austin]

On 8/17/2019 at 11:12 AM, erik2A3 said:

Helping rebuild/refurbish a forum friend's Welborne Labs Laurel II mono blocks.  A tremendous amount of cleanup; many feet of solder wick; complete recapping of all electrolytic, metallized film bypass, and film coupling caps.  There was terrible hum and noise in one channel, which turned out to be a heater to cathode short in one of the 6AC7 as well as a bad rectifier socket.  Many cracked wires, including capacitor leads, etc.  A main issue was over-heating in the 300B filament supply, which was replaced by a relocated full-wave bridge rectifier and CRC filter section (all rebuilt with paralleled resistors for DCR of 1ohm).  300B 5V filaments had been running 4.3VDC, and are now corrected.

 

Once wiring and parts replacement where needed was finished, there was still an inexplicable 60Hz hum in one channel.  I worked literally hours on this problem, and finally found the source:  hiding under a 3.3uf Solen cap was a wire from the 6.3 VAC heater supply (for the paralleled 6SL7 and 6AC7) leading directly into the grid.  That corrected, I have to say these Laurels are now probably the quietest single-ended amps I have.  I'm also going to take a picture of the MagneQuest 300B output transformer. They are absolutely huge, the largest output transformers of this kind I've seen.

 

And so, with one more step (new 4 pin finals sockets), I will be very sorry to see these shipped back home this coming week.  They are lovely sounding amplifiers; and if you have heard that the 300B is euphonic, syrupy, overly-warm, etc., this circuit thoroughly and comprehensively dispels that myth.  With the schematic in (basically) in my head, I'm going to build a pair after I finish up a couple of other project.

 

Forgot to mention: A large choke was used to replace the common resistor in the power supply between the main filter sections for CLCLC. Wonderful amps!

Nice stuff as always!

[avguytx]

I'm just impressed at how clean it is under the hood with no huge blobs of solder that look like they were done by a one armed 4th grader with a half empty butane lighter outdoors in a Kansas hay field.

 

Beautiful work, sir.