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Blackbird

Moondog 2A3 Monoblocks

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Here is a photo of the dead Moondog.  Notice the two busted caps (first and fourth).

 

1473974919_DeadMoondog.thumb.jpeg.396d2fb1f96d9921eb9fb2648c0cd4a7.jpeg

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This photo is the Moondog with static noise.  Interestingly enough I was able to make this Moondog produces music by slightly moving the rectifier.  This Moondog is still making slight static sound from time to time.  From what I can tell, caps one, four and five (starting from the bottom) are partially busted.

 

1541027648_StaticMoondog.thumb.jpeg.24ebc42a87a2800c436d6fe5dd811368.jpeg

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I would start replacing those swollen caps.... Dang...

 

 

 

 

 

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Yeah, but are they going to swell again, only this time sooner?  And if nothing else is amiss, maybe more suitable caps are in order?

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Posted (edited)

Well, I have had these pair of Moondogs for 18 years.  This is the first time I am having problem with them about two weeks ago.  Tonight was the first time I opened them in so long I don’t even remember.  I played music on these pair almost every day with my Klipschorns.

Edited by Blackbird

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Yours are much neater than mine underneath. Here's a link to my thread on getting them fixed. I didn't take a pic after the tech finished working on them.

 

 

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Posted (edited)

Since seeing so many busted (or partially busted) caps, I am inclined to think that my Moondogs are having problems with caps-attacked.  I have ordered a bunch of various new Solens caps.

 

It could also be that at one time or another, I have left my Moondogs powered on for such an extended period of time without ever power them down.

Edited by Blackbird

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Blackbird, there is no reason to leave these particular amps on constantly.  All you are doing is aging your tubes significantly and prematurely.  It is possible to install a standby switch on the B+ plate/anode supply so that ONLY the filaments receive current.  I am extremely familiar with this circuit and also own a pair.  I also know that, as designed, there is an aspect of the power supply that could use some slight alteration:  As Marvel mentioned, the 10uf cap is the input to DC ripple filter, and is the first capacitor immediately after the rectifier.  If you are using the GZ37 rectifier, that particular tube should ideally not see more than 4uf in this type of power supply.  While it most likely would not present a problem in cases where the amps are used for a few hours and powered off, leaving them on for extended periods as you say you have is simply not a good idea - particularly given the fact that this circuit does not have a high voltage standby switch.  If there is one thing that can damage components it's heat, and your amplifiers definitely show signs of that.

 

Given what you have, very helpfully said about them, I would stop using them (including switching them on and off to listen and look for problems) immediately and take or ship them to a good tech.  The 47uf Solen is the final stage cathode resistor bypass capacitor.  There could very likely be more than just one problem with them, and to just put in expensive new capacitors does not address causes beyond age alone.  More importantly, DO NOT poke around inside yourself unless you know exactly what you are doing.  That big black missile-like thing is the main high voltage filter capacitor (which these days sometimes sell for nearly $2,000 used — rather ludicrous IMO, but not my money).  Such capacitors can hold a serious charge for surprisingly long periods of time, and can deliver a painful shock.  

 

These are really nice amps, and I will also never sell mine.  It is worth the wait to have them properly checked out and repaired as needed. Damaged amps can also damage speakers, so I would really consider taking them out of service now until they are repaired and tested.  But take heart in the fact that they can most definitely be fixed and made to perform as new.

 

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erik2A3, thanks for the advice on the Moondogs.  I will not touch the internal of the amps and I will defer that to a qualified technician.  I have found a local tube amps expert.  I pointed out to him that many caps have been either busted or partially busted in my Moondogs.  I will leave the restoring work to him and let him replace any part necessary.  And I will also send him the schematics .  My Moondogs will be new again and hopefully it won't cost me an arm and a leg.

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Right and good! 👍

 

I would have been happy to help with this totally free of charge in terms of labor (you would buy necessary parts post-inspection).  I am in Arizona, so packing for safe shipment back and forth for two mono blocks would not be cheap.  Surely there should be a guitar amp tech not too far from you that could handle this very easily.  One aspect about the construction of this amp, is that the solder terminal strips are setup for reference during building.  Consequently, there are quite a number of connections that are indirect -- and subsequently increases length of leads, and so on.  As a result, someone not familiar with the circuit might take a bit longer to sort things out; which may add to cost if whomever takes this on charges by the hour. Hopefully there will be a reasonable flat-rate inspection fee,  to which of course would be added (the truly needed) parts and labor.

 

The Moondog is such a great amp, and has really become something of a classic.  It was built on the same chassis as their Laurel 300B mono blocks, and actually does not really need two 6SN7 input/driver tubes.   This is because only half of each section of the two tubes is used, and he did that partly as a way of finding something to do with the extra octal socket hole that was required by the Laurel 300Bs.

 

It might also be a good idea to confirm that the output transformer secondaries are configured for the nominal impedance of your loudspeakers.  Those leads are color-coded and clearly visible in the good pictures your took of the wiring.

 

 

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Posted (edited)

Each Moondog weighted about 25lbs.  So shipping them out would have cost quite a bit.  Don't worry about it, erik2A3.  Thanks for the offer.  I will send the amplifiers to my technician tomorrow as he is local to me.  I love the Moondogs and I would never give them away.  If I remember correctly, my Moondogs have Magnequest output transformers which is a favorite of many who own them.

Edited by Blackbird
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Definitely someone local is preferred, so that’s good.  Just for the record, the output transformer secondary winding color coding actually suggests Electra-Print as the maker of the OPTs.  I would point out that Electra-Print is arguably on par with MQ, and preference for one or the other being simply a matter of taste.  I’ve worked on both and heard both is THIS specific circuit, although I can say with confidence that A-B-ing these amplifiers can be difficult unless all aspects of it are equal:  same tubes, same resistors, caps, chokes, etc., and the only difference is the output transformer.

 

Whether MQ or E-P, I would say it’s a win-win either way.  I’m sure they’ll be barking happily again very soon.

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Thanks erik2A3!  I remembered they are Menaquest OPTs (as sold to me by an eBay's member) but I could be wrong since it had been so long.  I trust in your assessment, erik2A3.  But either MQ or EP, I am not going to argue.  I am just happy that my Moondogs will be back to normal again.  The Moondogs have quite a reputation, thanks to Mr. Welborne for an amazing product.  And they have been given me many happy years of music enjoyment.  I know that after they are restored, they will continue to give me many more years of happiness.

 

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  Some bits of advice. It was not current that killed your caps. It was voltage. 

  Maybe temperature played into it also. 

  How hot does the chassis normally get? Those poly caps may short out over time. But the caps are “self healing” and this blocks the shorts. 

  If the temperature is not excessive, replace the 400 volt caps. If the chassis does heat up, put some vent holes in it to keep the heat off those film caps. 

  Welborne built nice gear. Purchased some small items 30 years ago. Got a little shakey at the end.

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I remembered during the time I left the Moondogs on for some extended period of time without turning them off, they were never hot (not even to the touch) but only remained warm.  The amplifiers were well ventilated.

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  Might be the voltage surges. Could not tell if any of the 630 volt caps failed. Just like electro lyrics, guess film caps have a lifespan.

  I would replace all the film caps. Should improve the Sonics too.

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Today late afternoon I brought the Moondogs to my local tube amplifiers technician.  He took a look at one of the Moondog and told me that is it a piece of cake and won't cost me that much.  That was easy.  He will replace all of the bad caps.  He will also give me a thorough assessment of both amplifiers and replace any part necessary.  He is an expert in tube amplifiers and was very fun to talk to.  He told me my Moondogs are excellent in sonic and materials.

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Have a contact information for  tech in Fort Walton in case you need one in future. Meanwhile looking like you are good to go.. Enjoy...

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On 6/14/2019 at 12:38 AM, glens said:

It might've been the caps just went bad, but just as likely, or more, something else made them die.  A more thorough checkout should be in order.

 Find a reputable repair shop. Now is not the time to DIY. 

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