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Moondog 2A3 is Lagging


neo33

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my left Moondog took a minute longer to output audio.  My take is that one of the cap needed to be replaced but I don't know which one.  Does anyone know if this is the correct diagnosis and which cap do I need to replace?  Thanks.

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my left Moondog took a minute longer to output audio.  My take is that one of the cap needed to be replaced but I don't know which one.  Does anyone know if this is the correct diagnosis and which cap do I need to replace?  Thanks.

 

The first thing I would do is swap tubes (One at a time) from the left channel to the right channel and see if the problem follows. It may just be a weak tube. Anyway this is always my first step in diagnosing a problem with a tube amp. Thankfully, I rarely have to go beyond this first step.

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my left Moondog took a minute longer to output audio. My take is that one of the cap needed to be replaced but I don't know which one. Does anyone know if this is the correct diagnosis and which cap do I need to replace? Thanks.

The first thing I would do is swap tubes (One at a time) from the left channel to the right channel and see if the problem follows. It may just be a weak tube. Anyway this is always my first step in diagnosing a problem with a tube amp. Thankfully, I rarely have to go beyond this first step.

Yup good advice...

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Does this do this every time you start it up?

Yes.  It now takes from 3 to 5 minutes.

 

 

After it begins, does it sound normal?

Yes.

 

 

Does it do anything when you shut it down? Any noise?

 

No.  No noise.

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The first thing I would do is swap tubes (One at a time) from the left channel to the right channel and see if the problem follows. It may just be a weak tube. Anyway this is always my first step in diagnosing a problem with a tube amp. Thankfully, I rarely have to go beyond this first step.

 

This is the very first thing that I tried.  Nothing is wrong with the right channel.  The left channel exhibited the same behavior.

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Does this do this every time you start it up?

Yes.  It now takes from 3 to 5 minutes.

 

 

After it begins, does it sound normal?

Yes.

 

 

Does it do anything when you shut it down? Any noise?

 

No.  No noise.

 

 

 

I suggest as a troubleshooting step if you've not done so already is to note the time it takes for the filaments of the tubes to light up fully from a (cold start up) to see if there's a difference between the amps exhibited. If necessary perform this test in a darkened room.

 

If by chance V1 and V2 are slow to light up fully then I suggest looking at BR1 and/or C9 as possibly bad.

 

Also a bad solder joint in any of the tube's filament supply lines could be at fault also.

 

I hope this is the correct schematic for your amplifier I've posted to assist you in troubleshooting the problem.

 

 

miketn

post-12368-0-14980000-1421121450_thumb.j

Edited by mikebse2a3
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The filaments of all four tubes on both amps lighted up almost immediately after I flipped the power switch.  Both 6SN7 driver tubes on the troubled amp were slightly dim compare to the good amp.  But once the troubled amp outputted audio, the driver tubes remained slightly dim.  I swapped the driver tubes from the good amp to the troubled amp but the problem remained.  Both the rectifier (CV378) and the 2A3 tubes remained strong.

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First a word of caution: The schematic shows no bleeder resistors on the 350v B+ Supply and thus a potential shock hazard exist until the voltage disapates or is manually discharged. If you are uncomfortable working on the amplifier then you should seek out a reputable technician.

If the same tubes light up bright in the good amp but dimmer in the slow turn on amp then I suggest the following steps.

(1) Measure the 5.5v supply and if low proceed to next step.

(2) Inspect all wiring and solder connections in the 5.5v supply circuit and redo any in question.

(3) Inspect and measure/replace R14, BR1 and C9 if any are found in question.

miketn

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I hope this is the correct schematic for your amplifier I've posted to assist you in troubleshooting the problem.
It's correct, but there may be variations in construction depending on who built them or if they were factory built. Mine was built slightly differently, but I can still use the schematic for mine.

 

I ordered the assembly manual a while back, so I could have it, and maybe build another pair. It was a $10 download and worth have the whole thing.

 

Bruce

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When you swapped the preamp tube did the dim glow move with the tube or stay put on the troubled amplifier? If it stays in the troublesome amp I bet Mike's advice I is spot on, if the heater voltage for the preamp tube is low it could extend the warm up time before that tube starts to amplify the signal.

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When you swapped the preamp tube did the dim glow move with the tube or stay put on the troubled amplifier? If it stays in the troublesome amp I bet Mike's advice I is spot on, if the heater voltage for the preamp tube is low it could extend the warm up time before that tube starts to amplify the signal.

 

The dim glow stay put on the troubled amp when I swapped the preamp tubes.

 

 

Neo,

Are you able to measure voltages?

 

 

 

Yes. But I have not have time to open the amp to do voltage measurements.  One thing I noticed is that if I turned off the system at night time after retiring and turned it back on during the early evening everyday, there would be no delay in the audio output.  If I turned on the amp every other day, then it took from 1 to 3 minutes to output audio.  And if I turned on the system every two to three days, then it took 3 to 5 minutes to output audio.

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