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Could use some help Trouble Shooting my Welborne laurel 300B Monoblock ...............

107.2.79.94
Posted on August 28, 2020 at 12:30:03
Cut-Throat
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 I got a pair of Welborne 300B Laurel Monoblocks in my Back Room. I use them 3 or 4 times a week. Today I flip the power on everything, and 1 Channel is completely dead. All the tubes are cold, so I suspect a fuse has blown. I had not changed anything on that system in quite awhile.

So, I put the amp on my Bench and pulled the Fuse... Sure enough Blown. So, I do my usual put each of the 4 Tubes (1 at a time) in the other Monoblock, and power it up... All 4 Tubes are good. So much for an easy fix. I built SET amps before from Kits, but I did not Build These Amps

I could use some guidance on what I could look for, Measure Voltages etc? Schematics Attached. Thanks in Advance for any help.









 

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So did you replace the fuse?, posted on August 28, 2020 at 12:44:09
Ivan303
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Posts: 48887
Location: Cadiere d'azur FRANCE - Santa Fe, NM
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 Assume you did and it blew again?

If not, that'd be my first go.

Then look for heat damaged components in the bad amp, especially cathode resistor on the 300B.

Puffy caps, burnt resistors, maybe even a SS rectifier that looks like it's heat damaged?

That's about it if it keeps blowing fuses and the tubes are good.






First they came for the dumb-asses
And I did not speak out
Because I was not a dumb-ass

 

Not Yet - Picture of Amp .............., posted on August 28, 2020 at 13:09:04
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 My first thought was to replace the fuse, but then, I decided to be a bit more prudent before I chewed up another fuse. Something caused it to blow, and I wasn't moving speaker connections around etc.

I did look for some obvious heat damage, but didn't see anything. Here is a Pic of the Guts.







 

RE: Could use some help Trouble Shooting my Welborne laurel 300B Monoblock ..............., posted on August 28, 2020 at 13:18:08
Paul Joppa
Industry Professional

Posts: 7296
Location: Seattle, WA
Joined: April 23, 2001
 Pull out the rectifier and try again, checking to see if all the heaters / cathodes glow. If so, then check the high voltage winding AC voltages to be sure the power transformer is OK. If it is, then you know it's downstream of the rectifier.

You can then pull the driver tubes to see if the problem is in the output stage by checking its DC voltages.

Of course, you can always just re-flow the solder on every soldered connection - a huge majority of problems are solved that way without laborious diagnostics, and it takes only a few minutes.

 

RE: Not Yet - Picture of Amp .............., posted on August 28, 2020 at 13:32:32
rivervalley817
 where is that green wire at the top of the photo going? or not going?

that looks suspicious

regards,

 

RE: Not Yet - Picture of Amp .............., posted on August 28, 2020 at 13:42:32
Cut-Throat
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 Per the Assembly Manual...

"Locate the Green and yellow striped lead from T2. Cut this lead short as it will not be used. The end of the lead can be taped or covered with heat shrink tubing." Closeup Photo Attached.







 

RE: Not Yet - Picture of Amp .............., posted on August 28, 2020 at 14:04:06
rivervalley817
 yeah, that would have been too obvious I reckon

that amp wasn't run unloaded was it?

you mentioned speaker connections

 

RE: Not Yet - Picture of Amp .............., posted on August 28, 2020 at 14:09:41
Cut-Throat
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 No... I mentioned that I had NOT fooled with any connections.... Amps were sitting on shelf with the same speakers for over a year.



 

RE: Not Yet - Picture of Amp .............., posted on August 28, 2020 at 14:46:24
rivervalley817
 my bad. are the speakers fused?

if that opened up would it unload the amp?

just spitballing here

with regards,

 

RE: Not Yet - Picture of Amp .............., posted on August 28, 2020 at 14:48:49
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 Speakers not fused....



 

RE: Not Yet - Picture of Amp .............., posted on August 28, 2020 at 14:53:14
rivervalley817
 any recent paranormal activity? amp gremlins maybe?

[I'll just shut up and sit over here now]

with regards,

 

Sometimes, fuses just blow. When they blow on start up..., posted on August 28, 2020 at 17:03:43
Chip647
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 When they blow on start up and the fuse is 3 amp and the amp draws a steady 1 amp, the turn on surge can easily exceed the 3 amp. Over time those surges can stress the fuse and it will blow.

Easiest fix is a 5A fuse!!!!

Let another fuse blow before you poke around too much. Put it on a variac if you have one and monitor the current.

 

RE: Could use some help Trouble Shooting my Welborne laurel 300B Monoblock ..............., posted on August 28, 2020 at 18:38:18
bean
 Not likely, but I had a DHT develop a short that blew a fuse. Made for some interesting fireworks in the rectifier tube, too.

If you keep blowing fuses and you don't find anything obvious, you might try swapping a tube at a time from the other amp. Testing for shorts with a good tester is always a good idea, but they don't always show up on a tester.

 

was the fuse hard blown or soft, posted on August 28, 2020 at 19:30:45
Tre'
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Posts: 17304
Location: So. Cal.
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 is the fuse black inside the glass?

Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"

 

Here you go - Here's a Picture, posted on August 28, 2020 at 21:00:18
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 Looks Soft to me....







 

Already Swapped tubes as indicated in my first post.....nt, posted on August 28, 2020 at 21:01:20
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 nt



 

RE: LOL, forgot that by the time i read all the responses nt, posted on August 28, 2020 at 21:06:34
bean
 z

 

RE: Already Swapped tubes as indicated in my first post.....nt, posted on August 28, 2020 at 21:35:27
rivervalley817
 maybe route the input from the non working amp to the working amp?

if that fuse blows ... look upstream?

you know the fuses work so why not

then, install a new fuse in the dud and see what's what

just a thought

regards,

 

I think Tre and Chip nailed the problem ..... It was the Fuse - Thanks for all the Suggestions .............., posted on August 29, 2020 at 06:53:40
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 Looking at the Fuse that blew (1amp 250 volt), there was no Black Glass and just a disturbance of the element... So, Chip's explanation made sense. I stuck another 1 amp 250 volt fuse in place and the amp fired right up and has been running for over a half hour.

I did inspect closely every solder connection on the amp, which I had also done in the past when I bought them 10 years ago. The solder connections were all great looking, so I didn't mess with them.





 

1 amp fuse? The schematic shows a 3 amp fuse. nt, posted on August 29, 2020 at 07:25:47
Tre'
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Posts: 17304
Location: So. Cal.
Joined: February 9, 2002
 ,
Have Fun and Enjoy the Music
"Still Working the Problem"

 

And the pic of the fuse sure looked like a SB fuse to these old eyes..., posted on August 29, 2020 at 07:38:27
Ivan303
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Location: Cadiere d'azur FRANCE - Santa Fe, NM
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 Maybe at some point the normal 3 amp fuse, as shown on the schematic, was replaced with a 1 amp SB?






First they came for the dumb-asses
And I did not speak out
Because I was not a dumb-ass

 

You're correct .... The Amp came with 1 amp fuses ............., posted on August 29, 2020 at 07:39:02
Cut-Throat
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 I checked the parts list yesterday to make sure and all it said was 'Fuse'. Never thought about looking at the schematic....

I'll pick some up in case another blows --- Although this is the first time in 10 years for me. :-)



 

Yes, Slow Blow......., posted on August 29, 2020 at 07:44:23
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 The same fuse that came with my Welborne 45 SET amp Kit..... Although I just checked the schematic and it said 1.5 amp....

Well, I guess the amp is even safer with a 1 amp fuse.



 

So now that they work..., posted on August 29, 2020 at 19:40:24
Ivan303
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Location: Cadiere d'azur FRANCE - Santa Fe, NM
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 how do they sound?

Haven't heard Ron's 'Laurels' in years. Recall they sounded pretty good back in the day, with the right speakers.






First they came for the dumb-asses
And I did not speak out
Because I was not a dumb-ass

 

RE: So now that they work..., posted on August 29, 2020 at 22:02:29
Cut-Throat
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 Well, they've always worked except for that 1 day.. And they have always sounded great. I have used Welborne Amps for the Last 20 years or so. My main system had his final kit, which was called a Star Chief. 45 Tubes in a Mono block configuration. I also used to own his 2a3 Moondogs, which I sold. as I had too much gear. Once in awhile I'll swap the 300B Laurels in my Main System. I have been using these Amps on Oris Horns for the Last 6 years and had Altec Multicell horns before that.



 

RE: Could use some help Trouble Shooting my Welborne laurel 300B Monoblock ..............., posted on September 1, 2020 at 11:28:15
deafbykhorns
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Posts: 1067
Location: Florida
Joined: October 17, 2003
 Check power supply caps and bypass cap for shorts
Also check rectifier and OPT

 

Problem was solved days ago ..... See post below referencing Fuse ....nt, posted on September 1, 2020 at 11:37:08
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 nt



 

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