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Adrian R · 11-06-2009, 12:45 PM

Interesting story here..

Been experiencing a problem with my MKIII head the past month or so. When I got the head it needed a bit of work, mostly cosmetic. Anyway replaced the old tubes with a fresh set of matched EH6L6s...and biased the amp conservatively around 34mA.. things seemed fine at first..tone was good, punchy, articulate and warm....but..

The amp started to exhibit a humming noise that almost sounded like a ground issue...however all attempts to silence this hum failed...but it was strange as sometimes it would hum, sometimes quite loudly, and other times it almost was silent...I tried swapping out preamp tubes in addition with no success..

Then last night while practicing I noticed the hum was beginning to come back..and this time the overall tone quality was poor...Power was good, but the overall tone was harsh and sort of loose..especially in the lower frequencies..

I then looked at the back of the amp and noticed one of the tube failure fuse's LEDs was softly glowing....I also noticed that all four tubes were lit properly, but valve 3 didn't have that nice, softly lit, ultra-violet glow to it in play mode as the other 3 were. I shut down the amp and pulled the fuse...

The fuse didn't appear completely blown..as the element inside was still in tact, however one end of it appeared to be slightly brown..

I replaced the fuse and fired the amp back up. It didn't burn the replacement fuse, and the LED did not glow again...but the overall tone was still bad and that specific tube didn't seem to glow nicely like the other 3...

I called the Tubedepot and they are sending me a replacement free of charge with the same bias rating....hopefully this was the problem all along..and hope to have this amp up rockin' again!

In nearly 20 years of tube amp ownership this evidently seems to be the first OP tube failure I've experienced!

11-06-2009, 12:45 PM	#1 (permalink)
Adrian R Senior Member Join Date: Jan 2009 Location: Far North Chicago Burbs' Posts: 514	Apparent EH Tube Failure..Arrrrr..... Interesting story here.. Been experiencing a problem with my MKIII head the past month or so. When I got the head it needed a bit of work, mostly cosmetic. Anyway replaced the old tubes with a fresh set of matched EH6L6s...and biased the amp conservatively around 34mA.. things seemed fine at first..tone was good, punchy, articulate and warm....but.. The amp started to exhibit a humming noise that almost sounded like a ground issue...however all attempts to silence this hum failed...but it was strange as sometimes it would hum, sometimes quite loudly, and other times it almost was silent...I tried swapping out preamp tubes in addition with no success.. Then last night while practicing I noticed the hum was beginning to come back..and this time the overall tone quality was poor...Power was good, but the overall tone was harsh and sort of loose..especially in the lower frequencies.. I then looked at the back of the amp and noticed one of the tube failure fuse's LEDs was softly glowing....I also noticed that all four tubes were lit properly, but valve 3 didn't have that nice, softly lit, ultra-violet glow to it in play mode as the other 3 were. I shut down the amp and pulled the fuse... The fuse didn't appear completely blown..as the element inside was still in tact, however one end of it appeared to be slightly brown.. I replaced the fuse and fired the amp back up. It didn't burn the replacement fuse, and the LED did not glow again...but the overall tone was still bad and that specific tube didn't seem to glow nicely like the other 3... I called the Tubedepot and they are sending me a replacement free of charge with the same bias rating....hopefully this was the problem all along..and hope to have this amp up rockin' again! In nearly 20 years of tube amp ownership this evidently seems to be the first OP tube failure I've experienced! __________________ Adrian R. Amplification: Marshal JCM900 MKIII 2100 Marshall JCM900 MKIII 2100 (#2) Marshall JCM900 4100 (mods) Marshall MG15 Micro Stack Marshall 1960A 4x12 Cab. Marshall JCM800 1960A 4x12 Cab. MXR/Maxon Fx. Guitars: Gibson LP Standard (95') Gibson Flying-V 67RI (06') Gibson Flying-V V98 SE (01') Guild DV 6 Acoustic (00') "Only two tones apply: loud and LOUDER" Last edited by Adrian R; 11-06-2009 at 02:50 PM.