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Old 29th May 2006, 02:08 AM   #1
Trout is offline Trout  United States
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Default Don't you just hate it when

Don't you just hate it when you build a guitar amp, Run it for 2-3 hours, Get it seemingly working perfectly and then,

Loan it to a younger brother(age40) and it lasts under 45 minutes!

" I was playing, it sounded amazing, I had it running @ 7 on the clean channel, 8 on the master,(40-50W Amp)It just faded out sounding sort of distorted, Now nothing comes out"

To Be Continued:
trout

Edit, My brand new KT77 amp
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Old 29th May 2006, 02:23 AM   #2
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OK, how wierd is this? I just finished a new stereo SE amp. It uses any of the popular audio tubes. I played it for a few hours last night with 6L6's, it worked great. Tonight I put in a pair of Chinese KT-88's that I have used for at least 5 years, and cranked it up loud. After about 1 hour, 1 channel just faded away slowly. It lost some volume, then got distorted, and finally stopped making sound. It took at least 2 minutes to completely die. I put the 6L6's back in, everything is normal. One KT-88 is dead. After sitting around for another hour, the silvery getter is turning white. The tube sprang an air leak. There are no obvious cracks, but air got in.

I have seen the slow fading away symptoms caused by an open heater also. It only takes 10 to 30 seconds to die.
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Old 29th May 2006, 02:37 AM   #3
Trout is offline Trout  United States
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I Talked to him on the phone for about 30 minutes, The tubes all light, and a tiny bit of sound comes out the speaker, The unamplified guitar is louder.

These were all new JJ tubes, I know occasionally a new tube can die, but it never ceases to amaze me how he has this magic touch.

A couple years back, I started getting back into the whole amp thing simply because he gave me all his DOA's lol

He has very simple instructions when using my gear, DO NOT Touch ANYTHING.
So, I get the unit back tomorrow, He is about 1 hr away.

He was as impressed as I am with the sound of the new KT77's, I just hope they are not burnt.
Tomorrow tells the tale.
Gene
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Old 29th May 2006, 02:57 AM   #4
poobah is offline poobah  United States
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It has NOTHING to do with science or facts... Younger siblings OR children break ****. I buy 4 new remotes per year. They complain because the 27" Trinitron flat screen is upstairs...









They just don't get it!
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Old 29th May 2006, 08:03 PM   #5
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Quote:
Originally posted by Trout
I Talked to him on the phone for about 30 minutes, The tubes all light, and a tiny bit of sound comes out the speaker, The unamplified guitar is louder.

These were all new JJ tubes, I know occasionally a new tube can die, but it never ceases to amaze me how he has this magic touch.
Gene
Your brother wouldn't happen to be this guy:
Click the image to open in full size.

Quote:
Abilities

It is said that when Impmon appears, electronics temporarily go crazy.

http://en.wikipedia.org/wiki/Impmon
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Old 29th May 2006, 11:00 PM   #6
Trout is offline Trout  United States
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Quote:
Your brother wouldn't happen to be this guy:
HAHA,, that was GREAT!!


Well, I have it back, Ran the old digital VM around it, Looks like theres some sort of problem, A failure of some type.

Something looks very very wrong with the preamp voltages, And B1-4 are suddenly very high?

Working Schematic

I did not get a chance to write down the initial voltages but they were for sure 400V and lower.(B1 thru B4) Almost like there is no load.


All voltages below with tubes installed, Speaker plugged in on 8 ohm selection, No input source( Jacks closed )



V1-3 12AX7 (JJ-ECC83)

V1
Pin 1- 181V
Pin 2- 0V
Pin 3- 2V
Pin 6- 164V
Pin 7- 0V
Pin 8- 1V

V2
Pin 1- 181V
Pin 2- 0
Pin 3- 2V
Pin 6- 273V
Pin 7- 180V
Pin 8- 182V

V3
Pin 1- 85V
Pin 2- 0V
Pin 3- 2V
Pin 6- 272V
Pin 7- 24V
Pin 8- 62V

KT77 X2

Pin 1- 0V
Pin 3- 423
Pin 4- 425
Pin 5- -44

PS
B1 440V
B2 429V
B3 338V
B4 285V

As weird as the 12AX7 pin voltages are, This might be a nightmare to figure out. I get sound out of the amp, SORT of, Its very low volume and distorted and sounds like a bad compression effects pedal.

Thing is, It was easily pushing out in excess of 30W output prior to leaving. For no less than 2 hours.

I'll get a bottom view picture posted in a bit. I need to clear some FTP space.

Trout
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Old 30th May 2006, 12:42 AM   #7
lndm is offline lndm  Australia
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Pins 7 and 8 on V3 show some trouble (that's the driver isn't it?). Not sure where it is coming from but I would measure all resistances between V3 and its coupling caps on either side, and I would probably replace these coupling caps.

Then again maybe V3 is at fault.
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Old 30th May 2006, 12:57 AM   #8
Trout is offline Trout  United States
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Yep, V3 is the driver, I replaced all the tubes already with new ones, Pays to buy bulk sometimes.

Confirmed indeed these voltages are correct, well Not correct, but listed here correctly.

I got a bottom view picture

Bottom View And Top View

I would be surprised if its the caps or resistors, But anything is possible. The resistors are basically all 1W, All the film caps are 630V which is plenty over the needed ratings. Everything used was freshly ordered stock.

This was my first real attempt at an eyelet board type construction, Its the prototype for the turret boards I am planning to use on a few more.

Its looking more and more like a total nightmare now.

I did not put the board in with easy removal in mind, now its looking like I made a huge mistake not planning for serviceability
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Old 30th May 2006, 01:12 AM   #9
lndm is offline lndm  Australia
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Very nice looking amp

I can't see a reason why there should be any DC on the driver's stage grid, and the Vg-k is suspect.
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Old 30th May 2006, 01:17 AM   #10
poobah is offline poobah  United States
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Gene, can you remeasure V3 1,2,3 again? And use a lower scale for pin 3 (more accurate). Stage 2 looks weird too, but stage one has some voltage or current "missing".
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