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PostPosted: Tue Nov 04, 2014 12:18 am 
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Location: Ottawa
Hi all,

So I finished my trinity tweed complete with VRM a couple of weeks ago and it has worked and sounded excellent at all volumes and VRM levels... until a few days ago.

Here's a sequence of events and symptoms:

-Noticed that rectifier tube arced when I turned the amp on a couple weeks ago. It maybe happened again once or twice in the past few weeks but everything seemed to work fine.

-A few days ago I turned on the amp, heard no sound, checked guitar cables and volume knobs, all plugged in and turned up, strike the strings and hear a pop, and then it came to life and sounded as glorious as usual. Hmm, that was weird.

-Next day, same thing. Hmm, that was weird again, but it's still working. Perhaps a loose connection I thought.

-On the third day, no sound at all. It was then that I noticed when I strummed or struck the strings of the guitar, tubes V3 and V4 had a blue glow that seemed to dance with my strumming. Now, I've read that a blue glow is normal. Perhaps it was always there and I hadn't ever noticed it before, or perhaps not.

Here's how I've troubleshooted so far:

-Used ohmmeter to check for bad solder connections on all pins of V3 and V4, output jacks, speaker cable, and speaker cabinet. All measured zero or next to zero resistance, so no problems there.

-Checked the ohms of resistors R7,8,13,14,16,17,18,19,20,21,and 22. All were good.

-Used a wooden dowel to poke and wiggle some wires with the amp on, to no effect.

-Gave the amp a few gentle bumps and taps to see if a shoddy connection would respond. No effect.

-Measured voltages as follows: (JJ tubes, except V1 is electroharmonix)
-AC Mains: 120 VAC
-B+ all tubes in: 380 V
-V1
-Pin 1: 112 V
-Pin 3: 1.97 V
-Pin 6: 109 V
-Pin 8: 1.97 V
-V2
-Pin 1: 148 V
-Pin 3: 1.18 V
-Pin 6: 188 V
-Pin 7: 2.7 V (aha! should be 17 V. Also noticed that the V3 & V4 blue glow light show did a dance whenever I touched the lead to Pin 7)
-Pin 8: 46 V
-V3
-Pin 3: 375 V (a bit high? Manual says it should be 356 V with the JJ rectifier)
-Pin 4: 335 V
-Pin 8: 21 V
-V4
-Pin 3: 380 V (again, a bit high?)
-Pin 4: 334 V
-Pin 8: 21 V

-Since Pin 7 on V2 was the only one that seemed out of whack, I checked all of the solder connections around it with the ohmmeter, and all were good. I checked R12 and it was fine. (Note, I did not install the optional 470K resistor here.) I touched the leads of the ohmmeter to C6 and then checked the voltage across it and observed it to be slowly draining, which I believe means that it's good.

So, I do not know what to do next at this point. Is it safe to say that Pin 7 on V2 is likely the problem? I triple-checked the voltage on that pin and other locations on the same node, and got the same result every time. 2.7 Volts. Any idea what could lead to a low voltage on that pin? A bad tube perhaps? Does the blue glow in the output tubes that responds to the input signal indicate that the input signal is getting through the preamp to the output stage? If so, could it be a problem with the output transformer?

Any help would be greatly appreciated.

Thanks,

Greg


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PostPosted: Tue Nov 04, 2014 5:40 pm 
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Check the jumper on the board that goes from 1M/pin 7 to the other side of the board.

Replace the rectifier tube

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PostPosted: Tue Nov 04, 2014 9:58 pm 
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Joined: Thu Sep 18, 2014 10:20 pm
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Location: Ottawa
Thanks for the tip! I checked the resistance across the jumper you mentioned and it seems to be fine (0.3 ohms between pin 7 and the lead of capacitor C6).

Do you recommend to replace the rectifier solely because it was arcing? Or, is it instead because the symptoms I've listed can be caused by a bad rectifier? (Forgive me if some things aren't entirely obvious to me, it's my first amp build)

Here's a video of the blue glow flashing back and forth between V3 and V4. In this clip, I had the VRM, Tone, and both volumes all cranked to the max. I was touching the tip of the input patch cord with my finger which is enough of an input signal to cause the blue glow to flash back and forth. In the video, from about 0:01 to 0:06 I was repeatedly tapping the patch cord tip. With each tap V4 and V3 would alternately flash back and forth. Then at 0:06 I held my finger on the end of the cord during which V4 seems to be glowing brighter than V3. I should mention that my phone camera does a poor job of picking up the glow, it is actually a lot brighter and fuller to the naked eye.

http://youtu.be/4Z9f_BcpWa4

There is literally no sound coming out of the speaker cabinet, not even the background hiss that was there when the amp worked (especially with all knobs cranked). I remember there used to be a slight hiss when the amp was on standby, and then the hiss would get louder when I switched standby to on. Now, there's no hiss at all, even on standby. Could this be a fried output transformer? I never operated it without a load, but perhaps another problem such as a bad tube caused the output transformer to wear out over a few weeks of use?

My problem has some similarities with the following thread, with the exception that there is zero sound coming from my speaker cabinet.

viewtopic.php?f=11&t=3678&p=28912&hilit=video+tubes+glow#p28912


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PostPosted: Wed Nov 05, 2014 2:06 pm 
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Joined: Thu Sep 18, 2014 10:20 pm
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Location: Ottawa
Problem solved.

When troubleshooting and hitting dead ends, one tends to question the steps they've already taken. Turns out it was a faulty input jack on my speaker cabinet. I had originally checked the jack and it measured 4 ohms, then I checked the speaker cable and it measured zero resistance on both conductors. But I never checked the resistance through the speaker cable when it was plugged in to the speaker cab's jack. Well, I checked that now and it turns out it wasn't mating with the plug properly.

I'll swap out the jack shortly and hope I haven't wrecked the OT...

Regarding the low voltage on pin 7 of V2, I read on another forum that that voltage measurement can be erratic due to the impedance of some multimeters changing the characteristics of the circuit.


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PostPosted: Wed Nov 05, 2014 2:39 pm 
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Location: Ottawa
Well, it works but I definitely must have fried something during my endeavors. The volume is far far quieter than it used to be and it is incredibly distorted even at the lowest volume. Does this sound like the OT is toast? Or the tubes? Or both? Could operating the amp with no load harm the tubes? The tubes are still flashing blue, but again they may have always normally done that without me noticing.


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PostPosted: Wed Nov 05, 2014 8:16 pm 
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Sure sounds like OT. email sent with resistances to measure to see if it's OK. Confirm voltages still look high enough so not rectifier? suspecting the VRM?

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PostPosted: Wed Nov 05, 2014 9:37 pm 
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Location: Ottawa
Yeah, looks like the OT is caput:

Primary
Black-Yellow: 1.1 ohms - ok

Secondary
Brown-Red: 43.9 ohms - low
Blue-Red: 172.9 ohms - ok
Brown-Blue: 218 ohms - low

Most of the voltage measurements I did earlier seemed to be pretty close to what was in the manual, and they were done with the VRM set to max. When the new transformer arrives I'll check all of the voltages again. The VRM worked great beforehand, so I'm assuming it should be fine unless operating the amp without a load could have damaged it.

Thanks for your help!


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PostPosted: Wed Nov 05, 2014 9:43 pm 
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Brown Red in yours looks too low. Suspicious.

Should be:
Primary
Black - Yellow: 0.9 ohms

Secondary
Brown - Red: 150 ohms
Blue - Red: 173 ohms
Brown - Blue: 323 ohms

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PostPosted: Fri Nov 07, 2014 8:53 pm 
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Joined: Thu Sep 18, 2014 10:20 pm
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Location: Ottawa
Got the new transformer. Ohm readings matched what they should. I installed it and... it's back to its wonderful normal self again :-) Installed a new rectifier tube as well and I didn't notice any arcing. Thanks guys! The power tubes still glow blue, but everything seems normal and it kinda looks nice!

Greg


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PostPosted: Fri Nov 07, 2014 9:17 pm 
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Excellent news. Glad it worked out for you. This is a good time to take. measurements in case you need them later!

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PostPosted: Mon Jul 23, 2018 5:32 am 
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SPAM!!


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