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 Post subject: Re: 18 Watt Plexi Mk II
PostPosted: Thu Aug 24, 2017 2:03 pm 
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The Voltages with V4 and V5 Pulled are
R28: 417 V and 403 V
R22: 402 V and 329 V
R17: 199 V and 333 V
R16: 155 V and 335 V
There doesn't seem to be any wiring errors around V3, V4, or V5. I did some quick checks and it initially looks like the resistors are correct, but I will unsolder them later and check them properly.


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 Post subject: Re: 18 Watt Plexi Mk II
PostPosted: Thu Aug 24, 2017 6:35 pm 
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So I pulled the R31 and R19 to measure the resistance and found them to be close enough. (115K and 820) After i resoldered them in I checked the amp again and found that the amp didn't seem to be held back. There wasn't any obvious red plating. There was a very loud crackling sound. I plugged my guitar back in and got a equally loud sound through it. The crackling didn't change with the knobs. I took readings at pin 1 and 6 of V3 and Pin 2 on V4 and V5.
V3 Pin 1: 186
V3 Pin 6: 163
V4 Pin 2: 1.4
V5 Pin 2: 0.5
One weird thing that I observed is when I touched the V5 pin 2 with my oscilloscope set to resistance (from other tests) all the crackling stopped. When i removed it the crackling resumed. So I re-soldered both ends of C8. When I did a sound check the amp crackled very loudly for a couple of seconds, then went very scilent. When I played my guitar through it, the guitar sounded crackly and the V5 Tube started to overheat again.


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 Post subject: Re: 18 Watt Plexi Mk II
PostPosted: Thu Aug 24, 2017 9:19 pm 
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Is R31 really 820 ohms as you mentioned? It should be 8.2K. (8,200 ohms).


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 Post subject: Re: 18 Watt Plexi Mk II
PostPosted: Thu Aug 24, 2017 11:08 pm 
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Sorry, R31 reads 8410 ohms.


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 Post subject: Re: 18 Watt Plexi Mk II
PostPosted: Fri Aug 25, 2017 12:29 am 
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Well, if all the parts are good and there are no wiring errors or bad connections, that just leaves bad tubes. Try replacing V5 and maybe V4, too.


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 Post subject: Re: 18 Watt Plexi Mk II
PostPosted: Fri Aug 25, 2017 10:37 pm 
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One thing I've been noticing on my amp, is that as soon as I turn it on, as it I flip the first power switch to warm up the tubes, the V4 tube flashes. It gets pretty bright for a second then starts to warm up like V5.
I switched the output tubes around and the amp functioned like before and the V5 tube overheated.
I have a Vox Ac4C1 amp that has a EL84 output tube. This amp is stock and runs good enough (it hums a little, a challenge for another day). I switched out that output tube with the ones on my Trinity and nothing changed. I also plugged the two EL84's from the trinity into the vox and it functioned normally. I will state that one of my tubes flashed when in the Vox just like when in the trinity.
This leads me to think that the tubes are good and the problem is elsewhere.
I will also state, that I have replaced the EL84's ones before on this project, the most recent ones have only been in testing for a couple of hours at most.
Do you still think I should replace my tubes? And should I replace all of them or just the outputs?
Cooper


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 Post subject: Re: 18 Watt Plexi Mk II
PostPosted: Sat Aug 26, 2017 3:18 pm 
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Cmcordero wrote:
One thing I've been noticing on my amp, is that as soon as I turn it on, as it I flip the first power switch to warm up the tubes, the V4 tube flashes. It gets pretty bright for a second then starts to warm up like V5.
I switched the output tubes around and the amp functioned like before and the V5 tube overheated.
I have a Vox Ac4C1 amp that has a EL84 output tube. This amp is stock and runs good enough (it hums a little, a challenge for another day). I switched out that output tube with the ones on my Trinity and nothing changed. I also plugged the two EL84's from the trinity into the vox and it functioned normally. I will state that one of my tubes flashed when in the Vox just like when in the trinity.
This leads me to think that the tubes are good and the problem is elsewhere.
I will also state, that I have replaced the EL84's ones before on this project, the most recent ones have only been in testing for a couple of hours at most.
Do you still think I should replace my tubes? And should I replace all of them or just the outputs?
Cooper


Do you have the 100R resistor connected to both?
They both share the same cathode resistor so there is no obvious reason for one to behave differently. I would suspect only the power tubes.

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 Post subject: Re: 18 Watt Plexi Mk II
PostPosted: Tue Aug 29, 2017 10:27 pm 
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Yes I have R23, the 100R 2W, resistor in place. I measured the resistance and got 98.8 ohm's so it seems to be good to me. I guess I'll purchase another set of power tubes. Is there anything I should try and fix in the mean time? I there anything different that I should do once they show up? (Different ways of testing so i don't accidentally ruin them again)

Cooper


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 Post subject: Re: 18 Watt Plexi Mk II
PostPosted: Wed Aug 30, 2017 10:59 am 
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What is the cathode voltage on the EL84s? i.e. at the 150R / 50uF cap?

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 Post subject: Re: 18 Watt Plexi Mk II
PostPosted: Thu Aug 31, 2017 2:18 pm 
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At Pin 3 of V4 and V5 I read 12V. But When I turned on my amp today the V5 tube was flashing and the amp was making lots of popping sounds. It was kinda like a light show. So V5 clearly looks bad. I put a different tube into the V5 socket and it wasn't popping like the previous one.
What does this mean?


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 Post subject: Re: 18 Watt Plexi Mk II
PostPosted: Fri Sep 01, 2017 7:22 am 
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It means the tubes were biased correctly but apparently, V5 was failing. Assuming you are still reading 12V with the replacement tube in there, you are good to go.

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 Post subject: Re: 18 Watt Plexi Mk II
PostPosted: Sat Sep 02, 2017 6:14 pm 
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Alright, so I got the new EL84's in today and plugged them in and the overheating problem seemed to stop. Yay! I hope it stays this way this time.
There is still a loud crackling sound coming through the amp. The guitar sound through the amp is very clear.
So now it seems like the only problem is the crackling. Is this just from a bad solder joint or is there something else I should check?

Thanks for all the help so far, lets get her done!
Cooper C


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 Post subject: Re: 18 Watt Plexi Mk II
PostPosted: Sat Sep 02, 2017 8:30 pm 
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Glad the overheating problem has stopped.

The crackling could be caused a bad connection still lurking in the amp. Check all the connections again - turrets, pots, tube sockets, jacks. Make sure all your tube sockets are making good contact with their tubes.

It might be a bad pot, or it could be caused by a bad tube in the preamp or the phase inverter. Does the crackling stop if both volume controls are turned all the way down? Is the crackling on both channels?

We might have asked that before, but it's good to know the current state of the amp.


Last edited by mitch m on Sat Sep 02, 2017 8:38 pm, edited 1 time in total.

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 Post subject: Re: 18 Watt Plexi Mk II
PostPosted: Sat Sep 02, 2017 8:33 pm 
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The crackling is independent of the volume nobs. It is always the same loudness, no matter what setting the nobs are at,which channel I'm in, or if I even plug in my guitar. What does this indicate to you?


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 Post subject: Re: 18 Watt Plexi Mk II
PostPosted: Sat Sep 02, 2017 8:39 pm 
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Power supply trouble, bad connections to the output tubes, or trouble with V3 or its connections.


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 Post subject: Re: 18 Watt Plexi Mk II
PostPosted: Sat Sep 02, 2017 8:43 pm 
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I'll check around the output tubes for a bad connection. How would I test for a bad power supply?


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 Post subject: Re: 18 Watt Plexi Mk II
PostPosted: Sat Sep 02, 2017 8:47 pm 
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The best way is with an oscilloscope. Did you say you had access to one?


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 Post subject: Re: 18 Watt Plexi Mk II
PostPosted: Sat Sep 02, 2017 8:52 pm 
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Yes I have an oscilliscope


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 Post subject: Re: 18 Watt Plexi Mk II
PostPosted: Sat Sep 02, 2017 9:04 pm 
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OK. Make sure it can measure up to 500 volts or more. Set it on a high voltage range and hang a probe on the B+ that goes to the center tap of the output transformer. Ideally you will see a flat line around 400 volts, but you will see some ripple. You shouldn't see any spikes that correspond to the crackles. Also check the B+ at the anodes of V4 and V5.

You can also set the scope input to AC and use a lower voltage range for a closer look at the waveform.

I assume you know the basics of how to use a scope. Ie. setting vertical volts per division and horizontal time per division. If not, it would be best to get someone to help you.

Be careful!


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 Post subject: Re: 18 Watt Plexi Mk II
PostPosted: Sat Sep 02, 2017 9:44 pm 
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I read B+ at 356 Vdc when the amp is turned on. When the amp is in standby I read 290 Vac of a clean sine wave
At pin 9 of V4 and V5 I get 327 Vdc
Both the readings from when the amp is fully on, have a little fuzz to them, but other than that are very constant, no noticeable sine wave present.
As I was checking these measurements my amp fell silent and when I checked the V5 tube had started to red plate again so I turned the amp off. When I turned it on again the V5 tube had stopped overheating and the crackling came back.


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