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 Post subject: 18 Watt Plexi Mk II
PostPosted: Sun Sep 18, 2016 3:26 pm 
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Hey Guys,
I finished assembly of my 18 watt plexi mk ii, and I have run into some issues that I was hopping you could help me with. My amp seems to work, but just barley. What I noticed is the Normal channels work but sound very crackly and don't have any low decibel response. I really have to dig into my guitar to make a sound, but the sound it makes isn't quiet. Next, I noticed that the brown channels don't work at all. I have double checked the wiring and haven't found anything wrong. Finally, I noticed that my V5 output tube was glowing hotter than the V4 output tube. I swapped the tubes out with each other and still the V5 tube was glowing hotter than the V4. After some research I found that it looks like my tubes aren't biased correctly. I took voltage readings from ground to the heater wire on tube V5 and V4 and they where equal. I have also isolated the "biasing" problem to when the amp ins't in stand by. By this I mean that when I flip the power switch everything looks good, but when I flip the standby switch the V5 output tube quickly overheats, even when all nobs are at 0.
My finally, is how close should all the voltage readings be? I took readings from all the points indicated on the schematic diagram, and most where very close but some where further off. Some of the further off readings are, (listed as what it should be- then what I measured) 12-27.25, 340-286, 217-168, 180-223, 180-111, 155-113, 1-0.57. All these voltages were measured in DC, from ground to the point shown on the wiring schematic.

From,
Cooper Cordero


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 Post subject: Re: 18 Watt Plexi Mk II
PostPosted: Mon Sep 19, 2016 12:48 am 
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Might want to post a picture of the build


do you have a reference to voltage on pins of the tube's pins 4and5 would be AC readings on the el84's and 4 5 9 on the preamp tubes would also be ac. all others would be dc to ground there is a chart in the build guide if i recall correctly

would like to know what you have coming from the wall and what your b+ is at from the first capacitor

voltage have variance it depends on where it starts but some place between 10 to 15 % is reasonable

also note when you turn the amp on. only power to the heaters and rectifier tubes is supplied. when you take the amp out of standby the high voltage circuit is then supplied to the rest of the tubes


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 Post subject: Re: 18 Watt Plexi Mk II
PostPosted: Sun Sep 25, 2016 10:26 pm 
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Alright, time for an update.
Over this last weekend me dad, a friend, and me put in some time to try and figure this amp out. With keen eyes we found a couple of wiring errors. These were fixed as were all the solder joints in the amp. I went back over all of them and made sure that they were all had a more than generous amount of solder. Some of the same problems persist but others are now solved. The brown channels now work and i am starting to get more response out of the amp at lower volumes. The problems that are persisting are the sound crackles up at lower volumes and dies off quickly and my output tubes are very unbalanced. My v5 tube still glows well hotter(See pic below).

As for the requested voltage readings, I get 6.8 V AC on my heater wires. This is for all tubes except the rectifier. Secondly, I have my measured the voltage that i get from the wall and it comes in at 122 V AC.
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Any help is greatly appresiated and i hope i can fix this amp and make it sound great very soon

From,
Cooper Cordero


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 Post subject: Re: 18 Watt Plexi Mk II
PostPosted: Mon Sep 26, 2016 8:52 pm 
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Cant see the pictures yet, for some reason. Crackling is often solder joints. Not generois amounts of solder, but enough with proper heat. The joint should be pretty shiny.
But crackling can also be bad tubes, so you should eliminate that problem . Also get that power tube sorted out.

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 Post subject: Re: 18 Watt Plexi Mk II
PostPosted: Wed Sep 28, 2016 6:46 pm 
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Alright, I have put the pictures into a word doc attached to this post. Hopefully this allows you guys to view the pictures of my amp.

Thanks for all the help,

From,
Cooper Cordero


Attachments:
Tube Amp Pictures.docx [40.81 MiB]
Downloaded 428 times
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 Post subject: Re: 18 Watt Plexi Mk II
PostPosted: Thu Sep 29, 2016 1:01 pm 
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Cant get a good look until i get home but i can see that you do not have a wire going to the grid of what i am presuming is the glowing tube . (pin 2) . the output of the phase inverter has a yellow wire to one el84. and the output of the other side of the P.I has no wire going to the other el84. and i hope that that tubes survives that amount of red plating


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 Post subject: Re: 18 Watt Plexi Mk II
PostPosted: Thu Sep 29, 2016 5:42 pm 
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Tried to open that file at home with zero luck. At work was all messed up. Can you save the pictures in something other than an MS office file type


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 Post subject: Re: 18 Watt Plexi Mk II
PostPosted: Thu Sep 29, 2016 11:02 pm 
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Alright,
I've added each of the pictures as a separate attachment on this post (10 in total). Hopefully this allows you all to see these photos.
Thanks for all the help with the build thus far. I will make sure to keep you all posted on how it turns out.

From,
Cooper Cordero


Attachments:
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 Post subject: Re: 18 Watt Plexi Mk II
PostPosted: Fri Sep 30, 2016 3:22 pm 
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the purple scribble is the one your missing


Attachments:
20160925_172029.jpg
20160925_172029.jpg [ 4.79 MiB | Viewed 15112 times ]
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 Post subject: Re: 18 Watt Plexi Mk II
PostPosted: Mon Oct 03, 2016 6:22 pm 
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Alright, another weekend has come and gone so I have done some work on my amp.

First, I added in the wire from the turret board to pin 2 of V 5. I then proceeded to to a sound check and it appears that not much had changed. With help from a friend who has far more experience working on tube amps we checked the wiring of the amp and made sure that the component values were correct. All of the resisters and capacitors checked out. We also switched around the tubes to see if that could have been the problem. This didn't change anything. The V 5 tube was still glowing far hotter and the amp sounded very crackly and didn't make sound as low decibels.

Our final idea was that the output transformer may have been fried and making no resistance through the output tubes. This wasn't the case as we go 8k resistance through the output transformer.
After that we ran out of ideas and time. What we concluded was that something isn't prohibiting the voltage to the V 5 tube, but everything we measured checked out (Except of course the voltages).

This project is stumping me right now and I would greatly appreciate any feedback, suggestions or potential fixes.

From,
Cooper Cordero


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 Post subject: Re: 18 Watt Plexi Mk II
PostPosted: Mon Oct 03, 2016 8:36 pm 
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One question the 100 ohm resistor that is from the screen supply attaches at pin 9 of v5 wit a jumper to pin 9 of v4. Is that resistor touching the screw that holds the tube socket of v5 ?


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 Post subject: Re: 18 Watt Plexi Mk II
PostPosted: Mon Oct 03, 2016 9:48 pm 
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No, the resistor is attached the lug that isn't attached to the chassis, same with the jumper wire.


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 Post subject: Re: 18 Watt Plexi Mk II
PostPosted: Mon Oct 03, 2016 10:26 pm 
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What i meant was the screw on the tube socket it just looked in the picture that it was just touching the screw


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 Post subject: Re: 18 Watt Plexi Mk II
PostPosted: Mon Oct 03, 2016 10:40 pm 
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Ok I see what you mean. But no it isn't hitting the chassis at any point.


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 Post subject: Re: 18 Watt Plexi Mk II
PostPosted: Sun Oct 09, 2016 8:06 am 
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Still think were gonna need the full voltage readings for the tube pins (not the heaters)

somethin like this format

V1
1. 23v
2. 147v
3. 2.7v
4. -
5. -
6. 32v
7. 0v
8. 147v
9. -

v2.
1.
2.


so on and so on

something really messed up with the power tubes . V4 and V5 should be almost identical readings


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 Post subject: Re: 18 Watt Plexi Mk II
PostPosted: Tue Oct 11, 2016 1:39 pm 
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Alright, here are my voltage readings:

B+346 VDC 7.7 VAC

V1
1. 120 VDC
2. 24 mV
3. 0.821 VDC
4. 3.3 VAC
5. 3.3 VAC
6. 4.0 VDC
7. 2.3 Noisy
8. 1.9 VDC
9. 3.3 VAC

V2
1. 118.9 VDC
2. 189 mV
3. .74 VDC
4. 3.3 VAC
5. 3.3 VAC
6. 228.8 VDC
7. 199 VDC
8. 125 VDC
9. 3.3 VAC

V3
1. 165.8 VDC
2. 30.5 VDC
3. 64.1 VDC
4. 3.3 VAC
5. 3.3 VAC
6. 164.5 VDC
7. 28.5 VDC
8. 63.4 VDC
9. 3.3 VAC

V4
1. NC
2. 30 mV Noisy
3. 19 VDC
4. 3.3 VAC
5. 3.3 VAC
6. NC
7. 248 VAC 6.3 VAC
8. NC
9. 315 VDC

V5
1. NC
2. 12 VDC
3. 16.7 VDC
4 3.3 VAC
5. 3.3 VAC
6. NC
7. 320 VDC 6.0 VAC
8. NC
9. 300 VDC

V6
1. 307 VAC
2. NC
3. 349 VDC 5 VAC
4. 285 VDC GND
5. 304 VDC GND
6. NC
7. 307 VAC
8. NC
9. NC
Voltage read between pins 1 and 7 of V6 was 615 V

The biggest thing that jump out to me here is the difference on pin 2 between V4 and V5. Any suggestions for why I might be getting this reading?


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 Post subject: Re: 18 Watt Plexi Mk II
PostPosted: Tue Oct 11, 2016 3:01 pm 
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Are you sure you put the jumper to pin 2 of v5 to the right spot ?


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 Post subject: Re: 18 Watt Plexi Mk II
PostPosted: Tue Oct 11, 2016 3:07 pm 
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Yes


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 Post subject: Re: 18 Watt Plexi Mk II
PostPosted: Tue Oct 11, 2016 11:33 pm 
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v4 pin 7 is low as well . Just can't see how your getting 12v on the grid of v5. If the wire goes from 8.2K resistors straight too the grids of the power tubes. pin 2 .

Also check the voltage on both sides of the 8.2k grid resistors

The power tube voltages should be almost identical.



edit removed the part about out put wire swap


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 Post subject: Re: 18 Watt Plexi Mk II
PostPosted: Sat Oct 15, 2016 1:20 pm 
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So, I went ahead and switched the output transformer wires on V4 and V5. This helped a lot. My first sound check after this was playing though the brown channels with the normal channels set to zero. This was very good. I had complete range of sound through this channel. It didn't sound crackly or poppy at low volumes. Also the power tubes looked slightly overheated at first, but by the time I finished playing they looked normal. The only problem that I saw after that was a very noticeable hum and the occasional crackle. I then tested the normal channels and got nothing. There was no sound emitted through the speaker, except a small buzz. I also noticed as I increased the volume on the normal channels the V5 tube started to overheat. My dad theorized that I may have a bad pot for the volume on the normal channel. But we had no idea on how to test if that was true.
Any suggestions on how to test the pot?


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