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 Post subject: Re: 18 Watt Plexi Mk II
PostPosted: Wed Feb 15, 2017 11:01 pm 
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Sorry for the delay, I guess it's time for another update.
I when through my solder connections around v5 and didn't find anything that looked suspicious. I redid any of them that looked not perfect. When I plugged back in, it seemed that nothing had changed. My v5 tube was still overheating and the sound that the amp produced was extremely crackly.
Do you guys have any clue why this keeps happening? Should I cave and just buy new tubes? Do you guys think that might help?

Thank you for all your continued patients and help!

Cooper Cordero


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 Post subject: Re: 18 Watt Plexi Mk II
PostPosted: Thu Feb 16, 2017 12:39 pm 
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Sounds like a tube. Both tubes share a common cathode and anode so if that is OK snd the other tube is fine, it must be the tube. Swap tubes and see if the problem moves with the tube location!

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 Post subject: Re: 18 Watt Plexi Mk II
PostPosted: Sun Feb 19, 2017 6:11 pm 
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So i swapped the v5 and v4 tubes, and the v5 tube still overheated. It seams that the problem is on the v5 tube socket, and not the tube its self.
What does this indicate might be wrong?


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 Post subject: Re: 18 Watt Plexi Mk II
PostPosted: Mon Mar 20, 2017 10:02 am 
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Hey guys,
So I'm heading home next week for spring break and was looking to do some work on my amp. Do you guys have any suggestions? Is it time to start replacing parts? Or is it time to just let a professional handle it?


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 Post subject: Re: 18 Watt Plexi Mk II
PostPosted: Tue Apr 18, 2017 6:53 pm 
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Hello again,
My dad is convinced that the pots on my amp have gone bad. Is there a brand that you would recommend that I purchase to replace the ones that came with my amp?

Cooper Cordero


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 Post subject: Re: 18 Watt Plexi Mk II
PostPosted: Wed Apr 19, 2017 10:04 am 
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Cmcordero wrote:
Hello again,
My dad is convinced that the pots on my amp have gone bad. Is there a brand that you would recommend that I purchase to replace the ones that came with my amp?

Cooper Cordero


I Don't know how he came to that conclusion. It would be extremely rare (as in never happened to me), and in your case your issue was with the power tube?? Can he explain his logic and maybe we can help

We use good quality Alpha pots. You can spend a lot more (PEC, Bourns, CTS) but get marginal, if any, improvement.

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 Post subject: Re: 18 Watt Plexi Mk II
PostPosted: Sun Jul 23, 2017 9:24 pm 
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Hello,
Today my grandpa came over and we decided to take a look at my amp, which is still giving me troubles.
We found that the B+ isn't correct. We are reading 320 V DC with 7 V AC. We looked back on what other readings we had gotten throughout our time and found that they varied wildly. While taking these measurements we had almost no output, only a soft crackle and hum.
Should the B+ stay constant at the recommended voltage of 350? Also does the 7 volts of AC matter or is that negligible?

Thank you for your continued help!

Cooper C


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 Post subject: Re: 18 Watt Plexi Mk II
PostPosted: Tue Jul 25, 2017 8:59 am 
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The B+ depends on many things including the mains voltage, so posting that would be helpful. Also, it depend whether the amp is wired correctly because excessive high voltage current can make a big difference in B+. Maybe the power tubes are wired incorrectly?
My advice is to go back to startup procedure in the manual and check the power supply is working fine. If it is, then the next thing to check is the wiring and tubes of the output stage. Then the preamp checks.
Even at B+ of 320V, you should get a decent loud sound.

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 Post subject: Re: 18 Watt Plexi Mk II
PostPosted: Mon Aug 14, 2017 6:15 pm 
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The mains voltage is 122.2 V AC a very clean signal. I checked the voltages coming out of the power transformer and they all matched the printed voltages on the transformer.
What setup procedure are you talking about? I see a final checkout chart with voltages. Is this the chart you mean or is there something else? Also do I complete these measurements with or without the tubes in place?

Cooper C


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 Post subject: Re: 18 Watt Plexi Mk II
PostPosted: Wed Aug 16, 2017 5:12 pm 
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I did some work today and found/ fixed a couple of problems.
First, I redid almost all the solder joints on the circuit board. This helped and let me get a good sound from my guitar through the normal channel. I then focused on the plexi channel. I found that the co axial cable that goes from the gain pot was shorted to its shielding. I fixed this and checked the brown channel and found it gives off a great sound. During these tests there was a noticeable hum. This hum got increasingly worse as my playing continued. I went back and checked some more solder joints.
When I checked again the hum got INCREDIBLY loud (I had to put on head phones).
Why is this the case? There is no sound coming out of the speaker when the amp is on standby. But the second I fully turn on the amp, the amp starts to hum and crack. Why would this have happened? I still get my guitar to sound through the hum and crackling, but its very quiet compared to the hum. What should I check?
I should also state that the re-soldering of the circuit board fixed the overheating of the V5 tube.

Thanks for all the help!!
Cooper Cordero


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 Post subject: Re: 18 Watt Plexi Mk II
PostPosted: Fri Aug 18, 2017 1:46 am 
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Glad you got the V5 problem fixed.

Is the hum level on the Normal channel OK?

Sounds like you still have a wiring problem with the Brown channel. Make sure all the connections to the jacks, tubes and pots are OK and all the grounds are good.

Does the hum on the Brown channel go away if you turn its Volume control down all the way? Do the Treble, Mid and Bass controls have any effect on the hum?

Is the Brown channel fairly quiet if you don't have anything plugged into its input and you turn up its Volume control?


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 Post subject: Re: 18 Watt Plexi Mk II
PostPosted: Fri Aug 18, 2017 1:50 am 
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The hum problem is not related to either channel. When I turn the amp on (with all nobs at zero) the crackling is so loud I had to put ear protection on just to stand it. I checked with and without a guitar hooked up and it didn't change


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 Post subject: Re: 18 Watt Plexi Mk II
PostPosted: Fri Aug 18, 2017 2:01 am 
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OK. The problem is in the power amp or the power supply. What happens if you pull out V3? Do you have a good connection from the black wire on the output transformer to the output jack ground?

What's the B+ read?


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 Post subject: Re: 18 Watt Plexi Mk II
PostPosted: Fri Aug 18, 2017 2:16 am 
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When I disconnect the v3 tube and turn it on I get a soft hum that is completely acceptable and I also get some soft crackling. The crackling is greatly reduced from before and is kinda quiet. I could live with it like it is, but would like to eliminate it if possible.
I have a good solder joint from the output transformer to the output jack ground wired correctly with plenty of solder
My B+ reads 324 V a.c.
Also I noticed that there is a very small hum when I turn the amp on into standby, then once I completely turn it on the crackling begins


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 Post subject: Re: 18 Watt Plexi Mk II
PostPosted: Fri Aug 18, 2017 10:32 am 
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I would suggest that you touch up all your solder joints around V3 particularly. Crackling is almost always a bad solder joint or forgotten and subsequently not soldered joint or even a loose wire.

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 Post subject: Re: 18 Watt Plexi Mk II
PostPosted: Fri Aug 18, 2017 5:15 pm 
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One step forward, Two steps back.
I went a head and touched up the solder joints around the V3 tube. When I checked the amp the crackling had gone down quite a bit. (One step forward) But the guitar sound was very quiet, about the same level as the crackling. Also my V5 tube started to overheat again. (Two steps back)
It seems to me like the crackling problem hasn't changed. It's more like something is holding the entire system back. I think if the V5 tube would stop overheating I would return to an amp that is very loud with a good guitar sound, but lots of cracking.
I did also pull the V3 tube and check it again. This yielded no change. I still had a small amount of crackling with an equal amount of guitar sound and an overheated V5 tube.
What might have changed to cause the amp to fall back into its old ways?

Cooper


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 Post subject: Re: 18 Watt Plexi Mk II
PostPosted: Wed Aug 23, 2017 6:09 pm 
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Sorry for the delay. I went to a remote part of Oregon for the eclipse. No phone, no lights, no Internet!

Did you try swapping in a different tube for V3? You could try one of the 12AX7's from the preamp.

Keep the volume pots down all the way until the power amp is working without crackling, humming or the EL84's red plating. Then move on to testing the preamp.

Also check C8 and C9. Especially C8. With the volume pots turned all the way down, at C8 you should see around 220 VDC on one side and pretty much nothing on the other side. Same for C9. You should see pretty much nothing at the pin 2's of V4 and V5, too.

Problems that come and go are usually due to bad connections, bad tubes or bad capacitors.


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 Post subject: Re: 18 Watt Plexi Mk II
PostPosted: Wed Aug 23, 2017 9:46 pm 
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I'll start by explaining what my amp has desided it wants to do today. When I plug it in and power it on, I get very little sound through it. When I crank the volume and play my guitar I get a very small crackling sound through the amp. Also my V5 tube is overheating.
I swapped the V3 tube with the other 12AX7's in the pre-amp and nothing changed.
The voltages of the C8 and C9 are;
C8: 22V and 50-70V
C9: 1.5V and 135V
V4 Pin 2: 2.1V
V5 Pin 2: 29V
Those readings are all Vdc, but the signals are very noisy.
I soldered the C8 and C9 caps and checked their capacitance.
C8: .0228 uF
C9: .0228 uF
I soldered the caps back into place and turned the amp back on and nothing changed.
What's the next step?

Cooper C


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 Post subject: Re: 18 Watt Plexi Mk II
PostPosted: Wed Aug 23, 2017 10:39 pm 
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The voltages are way off at V3 and the 29 volts at pin 2 of V5 is why it's overheating (red plating).

What are the voltages at both sides of R28, R22, R17 and R16?

Are you sure there is no trouble on the board around these parts and R18, R19, R30 and R31? And the connections to V3, V4 and V5? Like accidental shorts, open connections, resistors in the wrong place, resistors the wrong value, etc?


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 Post subject: Re: 18 Watt Plexi Mk II
PostPosted: Thu Aug 24, 2017 2:16 am 
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Also V5 might be bad by now with all the overheating it has gone through. Pull V4 and V5 and do the voltage measurements listed in my previous post. Check the voltages at the pins of V4 and V5's sockets, too. V5 could be drawing too much current from the power supply and messing up the voltages at V3.


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