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 Post subject: Re: 18 Watt Plexi Mk II
PostPosted: Thu Oct 26, 2017 9:59 am 
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So I got a few caps from my schools electrical department and tried subbing them in.
First I replaced the with a cap that was rated at 100k, and I measured it with my scope at 1.4 nF. This didn't change anything. There was still crackling present, the crackling got worse when I turned up the volume on my normal channel and the V5 tube was still overheating.
I then tried a cap that I measured to be 1 nF. When I turned the amp on this time the crackling was still present and it was effected by the normal channel. There was no apparent red plating when this cap was in. I then pulled the tubes to check voltages. I should state that both tubes felt extremely hot when i pulled them, but the V5 tube felt a little hotter.
At pin 2 of V5 I read 2.0-5.0 V dc with a very noisy signal and on the other side of the cap I see 231 V dc with a very clean signal.

What does this seem to indicate? It seems like the C8 cap isn't the the bad component that is messing this up, but I don't really know


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 Post subject: Re: 18 Watt Plexi Mk II
PostPosted: Mon Oct 30, 2017 6:04 pm 
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what does a cap rated at 100k mean? or does it say 100k on it ?

http://www.robotoid.com/appnotes/electr ... kings.html


Can we get a current picture of the two output tubes and accosiated wiring


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 Post subject: Re: 18 Watt Plexi Mk II
PostPosted: Thu Nov 02, 2017 10:54 pm 
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On the first cap I tried, has 103K written on it. From the website you linked I see that means .01, the second cap I tried had 222 written on it so that corresponds to .0022. I do have a couple caps that are marked 104 which means .1 I didn't try these in the circuit cause the terminals on them are very small and seemed like it would be a pain, but I obviously could try if you think that could help.
Neither of these caps seemed to change anything.
I have attached some pics of the current build. I redid the "jumper cables" on the turret board so I could see them. I also have the 222 cap still in the circuit because it was the last one I tested. When I turned the amp on today, there wasn't much crackling through the amp until the V5 tube got hot them the crackling got very bad very quickly.

What should I do?


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 Post subject: Re: 18 Watt Plexi Mk II
PostPosted: Fri Nov 03, 2017 9:41 am 
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Its hard to tell but it looks like pin 2 and 3 are wired backwards on v 5 in comparison 2 v4


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 Post subject: Re: 18 Watt Plexi Mk II
PostPosted: Fri Nov 03, 2017 5:02 pm 
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Yes I see what you mean, but It is wired correctly. The blue wire goes to pin 3 and the yellow wire goes to pin 2


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 Post subject: Re: 18 Watt Plexi Mk II
PostPosted: Sat Nov 04, 2017 10:46 pm 
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The only think I can think of is that pin2 and 3 are somehow shorted in the socket (almost looks like your getting the cathode voltage on pin 2)

Unplug amp and pull v5. disconnect the yellow and blue wires from the board. and use an ohm meter to check continuity between pins 2 and 3 on v5 .
Zero resistance bad . no resistance good


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 Post subject: Re: 18 Watt Plexi Mk II
PostPosted: Fri Nov 10, 2017 12:55 am 
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So I pulled V5 and the blue wire and measured the resistance and it was infinite resistance, and open circuit between the two pins.


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 Post subject: Re: 18 Watt Plexi Mk II
PostPosted: Sun Nov 12, 2017 8:19 pm 
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Since that doesn't seem to be the problem, what else could it be?


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 Post subject: Re: 18 Watt Plexi Mk II
PostPosted: Wed Nov 15, 2017 5:08 pm 
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Sorry i miss read something and missed the fact that when you replaced the grid cap for v5 that the voltage came down on pin 2.

But im at a loss to why your getting tube over heating at this point if the votages are in check . maybe remove c 8 from the circute and see what voltages you have on the grid of v5

Im going to recheck the voltages on my amp in way of comaprison


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 Post subject: Re: 18 Watt Plexi Mk II
PostPosted: Tue Nov 21, 2017 6:58 pm 
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So I removed the C8 capacitor and took measurements on the V5 tube. They are as follows,
V5:
Pin 1: 0 vdc
Pin 2: 25 vdc
Pin 3: 26 vdc
Pin 4: 3.3 vac
Pin 5: 3.4 vac
Pin 6: 0 Noisy
Pin 7: 290 vdc. Very noisy, couldn't get a good reading. meter was jumping around
Pin 8: 2.3 vdc Noisy
Pin 9: 303 vdc

During this process the V5 tube still overheated and there was lots of static/ crackling coming through the amp.
What does this seem to indicate?

Cooper C


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 Post subject: Re: 18 Watt Plexi Mk II
PostPosted: Wed Nov 22, 2017 1:27 am 
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I don't have an 18 watt amp here right now to compare, but I would think with C8 removed you shouldn't see much voltage at pin 2 of V5. Not 25 volts anyway! Maybe someone with an 18 watt Plexi can verify this.

Pins 1, 6 and 8 have no connections so disregard anything you measure there.

The voltage at pin 7 should not read "noisy." But with the pin 2 problem it's hard to say.

Are you 100% sure there's nothing wrong with the V5 socket itself, your wiring to pin 2 or the board in the C8 / R31 area? Some of your wires look burnt and nicked in the photos. Check the bottom side of the board, too. Sometimes stray wires that cause shorts are hard to see. Try cleaning the board with alcohol or flux remover and a Q-tip swab.

Try removing the wire at pin 2 of the V5 socket and see if you still read 25 volts there.

As sazafraz said, we are pretty much out of ideas here, and it's hard to troubleshoot a problem like this without having the amp on our benches.


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 Post subject: Re: 18 Watt Plexi Mk II
PostPosted: Thu Nov 30, 2017 11:41 pm 
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The socket for pin 3 on the V5 tube does look a little damaged. It doesn't look like there isn't a connection but the socket has pushed up higher than the others.
I took the wire off of pin 2 and measured 9 mv dc at the pin. I then noticed that the wire was connected to the wrong turret on the board. It was connected with the 0.022 cap and the 100pF cap. When I moved this back to the correct location lots of the crackling stopped. I measured the voltage at pin 2 now and it measured in the range of 100 mv.
I'm not exactly sure why the wire was in the wrong place and how long it was there for. I should also state there is no more obvious red plating in the V5 tube. It looks just like the other tube.
The level of crackling that is currently in the amp is low enough that I could live with is, but I feel is more than there should be. Would ruin any recording through the amp.
Finally when the normal channel's volume is increased the crackling get incredible loud very quickly. This seems like there's a grounding issue between the normal channel inputs and the V3 tube as this problem doesn't occur with the plexi channel.

Were should I start to check the grounding in the normal channel? Is the slight deformity in the V5 socket something to be worried about? Is there anything I can do about the ambient crackling sound? And to address your last comment, do you think it's time to let someone else solve this problem for me?
Thanks for all the help! I hope we are on the home stretch.
Cooper


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 Post subject: Re: 18 Watt Plexi Mk II
PostPosted: Fri Dec 01, 2017 12:01 am 
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Quick update,
I was cleaning the flux off the turret board and turned on the amp afterwards and found that the crackling that comes from the volume pot of the normal channel has decreased greatly, but is still there. Also the crackling gets very out of hand if the tone nob is turned up.
I then noticed that the V4, yes V4, tube was red plating. I measured the voltage at pin 2 and got 20 v dv.
What could have caused this switch?


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 Post subject: Re: 18 Watt Plexi Mk II
PostPosted: Mon Dec 04, 2017 2:32 am 
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I checked your photos several posts ago to see if the pin 2 wire for V5 was on the proper turret and it did appear to be. If it was connected to the the C21/C8 turret as you said, that's the wrong end of C8 and the voltage there is pretty high.

You can check the grounds by connecting one probe of your ohm meter to the chassis and the other to the various components that should be grounded. Check the input jacks and the volume pots and all the components along the ground side of the circuit board.

Have you looked on the bottom of the circuit board to make sure everything is OK?

But it sounds like there are still some flaky things going on with your amp the way the problems keep changing. If you can't find them based on everything we have posted so far, it's time to take the amp to a professional amp technician.


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 Post subject: Re: 18 Watt Plexi Mk II
PostPosted: Thu Dec 14, 2017 1:24 pm 
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As far as seeking professional help, should I ship it back up to the north so the real experts can take care of it? or should I just find a local professional?


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 Post subject: Re: 18 Watt Plexi Mk II
PostPosted: Fri Jan 19, 2018 9:39 am 
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There is a lot of flux and splatter on the board so I really suspect solder joints.
You should try to clean up the board with acetone and a toothbrush first.

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 Post subject: Re: 18 Watt Plexi Mk II
PostPosted: Wed Oct 03, 2018 5:36 pm 
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Hello again,
Still not done sadly, but progress is being made.
Last week I took the amp into a professor at my school who has experience with tube amps. He educated me greatly in how the amp works, specifically what the tubes do. We then figured out that the grid voltage on the V4 and V5 tubes where way to high.
We consistently read 22V on V4 and 36V on V5. When the amp is left on V5 is red plating but V4 is not visibly red plating. We pulled the two caps that are in the biasing circuit, C8 and C9 I think, to make sure they weren't shorted. We found that they showed no connectivity through the cap, so they seemed good. We measured the resistance from ground to pin 2 and got the expected 108 KOhms.
This lead us very stumped as to why the tubes where out of bias. I just recently replaced the output tubes so they shouldn't be broken unless the circuit fried them instantly.
It is also worth mentioning that since I last posted I determined the tube sockets for V4 and V5 where bad. There was continuity between pins 7,8 and 9. I have replaced them both.
Any tips or tricks about how to properly bias the output tubes? Seems like everything checks out but the voltage at the grid pins is far too high.

Cooper C.


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 Post subject: Re: 18 Watt Plexi Mk II
PostPosted: Wed Oct 03, 2018 11:14 pm 
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Voltage at grid being high might be due to parasitic oscillation. It can drive tubes wild. try moving the wires to the power tubes around and see if It changes. You can disconnect the grid leads and confirm the bIas is correct as well. Then you will know.

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 Post subject: Re: 18 Watt Plexi Mk II
PostPosted: Fri Oct 05, 2018 1:20 pm 
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Not sure if a typo or not but, what are the wores and where should I move them?


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 Post subject: Re: 18 Watt Plexi Mk II
PostPosted: Fri Oct 05, 2018 7:03 pm 
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Believe he meant wires, "i" and "o" are next to each other, so a typo.


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