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 Post subject: Re: 18 Watt Plexi Mk II
PostPosted: Sat Sep 02, 2017 10:25 pm 
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Holy Ghost
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Suggest you Redo or touchup the solder joints on. the power tubes and power tube components on the turret board.

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 Post subject: Re: 18 Watt Plexi Mk II
PostPosted: Sun Sep 03, 2017 7:05 pm 
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I have touched up some of the solder joints around V4 and V5 and the crackling problem hasn't gone away. At one point the crackling stopped but then it came back very quickly.
Is there a special trick to soldering on the tubes?


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 Post subject: Re: 18 Watt Plexi Mk II
PostPosted: Tue Sep 05, 2017 9:46 pm 
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Cmcordero wrote:
I have touched up some of the solder joints around V4 and V5 and the crackling problem hasn't gone away. At one point the crackling stopped but then it came back very quickly.
Is there a special trick to soldering on the tubes?


I use lots of heat and hold until it flows easily. On lugs, I fill the holes with solder. But there may be other problems. Is the power tube still flashing?
One way to determine where the crackling is coming from is to pull V1 and see if crackling continues. If it does, pull V2 and repeat. If you get to the power tubes and there is still crackling, you know you've found the area to work on. Thisis a way to divide an conquer.
Crackling is often a bad solder joint but it can be a component as well.

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 Post subject: Re: 18 Watt Plexi Mk II
PostPosted: Thu Sep 07, 2017 10:54 am 
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Alrighty,
So I started pulling tubes to find where the crackling was and found that there was still crackling when I had pulled V1 V2 and V3. I then pulled V4 and V5 (With 1,2 and 3 still pulled) and there was no crackling, That makes sense the only tube that was in was the rectifier. I then put in only the V5 tube, this resulted in the same level of crackling as normal. I then only put the V4 tube in and the amp made more of a static sound. This sound was quieter and overall higher pitched.
No the V5 tube is not still flashing. I believe that was just one one singular tube that had been worn out and I have since replaced both power tubes.
It seems to me that the problem is in the V5 tube area, not really a surprise as its been the problem child of this project. I also noticed from me pulling the tubes, that the V5 tube feels hotter than the V4 tube. There is no noticeable red plating in the V5 tube but it was something I noticed.
I also noticed that some excess solder had dripped from the turret board to the frame, so I pulled the board up and cleaned up some of the remains. One of them even seemed like it was touching the case itself. When I turned it on after this, it seemed that the crackling had become more faint but was still present.
My plan is to redo some more solder joints around V5 until the crackling goes away. Is there something else I should check or fix?

Cooper


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 Post subject: Re: 18 Watt Plexi Mk II
PostPosted: Wed Sep 13, 2017 11:58 pm 
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So I've been touching up solder joints around the amp and I've got the crackling down pretty low. But through my testing I found that the crackling will increase with the normal channel volume and tone knobs. It seems that as the volume knob increases the cracking builds upon its self and gets loud. Also When the plexi channels are maxed volume and gain, the crackling increases, but if I was actually playing at this would be negligibly loud but I thought I would note it.
I touched up the solder joints on the normal channel volume and tone pots and a couple random ground connections in the preamp and I didn't notice any big change in the level of crackling.
What should I check next? I think the problem may be isolated in the normal pre-amp.

Cooper


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 Post subject: Re: 18 Watt Plexi Mk II
PostPosted: Sun Oct 01, 2017 10:42 pm 
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So last night I was redoing some solder joints around the pre-amp and when I went to check the amp my V5 overheating problem came back. I didn't check the voltages at pin 3 of V4 and V5 but my guess is V4 is fine and V5 is a little high.
What could have caused this? Is there anything that I should check on to try and fix this?

Cooper C


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 Post subject: Re: 18 Watt Plexi Mk II
PostPosted: Wed Oct 04, 2017 8:47 pm 
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If the voltages at the anodes of V4 and V5 aren't reasonably equal and a good tube in V5 runs hot, it could be a problem with the output transformer. And it would be an intermittent one judging by all the crackling and the way the problems come and go.

Pull out V4 and V5 and with the power off measure the resistance from the center tap of the output transformer to each of its primary leads. You can get the center tap at the standby switch. One primary will be at pin 7 of V4 and the other primary will be at pin 7 of V5. Both readings should be pretty much equal.


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 Post subject: Re: 18 Watt Plexi Mk II
PostPosted: Thu Oct 12, 2017 12:42 pm 
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With V4 and V5 pulled and the amp off, I get 267 ohms from B+ to pin 7 of V4 I get 267 ohms. On pin 7 of V5 I get 321 ohms.
Since these measurements are off by 54 ohms of each other, should I consider replacing the output transformer? Could that be whats been tormenting me these last 14 months?

Cooper


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 Post subject: Re: 18 Watt Plexi Mk II
PostPosted: Thu Oct 12, 2017 8:32 pm 
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Transformer problems are rare, but they do happen. Trouble with the wiring with kits or flaky tubes are more common.

However, I think we have eliminated tubes and wiring as the culprits. And the problem always seems to be with V5 no matter what you do. So I would say it's the output transformer. Maybe someone else can chime in with their opinion.


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 Post subject: Re: 18 Watt Plexi Mk II
PostPosted: Thu Oct 12, 2017 8:40 pm 
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Is there any more test I should run before buying a new transformer?


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 Post subject: Re: 18 Watt Plexi Mk II
PostPosted: Thu Oct 12, 2017 9:39 pm 
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Just to verify, next time V5 is overheating, measure the voltage at the anodes of V4 and V5. Check the cathode and screen voltages, too. There shouldn't be much difference between the two tubes if the amp is working properly.

Swap V4 and V5. If the tube in the V5 socket still overheats that's a good indication the OT is bad.


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 Post subject: Re: 18 Watt Plexi Mk II
PostPosted: Thu Oct 12, 2017 10:47 pm 
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Here are the measurements I got with V5 overheating.
V5 Pin 2: 27.1
V5 Pin 3:26.4
V5 Pin 7:276
V5 Pin 9:282
V4 Pin 2: 4.4
V4 Pin 3:26.3
V4 Pin 7:320
V4 Pin 9:282

After I switched the V4 and V5 tubes I got
V5 Pin 2:26.6
V5 Pin 3:26.3
V5 Pin 7:0.2
V5 Pin 9:285
V4 Pin 2:5.3
V4 Pin 3:25.6
V4 Pin 7:320 noisey
V4 Pin 9: 285

When I switched the tubes the overheating problem continued and looked worse.
So is it decided that the OT is bad?

Cooper


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 Post subject: Re: 18 Watt Plexi Mk II
PostPosted: Thu Oct 12, 2017 11:54 pm 
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I assume the 0.2 volts you said you got at pin 7 of V5 after swapping the tubes isn't right since the tube must have been conducting if it was overheating. Maybe your probe wasn't making good contact.

I'm still worried about the 27 volts showing at pin 2 of the V5 socket. That's way too high. That could mean C8 is leaky but I think you tried replacing it and nothing changed. I think you verified that R19 and R31 are OK, too.

Just out of curiosity, what are the voltages at pin 2 of the V4 and V5 sockets if V4 and V5 are removed?


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 Post subject: Re: 18 Watt Plexi Mk II
PostPosted: Thu Oct 19, 2017 7:57 pm 
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With V4 and V5 pulled on pin 2 I get
V4: 1.5
V5: 30.0
The values seemed to jump around a lot but they seemed to stay around those values. The oscilloscope also shows a pretty noisy signal.

When I plug V4 and V5 back in at pin 2 of V5 I get about 300V. This is a very noisy and it jumps around too. For accuracy's sake I measured V4 and got 341 V. When I turned the volume up on my normal channel the signal the signal became very noisy and I couldn't get an accurate measurement.

Also, I never replaced the C8 cap I only every removed one end and tested that the capacitance was what it should be. Same thing I did with the R19 and R31 resistors.

What does all this mean? I think trying to change the C8 cap might be a good idea. Should I try and switch the C8 and C9 caps and see if the overheating goes to the V4 tube?

Cooper C


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 Post subject: Re: 18 Watt Plexi Mk II
PostPosted: Thu Oct 19, 2017 11:01 pm 
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Replace C8 with a new one. I wouldn't bother swapping it with C9.


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 Post subject: Re: 18 Watt Plexi Mk II
PostPosted: Thu Oct 19, 2017 11:13 pm 
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Where would you recommend I buy a new cap? Or does any old cap work?
I should state that I did get the sozo mustard caps if that changes anything

Cooper


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 Post subject: Re: 18 Watt Plexi Mk II
PostPosted: Fri Oct 20, 2017 2:54 am 
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I don't know where you live, but if it's somewhere that has a good electronics store you can probably get the capacitor there. The Sozos are good capacitors, but really, anything with the same capacity and voltage rating will work. It doesn't hurt if the voltage rating is higher.

If you want to order on-line, a couple of good places are Antique Electronic Supply in Arizona and Watts Tube Audio in Florida. AES has the Sozos. The Mallory 150 series are good too.

The prices are pretty good at the on-line stores, but the shipping is what gets you. Browse around the sites and see if there is anything else you might want to help justify the shipping charges!


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 Post subject: Re: 18 Watt Plexi Mk II
PostPosted: Fri Oct 20, 2017 8:01 am 
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If you want to test your transformer, Testing 18 OT

If you suspect your OT for some reason, you can do a resistance check to get an idea of it's health.

Disconnect all the wires from the circuit, then use a digital ohm meter to measure the following resistances. Clip the probes to the leads, do not hold them with your fingers.

Primary
Brown - Red: 275 ohms
Brown/White - Red: 160 ohms
Blue - Red: 331 ohms
Blue/White - Red: 166 ohms
Brown - Blue: 606 ohms
Brown/White - Blue/White: 326 ohms

Secondary
Black - Orange (16): 1.0 ohms
Black - Green (8): 0.7 ohms
Black - Yellow (4): 0.6 ohms
Note, it is very hard to measure low resistance.

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 Post subject: Re: 18 Watt Plexi Mk II
PostPosted: Fri Oct 20, 2017 8:11 am 
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Holy Ghost
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Cmcordero wrote:
With V4 and V5 pulled and the amp off, I get 267 ohms from B+ to pin 7 of V4 I get 267 ohms. On pin 7 of V5 I get 321 ohms.
Since these measurements are off by 54 ohms of each other, should I consider replacing the output transformer? Could that be whats been tormenting me these last 14 months?

Cooper


Take a look at the post on the OT resistances . They are not the same on each side of the OT And that is OK.

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 Post subject: Re: 18 Watt Plexi Mk II
PostPosted: Fri Oct 20, 2017 5:30 pm 
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Holy Ghost
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You should not have much DC at the grids of any of the tubes, otherwise you could get nose, static and overdriven tubes.
Check for DC before the cap (correct) and after (incorrect) .
Looks like a bad cap. We can send you one. To test you need something in the .01-.1 polyester 400VDC range.
if you swap caps, you should will DC on the other grid.

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