Blown PT?

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Riffmonster
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Blown PT?

Postby Riffmonster » Fri 06/07/19 5:05 am

Hey guys,

I did not post so much here yet, so I want to thank you all first for all the info available here. Big respect from my side!

I built a 18 Watt lite 2b during the last days and it was working fine until yesterday. I noticed blue arcing inside of the rectifier tube. I turned it off and checked for any visual flaws inside but could not find anything. I turned it on again and there is no sound coming out of the speaker anymore. I tried several rectifier tubes but the problem is still there.

I then decided to check the power transformer, since I could not measure any voltage output from the rectifier tube. There is no resistance from the PT secondaries to ground. This indicates that the PT is bad with an open winding, right? I just can not figure out, what has led to the failure of the PT. The whole thing happened when there was a little thunderstorm outside, but I do not think that this is the cause for the failure.

Any ideas would be much appreciated.
Thanks and cheers!
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Re: Blown PT?

Postby zuceno » Fri 06/07/19 5:58 am

Riffmonster wrote:
Fri 06/07/19 5:05 am
I then decided to check the power transformer, since I could not measure any voltage output from the rectifier tube. There is no resistance from the PT secondaries to ground. This indicates that the PT is bad with an open winding, right?
It doesn't tell you much - but you are not telling us enough about how you tested that, anyway.

There are two obvious possibilities. One is that it looks like the PT may have died from "natural causes". The other is that it may have died because it was stressed by other components trying to draw too much power from it, possibly because another component has failed.

From what you say, I would suspect that one of the smoothing caps has shorted out. Start by disconnecting each in turn from the B+ and checking the DC resistance, which should be very very high. Connect the red (+) lead of the meter to the (+) side of the cap when you do this. If any one of them gives a low resistance, you're beginning to sort out the problem.

How big are the smoothing caps - particularly the one nearest to the rectifier?

While I'm asking questions - did you notice any heaters glowing when you tried the alternative rectifiers? Don't go back and turn it on to check.
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Re: Blown PT?

Postby Riffmonster » Fri 06/07/19 8:19 am

Thanks for the quick reply and the advice!
I won't be able to check the caps until tomorrow but I have a 32 uf + 32 uf 500V can cap in the first place and then followed by a 15 uf 500 cap.
The heaters were all working fine on all tubes. I did not smell something burned and did not saw smoke.

I will check the caps tomorrow and provide some photos.
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geoff 1965
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Re: Blown PT?

Postby geoff 1965 » Fri 06/07/19 8:47 am

hello,
with the amp unplugged and filter caps drained a simple test you can do is set your multimeter to "ohms" and connect to both HT wires going to the rectifier tube,you should have a resistance reading "continuity" if the secondary is good.
has it blown the fuse as well?
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Re: Blown PT?

Postby zuceno » Fri 06/07/19 3:17 pm

Riffmonster wrote:
Fri 06/07/19 8:19 am
I have a 32 uf + 32 uf 500V can cap in the first place
That is too much! One of those will do. Two is more than the 50 MAX recommended by the tube manufacturers. That will overload the diodes. Ah well, at least we have some insight.

http://www.mif.pg.gda.pl/homepages/fran ... e/EZ81.pdf
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Re: Blown PT?

Postby JMPGuitars » Fri 06/07/19 4:33 pm

zuceno wrote:
Fri 06/07/19 3:17 pm
Riffmonster wrote:
Fri 06/07/19 8:19 am
I have a 32 uf + 32 uf 500V can cap in the first place
That is too much! One of those will do. Two is more than the 50 MAX recommended by the tube manufacturers. That will overload the diodes. Ah well, at least we have some insight.

http://www.mif.pg.gda.pl/homepages/fran ... e/EZ81.pdf
32/32uf is standard for the Lite IIb (the amp he's referring to). You can see the schematic in the downloads section.
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crgfrench
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Re: Blown PT?

Postby crgfrench » Fri 06/07/19 5:43 pm

According to the EZ81 datasheet posted, that is 50uF max for the FIRST cap in the filter. Not for the entire filter. At least that's how I am reading it. So 32 is fine, with the next 32 following later in the filter sequence.
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Re: Blown PT?

Postby zuceno » Fri 06/07/19 6:03 pm

crgfrench wrote:
Fri 06/07/19 5:43 pm
According to the EZ81 datasheet posted, that is 50uF max for the FIRST cap in the filter. Not for the entire filter. At least that's how I am reading it. So 32 is fine, with the next 32 following later in the filter sequence.
Exactly. There is a resistor in the B+ line between the first and second cap of every schematic I looked at, and the second 32uF is connected to the screen supply. But that is not what the OP said.
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Re: Blown PT?

Postby crgfrench » Fri 06/07/19 6:22 pm

JMPGuitars wrote:
Fri 06/07/19 4:33 pm
zuceno wrote:
Fri 06/07/19 3:17 pm
Riffmonster wrote:
Fri 06/07/19 8:19 am
I have a 32 uf + 32 uf 500V can cap in the first place
You can see the schematic in the downloads section.
Josh where is the downloads section -- all I seem to see here are Q&A forums? Thanks
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crgfrench
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Re: Blown PT?

Postby crgfrench » Fri 06/07/19 6:25 pm

Sorry I think I found it under "Navigation" -- doh!
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Re: Blown PT?

Postby Riffmonster » Sat 06/08/19 5:00 am

Hey guys,

thanks for the answers! Sorry if my language was not clear, but yes I have the 32uf cap first, then the resistor and then the other 32uf cap exactly like in the schematic. I checked the caps and they are all good.

Good news is, the PT is not blown. I did a noob mistake and did not check the HT fuse yesterday, which was blown. I get correct voltage readings and sound again with a new fuse and a different rectifier tube.
Still, I have not figured out what caused the HT fuse to blow. Maybe it was just a bad rectifier tube after all (was a new JJ)?

I attach a photo of the power supply.
You do not have the required permissions to view the files attached to this post.
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Re: Blown PT?

Postby JMPGuitars » Sat 06/08/19 7:14 am

Two things:

1. When making solder connections, whenever possible, you want a mechanical connection as well as a solder connection. In the case of solder tabs (like your IEC socket, & switches), that means bending the wire around the solder tab (rather than just sticking it through). After the wire holds itself in place on the solder tab, then you solder it. This helps make sure that if something is overheated, the solder won't let the wire go.

2. What's up with that second fuse? It looks like you have a grey wire from the PT going to a fuse, and then to ground?

Thanks,
Josh
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Riffmonster
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Re: Blown PT?

Postby Riffmonster » Sat 06/08/19 7:38 am

Hi Josh,

thanks for the tip with the soldering. I will redo them!

The second fuse is the HT fuse. My transformer has center tap (the grey one) on the secondary high voltage side.

Cheers
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geoff 1965
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Re: Blown PT?

Postby geoff 1965 » Sat 06/08/19 9:08 am

good!
glad you sorted it and it was the fuse and not your PT.
yeah Josh i had to look twice at that fuse wiring! it's the same as the HT fuse in the valvestorm layout skydog posted.
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Re: Blown PT?

Postby Riffmonster » Sat 06/08/19 10:00 am

Yeah I am happy it is not the PT too. I had it running for an hour without problems now. Strange...

The other thing I can not solve is that the amp is pretty much quiet without a guitar plugged in/ touching the metal parts of the guitar but with the guitar plugged in and not playing there is a pretty loud hiss. I checked all ground connections already and chopsticked the amp without results. I thought it must be a bad ground/ bad shielding?

Cheers
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