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Old April 16th, 2009, 10:30 AM   #1 (permalink)
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Help. Are these symptoms of a bad rectifier tube?

I just got a Marsh built 5F1 clone off ebay and the seller was nice enough to pack the tubes nice and neat in the box, but I think the Sovtek 5Y3GT rectifier got damaged in transit. The 5Y3 doesn't light up when I turn the amp on. The preamp and power tubes look like they are working OK. No sound is coming from the amp, arrghh! So I'll have to stop by Guitar Center after work and get a new rectifier tube.

But now I wonder, if a bad rectifier tube is the only problem. I always assumed that if a rectifier tube went bad that the fuse would blow and the amp would be dead. But the amp powers on, the jewel lamp lights up, the preamp and power amp tube heat up and look good, fuse is good, all connections including speaker look good. Just no sound is coming from the amp. Is this what normally happens when a rectifier tube goes bad?

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Old April 16th, 2009, 10:41 AM   #2 (permalink)
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I just got a Marsh built 5F1 clone off ebay and the seller was nice enough to pack the tubes nice and neat in the box, but I think the Sovtek 5Y3GT rectifier got damaged in transit. The 5Y3 doesn't light up when I turn the amp on. The preamp and power tubes look like they are working OK. No sound is coming from the amp, arrghh! So I'll have to stop by Guitar Center after work and get a new rectifier tube.

But now I wonder, if a bad rectifier tube is the only problem. I always assumed that if a rectifier tube went bad that the fuse would blow and the amp would be dead. But the amp powers on, the jewel lamp lights up, the preamp and power amp tube heat up and look good, all connections including speaker look good. Just no sound is coming from the amp. Is this what normally happens when a rectifier tube goes bad?

Hopefully its just the tube, Those Sovteks are fairly flimsy.

I would avoid using the Sovtek if possible, Maybe temporary until you can get an NOS 5Y3, They are plentiful and only $2-$5 more but last decades in most cases.

The Rectifier runs on its own tap, ( 5V AV ) So the pilot and other tubes will light even if the tube or transformer (open) are damaged in most cases.

You can test the transformer by plugging voltmeter probes into pin 2 & 8 of the rectifier socket. Set the Meter for AC.
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Old April 16th, 2009, 11:10 AM   #3 (permalink)
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Thanks for the info Ed.

Yeah I'll avoid Sovteks. A google search turned up a lot of bad comments on their reliability. I can even hear a nasty rattle inside the Sovtek tube when I pick it up. I assume rectifier tubes are not supposed to rattle.
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Old April 16th, 2009, 08:42 PM   #4 (permalink)
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Picked up a new 5Y3 at GC. All they had were Mesa-Boogie labelled tubes to choose from for rectifier tubes. Oh well, it's better than nothing and it doesn't rattle like the Sovtek. Popped it in and started rocking! Whew, I'm glad that was an easy fix.
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Old April 17th, 2009, 01:40 AM   #5 (permalink)
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I would start looking for a better tube them mesa boogie labeled tubes are not great either.
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Old April 17th, 2009, 02:55 AM   #6 (permalink)
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I just got a Marsh built 5F1 clone off ebay and the seller was nice enough to pack the tubes nice and neat in the box, but I think the Sovtek 5Y3GT rectifier got damaged in transit. The 5Y3 doesn't light up when I turn the amp on. The preamp and power tubes look like they are working OK. No sound is coming from the amp, arrghh! So I'll have to stop by Guitar Center after work and get a new rectifier tube.

But now I wonder, if a bad rectifier tube is the only problem. I always assumed that if a rectifier tube went bad that the fuse would blow and the amp would be dead. But the amp powers on, the jewel lamp lights up, the preamp and power amp tube heat up and look good, fuse is good, all connections including speaker look good. Just no sound is coming from the amp. Is this what normally happens when a rectifier tube goes bad?
Depends on how the tube fails, if it shorts out that would be what would happen, probably taking the PT with it if you haven't got diodes inline with the rectifier tube. Then what will happen is that the B+ voltage will rise and your amp will sound louder. :D

If it fails open then nothing will happen, the tube will just not pass current.

In your case I would suspect a failed heater, as the heater didn't power up, which it would do if it was an open failure.

Glad the Mesa Tube fixed it, but you might wanna measure B+ if you have the skills, as the newer 5Y3's doesn't drop the voltage as the NOS ones do, which in an unmodified circuit could lead to an altogether too high B+. If the amp did come with a Sovtek tube (which does not provide this voltage drop), then there's a chance the builder did know what he was doing and used a smaller HV secondary to lower the output from the tube. But there's also a chance he wasn't aware of this little quirk with 5Y3's, so if you have the skills to safely check the B+ voltage I really suggest you do so. Post it here if you do as I can't really remember what it should be right if the bat. Anything over 400V is definitely wrong though.
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Old April 17th, 2009, 12:55 PM   #7 (permalink)
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Depends on how the tube fails, if it shorts out that would be what would happen, probably taking the PT with it if you haven't got diodes inline with the rectifier tube. Then what will happen is that the B+ voltage will rise and your amp will sound louder. :D

If it fails open then nothing will happen, the tube will just not pass current.

In your case I would suspect a failed heater, as the heater didn't power up, which it would do if it was an open failure.

Glad the Mesa Tube fixed it, but you might wanna measure B+ if you have the skills, as the newer 5Y3's doesn't drop the voltage as the NOS ones do, which in an unmodified circuit could lead to an altogether too high B+. If the amp did come with a Sovtek tube (which does not provide this voltage drop), then there's a chance the builder did know what he was doing and used a smaller HV secondary to lower the output from the tube. But there's also a chance he wasn't aware of this little quirk with 5Y3's, so if you have the skills to safely check the B+ voltage I really suggest you do so. Post it here if you do as I can't really remember what it should be right if the bat. Anything over 400V is definitely wrong though.
Thanks Stuggi. I'll measure B+ for peace of mind. But here's a stupid question - Do I pull the rectifier tube (or any tube for that matter) when measuring B+ at that tube? And B+ is measured at pin 3, right? I've heard the term "B+" for years now, but never bothered with it until now. Thanks.
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Old April 17th, 2009, 01:16 PM   #8 (permalink)
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Thanks Stuggi. I'll measure B+ for peace of mind. But here's a stupid question - Do I pull the rectifier tube (or any tube for that matter) when measuring B+ at that tube? And B+ is measured at pin 3, right? I've heard the term "B+" for years now, but never bothered with it until now. Thanks.
Tube in, and in most cases measure at pin #8, or the + side of the first power supply cap which is often easier to access that the tube socket lug..
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Old April 17th, 2009, 01:38 PM   #9 (permalink)
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Tube in, and in most cases measure at pin #8, or the + side of the first power supply cap which is often easier to access that the tube socket lug..
Oh, so that's where B+ is. I've seen it before on Weber's layout diagrams. Now I'm getting the hang of this. Thanks!
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Last edited by metropolis74; April 17th, 2009 at 02:11 PM.
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Old April 19th, 2009, 10:13 PM   #10 (permalink)
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Tube in, and in most cases measure at pin #8, or the + side of the first power supply cap which is often easier to access that the tube socket lug..
B+ is around 370V. I guess I'm not in the danger zone. Will hunt down a NOS 5Y3 though.

I measured the B+ at the + side of the first filter cap (touching one probe to the lead of the cap while the other probe was clipped to the transformer bolt ground). Went pretty easy, but now it might be my imagination but my amp seems to have more 60 cycle hum now. There's no danger of messing up the caps measuring B+ this way, is there?
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