LSS Rectifier Issue - Info

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ja22y
Mark III
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Joined: Fri Oct 05, 2007 3:51 pm

LSS Rectifier Issue - Info

Post by ja22y » Tue Jan 03, 2012 2:32 pm

I've not been on this board for a while thought I stop by. I still see a lot of question regarding the rectifier issue/fix. The official Mesa fix is to snip off the two 470 ohm (R101 R103) located on either side of the ribbon cable connecting the main board to the small power board on the back. What this does is it increases the power tube cathode resistance and thus running the tubes a little cooler and thus less demand on the rectifier tube. I took a picture of mine with the resistors snipped:
Image

The mod and a good rect tube should help
Now if you did the mod and still having problem, the real issue is Mesa uses two 220uF caps in series (110uF) for the first power filter (you can see them in the pic, the two big blue things). The 5Y3 specs calls for a max of 20uF. The large cap over-worked the rect tube, especially during startup. This is why you often see the fuse blown when you switch on the standby switch.
You can opt to change the filter cap to something smaller. I changed mine to two 80uF in series (40uF) and didn't notice any change in tone, feel, or hum at all.
Last edited by ja22y on Wed Jan 04, 2012 10:49 am, edited 1 time in total.

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plan-x
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Re: LSS Rectifier Issue - Info

Post by plan-x » Tue Jan 03, 2012 8:31 pm

Thanks ja22y, this looks like some very good info for alot of people around here.
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Plan-x

Jared Purdy
Mark II
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Re: LSS Rectifier Issue - Info

Post by Jared Purdy » Sun Jan 08, 2012 2:33 pm

I simply switched to a Sovtek 5Y3 and the problem is solved, so far at least. I've had the amp about 2 months now and within 2 days of getting it (it was used, but less than a year old and the previous owner didn't use Mesa tubes in it) it blew the Mesa 5Y3. After speaking to the techs at the shop where the amp was purchased they said to use the Sovtek, and they'd still honour the warranty. An easy fix.

Dmatrix
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Re: LSS Rectifier Issue - Info

Post by Dmatrix » Thu Nov 08, 2012 3:18 pm

Has anyone tried and confirmed this fix?

I'm thinking about trying it. My LSS started acting up about 6 weeks after buying it used. It came with a Sovtek 5Y3, Mesa EL84's, and a mix of 12AX7 brands in the pre-amp.

The symptoms I'm facing: Any EL84 that I put in the left-most slot (nearest to mains transformer and fan) will red-plate after about a minute. Depending on how I swap my power tubes around, channel 2 overdrive either does not work at all, or works but sounds a bit muddy. I'm thinking about swapping out the rectifier tube and all power tubes first to see if that solves the problem. In the 6 weeks that I played it, I almost exclusively played it in the 5W class A setting.

Any replies would be appreciated, especially from those who have tried the above fix. Thanks to everyone who has been actively investigating the issue so far.

ja22y
Mark III
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Re: LSS Rectifier Issue - Info

Post by ja22y » Thu Nov 08, 2012 6:48 pm

Power tube red plating has little to to with rectifier tube. However you can try to see if it's a bad tube or a bad tube position.
Use a tube from another socket that does not red plate and swap that into the trouble socket. If it doesn't red plate then it's the tube. If it's red plated then most likely that tube position is bad. You can try several good tubes to make sure. Most likely the screen grid resistor for that position is bad (burnt). If you have a bias probe you can check the bias. Hope that it's just a bad tube, else you need to bring it to a tech if you have not worked with tube amp. good luck

Dmatrix
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Re: LSS Rectifier Issue - Info

Post by Dmatrix » Thu Nov 08, 2012 9:11 pm

ja22y wrote:Power tube red plating has little to to with rectifier tube. However you can try to see if it's a bad tube or a bad tube position.
Use a tube from another socket that does not red plate and swap that into the trouble socket. If it doesn't red plate then it's the tube. If it's red plated then most likely that tube position is bad. ...
Yeah I gave this a try and it's the left most position that red-plates all the time no matter which tube I put in there. Something must have gone wrong with that socket's circuitry. Shoot. This thing sounds *amazing* when it's working perfectly. I'll seek out a good tech. Thanks.

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