# Screen filter replacement



## SAURUS (Apr 18, 2017)

I have a Mesa nomad 55 and I redplated an el34 last week. My amp sounded pretty bad afterwards so I swapped a set of 6l6's and until today they worked fine. I started to get volume swells and did some troubleshooting to figure out that a screen filter will be in need of replacing. For the hell of it I swapped the El34's back in and it sounded great for about 5 minutes until I started redplating one of the tubes again. Clearly one side is running hotter than the other. I am fairly confident on the screen filter being the issue but I won't be able to get parts until next week. In the meantime I was hoping to see if anyone could offer any insight? Should I change the screen filter in both sides? Could it be more than just one problem?


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## sammyr (May 7, 2013)

Only one of the tubes is redplating, if you switch the positions of the output tubes does the redplating stay with the tube or with the socket?

It's hard to make a diagnosis without more information. Have you checked for bias voltage on the the grids of the output tube sockets?


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## SAURUS (Apr 18, 2017)

sammyr said:


> Only one of the tubes is redplating, if you switch the positions of the output tubes does the redplating stay with the tube or with the socket?
> 
> It's hard to make a diagnosis without more information. Have you checked for bias voltage on the the grids of the output tube sockets?


It stays with the socket. the 6l6's don't red plate, only the right side when the el34's are in. When the 6l6's are installed the left socket hardly warms the tube, I tried the amp with only the left side 6l6 in and it hardly made any sound at all. I have yet to check bias voltage because it is a mesa and they come at a fixed bias.

It volume swells with the 6l6's and it sounds great with the el34's until it starts to red plate and lose volume and clarity.


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## sammyr (May 7, 2013)

If one tube is red plating you should pull the output tubes out and and check for negative voltage on pin 5 of the sockets. This may provide some information that can help the troubleshooting. Negative voltage at pin 5 of the output tubes would rule out a major bias problem. No negative voltage on the offending socket would likely cause red plating. If there is negative voltage then the screens are next on the order of suspicion.


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## SAURUS (Apr 18, 2017)

sammyr said:


> If one tube is red plating you should pull the output tubes out and and check for negative voltage on pin 5 of the sockets. This may provide some information that can help the troubleshooting. Negative voltage at pin 5 of the output tubes would rule out a major bias problem. No negative voltage on the offending socket would likely cause red plating. If there is negative voltage then the screens are next on the order of suspicion.


ok, thanks. I'll pick up some diagnostic tools this afternoon and see what I can figure out.


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## nonreverb (Sep 19, 2006)

Based on your description, there are two independent issues at play. First if you have a socket where the tubes are red plating, it's not the screen at fault but rather the lack of negative voltage on the grid pin. Question: Did you ascertain that both EL34's red plate in that socket?
As for the 6L6 problem, based on your description, the 6L6 socket with no output might have an open screen resistor or open B+.
Two seemingly unrelated problems that "might" be related?
The more detailed info you can give, the better.


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## jb welder (Sep 14, 2010)

When you check that pin5 voltage that sammyr mentioned, check for all the power tube sockets.
When you are changing from 6L6 to EL34 you are also changing the bias switch on the back?
Sounds like one side may have a coupling cap issue and the 6L6's are more 'in range' than the EL34's.
Or the screen resistor is gone on the 'cold' side like nonreverb mentioned. Or both issues.


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## nonreverb (Sep 19, 2006)

jb welder makes a good point. A leaky coupling cap on the phase inverter output would cause a similar symptom on the tube grid. If you have little to no negative voltage....or worse, a positive voltage on the grid, it could be that or an open bias resistor.


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## SAURUS (Apr 18, 2017)

yes, both el34's redplated and I have been using the el34 and 6l6 switch.

I just picked up a multimeter and will be checking out pin 5 once I gather enough information about doing it correctly and safely. A couple google searches should suffice but feel free to give me step by step instructions.


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## SAURUS (Apr 18, 2017)

I opened up the chassis and discovered that I am completely missing a part! 470 2w resistor if I am not mistaken. I couldn't find the resistor in the amp so I'm assuming it's been gone for the entirety of my 1 year ownership. It had a bad hum when I got it but ignored it until last month when I got all new tubes, pre amp and power. I swapped my phase invertor from a 12ax7 to a 12at7 and have been running it louder in general. Would these factors have something to do with it finally failing or do you think there would still be an issue once I put a new resistor in?


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## sammyr (May 7, 2013)

Before you put in that resistor are you absolutely sure that it's "missing". the nomad series had a few different models its possible they all use the same pcb but with different part layouts. I would check against the schematic.


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## SAURUS (Apr 18, 2017)




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## SAURUS (Apr 18, 2017)

sammyr said:


> Before you put in that resistor are you absolutely sure that it's "missing". the nomad series had a few different models its possible they all use the same pcb but with different part layouts. I would check against the schematic.


There is solder within it's place in the PCB and it is written 470 2w. I am pretty certain but I will double check.


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## sammyr (May 7, 2013)

looking at the picture and the schematic that appears to be your screen resistor for your left power tube socket.


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## sammyr (May 7, 2013)

I'm wondering if that space is for a 30uf/500v electrolytic is your screen supply filter that comes after the choke/bias switch


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## SAURUS (Apr 18, 2017)

The 30uf/500v is a big blue guy on the backside of the PCB I am holding...If that's what you are asking.


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## jb welder (Sep 14, 2010)

That 470R screen resistor may be rolling around in there somewhere, or may have got shaken out. You need to replace it.
Then you will still need to find out why the EL34 is red-plating in the other socket, or probably both sides once that resistor is installed.


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## nonreverb (Sep 19, 2006)

I'm assuming you bought this used?


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## SAURUS (Apr 18, 2017)

I bought it used about a year ago. I looked into the amp and couldn't find the resistor anywhere. Once the resistor is installed shouldn't it distribute the load evenly to both tubes? And if that is true, shouldn't they both run at a decent temp instead of one red plating? If the resistor isn't the problem than what is? 

Anybody know an electronic store for parts in Sudbury Ontario? Or a website with quick shipping? I'm up in the bush until May 1st.


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## nonreverb (Sep 19, 2006)

I find it irritating when people sell stuff like this in this condition. Although I must admit, I make part of my income fixing others messes. 
Best to go over the amp in detail to find out what if anything else is missing.


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## SAURUS (Apr 18, 2017)

nonreverb said:


> I find it irritating when people sell stuff like this in this condition. Although I must admit, I make part of my income fixing others messes.
> Best to go over the amp in detail to find out what if anything else is missing.


I double checked the chassis a bit harder and found the resistor underneath the reverb tank! I soldered it back and it the amp is up and running again  I played it for about a half hour and the tubes were both performing like they were supposed to. I'm crossing my fingers that it holds up. 

I got it for $700 which I think is a good deal, however there are some issues that may bother some people. The reverb is always on, switching to channel 3 has a delay and the issue with the resistor. The seller said it was behaving strangely but he "changed the power tubes and it was fine", either he was lying or just unaware of the actual issue. 

Thanks for everybodys input, hopefully there is no longer any issues. During my research I found some mods for my amp so I'll be starting a new thread in the coming weeks.


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