One Tube Red Plates
Moderators: pompeiisneaks, Colossal
One Tube Red Plates
I'm working on a 100W Plexi clone. The tubes on the left red plate, but the tubes on the right are fine. I've changed the tubes, tried it with just one tube on each side in all socket combinations, etc. I have also reheated all the solder joints. I've checked all the connections a million times. Any ideas as to what I am missing?
Re: One Tube Red Plates
how does the bias votage and draw compare between the two pairs?
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chief mushroom cloud
- Posts: 429
- Joined: Tue Jan 18, 2005 5:42 pm
- Location: Peenemunde CA
Re: One Tube Red Plates
might be a bad screen resistor
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chief mushroom cloud
- Posts: 429
- Joined: Tue Jan 18, 2005 5:42 pm
- Location: Peenemunde CA
Re: One Tube Red Plates
marshalls are crap anyway
re
Now, now...to each their own.chief mushroom cloud wrote:marshalls are crap anyway
It could be bad screen resistors...but that wouldn't cause this big a problem...and the odds are against both tubes having leaky screens at the same time. Somebody already mentioned an issue with the bias supply...that's much more likely the cause of the problem. If you've swapped the tubes around and the problem persists on the same side, then it's a bias supply problem. Make sure the 220k and 1.5k control-grid resistors are not open (showing infinite resistance) or shorting (showing little or no resistance). But first measure the voltage at the control-grids on each side for all tubes. It should be very close on all of them...say within a volt or so.
Re: One Tube Red Plates
Steven
I suppose my first post here ought to have been Trainwreck-related but there you go.
Marshall plexis are fairly prone to parasitic oscillation and I wouldn't be surprised if that's what you are seeing here. If you can get a scope on the grid or the plate of one of the tubes on the redplating side you should be able to see it as a very high frequency signal often well outside the audible range.
It's likely to be lead dress between the PI and the output tubes as it's only affecting one side (I'm presuming here .
Have you loosely twisted the orange and green grid wires together, or are they running independently to the output tubes? Does your NFB wire to the presence control run anywhere near, and particularly parallel, to either the green/orange wire? With its high voltage swings this is a rather effective transmitter inside the amp. These are the areas I'd look at first.
You can also increase the grid stopper resistors on the EL34s which will help. Going up as high as 10K shouldn't have any audible effect (you need to go up above 20K apparently, although I've stopped at 10K) but will contribute to suppressing supersonic noise. Also, make sure you have all 4 grid stoppers and not one each on the outside tubes only as Marshall sometimes mistakenly did.
Hope this helps.
Paul
I suppose my first post here ought to have been Trainwreck-related but there you go.
Marshall plexis are fairly prone to parasitic oscillation and I wouldn't be surprised if that's what you are seeing here. If you can get a scope on the grid or the plate of one of the tubes on the redplating side you should be able to see it as a very high frequency signal often well outside the audible range.
It's likely to be lead dress between the PI and the output tubes as it's only affecting one side (I'm presuming here .
Have you loosely twisted the orange and green grid wires together, or are they running independently to the output tubes? Does your NFB wire to the presence control run anywhere near, and particularly parallel, to either the green/orange wire? With its high voltage swings this is a rather effective transmitter inside the amp. These are the areas I'd look at first.
You can also increase the grid stopper resistors on the EL34s which will help. Going up as high as 10K shouldn't have any audible effect (you need to go up above 20K apparently, although I've stopped at 10K) but will contribute to suppressing supersonic noise. Also, make sure you have all 4 grid stoppers and not one each on the outside tubes only as Marshall sometimes mistakenly did.
Hope this helps.
Paul