The thing that motivated me me to build an amp was my Marshall TSL122, which sounds just slightly better than miserable even with totally expensive NOS. And now its got a new problem.
After being on for about twenty minutes it red-plates on one side of the push-pull only . First time it happened I thought it was a bad tube (they were pretty old). The obvious guess is that the weeny bias pot for that side is crapping out but it seems odd that this consistantly starts after about 15 to 20 minutes
Any thoughts would be much appreciated
Thanks
WL
One side red-plating
Moderators: pompeiisneaks, Colossal
Re: One side red-plating
Sounds EXACTLY like a problem I had with a build I did a year or so back. Turned out being a problem with a wire (can't remember exactly which one at this point) in the bias area that was stripped just a smidge to long. It extended too far through the eyelet and when things heated up it grew just long enough (due to thermal expansion) to touch the chassis and ground out the bias voltage to it's associated tube.
I would stick a meter on the offensive tube's grid, fire up the amp and gently push on the board (assuming you have one) with a non-conductive rod (e.g., wooden chopstick) in the bias wiring area just enough to flex it slightly. If the bias voltage bottoms out...you have found the problem.
Good luck,
Dave O.
I would stick a meter on the offensive tube's grid, fire up the amp and gently push on the board (assuming you have one) with a non-conductive rod (e.g., wooden chopstick) in the bias wiring area just enough to flex it slightly. If the bias voltage bottoms out...you have found the problem.
Good luck,
Dave O.
Re: One side red-plating
Thanks Dave
I think you're right. It's got to be thermal
I think you're right. It's got to be thermal
Re: One side red-plating
A critical observation in my situation was that it NEVER occured when I had the chassis out of the cabinet sitting on the bench in the same orientation as in-vivo.
Once I recognized that and coupled it with the fact that it was the first eyelet style fab that I had done (only turret style up until that point) it was pretty clear what was occuring.
I went back and measured temps and did a couple of quick calcs when all was said and done and determined that the offending wire was pretty darn close to the chassis when cold. A couple of 10 thousands of an inch if I remember correctly.
For the life of me though, I can't remember why the whole bias supply wasn't sunk to ground.
One quick clarification: I said "...bias voltage bottoms out...." in the previous post. I mean....goes to zero. Sometimes I lose trach of way is really up!!!
Good luck,
Dave O.
Once I recognized that and coupled it with the fact that it was the first eyelet style fab that I had done (only turret style up until that point) it was pretty clear what was occuring.
I went back and measured temps and did a couple of quick calcs when all was said and done and determined that the offending wire was pretty darn close to the chassis when cold. A couple of 10 thousands of an inch if I remember correctly.
For the life of me though, I can't remember why the whole bias supply wasn't sunk to ground.
One quick clarification: I said "...bias voltage bottoms out...." in the previous post. I mean....goes to zero. Sometimes I lose trach of way is really up!!!
Good luck,
Dave O.
Re: One side red-plating
aaaarrrrghhhhh these truly suck to work on. It's no wonder the assembly folks miss stuff.
I found cold solder (and not much of it) at a through-hole on the trace
between the bias pot and the grid pin. Guess it's gradually become a
semi conductor over the years. Seems like it should have been plated through anyway
Next time I'll just drive a stake through the fucker.
I found cold solder (and not much of it) at a through-hole on the trace
between the bias pot and the grid pin. Guess it's gradually become a
semi conductor over the years. Seems like it should have been plated through anyway
Next time I'll just drive a stake through the fucker.
-
Lonely Raven
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Re: One side red-plating
Thanks for the morning laugh...I needed that.jurgen wrote: Next time I'll just drive a stake through the fucker.
Jack of all Trades,
Master of None
Master of None
Re: One side red-plating
I'm here to helpLonely Raven wrote:Thanks for the morning laugh...I needed that.jurgen wrote: Next time I'll just drive a stake through the fucker.
Re: One side red-plating
The thing that motivated me me to build an amp was my Marshall TSL122, which sounds just slightly better than miserable even with totally expensive NOS. And now its got a new problem. After being on for about twenty minutes it red-plates on one side of the push-pull only. First time it happened I thought it was a bad tube (they were pretty old).
The TSL 122’s as well as the TSL-100’s have their share of common occurances. I’m not certain where the amp is presently biased but I would not recommend following the factory Marshall bias sheets as they are incorrect.
The obvious guess is that the weeny bias pot for that side is crapping out but it seems odd that this consistantly starts after about 15 to 20 minutes
The bias will run away causing one side to go cherry red if the amp is not properly biased. I would recommend taking a plate voltage reading, crunching the math and setting the bias accordingly (again do not use the suggested Marshall factory bias sheets). What seems to work best would be to use two DMM’s so that you can monitor each side simultaneously. More times than not, as you are slowing turning one of the pots the other side will sky rocket on you.
Another common issue is when a power tube fails there is a capacitor that also tends to short/fail and if you have not come across this before it can be frustrating to properly diagnose/track down (we have also seen this when a tube goes cherry for an extended period of time). I believe the cap is C46/22pF and is rated at approximately 500VDC or there abouts (going from memory). With the controls facing you this is located on the main PCB (that houses the power tubes) and is on the lower right hand side. When replacing the cap we usually increase the voltage rating for added insurance, though not full proof it never hurts to go the extra mile.
I found cold solder (and not much of it) at a through-hole on the trace
between the bias pot and the grid pin.
We often find several when we work on these amps. The boards are wave soldered and it's not uncommon to see some pads where the component is making physical connection although there is no solder on the pad. Hopefully you have a model that is not RoHS compliant as the RoHS models can be difficult to tell a good solder joint from a cold one.
Hopefully this will be of some help to you, best of luck and have a very Merry Christmas!
Happy holidays
Trace
The TSL 122’s as well as the TSL-100’s have their share of common occurances. I’m not certain where the amp is presently biased but I would not recommend following the factory Marshall bias sheets as they are incorrect.
The obvious guess is that the weeny bias pot for that side is crapping out but it seems odd that this consistantly starts after about 15 to 20 minutes
The bias will run away causing one side to go cherry red if the amp is not properly biased. I would recommend taking a plate voltage reading, crunching the math and setting the bias accordingly (again do not use the suggested Marshall factory bias sheets). What seems to work best would be to use two DMM’s so that you can monitor each side simultaneously. More times than not, as you are slowing turning one of the pots the other side will sky rocket on you.
Another common issue is when a power tube fails there is a capacitor that also tends to short/fail and if you have not come across this before it can be frustrating to properly diagnose/track down (we have also seen this when a tube goes cherry for an extended period of time). I believe the cap is C46/22pF and is rated at approximately 500VDC or there abouts (going from memory). With the controls facing you this is located on the main PCB (that houses the power tubes) and is on the lower right hand side. When replacing the cap we usually increase the voltage rating for added insurance, though not full proof it never hurts to go the extra mile.
I found cold solder (and not much of it) at a through-hole on the trace
between the bias pot and the grid pin.
We often find several when we work on these amps. The boards are wave soldered and it's not uncommon to see some pads where the component is making physical connection although there is no solder on the pad. Hopefully you have a model that is not RoHS compliant as the RoHS models can be difficult to tell a good solder joint from a cold one.
Hopefully this will be of some help to you, best of luck and have a very Merry Christmas!
Happy holidays
Trace
Re: One side red-plating
Thanks Trace
Re: One side red-plating
Trace wrote:The thing that motivated me me to build an amp was my Marshall TSL122, which sounds just slightly better than miserable even with totally expensive NOS. And now its got a new problem. After being on for about twenty minutes it red-plates on one side of the push-pull only. First time it happened I thought it was a bad tube (they were pretty old).
The TSL 122’s as well as the TSL-100’s have their share of common occurances. I’m not certain where the amp is presently biased but I would not recommend following the factory Marshall bias sheets as they are incorrect.
The obvious guess is that the weeny bias pot for that side is crapping out but it seems odd that this consistantly starts after about 15 to 20 minutes
The bias will run away causing one side to go cherry red if the amp is not properly biased. I would recommend taking a plate voltage reading, crunching the math and setting the bias accordingly (again do not use the suggested Marshall factory bias sheets). What seems to work best would be to use two DMM’s so that you can monitor each side simultaneously. More times than not, as you are slowing turning one of the pots the other side will sky rocket on you.
Yea Marshall says 90ma per side which I thought was nuts and sounded
stiff to boot. I figured they were doing this for the clean channel as
They recommend 80ma for the DSL. I thought the amp
sound much better at about 76ma. Never saw that interaction but it's
food for thought -
Another common issue is when a power tube fails there is a capacitor that also tends to short/fail and if you have not come across this before it can be frustrating to properly diagnose/track down (we have also seen this when a tube goes cherry for an extended period of time). I believe the cap is C46/22pF and is rated at approximately 500VDC or there abouts (going from memory). With the controls facing you this is located on the main PCB (that houses the power tubes) and is on the lower right hand side. When replacing the cap we usually increase the voltage rating for added insurance, though not full proof it never hurts to go the extra mile.
I've heard of noise problems with that cap as well. Some people are just
pulling them out
I found cold solder (and not much of it) at a through-hole on the trace
between the bias pot and the grid pin.
We often find several when we work on these amps. The boards are wave soldered and it's not uncommon to see some pads where the component is making physical connection although there is no solder on the pad. Hopefully you have a model that is not RoHS compliant as the RoHS models can be difficult to tell a good solder joint from a cold one.
Yea I'm still chasing an intermittent hum that is worse on the clean channel but apparently occurs somewhere past the masters
Hopefully this will be of some help to you, best of luck and have a very Merry Christmas!
Thanks Trace I appreciate the help
Happy holidays
Trace
Re: One side red-plating
Yea Marshall says 90ma per side which I thought was nuts and sounded
stiff to boot. I figured they were doing this for the clean channel as
They recommend 80ma for the DSL.
Setting the bias in the matter they recommend is less than ideal. They are several other methods/ways to give more headroom to the clean channel without biasing the amp beyond where it should be.
I thought the amp sound much better at about 76ma. Never saw that interaction but it's food for thought -
Personally speaking I feel the EL34B's are nice tubes for these amps and we've been using them for years. The Russian Svets sound pretty good in the non RoHS versions.
I've heard of noise problems with that cap as well. Some people are just
pulling them out
It's personal preferance so let your ears be the judge. For stock amps we tend to upgrade the cap (see above) so the amp is still stock but if it's a Modified amp then it varies (so long as the customer is happy--ha, ha)
Yea I'm still chasing an intermittent hum that is worse on the clean channel but apparently occurs somewhere past the masters
Feel free to email me at voodooamps@yahoo.com. It's not my main email however it's the one I check from home for the tech/DIY sites such as this one. I’m happy to help so if you like shoot me an email with what you've already tried and we can proceed from there.
Happy New Year!
Trace
stiff to boot. I figured they were doing this for the clean channel as
They recommend 80ma for the DSL.
Setting the bias in the matter they recommend is less than ideal. They are several other methods/ways to give more headroom to the clean channel without biasing the amp beyond where it should be.
I thought the amp sound much better at about 76ma. Never saw that interaction but it's food for thought -
Personally speaking I feel the EL34B's are nice tubes for these amps and we've been using them for years. The Russian Svets sound pretty good in the non RoHS versions.
I've heard of noise problems with that cap as well. Some people are just
pulling them out
It's personal preferance so let your ears be the judge. For stock amps we tend to upgrade the cap (see above) so the amp is still stock but if it's a Modified amp then it varies (so long as the customer is happy--ha, ha)
Yea I'm still chasing an intermittent hum that is worse on the clean channel but apparently occurs somewhere past the masters
Feel free to email me at voodooamps@yahoo.com. It's not my main email however it's the one I check from home for the tech/DIY sites such as this one. I’m happy to help so if you like shoot me an email with what you've already tried and we can proceed from there.
Happy New Year!
Trace