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amp hum after mod - help!


gusfinley

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Okay, so I modded my amp (DSL401) - adding birght switches to both of the channels, removing a unnessasary capacitor that was limiting the bass response and added pin jack to the back of the amp so I could check the bias without taking the amp apart.

 

So i put it all back together and fired it up - no sound - then the sound faded in and there was all this crackling and stuff and then I watched on the 1/2W screen resistor burn up!! YIKES!! I looked around and found that I hadn't connected the HT to the output transformer - it was way in the back and close enough to the place it was supposed to go that it looked cennected when I double checked.

 

Well, I was only running my amp with two tubes anyway, so I put the two tubes in the outer sockets. I turned it on and then I see lots of blue sparks in the tube that was in the socket when the screen resistor burned up. WOAH!! Well, I have like six spare tubes so I put in another set and everything seems ALMOST alright....

 

One strange thing is that when I turn down the master volume all the way now, I still get a signal coming through the speakers..

there is also a permanant hum. The master volume seems to funstion more as a presence knob now, because when I turn it down the sound get thin and when I turn it up it gets really think and bassy....

 

I am assuming that somewhow somewhere the signal from the phase inverter is sneaking past the master volume and into the grids...

 

one more odd thing.. I was cheking the differences in the circuitry between the two tubes. and noticed that when I connected the lead of my multimeter to pin 2 of the leftmost tube there was a loud pop. When I connected it to pin 2 of the rightmost tube there wasn't a pop. This was the same for the inner sockets that contained no tubes (including the one that had failed previously) The left one popped and the right one didn't..

 

anybody have any suggestion as to what might be the problem here? It is baffling me!! Thanks for any input

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those tube sockets are wired in parallel. if you bugger something up on one of the left ones, then the other left one probably isn't gonna work right either.

 

the problem is you're getting in over your head. take it to a tech before you kill it.

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yup, they are wired in parallel alright.... thats why it still works!! The burned out resistor isn't connected to anything and therefore shouldn't effect anything.

 

For what its worth the bias is 12V on the left side and 17V on the right - another odd symptom...

 

anybody have any ideas as to what could be happening here?

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After poking around with my multimeter I found something VERY different between the left and right sockets. Measuring the resistance from the contol grid to ground on the lft side gave 9-10M ohms while the right side it measured between 25K -200K (by varying the master volume control) It may be a bad master volume pot, or just a bad solder joint. That PC board is pretty flimbsy when you pull it out of the chassis...........

 

Looks like I just have to find where it isn't connected and fix it, while I'm at I'll I'll replace the screen grid resistor, too, and it should be good as new...........

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I appreciate the "help" so far, but let me just let ya'll know that I am going to work this out myslef. I have been studying tube circuits and such for over a year and am just about to graduate in Electrical Engineering, so this IS up my alley.... and I've already traced the problem to a specific area.... I AM gogin to fix this myself and I see absolutely no danger in trying to do so.... If I screw up my amp, so be it. It would be worth the cost to learn from the experience......

 

Unfortunately I don't have too much time to do a lot of diagnostic work because I have a lot of projects at school, too... so What I am looking for is someone who may be able to give me some information that could quicken the process - since I won't be able to fix it for another week or so.... but at this point I think I've done all I can do without taking it apart and finding the bad connection.

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Originally posted by gusfinley

I appreciate the "help" so far, but let me just let ya'll know that I am going to work this out myslef. I have been studying tube circuits and such for over a year and am just about to graduate in Electrical Engineering, so this IS up my alley.... and I've already traced the problem to a specific area.... I AM gogin to fix this myself and I see absolutely no danger in trying to do so.... If I screw up my amp, so be it. It would be worth the cost to learn from the experience......


Unfortunately I don't have too much time to do a lot of diagnostic work because I have a lot of projects at school, too... so What I am looking for is someone who may be able to give me some information that could quicken the process - since I won't be able to fix it for another week or so.... but at this point I think I've done all I can do without taking it apart and finding the bad connection.

 

 

If you want my help please post a schematic and use it to explain the changes you have made and as reference to where you are taking your measurements. For now if the amp had no serious problem before your mod I would switch it back and go from there. Also a good thing to remember if you are doing more than one mod to an amp, do one mod at a time and check results before going on.

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I'm sure that the shipping on any repairs will cost more than the repairs themselves.... a few cents for a new resistor and about 50 cents each for the little caps I'm going to replace...

 

I've decided that I don't like the new bright cap anyway... I'm going to just put 1 pin terminals (socket, aka "headers") where the cap leads where and then I can easily switch back and forth between mod/stock values and see which one I want to put on the switch.

 

I'll post the schematic a bit later.... the main thing I will need to do is figure out how all this stuff is connected on the PC board traces... there are a lot of "links" running from one trace to another so Its hard to follow the circuit path on the upper portion of the board...

 

I'll also post measured values on the effected part of the amp.. branch values for voltage, capacitance, etc... but it maight take a day or two....

 

THANKS for the offer of your help guitarzan!!!

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Originally posted by gusfinley

I'm sure that the shipping on any repairs will cost more than the repairs themselves.... a few cents for a new resistor and about 50 cents each for the little caps I'm going to replace...


I've decided that I don't like the new bright cap anyway... I'm going to just put 1 pin terminals (socket, aka "headers") where the cap leads where and then I can easily switch back and forth between mod/stock values and see which one I want to put on the switch.


I'll post the schematic a bit later.... the main thing I will need to do is figure out how all this stuff is connected on the PC board traces... there are a lot of "links" running from one trace to another so Its hard to follow the circuit path on the upper portion of the board...


I'll also post measured values on the effected part of the amp.. branch values for voltage, capacitance, etc... but it maight take a day or two....


THANKS for the offer of your help guitarzan!!!

 

 

You are most welcome! I check in most everyday at about noon(EST). If you post your schematic I will be sure to help any way I can. I will not go into all my experience but lets just say my work forces me to play with voltages that make a tube guitar amp look minuscule and that my own DIY amp is pretty much perfect except to look at : ) One day I will improve on that too.

Remember safety first, always!

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Originally posted by guitarzan



How is he to learn if he just jobs out? I think DIY is a great way to understand more about your amp, as long as you follow basic safety procedures.

 

 

DIY is a great thing if you have a clue about what you're doing. To go in recklessly and start clipping out caps because you don't think they should be there is foolish. He took out caps that limited his bass and then added bright switches. Sounds like those two things work against each other. Those caps are there for a reason. To change them you should at least know their function first and then make changes because you have an idea of what the results will be. Watching changes on a scope and with meters is the right way to mod amps, not clipping things and watching for smoke.

Then he powered up an amp with a known failed resistor without any care or thought about what more damage that would cause. I'm sorry, I'm all for people learning and trying things but it seems quite obvious to me that this guy is in too deep. I help people everyday when I can and it doesn't look like they're gonna get hurt or destroy their amps.

Jerry

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For those of you have been cheering me on rather than scorning me, you'll be happy to know that I tracked it down to the master volume pot itself. There is an open circuit somewhere in there (bad solder joint probally) and when I take it apart again I'll be able to diagnose it

 

"Then he powered up an amp with a known failed resistor without any care or thought about what more damage that would cause"

 

If this person would have paid attention to my post they would have seen that I changed the tubes over so they were operateing on different sockets.... so the failed resistor wasn't in the circuit at that point....

 

ChiLL people!!

 

Let the kids learn the hard way!!

 

"To go in recklessly and start clipping out caps because you don't think they should be there is foolish. He took out caps that limited his bass and then added bright switches. Sounds like those two things work against each other. Those caps are there for a reason. To change them you should at least know their function first and then make changes because you have an idea of what the results will be. "

 

Again, I had a knowledge of what the caps were doing and which were unecessary, I didn't just hack at it with a chainsaw as you seem to be implying......

 

Everything is gonna be okay..... just have a little patience!!

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Originally posted by gusfinley

For those of you have been cheering me on rather than scorning me, you'll be happy to know that I tracked it down to the master volume pot itself.

 

 

Good for you! See it wasn't rocket science after all;)

First,

To be sure it is not a great ID for anyone to poke around in an amp.

Second,

To be sure you will not cover tube therory in EE class except maybe one afternoon outlining history.

Third,

Be sure to pick up some worthy text. At least an old RCA tube manual. The info in that would be far better than most you PU on the net. Not refering to JerryP, he is usually right on but several others I see have little or no ID what they are talking about. Suggestions for mods given out on the net can be very wrong and some are down right stupid.

Lastly,

Mistakes can be costly as you know but the ones that kill are the {censored} to always look out for. Always work with one hand only in a live amp. Always wear shoes, runners are fine. Never under estimate the damage 500ma @ 450V can do to your heart and other organs. No matter what anyone tells you the CURRENT in a tube amp is enough to kill you! Voltage just sits there, that's why it is defined as potential difference but if you become part of the circuit and allow that much current threw you it will hurt a lot, if not kill:eek:

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Good luck with your work. I hope you get it all figured out. I have also done what you did, but on a $100,000 piece of Air Force communication equipment. And I'm still in the Air Force 14 years later.

 

You will get good and you won't make the same mistake twice. You have to learn something and along the way all of the VOODOO will go away.

 

Remember the guys in white coats in the glass lab working on computers....its just the same voodoo.

 

Some folks will spend over $5000 a year recycling amps, paying techs $40 to bias tubes, $25 to replace pick ups and so what if you blow your {censored} up. Your right, if you learn, then you done good.

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Gus, I read about that cap that limits the bass on another forum. I also understand that theres bright caps that can be clipped that smoothes out the treble response(less buzzy). I have modded the effects loop so that it works as a series loop. Now I can use my DSL401 as a slave amp taking a signal from my 2204's preamp. That EL84 power section sounds pisser with the 2204's preamp going into it. I think if someone could mod the existing DSL401 preamp for a more classic vibe this amp would be the ticket. Also upon removing the chassis one would do good to inspect every solder joint. These amps are notorious for cold solder joints. I had mine gone over 4 1/2 years ago and no problems yet. Good luck with your mods and if you feel up to it feel free to post some. :D

 

P.S. I've read a few articles that claim that the OD channel volume is a Post Phase Inverter Master. Can you confirm this?

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Originally posted by guitarzan

...you will not cover tube therory in EE class except maybe one afternoon outlining history. ..... Be sure to pick up some worthy text. At least an old RCA tube manual.

 

 

No kidding! Tubes don't get any respect these days!! About the only time they get any mention if CRT for TV's and computers... it kinda sad really... There's just something about the glow of a tube that make my heart warm....

 

I have two tube manuals that I downloaded ftom the internet... I think I actually understood them the third time around. I just finished Dan Torres's book, which got me started and really helped me narow down components to use on my next completely crazy DIY amp project (512 different tone cominations). I just ordered The Ultiimate Tone, and that seems to have a lot of great stuff in switching, etc.

 

If I want to get really technical and learn about tubes I general, I have checked out a few books from the 1940s-50s from the tech library about tubes (they were WAY over my head then), and there are books on tubes circuits and even Morgan's Valve Amplifiier books. Turns out my Grandpa's brother wrote a few of those books - too bad his health is failing as well as his mind - he could teach me TONS!!

 

Sadly the tech Library somehow lost the Radiotron designers handbook, so I won't be able to look at that one for while...

 

anyone else suggest more good reading material as far as tube circuits go?

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Originally posted by cap'n'crunch

Gus, I read about that cap that limits the bass on another forum. I also understand that theres bright caps that can be clipped that smoothes out the treble response(less buzzy). I have modded the effects loop so that it works as a series loop. Now I can use my DSL401 as a slave amp taking a signal from my 2204's preamp. That EL84 power section sounds pisser with the 2204's preamp going into it. I think if someone could mod the existing DSL401 preamp for a more classic vibe this amp would be the ticket. Also upon removing the chassis one would do good to inspect every solder joint. These amps are notorious for cold solder joints. I had mine gone over 4 1/2 years ago and no problems yet. Good luck with your mods and if you feel up to it feel free to post some.
:D

P.S. I've read a few articles that claim that the OD channel volume is a Post Phase Inverter Master. Can you confirm this?

 

The cap that limits the Bass (C92) isn't necessary - supposivley it was put in there to elimate any pops from occuring when the relay switches the channel - I didn't notice any pops with it out - probally die to the circuits that "ducks" the volume when the channel is switched... Just remember to replace it with a wire if you take it out or you won't be hearing anything!!

 

The bright caps (C100, C97) could be clipped out..... it turns out that my amp had the bright cap (C97) on the clean section modded it was replaced with a 100pF - it sounded beautiful!! Resotring to the 470pf stock value made it REALLY shrill!! Woah!! Changing that one to a 100pF will still keep the channel bright without the harshness of the 470pF. I'm definately going to change mine back!! No wonder people hate this amp when they hear it, and no wonder the ones at the guitar shop didn't sound as good as mine!!

 

I haven't done any experiments with the cap on the overdrive, but it does sound good without it in there ( My wire was about 5mm to short and so it pulled out when I put everything back together...) I'm not sure if this one got modded or not...

 

By they way, You don't need a serial effects loop to use the DSL401 as a slave power amp. You can plug it into the effects return and use the FX mix as a volume control. The GREAT thing is that it comes into the amp before the tonestack if you put your amp on the OD1 channel (OD2 will work, but might cause noise). So you can tweak your tone even more- if you want to run it straight through with no volume contol or whatever you can just use the clean channel...

 

Yes, the DSL401 has a post-phase inverter master volume control- this is my problem... one side has a volume control and the other doesn't know what is going on at this point!!

 

The person who had my amp before me also modded it with a fan, and probally touched up the cold solder joints, too...

 

Within the next few months I will be posting a website dedicated to the DSL 401... I will include, tips, tricks, mods, sound clips, etc. I really think that this is a great amp... just a few things that can be done to improve it if you have the patience.

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Originally posted by gusfinley



No kidding! Tubes don't get any respect these days!! About the only time they get any mention if CRT for TV's and computers... it kinda sad really... There's just something about the glow of a tube that make my heart warm....


I have two tube manuals that I downloaded ftom the internet... I think I actually understood them the third time around. I just finished Dan Torres's book, which got me started and really helped me narow down components to use on my next completely crazy DIY amp project (512 different tone cominations). I just ordered The Ultiimate Tone, and that seems to have a lot of great stuff in switching, etc.


If I want to get really technical and learn about tubes I general, I have checked out a few books from the 1940s-50s from the tech library about tubes (they were WAY over my head then), and there are books on tubes circuits and even Morgan's Valve Amplifiier books. Turns out my Grandpa's brother wrote a few of those books - too bad his health is failing as well as his mind - he could teach me TONS!!


Sadly the tech Library somehow lost the Radiotron designers handbook, so I won't be able to look at that one for while...


anyone else suggest more good reading material as far as tube circuits go?

 

 

I have not seen any Torries books but have heard some good things about his FX loop. Ultimate tone is a super book as are any of the London Power series and Kevin is a great guy a I mentioned before. I have access to a military library just stacked full of tube stuff if the need be. Old books rule and a library that stocks them is my kind of place. Too bad the Radiotron is gone from yours, you may want to buy one if you get real serious. One other very famous book for tubes is Jack Darr's "Guitar Amplifier Handbook" It covers more about servicing than design but it is great and easy to read. I have no schematics for newer Marshalls. I assume the preamp in a DSL/TSL is simular to a 800. Would still like to see it if possible. Is there a link for them? At least then I would know exactly what you are doing or intending to do...

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