Biasing v3 L120 Help

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Biasing v3 L120 Help

Postby ep1966 on Thu Feb 13, 2014 7:19 pm

First let me say thanks to all that contribute to this forum. My hat is off to all of you this is the best forum hands down.
I would have not gotten my L120 working without the good people here. I have try my best to find this on here I sure its been posted before so sorry for starting a new tread.

Thanks to the good people here that has help me I have had my L120 working for a little while. recently it stated making a frying sound through the speakers. It finally blew the 3 amp mains fuse. I removed the chassy and started checking things over. I found if I placed the gain switch on lower side the amp would work and not blow the main fuse. if switch to the higher gain setting it would start making the frying sound and then blow the fuse. I checked the
V-resistors the 10k seem fine but when I checked the other which I'm assuming is a 22K due to it having 2 red dots and 1 orange dot it shows no resistance no matter how you turn it. just wonder if this would cause the current in v3 to rise enough to blow the mains fuse or if it causing the current in my power tubes to rise from all the gain due to the bad
V-resistor and make them draw to much current.
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Re: Biasing v3 L120 Help

Postby Unit_1 on Thu Feb 13, 2014 11:21 pm

checking components can be tricky if there is a path in the circuit that allows the dmm to find an alternate route because of a capacitor in circuit, or maybe go through the transformer to ground.

the only reliable way is to desolder one end and then check. kind of a pain....

not really familiar with your amp, but have you checked the bias?

i just did this last night and blew a fuse because of a DOA EL34 :POd:

18 bucks down the drain.

if you don't know how to check the bias, there's a pretty good explaination of how to in the metroamp "how to build a plexi jtm50" manual you can find at:

http://metroamp.com/wiki/index.php/Cate ... structions

download the 50 watt kit instructions, the how to bias section starts somewhere after page 27 or so...

you'll need to find the power rating for the type of tube used in your amp if it's not an EL34

:jam:
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Re: Biasing v3 L120 Help

Postby Papa Dog on Thu Feb 13, 2014 11:50 pm

got your PM and decided to answer here just in case anyone else is having the same issue.

After I took over managing the site the links changed a bit. Please update your bookmarks.

The Sound City Site

schematics page

LB 120 Mk4 11-73 factory updated schematic

You may have a shorted trimmer on V3, but blowing the fuse seems to indicate a dead short to ground.
If the sensitivity switch is what makes it happen, then the fixed resistor in that chain might have failed...but it hardly seems likely that cathode shorted to ground would cause that big an issue.

You might have a bad tube, or rather a marginal one, which can work with the 10k resistance in the cathode, but which cannot handle more gain without arcing inside. The "frying" sound could indicate that the tube has arced and has a grid to heater or cathode to heater leak. I would try repalcing the tube, and check or repalce the resistor on the cathode.
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Re: Biasing v3 L120 Help

Postby ep1966 on Fri Feb 14, 2014 7:33 am

Unit_1 wrote:checking components can be tricky if there is a path in the circuit that allows the dmm to find an alternate route because of a capacitor in circuit, or maybe go through the transformer to ground.

the only reliable way is to desolder one end and then check. kind of a pain....

not really familiar with your amp, but have you checked the bias?

i just did this last night and blew a fuse because of a DOA EL34 :POd:

18 bucks down the drain.

if you don't know how to check the bias, there's a pretty good explaination of how to in the metroamp "how to build a plexi jtm50" manual you can find at:

http://metroamp.com/wiki/index.php/Cate ... structions

download the 50 watt kit instructions, the how to bias section starts somewhere after page 27 or so...

you'll need to find the power rating for the type of tube used in your amp if it's not an EL34

:jam:

hey thanks for the response. Basicly it's two variable resistor in series with there wiper connected to one side of themselves. one end of the series is connected to ground and the other end of the series is connected to the cathode, I/2 of a titrode in this case a 12ax7. there is a spst switch that basicly takes one out of the series. When the switch is open (Low Gain), the 22k is in series with the 10k. When the switch is close it jumps the 22K out series and connects the 10k directly to the cathode. sorry the title was misleading.
Last edited by ep1966 on Fri Feb 14, 2014 8:17 am, edited 1 time in total.
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Re: Biasing v3 L120 Help

Postby ep1966 on Fri Feb 14, 2014 8:14 am

Papa Dog wrote:got your PM and decided to answer here just in case anyone else is having the same issue.

After I took over managing the site the links changed a bit. Please update your bookmarks.

The Sound City Site

schematics page

LB 120 Mk4 11-73 factory updated schematic

You may have a shorted trimmer on V3, but blowing the fuse seems to indicate a dead short to ground.
If the sensitivity switch is what makes it happen, then the fixed resistor in that chain might have failed...but it hardly seems likely that cathode shorted to ground would cause that big an issue.

You might have a bad tube, or rather a marginal one, which can work with the 10k resistance in the cathode, but which cannot handle more gain without arcing inside. The "frying" sound could indicate that the tube has arced and has a grid to heater or cathode to heater leak. I would try repalcing the tube, and check or repalce the resistor on the cathode.



Thanks papa dog for the info. No matter how I switch the gain setting it shows the 22k has no resistance so guess its bad. My Amp doesn't have a fix resistor in this circuit it's a 10k and a 22k both are variable resistors( May be reading the color codes wrong guess that could be a 2.3k ). Guess Soundcity ran out and dropped in what they had on hand LOL that would go along better with the schematics. so i'll start there. Maybe I did have a power tube go bad, dam shame just put them in not long ago. there's another 100.00 or so. I replaced the 12ax7 first thing when it started doing it, same thing. Power tubes right now are Bias on the cold side at 30ma each 90ma per side. I set the bias on the coldside to see if that would stop the current drain for blowing the fuse. I hope I explained that right but I think you got the picture of what I was trying to do.
Thanks Again Eddie
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Re: Biasing v3 L120 Help

Postby Papa Dog on Fri Feb 14, 2014 4:28 pm

ep1966 wrote:Thanks papa dog for the info. No matter how I switch the gain setting it shows the 22k has no resistance so guess its bad. My Amp doesn't have a fix resistor in this circuit it's a 10k and a 22k both are variable resistors( May be reading the color codes wrong guess that could be a 2.3k ). Guess Soundcity ran out and dropped in what they had on hand LOL that would go along better with the schematics. so i'll start there. Maybe I did have a power tube go bad, dam shame just put them in not long ago. there's another 100.00 or so. I replaced the 12ax7 first thing when it started doing it, same thing. Power tubes right now are Bias on the cold side at 30ma each 90ma per side. I set the bias on the coldside to see if that would stop the current drain for blowing the fuse. I hope I explained that right but I think you got the picture of what I was trying to do.
Thanks Again Eddie


Hey Eddie,

The cathode reistance on that side of V3 is generally much higher than for any other tube in that amp.
Nost of them are 2k2...that one is supposed to have a fixed value (c.10k) on the board and then either one or two variables, depending on the year/version/issue. In the factory mod schematic I linked to there is an instruction to turn one of therm up all the way, and to set the other to taste. Trust me that putting to low a resistance on that cathode will cause all manner of noise and artifacts in the signal...by pushing the gain on that stage way "over the top".
If the trimmers(s) are bad you can remove thre switch and the trimmers, make sure you have a minimum fixed resistor and put a 20k or 25k linear pot where the switch was, wired as a rheostat...give a person a bit more "granular" control over the gain on that tube.
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Re: Biasing v3 L120 Help

Postby ep1966 on Sat Feb 15, 2014 6:33 am

Thanks Papa Dog for the tip. Believe I give that a try think I would like that setup better then the stock one. Buy your responce guess I did read the color codes right on the 22k variable resistor. :D
Eddie
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Re: Biasing v3 L120 Help

Postby ep1966 on Tue Feb 18, 2014 6:10 pm

Papa Dog wrote:
ep1966 wrote:Thanks papa dog for the info. No matter how I switch the gain setting it shows the 22k has no resistance so guess its bad. My Amp doesn't have a fix resistor in this circuit it's a 10k and a 22k both are variable resistors( May be reading the color codes wrong guess that could be a 2.3k ). Guess Soundcity ran out and dropped in what they had on hand LOL that would go along better with the schematics. so i'll start there. Maybe I did have a power tube go bad, dam shame just put them in not long ago. there's another 100.00 or so. I replaced the 12ax7 first thing when it started doing it, same thing. Power tubes right now are Bias on the cold side at 30ma each 90ma per side. I set the bias on the coldside to see if that would stop the current drain for blowing the fuse. I hope I explained that right but I think you got the picture of what I was trying to do.
Thanks Again Eddie


Hey Eddie,

The cathode reistance on that side of V3 is generally much higher than for any other tube in that amp.
Nost of them are 2k2...that one is supposed to have a fixed value (c.10k) on the board and then either one or two variables, depending on the year/version/issue. In the factory mod schematic I linked to there is an instruction to turn one of therm up all the way, and to set the other to taste. Trust me that putting to low a resistance on that cathode will cause all manner of noise and artifacts in the signal...by pushing the gain on that stage way "over the top".
If the trimmers(s) are bad you can remove thre switch and the trimmers, make sure you have a minimum fixed resistor and put a 20k or 25k linear pot where the switch was, wired as a rheostat...give a person a bit more "granular" control over the gain on that tube.


Hey Papa Dog took your advised removed the old 10k and 22k varible resistors and put a fixed 10k resistor on the broad pulled the spst switch and placed a 23k pent. in it place. wired it up and the frying sound is gone most of the noise is gone. to the point that now i believe I can hear some AC hum in it. I had read some were that the diodes in these old amps would break down over the years start to have some reverse leakage. think this could be the case and cause it to blow the mains fuse when it being drived harder? she is still blowing fuses. By the schematics I have found the 22k resistor is on the broad and the 10k is a varible that gets switched out when the spst switch is closed. I took My DMM and Measure both the fix 10k and the 23k pent. in series set them to 22k tried it again sounded much better but it blew another fuse. I have aready replaced all the filter caps in the amp but i guess that doesnt mean i didnt get a bad one LOL.
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Re: Biasing v3 L120 Help

Postby Papa Dog on Tue Feb 18, 2014 10:25 pm

when my 200+ was shelved 30+ years ago it was for 2 reasons: (1) it was blowing fuses and the only tech I could find at the time could find nothing worng. (2) one of the filter caps had been replaced with a cludge...so nothing was working.

I pulled the amp from storage intending to send it off for service, but found this board and the courage to at least attempt the repairs myself.
New filter caps, had to replace the choke, as it had gone missing in one of the trips to the shop all those years ago.
Decided that it would be cheap and not hurt a thing to replace the diodes, so that happened as well.

Fired it up and took out a wall I was wanting to remove, drove all the vermine off the place and was heard several blocks away by a patroling policeman. :twisted:
zero noise, zero hum, just incredible deapth of tone and huge amp feel, even at the lowest of volumes.

So...yes, if I had that amp on my bench I would likely change the diodes, just in case. Might halp, and can't hurt, so long as they are put in correctly.

Most any other changes or mods I might suggest on this, or any other board, are the result of my own tinkering and experimenting. I did the sensitivity change on my 200+ because I wanted to be able to vary it and not just have 2 choises...it just works...
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Re: Biasing v3 L120 Help

Postby ep1966 on Wed Feb 19, 2014 7:11 am

Papa Dog wrote:when my 200+ was shelved 30+ years ago it was for 2 reasons: (1) it was blowing fuses and the only tech I could find at the time could find nothing worng. (2) one of the filter caps had been replaced with a cludge...so nothing was working.

I pulled the amp from storage intending to send it off for service, but found this board and the courage to at least attempt the repairs myself.
New filter caps, had to replace the choke, as it had gone missing in one of the trips to the shop all those years ago.
Decided that it would be cheap and not hurt a thing to replace the diodes, so that happened as well.

Fired it up and took out a wall I was wanting to remove, drove all the vermine off the place and was heard several blocks away by a patroling policeman. :twisted:
zero noise, zero hum, just incredible deapth of tone and huge amp feel, even at the lowest of volumes.

So...yes, if I had that amp on my bench I would likely change the diodes, just in case. Might halp, and can't hurt, so long as they are put in correctly.

Most any other changes or mods I might suggest on this, or any other board, are the result of my own tinkering and experimenting. I did the sensitivity change on my 200+ because I wanted to be able to vary it and not just have 2 choises...it just works...


Hello Papa Dog,
Well guess I'll replace the diodes why im at it I'll replace the bias diode to. hope it does the job. Now I know why it had a 30 amp fuse there instead of the a 3 amp. I can see a mistake at reading a fuse wrong Im blind without my reading glasses but it really makes me wonder.
I have around 30 of 1n40007 diodes on hand would these work? there 1000v reverse voltage but there only 1 amp. but hey I got enough to double them. If not what would you suggest.
The Caps thats solder across the diodes think they should be replaced? I think the schematic has the value of them but not the voltage.
I have ruled out that the Power Tubes being bad. Placed them in another amp set them to 40 ma and jam them for 40 mins plus with no problems.
I really liked that mod to the gain switch would like to hear more that you done.
Thanks
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Re: Biasing v3 L120 Help

Postby Papa Dog on Wed Feb 19, 2014 3:35 pm

1n4007 are what I use...some folks use different things.

I changed the caps going to V2 from .02 and .001 to both .022. dont care for "bright" channels, and took out the "bright" cap from the "normal" channel.
I tried cascading the pre-amp channels.
used the "attenuated" jack as a switch .
didn't work out too well, as the gain on V1 is fairly low on the Mk4 circuit.
there are a couple ways to get higher gain on V1, such as, when changing the cathode bypass cap you can lower the resistance on the cathode resistor. I don't recomend doing it that way.
You can also change the grid leak on one or the other channels, or both, to 500K or 1 Meg. That affords a bot more crunch on the front end with the guitar wide open.
The "secret" (if there is one) is to add a bit of gain accross the board and not too much at any one point. In fact, with the Mk4 pre-amp one can overdrive any one, or all 3 bands with some interesting results.
But, those amps were designed to be "clean machines" and shine best when not pushed too hard for overdrive.
There are some mods in other peoples schematics, but most are not what I would suggest...one mod leads to another and requires tweaking to control squeal, which results from pushing the gain "over the top".

IMHO best practice is to "clean it up" maybe boost the gain on V1 a bit and use the sensitvity mod as a "master volume" by turning it down a good bit and opening up the channel volume. If that doesnt give you the crunch you want, then a switchable booster ahead of the input will give you what you want. These amps are known to take pedals very well, and if , like me< "fuzz" is not your thing, then just boosting the input will give you a less abrasive mellower and more pleasing "crunch".
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Re: Biasing v3 L120 Help

Postby ep1966 on Thu Feb 27, 2014 8:09 am

Papa Dog wrote:1n4007 are what I use...some folks use different things.

I changed the caps going to V2 from .02 and .001 to both .022. dont care for "bright" channels, and took out the "bright" cap from the "normal" channel.
I tried cascading the pre-amp channels.
used the "attenuated" jack as a switch .
didn't work out too well, as the gain on V1 is fairly low on the Mk4 circuit.
there are a couple ways to get higher gain on V1, such as, when changing the cathode bypass cap you can lower the resistance on the cathode resistor. I don't recomend doing it that way.
You can also change the grid leak on one or the other channels, or both, to 500K or 1 Meg. That affords a bot more crunch on the front end with the guitar wide open.
The "secret" (if there is one) is to add a bit of gain accross the board and not too much at any one point. In fact, with the Mk4 pre-amp one can overdrive any one, or all 3 bands with some interesting results.
But, those amps were designed to be "clean machines" and shine best when not pushed too hard for overdrive.
There are some mods in other peoples schematics, but most are not what I would suggest...one mod leads to another and requires tweaking to control squeal, which results from pushing the gain "over the top".

IMHO best practice is to "clean it up" maybe boost the gain on V1 a bit and use the sensitvity mod as a "master volume" by turning it down a good bit and opening up the channel volume. If that doesnt give you the crunch you want, then a switchable booster ahead of the input will give you what you want. These amps are known to take pedals very well, and if , like me< "fuzz" is not your thing, then just boosting the input will give you a less abrasive mellower and more pleasing "crunch".



Hey Papa Dog
Replaced all the diodes in the amp. Samething blowing fuses. But found out a few things. I placed the caps back on the diodes. found they were causing a major noise issues. First let me say Power supply section is still a bit greek to me. So this was all on a assumption that the caps could be breaking down under a load shorting and poping the mains fuse. I clipped them out and the noise lower at least by 80% at very playable levels so guess they were bad to a point all the schematics I've seen dont show them at all.
I set back took a fresh look at it agian. Been top to bottom trying to find whats causing it to blow fuses. Today got a wild hair and decided I was going to froce the problem to show itself. I would have not done this if it wasn't my amp. but guess I'm getting a lettle frustrated with it. I took a 30 amp fuse and placed in it and fired it up. played it at low volume and no problems like it would if had the 3 amp fuse. Actually it sounded great and i was amased at the tone flowing in those Vintage 30's.
Had my brother to watch the power tubes and I watched the preamp tubes set the sensitvity to high and begain rolling the volume up a notch or two at a time. got her a little over half and shaking the stuff off the walls at the other end of the house i may add LOL and there it was. One side of the power section began to red plate. Turned her off quickly. So I'm assuming the current in one half of the power sections is drifting way to high when it being driven hard. With the 3 amp fuse in her it would blow before it would reach this condition. I now this was extreme but i have tried every thing i could think of to figure it out.
As long as the amp is idle or low gain its stable I let the amp cool off turned it back on checked the bias and both sides are still on the cold side at 90 mA Now I'm going to completely replace everything in the bias circuit fully rebuild it just to be on the safe side. I have found the worst drifted resistors in this amp i have ever seen. Any other ideas on what may be causing the problem? May even replace all the 10k grid resistors.
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Re: Biasing v3 L120 Help

Postby ep1966 on Thu Feb 27, 2014 5:01 pm

I was just thinking if the coupling cap from the PI is bad could it be leaking positive dc voltage into the grid and if so could that voltage go up and down as the AC signal rises up and down through the cap. I know i'm throwing a wild thought out there but if it is this could explain why my bias is holding at what I set it at idle and when its driven harder the current on that side is rising to much. If it had a positive voltage leak into it wouldn't this effect the bias voltage by rising it more postitive and allowing the power tubes to drawn more current?

Just a thought!
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Re: Biasing v3 L120 Help

Postby Papa Dog on Fri Feb 28, 2014 2:52 pm

I thought about the last two posts a lot yesterday and last evening.
Missed the "snubbers on the diodes" early on. Should have mentioned that they are not factory, or at least not on the schematics at all. Some folks actually add those to reduce noise. Your experience would indicate that, if not done properly they would add more noise than they remove.

Interesting way you found the "red plating" on the one bank. That one bank is plling so much current when "pushed" is a head scratcher.
Youcould check those caps, or replace them and see if it makes a difference. But, I will have to do some reading and searching to see if i can find any other mention of that happening other amp.

Seems as though that if there were an issue with the output transformer it would show up at lower volumes, but it is possible that is where the issue lies.
If there are shorted windings in the output transformer one side would tend to run "hotter" than the other.
But it could also be a failing resistor on the screens. Bottom line is that something somewhere is getting hot or failing under load.

I will give it mor ethought and see if I can find anything anywhere that mentions this.
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Re: Biasing v3 L120 Help

Postby Mr. Foxen on Fri Feb 28, 2014 4:08 pm

I've seen same snubber install on a few amps so I reckon it was done at the factory. Another inconsistency with the amps.
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