60 Hz Hum On 2008 Marshall Randy Rhoads Tribute Super Lead Mkii.

[anton1117]

Hi All.
 I've replaced *all* the tubes with new ones. There was 1 bad power/output tube, that was causing the HT fuse to blow. I'm using a known-good speaker cable. Hums even with no guitar plugged in, and hum increases when volume is turned up. It sounds great, otherwise (with a guitar plugged in and playing!)! I've poked around inside the chassis with a wooden stick - looking for a possible bad ground, somewhere - but I'm not sure what my next step is. The hum sounds like when you touch the end of a guitar cable. This amp head appears to be 'barely used' - as everything on it and in it - looks pristine! Where/how do I start to troubleshoot this issue? Please advise. Thanks!

[VMS]

Does it hum with old preamp tubes?

[Ed_Chambley]

Pull one tube at a time and put it back.  If one removes the hum, this will give you the general area of the problem.  Start with the first preamp tube.


Next, look for a possible "repair" or Modifiaction that may have been done.  Solder that looks different.  Maybe a wire or component that looks different.


Find the schematic for the correct amp.  Attach it here.


Take voltage measurements of each tube and post them.

[anton1117]

Thanks for responding! Yes - it hums with 'old' preamp tubes in. I've tried tube-swapping the preamp tubes - 1 at a time, and then just replaced all with brand new tubes. It hums without even having a guitar plugged in.
The amp looks pristine inside - like it just came from Marshall manufacturing.
I've been told I could check for a 'ground issue' with a multimeter. Can you explain that?
I will do my best to get the schematic, and take measurements. Thank you!

[anton1117]

This is the link to this amp's schematic:
http://el34world.com/charts/Schematics/files/marshall/Marshall_1959mkII-volt-chrt.pdf

[Ed_Chambley]

the 1959RR, or Randy Rhodes MKii has 2 inputs that are cascaded if I remember.  I have only worked on 1, but if memory serves it is not a stock 1959 Super Lead.  These amps had white tolex and how it looks inside is great, but it still only takes a littls something wrong.


This amp was not embraced that well since it was marketed towards Randy Rhodes tone fans and Randy Rhodes dod not use a cascaded gain stage.  I believe it was designed to plug into hole 1 for a traditional low gain Marshall and hole 2 added a cascaded stage.


I do not believe you have the correct schematic.  The only one I repaired was when someone else tried to lower the gain and the resistor change was not made correctly and the amp had noise.


Sometimes you can check grounds in an amp with a meter, but most times this will not really tell you where the problem is.


You may want to find the correct schematic for the Randy Rhodes MKii.  If the amp arrived to you with the noise, it could be simple as the grid of the first stage not grounding or even wired wrong.  Is the amp motorboating.  IOW, how do oyu know it is a 60 cycle hum?

[anton1117]

The hum sounds like 'touching the end of a guitar cable'. Which my understanding - is known as the 60 Hz hum. But the hum happens even with no guitar cable plugged in. And the hum increases when you turn the volume up.

[shooter]

the hum increases when you turn the volume up.

that indicates your problem is most likely left of the volume pot.  can you get the sound to change as you move your hand close, (DON'T TOUCH), around that section, input jack, V1, etc

[anton1117]

I'll try that. If you know of a good link for the 1959rr schematic - that would be appreciated! I google-searched the heck out of - and couldn't locate a schematic. :p

[anton1117]

The sound does not change if I put my hand around any of the inputs or pots. It's a 60 Hz hum at almost nil volume - but the hum starts to turn to a buzz as I turn the volume up. Again - no guitar or guitar cable attached to input. I also tried a 'ground lift' adapter - to eliminate the possibility of it being a ground hum/ground fault with my home grounding. No change to hum/buzz when I used the ground lift adapter.

[shooter]

your subject line says 2010...Is this a pcboard re-issue internals?, cliff jacks?
what happens when you pull V1 and leave it out?

[Ritchie200]

Retention/clean the input jacks grounding lugs.


Jim

[anton1117]

Sorry - that should have read 2008.

[Ed_Chambley]

Schematic Attached.  Marshall sent it to me.  What Ritchie200 is telling you is you could have a open grid on your first tube and this is either pin 2 or pin 7.  In you case it is both since the stages are cascaded.  Funny, this is nothing more than another verision of making a marshall sound like mud.  Sorry, just an opinion.  Randy Rhodes used a MXR Distortion Plus into a Super Lead, which may have had a cascaded stage but when I saw him it did not sound like this amp sounds.  Just an opinopn and has nothign to do with fixing the amp.


If you pulled the first tube and still have the noise, it cannot be input jacks simply because you are disconnecting them from the rest of the circuit.  If this is the case, I would look closley at R31, R32 and the balance pot which is 100k.  I have had problems with this and this is testing my memory, but I believe I replaced R31 and R32 with 2 watt 220K resistors because the humbalance pot did not center correctly.  So pull V1 tube. If oyu still have the noise, measure with your meter WITH THE AMP OFF! R31 and R32.  If one happens to be burned, you may be done just by replacing this.


Gotta start somewhere.  Here is the idea.  Pulling one tube, not all of them, at a time.  If you are lucky, one will make the noise disappear.  If it does, let us know.  If not say this too.


I know I changed the mix resistors to 220K, but this should not be your problem.  Just my preference.


Take some good photos of the inside and post them.  Having more is better than 1or 2.  Take you photos of the inside close and move and take another until you get to the end and then turn it around and do it again.  Everyone will want to see the chassis front inside and back inside.


We have a schematic and a few good techs around.  These amps do not have a good history, but it is not a complicated amp.  It is very common to revise the cascaded gain of input 2 as this is really a Super Lead and when the output stage distorts with the preamp it loses the Marshall Mid Punch.  This is if you plan to be able crank it.


What I am saying is your problem should be repairable.  Review this schematic until you can see how input 2 works.  Check the schematic to component values in the amp and insure someone has not changed them.  This should get you started.

[anton1117]

WOW! Thank you - Ed - for the invaluable insight and advice! I'll check everything you said - and check back in - asap. Thanks again!

[anton1117]

I pulled V1, and the hum/buzz is decreased in volume - but still noticeable. Pulled V2 and the hum decreased further. All 3 preamp tubes pulled, and (of course) the noise is gone. Was I supposed to replace V1, and then pull V2, then replace V2 and pull V3? I also checked R31 and R32 - and the both checked good - at or near 100 ohms.
The inside of the amp looks pristine, and untouched - like the day it was birthed at Marshall manufacturing. lol It doesn't appear as if any other worked had been done on it previously.

[anton1117]

More pics.

[Ed_Chambley]

Yes, you should pul one and replace.  Pull the second one and replace it.  The Phase inverter, V3.  Which one made the greatest change in the noise?

Since I do not know your experience with electronics, I am considering you still learning tube amps.  Have you confirmed your bias on power tubes since installing new ones?


It is common for ALL v1 tubes to make an amp more quite, but it is you specific hum you want to listen for.


After you have a good ear for what happens, go here:
http://www.geofex.com/ampdbug/ampdebug.htm


There is a list of symptoms.  Check each one and let us know one(s) seem spot on.  Since the amp is 9 or 10 years old, it is possible to have a bad filter cap, but they usually last longer unless someone damaged it in some way.


Do not let the condition of the inside fool you, if it were really perfect you wouldn't have a problem.  I have seen quite amps that look bad and loud amps that look great.  Just get a filter cap Positive near a pot and you will be humming.  Doesn't take much.  There is a lot of current inside the chassis.


Next, you want to get voltages.  Have a DMM I guess, if not go get one.  The cheap one is fine, with tubes we are looking for close enough, not perfect.  And a Cheap one is fine until one day you change a cap value and prefer the tonal change.  Then you get the Amp Virus and you will probably start collecting old crap, but I digress!


Connect the black lead to ground.  Chassis lip is fine.  You must be careful as the amp must be running all on into a dummyload or speaker.  If the speaker, you will hear a pop sometimes.  Doesn't hurt anything.


Measure
V1
Pin 1
Pin 2
Pin 3
These are DC
The other half
Pin 6
Pin 7
Pin 8


Switch to AC and put the black on 4-5 and red on 9 or the other way, doesn't matter. Should be close to 6.3vac

Pin 1 generally reads high as it is the plate and so does pin 6.

Move to V2 and B
Do the same

V3 A and B

Do the same.

Power tubes are V4.V5, V6 and V7.

Measure
Pin 3 (This one should be the highest on all Power Tubes.)
Pin 4
Pin 5

[Ed_Chambley]

Also, plug into input jack 1 for testing. This is the low gain patch and should have less noise.

[anton1117]

Hi Ed.

I'm a novice with tube amps, and electronics, generally. But not entirely 'unskilled' - as I've built a gain stage 'pedal' using 1 12ax7 - lol . I know how to use a voltmeter, and know about the need to discharge filter caps (Danger: High Voltage - potentially deadly!), and the '1 hand rule'. :) And I know I need to keep a 'load' (speakers) connected to the output - when the amp is on. I am most definitely still learning about tube amps, and am extremely thankful for your input! I'm not entirely sure how to bias the new EL34 power tubes I bought for this amp. Can you provide a link to help me with that?
The amp hums with *nothing* connected to the inputs. Do you still want me to connect a guitar/cable into input jack 1?

[Ed_Chambley]

Hi Ed.

I'm a novice with tube amps, and electronics, generally. But not entirely 'unskilled' - as I've built a gain stage 'pedal' using 1 12ax7 - lol . I know how to use a voltmeter, and know about the need to discharge filter caps (Danger: High Voltage - potentially deadly!), and the '1 hand rule'. :) And I know I need to keep a 'load' (speakers) connected to the output - when the amp is on. I am most definitely still learning about tube amps, and am extremely thankful for your input! I'm not entirely sure how to bias the new EL34 power tubes I bought for this amp. Can you provide a link to help me with that?
The amp hums with *nothing* connected to the inputs. Do you still want me to connect a guitar/cable to input jack 1?

I was just going by your first photo.  No need to plug in if the noise is already there.


Look here http://www.aikenamps.com/the-last-word-on-biasing


The 1-ohm resistor replacing the cathode ground is how a lot do this.  The schematic shows one per pair per side.  This may be what you need.

[Ed_Chambley]

Hey, is your amp doing this?

[Ed_Chambley]

Read this thread.
http://www.marshallforum.com/threads/loud-static-hum-from-1959rr.59170/


The heater balance pot marked PR2 on teh schematic, but on the board it is VR2 and has 2 100 ohm (R31 and R32) resistors near it. is a common failure and is what I had to fix.  Not PR1 or as the board notes VR1, this is your bias pot.


Turn it with the amp running and listen to hear if anything changes. Tap it and wiggle it a little too.  It is possible if it crapped out it made the heater string unblanced.  You do not need a adjustable pot to balance heaters, however it is convenient. 


This is a 100r pot in series with 2, 100r resistor.  The pot is designed to BALANCE the heater string and is there to reduce hum.


I actually removed it in the one I repaired and simply used resistors only which is a common method of an Artificial Center Tap.

[anton1117]

Yes! The amp sounds just like that video! I read that if a power tube blows (which it did on this amp) - it can take out that hum balance pot! When I turned/adjusted VR2 - the hum reduced significantly. It's still got a hum, but not as 'pronounced'. Do you think I still need to replace that pot? I tapped it and wiggled it - but it did not affect the noise. The noise only changed when I adjusted it with a screwdriver.
I took measurements on V1 pins:
1 = 132.1 v
2 = 0.0
3 = 0.9
6 = 185
7 = 0.0
8 = 1.7

Pin 9 = 5.7 v  I'm concerned that it's not at the 6.3vac, as you stated it should be. ?
I'll try to post video of the noise I get, and when I turned/adjusted VR2.

[Ed_Chambley]

No video needed.  We have one.

When you find something that makes a change it is easy to get excited because we feel close.  Step back, you need to know a few things.

First, the Power Transformer is NOT like a 1969 Marshall Super Lead and I am not sure if this is good or bad.  Since I generally rate a tranny on size, this one is smaller.

5.7 vac is on the edge of acceptablilty and would not be acceptable to me.  Pull all your tubes and measure your heater voltage again. BTW the power tubes heaters should be checked across pin 2 and 7.  This time do them all.  What do your get?

When you purchased new Power tubes, did you get a matched quad?  Where did you get them?

This amp is noisy from the factory and Marshall knew it and were supposed to offer a recall, but instead simply repaired under warrenty.  That little pot your are turning cannot withstand a heater short and those 2 resistors that connect, the 100 ohms, look different.  Measure the resistance of each while you turn the pot.  Make them equal if possible and leave it.

Power tubes not balanced and biased hot will add to noise.  The first stage does too.

It was 8 years ago I repaired one, but I think the plate voltage is about 450vdc.  To keep from writing a book here, a good place to put the amp for bias is about 34ma for each tube.

So lets say you do have 4 and they are somewhat diffferent.  Think of them as 2 per side.  So lets say you end up with this

V4-34ma
V5-31ma
V6-41ma
V7-46 ma

Since V4 and V5 are parallel, this will draw less current and will generally cause someone to increase current, but when they do, V6 and V7 run hot and will make noise.  The thing to do first is in the example is swap V7 for V4 and it will be better balanced to the OT at idle.

So here is the deal.  The cascaded tube will add noise, but you can get this amp quiet.  We will ground a portion once other common noise issues in this amp are checked.

[anton1117]

The EL34's were 'matched pairs' - and I got them from ApexMatching.com (from Amazon).

V4 - bias reads: 35.1 ma
V5 - bias reads: 35.1 ma
V6 bias reads: 38.7 ma
V7 bias reads: 38.4 ma

How can they individually be bias-adjusted to 35 ma - if only VR1 can be adjusted? Turning VR1 affects all 4 tubes' bias voltage together - doesn't it?

[shooter]

take one tube, V6 or 7, and swap it with one tube, either V4 or 5

that will kinda sorta balance out, but I suspect 4mA isn't gonna make a noticeable difference

[Ed_Chambley]

The EL34's were 'matched pairs' - and I got them from ApexMatching.com (from Amazon).

V4 - bias reads: 35.1 ma
V5 - bias reads: 35.1 ma
V6 bias reads: 38.7 ma
V7 bias reads: 38.4 ma

How can they individually be bias-adjusted to 35 ma - if only VR1 can be adjusted? Turning VR1 affects all 4 tubes' bias voltage together - doesn't it?

well yes, and this is idle bias. Similar to car idle. Just like a carburetor. Once signal is applied things change.


Scooters suggestion is valid, I too do not believe 4ma is the problems. I have created an imbalance larger on purpose! Truth tho, with high watt Marshalls it is best to balance.


It appears you selected a good tube vendor. If they hold this is the best matched set I have seen in a while.

[Ed_Chambley]

Do you or can you use an oscilloscope?

[Ed_Chambley]

Ok, I feel better seeing your bias. I cannot find the link to how to make am listening amp. I will post it when I do.

[shooter]

here's something that works for signal tracing, couldn't find the link here;

http://billmaudio.com/wp/?page_id=1254

fwiw

[anton1117]

Thanks.
I can use an oscilloscope - but unfortunately, I don't have one, nor do I have access to one. Do you have any 'next step' instructions for me - or do you think that, after seeing my bias readings, that it's as good as it's going to get?
Thanks again for all of your help thus far!

[shooter]

without a scope the next best is, build the adapter probe, plug into some powered computer speakers and inject a signal.  walk it from input jack to grid of PA tube, figure out which stage is getting ugly.

[Ed_Chambley]

Yes, you can get a scope for 20. Dollars here, http://el34world.com/Forum/index.php?topic=22702.msg242560#msg242560


 Building a listening amp if you question a 20 scope. It does work!


Yes, did you remove your tubes and see if your heater goes over 6,3 vac as a matter of fact I asked you to do a bunch of things that you have not done yet,


Go back and reread. You have one of 2 choices. Start replacing parts until you find one that might fix it or locate the problem,


Sure they us more, it is not right. Here is one get a 6volt lanter battery and run your preamp tubes with it,  if the hum remains it is not the heaters. Eliminate, and move on or replace parts.

[anton1117]

Since I have some powered computer speakers - I'll likely build the probe (and I'll probably get that little oscope too!)
Pull *all* the tubes - preamp and power tubes - and then take measurements?

[shooter]

Pull *all* the tubes

umm, NO, if it makes guitar sound with hum, you're trying to get rid of hum and keep guitar, that can only happen WITH tubes.

take the schematic, start on left, I like to probe just right of each stages coupling caps for the main signal path.  I also like to inject an 800hz sin wave instead of a guitar, frees up my hands for probing

[anton1117]

Hi Shooter.
I'm not sure how a 'signal injector' is going to help me. The amp sounds amazing when you plug a guitar in to it - except for the 'hum'. Lol It's not 'losing' a signal anywhere. I'm just not quite sure how to track the hum down. I'll need to refer to Ed's previous posts, and start 'digging deeper'. :p If you have any additional insight - I'm all ears! lol Thanks!

[shooter]

except for the 'hum'.

you probe the 1st stage, sounds amazing, no hum, you probe the 2nd stage, sounds amazing, but hum, you have now eliminated 90% of the places hum ain't

[Ed_Chambley]

Pull *all* the tubes

umm, NO, if it makes guitar sound with hum, you're trying to get rid of hum and keep guitar, that can only happen WITH tubes.

take the schematic, start on left, I like to probe just right of each stages coupling caps for the main signal path.  I also like to inject an 800hz sin wave instead of a guitar, frees up my hands for probing

Scooter, he is gettting 5.7 vac on his filaments.  I am willing to bet you 7 doughnut holes this is not what it should be.


If he doesn't check with no load on the tubes and see if the voltage responds accordingly, how are we going to know if filiment supply problem?  We know there is Hum and we know it changes with that trimpot.  Think that little pot coud handle a filiment short from the power tube that went south?


I agree to find the source of the hum, but I know this is not correct. 

[shooter]

he is gettting 5.7 vac on his filaments.

I won't take that bet, since I believe you're correct.  Also in my troubled youth days I helped out making doughnuts and getting the munches, then eating them straight outta the vat, made me hate doughnuts AND their holes for many , many years

[Ed_Chambley]

he is gettting 5.7 vac on his filaments.

I won't take that bet, since I believe you're correct.  Also in my troubled youth days I helped out making doughnuts and getting the munches, then eating them straight outta the vat, made me hate doughnuts AND their holes for many , many years

Troubled youth days!  Are they over?


Doughnuts right out of the fryer.  Now that is good stuff.

[anton1117]

I'm going to build the signal injector probe. Would you say - that should be my next step - to trace an audio signal through the circuit? Ed - I'm not trying to avoid the steps you've spelled out for me - I'm honestly just not sure what my next 'troubleshooting step' - should be. Some 'next step' advice would be extremely helpful for me. Please don't give up on me! :)

[shooter]

'next step'

do what ED suggested, especially the area of filament volts

Are they over?

NO, that's why I'm here

[Ed_Chambley]

I am not giving up, you did.

Everyone has slightly different troubleshooting methods.  I have never used a listening amp, but I guess they work. 


Here is what I would do since it is easy.  Measure all your voltages with the tubes in.  Write them down.  All of them, every single one.  Remove the tubes and do it again.  Now oyu have loaded voltages and unloaded.  Now lets say your Power Tube Plates raise up to 490vdc, but you filiments (heaters) stay lower than 6.3 vac.  Well then by golly you know something is screwing with this winding.


If the voltage jumps up to 6.6-6.8 vac, but your tubes load it to 5.7 vac that is a problem.  You have already found this one, you just have not confirmed what is causing it.  A lot of folks seem to ignore the lowley 6.3 volts on the cathode, but this is where ALL your good old electrons get snatched from.


I am not saying this is the issue, but what you described I posted a video for the hum.  You said this is the noise.  Here is what happened to the one I fixed.  If you will look at the schematic, you will see the trim pot is the artificial center tap ground.  One of the lugs on the pot is connected to one resistor and one to another.  The third goes to ground.


When the power tube in the amp I repaired failed, the trimpot began to arc across the lugs internally.  What was happening was some of the current was passing to ground.  Guaranteed hum city.


You could simply rule it out by lifting the ground side of R31 and R32, this would be the side closest to the pot.  It looks as if you could actually clip them and solder them back since whoever built the board made them stand tall and left a lot of lead and put them in in opposing directions, which is not a problem per say as a resistor does not really care, but I do.


Anyway, if you clip them, you can clip that end to chassis ground for the test.  This will eliminate the pot and let you know if it is ok.  When I did it, I used new 100ohm resistors and when I grounded them the amp was silent.  I made the work look neat, removed the pot from the board and put the artifical center tap on V7, pin 2 and 7 to a common ground. 


If ok, then move on to circuit areas.  The only reason I would use this method is because it worked before, but doesn't mean it will work this time.  I will say whatever has your filiment voltage reading 5.7 vac is a problem.  It may not be the source of your hum, but it is a problem.  Got any idea what they may drop to when the amp is running hard?  Me neither.

[Willabe]

'next step'

do what ED suggested, especially the area of filament volts

Agree, the filament voltages 1st!!!!!!   

[anton1117]

The pot is bad! THANK YOU! I did as you suggested - clipped the leads on R31 and R32, tied them together, and grounded. Peace and quiet! LOL

[anton1117]

Since the '3rd leg' of the pot ties to ground - couldn't I just solder a jumper wire from the end of those two resistors and connect the other end to the ground post that the 3rd leg ties to?

[anton1117]

I want to 'do the right thing' here. I'd have to disconnect a *whole bunch* of wires to be able to get to the bottom of that circuit board - which I'd prefer to avoid - if possible. ;) Will my above-mentioned fix - be ok?

[PRR]

Yes. Two equal small resistors from each side of the 6VAC to any decent ground, done.

[Ed_Chambley]

Just in-case you are not certain, PRR is one to listen to.


Here is what I did.  I removed the pot from the board as not to confues anyone who may work on it next.  I wrote on the board where it was, removed trim pot, not needed.  I removed the resistors from the board.


Considering a Tech should know what Artificiial Center Tap is, where the resistors were I wrote relocated to V7.  I like to use 3 watt, 100 ohm.  I connect one to pin 2 and one to pin 7 and place a grounding lug under the closest nut and solder both resistors here.  A 3 watt resistor will  usually live through a heater short.


The resistors need to be equal values and can be 100-220 ohms, but must be the same.  Then print a copy of the schematic and note your changes on it.  I use plastic pouches and fold the schematic and stick the pouch in the inside of the head.


I am gald you  hum is gone.  Even though I know the amp sounded good, haveing a hum is annoying, plus you saved your Power Transformer, made your tubes run under better conditions.