Marshall SUper Bass Clone noise/hum through speakers on power up.

[tfrost]

Hopefully this is an easy question?  basically I am getting a hum/noise through the speakers when I flip the power toggle to on.  This is with the standby off.  Any ideas?  I am assuming a ground issue, but unsure of where to start?  The amp is a Marshall Super Bass Clone per 67' era specs.

thanks

[SILVERGUN]

The amp is a Marshall Super Bass Clone per 67' era specs.

Did you build it?

Post a schematic, if you have one (or post a link) so guys won't have to start trying to help you by searching for one....
I mean this in the nicest possible way. 

Is this it?:
http://el34world.com/charts/Schematics/files/marshall/Marshall_jmp_super_bass_100w_1992.pdf

[tfrost]

Yes I built it a couple of years ago...  All worked perfectly up until a couple days ago when I noticed this problem.  the hum stays when you flip the standby to on as well. 

I used that schematic as best I could, but I mostly went with the metroamp 100 watt plexi kit instructions but I transferred them to a 67 era spec build.  So cap board inside instead of on the chassis.  Is there a common problem that could cause this all of a sudden.  I have tried many different cords and all are good with the same results, so I am sure its the amp.

thanks again

[HotBluePlates]

Yes I built it a couple of years ago...  All worked perfectly up until a couple days ago ...

What else changed a couple days ago?

Couple-years is too-short life out of filter caps (unless you used 30 year old caps in your build). "Worked perfectly" for couple-years suggests you may not have been s shoddy builder, unless couple-years-ago was the last time the amp was on.

So what new thing did you try, or new tube installed, or new mod, or new cord, or new guitar, or what new accident did the amp suffer, o what friend did you loan it to a couple days ago?

[tfrost]

thanks for the reply HotBluesPlate...  The only change I can think of is that I quit using my Ultimate Attenuator with the amp.  But that has been a few weeks now.  This problem occurred just a couple of days ago.   I have been messing around with the amp a little bit today, to see what other symptoms I see.  Caps were all brand new, NOT nos caps.  I do have NOS glass through out but tapping on the tubes yields no results to indicate bad tubes, nor does tapping on any of the tube socket connections. 

I would change tubes, but I am curious what can cause such a noise through the speaker cab when the amp is on, but also on standby

First thing I notice is that in play mode meaning amp is on and standby is off ie... amp is ready to play/make sound.  when I turn the volume up the hum get very loud.  Not a normal 100 watt marshall hum or hiss.  But very loud, like some sort of ground hum. 

the other thing I noticed and something I will check through out the week as time allows, is now I am getting a loud audible POP when I turn the amp off.  Not the standby, just the on/off switch when turned off.

Something happened to this amp that otherwise worked perfectly fine a week ago.  Amp gets regular use, but I am no longer using this amp to gig since I recently built that 5E8A amp...

[HotBluePlates]

... I am curious what can cause such a noise through the speaker cab when the amp is on, but also on standby ...

To make sure I understand, the amp is in standby so if you try to play you get no sound. Right?

Can you carefully measure voltage inside the amp and verify that the B+ to the center-tap of the output transformer is cut off when standby is engaged? Doing this tells you something important: the hum can't be coming from any of the amp circuit, because the output tubes are unable to conduct (effectively turning off all the audio path of the amp).

If that happens, there's only one way to get hum to the speakers: the power transformer radiates hum into either the speaker cabling or into the output transformer, which remains connected to the speakers.

Can you post a photo of the power and output transformers? If the core laminations of both transformers sit in the same plane (and are close enough), the power transformer can radiate hum into the output transformer. Often, the PT is a laydown type while the OT stands upright, which puts the laminations at 90-degree angles. This minimizes hum coupling. Or two upright transformers are arranged with one rotated 90-degrees.

...  The only change I can think of is that I quit using my Ultimate Attenuator with the amp.  ...

If you were using an attenuator, it might have been reducing the hum (along with everything else) enough for you to not notice. If you only played loudly with the attenuator removed, you might have again covered up the hum. *IF* the PT is coupling hum into the OT, it is a set level regardless of how loud you're playing. So if you play softer, you'll hear the hum more prominently. But if you use an attenuator, it's knocking down the level of everything traveling on the speaker cables, hum included.

Assuming I've guessed correctly, you see why it's important to know about everything that changed once the problem starts?

[stratele52]

Check DC negative bias voltage , you may have too much AC voltage from bias circuit  ( bias filter caps , ss rectifier )

[tfrost]

... I am curious what can cause such a noise through the speaker cab when the amp is on, but also on standby ...

To make sure I understand, the amp is in standby so if you try to play you get no sound. Right?  Correct

Can you carefully measure voltage inside the amp and verify that the B+ to the center-tap of the output transformer is cut off when standby is engaged? Doing this tells you something important: the hum can't be coming from any of the amp circuit, because the output tubes are unable to conduct (effectively turning off all the audio path of the amp).  Ok, so I have the Heyboer 1202-132, its the metro spec output transformer.  With the amp on, and in standby (or rather standby engaged) I see no B+ at the output transformer center tap.  When I flip the standby off (to play mode) I see the 490VDC B+. 

If that happens, there's only one way to get hum to the speakers: the power transformer radiates hum into either the speaker cabling or into the output transformer, which remains connected to the speakers.

Can you post a photo of the power and output transformers? If the core laminations of both transformers sit in the same plane (and are close enough), the power transformer can radiate hum into the output transformer. Often, the PT is a laydown type while the OT stands upright, which puts the laminations at 90-degree angles. This minimizes hum coupling. Or two upright transformers are arranged with one rotated 90-degrees.  I do have the metro spec from Heyboer, Laydown PT, off the top of my head I believe it is the 1203-80, if my memory serves me correctly.  And the lamination are at 90 degree's or perpendicular, if you will, to the OT.

...  The only change I can think of is that I quit using my Ultimate Attenuator with the amp.  ...

If you were using an attenuator, it might have been reducing the hum (along with everything else) enough for you to not notice. If you only played loudly with the attenuator removed, you might have again covered up the hum. *IF* the PT is coupling hum into the OT, it is a set level regardless of how loud you're playing. So if you play softer, you'll hear the hum more prominently. But if you use an attenuator, it's knocking down the level of everything traveling on the speaker cables, hum included.

Assuming I've guessed correctly, you see why it's important to know about everything that changed once the problem starts? YES! 

My B+ is 490VDC

I have NOS Tesla's from the 60's in this amp.  They were sold to me as matched quad.  One thing I find interesting is that my hottest tube is 38mV, my coldest is 29mV.  Seems like quite a major difference.  I know they are not going to be the same, but is this something to be concerned about?  I realize this may not be why I am having my problem.

OK So that may have been a bit off topic.  Is there anything else I can check to see what may be causing the hum?  I have the higher end Monster cables, however my other amps don't have this issue.  So I am thinking that it could be this hum coupling you speak of?  I am noticing with more and more frequency, my on/off toggle is popping loudly as I turn the amp off (through the speakers that is)

thank you for the help so far.

[tfrost]

Here are a couple internals

[tfrost]

And another.  Let me know if close ups are needed?

[stratele52]

Is there anything else I can check to see what may be causing the hum?   

thank you for the help so far.

Why don't you check  what I write  May 12 ?


How many AC volts ( ripple ) on you -DC bias voltage ?

[tfrost]

Is there anything else I can check to see what may be causing the hum?   

thank you for the help so far.

Why don't you check  what I write  May 12 ? Sorry Stratele52, I only had so much time last night.  I am planning on doing that tonight though!  I will post results after work.....   Thank you!


How many AC volts ( ripple ) on you -DC bias voltage ?

[stratele52]

+1

[HotBluePlates]

Ok, so I have the Heyboer 1202-132, its the metro spec output transformer.  With the amp on, and in standby (or rather standby engaged) I see no B+ at the output transformer center tap.  When I flip the standby off (to play mode) I see the 490VDC B+.

I do have the metro spec from Heyboer, Laydown PT, off the top of my head I believe it is the 1203-80, if my memory serves me correctly.  And the lamination are at 90 degree's or perpendicular, if you will, to the OT.
[/color]

You are correct: with the laydown PT, the OT is automatically at 90-degrees by virtue of standing up. But when you engage the standby, you get 0vdc at the OT center-tap.

Pretend your PT is a car's engine, the OT (and the connected speakers) are the wheels. The output tubes then are like the transmission. When you go into standby and there's no voltage applied to the output tubes through the PT, none of the engine's power gets to the wheels.

What I mean is you could inject 150vac into a preamp stage of a properly-functioning amp and if you kill the B+ (say, with the standby switch), nothing gets to the speaker because the output tubes aren't swinging current through the OT.

So if there is truly hum in standby, and the standby switch is cutting the B+, it can only be caused by direct radiation from the PT to the OT. OR, the PT itself is vibrating and generating an audible buzz;  however, this doesn't square with your previous experience with a quiet amp (the PT would still buzz with an attenuator, maybe even more noticeably because the speaker output is quieter).

Do you have enough slack in the OT leads to rotate it? If so, unbolting it and turning it (with the amp on) may reduce the hum. Or using a [urlhttp://billmaudio.com/wp/?page_id=882]shield plate as Bill M did to quiet a Blues Jr[/url] (which also has a transformer orientation that shouldn't require such tricks, but did).