super lead noise

[Tipto]

Hello, Tipto from Italy ! This is my first thread !
My Marshall 100 W super lead head from 1972 (recapped and totally hoverhauled four mounth ago) make some strange noises as soon as
I switch on the standby, but after 3-4 seconds of "hum" all seems to stabilyze and all become quiet and the amp sounds good and loud as always it does. If I put the amp on standby again and then switch it on again, the same thing happens.
What to do ?: changing the standby switch is obvius, but my technician told me he has replaced it just four month ago when the amp was overhauled! It' s possibile it' s became bad again ? It not show anything strange, not bad soldering or burned traces . perhaps something is bad in the circuit ? Preamp tube and sockets seems ok and clean as output. Any suggestions.........! Thank you and sorry for my bad english !

[SatelliteAmps]

I would doubt it was the standby switch causing a hum. It is probably just the B+ rising. If it isn't causing any problems, I wouldn't worry about it.

[Tipto]

I would doubt it was the standby switch causing a hum. It is probably just the B+ rising. If it isn't causing any problems, I wouldn't worry about it.

I don' t understand if the hum coming from the preamp or from the output stage. I' ll check all tubes and clean all contacts in the sockets again. Maybe an oxydated contact pin.......

[Blue Strat]

I don' t understand if the hum coming from the preamp or from the output stage. I' ll check all tubes and clean all contacts in the sockets again. Maybe an oxydated contact pin.......

To find out, remove the phase inverter tube (V3) and try again.

Not sure what you mean by "hum" exactly, but some new production preamp tubes will make noise when first switched from standby to run.

[SatelliteAmps]

When an amp is on Standby in that particular amp, the B+ (high voltage) is lifted from the circuit. That means that the tubes are basically sitting at idle. When you flip the Standby switch, the B+ comes rushing in, letting the amp work. This happens fast, but not instantly. As the B+ rises, there might be a slight bit of hum. If it is something other than that, please explain more so we can help understand.

[donnyjaguar]

Wild guess. There's either too much capacitance on the power supply or one of the caps has high ESR and will eventually vent. It could also be too much capacitance on the bias circuit and its its voltage is not falling quickly enough when you switch to RUN. This assuming that the Standby/Run switch also kills the bias voltage when on Standy.

Ask your tech if he increased the capacitor values during the overhaul.

[Tipto]

Thank you friends for all your precious suggestions.
I' ve tried to swap the phase inverter, change the tube but nothing happens.
The "hum" it' s not a normal hum, it' s a mixture of hum and scratches that go out about 3-4 second after the standby is disengaged. The more the volume has got the amp, the more are the scratches-hum until they don' t stopped.
I' don' t know if the tech has changed the capacitors value during the overhaul. Seems the amp has replaced byt F/T capacitors the same value of the previous old ones.
At any rate, I think that the only thing to do is make an accurate checking by the technician of the ALL circuit to found what bad component make noises

[donnyjaguar]

Another wild guess. If there's an electrolytic capacitor bypassing the cathode of the first voltage amplifier, check to see that it wasn't installed backwards. The negative "-" end should be common with ground.

[RedMan]

Sounds like a good tube getting ready to go bad. Just like a spaghetti western.

[Blue Strat]

Thank you friends for all your precious suggestions.
I' ve tried to swap the phase inverter, change the tube but nothing happens.
The "hum" it' s not a normal hum, it' s a mixture of hum and scratches that go out about 3-4 second after the standby is disengaged. The more the volume has got the amp, the more are the scratches-hum until they don' t stopped.
I' don' t know if the tech has changed the capacitors value during the overhaul. Seems the amp has replaced byt F/T capacitors the same value of the previous old ones.
At any rate, I think that the only thing to do is make an accurate checking by the technician of the ALL circuit to found what bad component make noises

I didn't mean to swap the PI, I meant to PULL IT and leave it out. This will help narrow down where the hum is coming from...preamp or power amp.

[nek]

If it turns out to be preamp noise, try running a jumper from the common ground wire connection on the back of the potentiometers to a good chassis ground point. Real common problem with Super Leads of this era.

[Tipto]

I didn't mean to swap the PI, I meant to PULL IT and leave it out. This will help narrow down where the hum is coming from...preamp or power amp.


Without the PI tube or ANY PREAMP TUBE PULLED OUT the hums stop, but (obviously) also the sound !

[Tipto]

If it turns out to be preamp noise, try running a jumper from the common ground wire connection on the back of the potentiometers to a good chassis ground point. Real common problem with Super Leads of this era.

I' ve tried to do as you recommend, but the hum-scratch don' t stop.
I have tried anything with preamp tube but without results. I suppose is an output tube that' s about to go bad. I' ll try to change all of them, rebias ad then see what happens....

[Blue Strat]

I' ve tried to do as you recommend, but the hum-scratch don' t stop.
I have tried anything with preamp tube but without results. I suppose is an output tube that' s about to go bad. I' ll try to change all of them, rebias ad then see what happens....

If you pulled the PI tube and the problem stopped, it's not a power tube.

The problem is in the preamp or the part of the power supply that services the preamp.

On the other hand, since the hum stops after a few seconds there may be nothing wrong at all.

[SatelliteAmps]

If you pull the PI or any preamp tube and the noise stops, then the problem would be in the preamp section, not the power tube section.

The next step is probably going to be to take it back to the tech to have them diagnose it.

[SatelliteAmps]

On the other hand, since the hum stops after a few seconds there may be nothing wrong at all.

My thoughts all along.

[Tipto]

......On the other hand, since the hum stops after a few seconds there may be nothing wrong at all.

Ok, perhaps, at all, nothing will be wrong (in fact the amp sounds good after the hum stop) but something in the amp not function as it would has to do ! If I don' t find this little, (I hope !) trouble and leave it to stay or ignore it, I think that the problem will become more serious.... There' s no chance: I' ll take it back to the tech and .....buonanotte !

[Blue Strat]

One thing is for sure. Posting about the amp here isn't going to fix it (if it needs fixing).

If you're concerned, take it to your tech.

[Tipto]

One thing is for sure. Posting about the amp here isn't going to fix it (if it needs fixing).

If you're concerned, take it to your tech.

Oh yes, I have understood. I don' t pretend to solve the troubles by simply posting about the amp. thank you for the explanation. I have only ask some help to the members of this forum. Perhaps someone has got the same issue. If the problem I have exposed is not a problem for you, I respect your opinion. But not reply to me as I' m a stupid that' s concerned about the trouble (that is not a trouble for you) and want to fix it ! I' m not an amp expert or a technician but it' s very strange that someone tell me that what happens to my amp has nothing of wrong ! Anyway, excuse me all if I have posted about a no-problem and forced you to solve it !

[SatelliteAmps]

I think you are taking things a lot more personally than anyone intended. What we are trying to do, is to help diagnose a potential problem, for free. When we are asking about the problem, and the condition, and everything seems to sound like it is functioning properly, then there isn't much more we can do.

I've explained a few times what happens when you flip a standby switch. What you are describing fits that very well. If the amp functions perfectly otherwise, then there is really nothing to worry about. It's not something that will get worse, or cause anything else to fail, because it is working properly.

[Tipto]

I think you are taking things a lot more personally than anyone intended. What we are trying to do, is to help diagnose a potential problem, for free. When we are asking about the problem, and the condition, and everything seems to sound like it is functioning properly, then there isn't much more we can do.

I've explained a few times what happens when you flip a standby switch. What you are describing fits that very well. If the amp functions perfectly otherwise, then there is really nothing to worry about. It's not something that will get worse, or cause anything else to fail, because it is working properly.

So, you try to tell me that the scratches and hum after disengaged the standby it' s a NORMAL thing in any Marshall tube amp ! Well, now I' m going to get a new one in the store and it HAS to scratch and noise to sound good ! If not, I' ll return it to the vendor as a BAD gear !
I think that the things don' t stay exactly as you say.
There is a TROUBLE in the amp: probably, a little problem that will not get worse as you say, but I want someone to help me to fix it, not someone that tell me " Ignore it, don' t worry about it !!" also if the problem is a stupid problem.
I hope you have understood my bad english. I' don' t take anything personally. I have joined to the forum for asking an help and exchange opinions. I don' t pretend that my problem MUST be solved by anyone of you. But I not accept who minimyze the problem: that' s the wrong way to obtain any good result. It' s more honest to say: " I m not able to solve your problem" than say "don' t worry your problem is not a problem " . This is my opinion. But seems I' m alone.
Stop talking. I ' ll' take back the amp to the tech and watching is the problem can be fixed or not.

[donnyjaguar]

Tipto, *I* think your amp has a problem and I've given you a few things to check out. There are some very clever and knowledgeable people on this discussion group, but most being musicians too, their opinions often differ. :) On the plus side, nobody knows it all and when you get a group of right-brained, creative lateral thinkers together many options can be explored quickly.

It reminds me of the joke: How many drummers does it take to play a part in a tune? Answer: 5. One to play the part and 4 to comment on how much better they could've done it.