Marshall 6100 question

Hi Guys

I am no tube amp wizard, although having ordered the PM670 PCBs recently I had better get learning Anyway I digress...... I am asking for some help debugging my Marshall 6100 amp which has developed a problem ......

The puzzle: I get a steading buzzing sound when the amp is switched on, with a guitar plugged in but not playing. The amp has a "standby" mode which seems to disable the ouput stage. The buzzing occurs even when in standby mode. My initial thought was that it was a preamp issue, and that I need to replace the preamp tubes. But now I am not so sure. Schematics:

Preamp
Power Amp
PSU

Any ideas?

The power amp tubes are a few years old, but frankly I only use the amp a few times a year. I dont know if the preamp tubes have ever been changed.


thanks

Steve

With the standby on and still problems i'd think it may be a power supply
issue [ leaky caps that need to be replaced ] or maybe simple a grounding issue
[ are the mains ground connected ]
does it make noise with no gtr cable plugged in ?

standby turns off the hi voltage to all tubes. you shouldnt get any sound from the amp.

take all tubes out and turn it on. if it goes away put back power tubes first, then phase splitter, then preamp tubes till buzz returns. and there;s your bad tube. thats what im guessing. and easy enough to check.

remember that the power tubes are in pairs, so put them back in pairs.

can you describe the buzz or better can you post a sound clip of it?

buzz could be one of many possibilities but usually it's a bad tube somewhere. If there was hum then it's ground related.

A Marshall with FETs and ICs... SIGH :sad:

remember that the power tubes are in pairs, so put them back in pairs.

Click to expand...

It is important to remember that the pairs are outside and inside. For instance, power tube #1 & #4 are a pair, and power tube #2 & #3 are a pair. Amorris is correct, in standby, you should get zero noise, as all power to all tubes has been cut. Sometimes,you will see the standby after the first cap though. Actually, standby is not really needed. Ever see a standby switch on high end tube audiophile gear (for that matter, right here with the G9)? Standby switches come from the old radio days when they had over 1000V on the tubes and a standby was actually needed while the heaters came up to temp.

I corrected the tube #s, as I mistyped the first time.

Guys, thanks very much for your suggestions, I'll give it a try at the weekend & report back.

Thanks!!

Steve

Is the buzz coming from the speaker(s)? or is it a transformer Buzz?Is the standby switch good? You might have more than one problem

If it has plastic PCB mount jacks check for loss of tension at the contacts

> standby is not really needed.

Paying gig. You take a 10 minute break. You must silence the amps so background noise does not disturb the patrons. 20 minutes later the barkeep yells at you to "PLAY! NOW!" He does not want to wait another 11 seconds for your heaters to redden the cathodes.

> The amp has a "standby" mode which seems to disable the ouput stage. The buzzing occurs even when in standby mode. My initial thought was that it was a preamp issue

Can't be preamp.

The Standby on this amp cuts the AC to the rectifier which feeds plates, screens, driver, tubes in preamp. AC still goes to the Bias rectifier.

I agree: you needa figure out if the buzz is in the speaker or coming direct off the chassis. At dead idle, this amp should be safe with speaker unplugged. If it still buzzes, put your ear to the chassis but NOT your nose on hot glass or wires.

OK, thanks for all the suggestions. Right now I have all the tubes removed, and the *hum* (thanks Gustav) is still coming fron the speaker. Definitely the speaker.

However, there is also a hum coming from the chassis, which persists whether the speaker is plugged in or not.

Having taken the wall socket to pieces, it seems that although the house wiring is old, it has an earth connection.

Having pulled the amp out of the cabinet and eyeballed the PCBs on the component side, there are no obviously leaky caps. To take the PCBs off the chassis to inspect the solder side is a bigger job.....

Any more ideas? :?

if the transformer makes a audible hum by itself , you can tighten
the end caps and that may help a little ,
speaker hum with no power tubes ? being carefull of course get a high voltage cap
and clip lead it across one the the big power supply caps ,
if they are going bad that should reduce some of the hum and tell you
if that is the problem .
Does the hum behave the same at other places ? [ grounds ]

PLEASE! use a sharpie to mark all the little "molex" style connectors so that re-assembling the amp is a little easier. You will come up with some very creative swear words if you don't. I will also check the Korg/Marshall tech bulletins to see if they say anything about this particular symptom.

[quote author="stinky"]PLEASE! use a sharpie to mark all the little "molex" style connectors so that re-assembling the amp is a little easier. You will come up with some very creative swear words if you don't. [/quote]

:grin:

I have only taken the PSU board off and only had to remove 2 connectors to do that.