EDIT- POWER AMP PROBLEM?? Reverb Ground Fix- Help??

[alistair]

Hey guys,

just to update- I have spent a lot of time fiddling with the amp (havent got a chance to try this reverb ground yet) but I've realised that most of the noise coming off my amp is actually due to the master 1 knob. On 1-2, the buzz is probably 50% of the amp volume. This is worsened by using the presence and reverb knobs, but they are certainly not generating it.

Has anybody come across this before? I assume it has to be a power amp problem?

cheers!


old post--------------------->

Hi guys,

there was a post up here a few weeks ago about a reverb ground fix. I have a very very noisy reverb circuit on a IIC, and really want to pass this onto my tech to try it, however the info is no longer on this site.

If anyone has saved the post/info, it would be great if you could repost or PM it to me! It would really get me out of a pickle and hopefully get my sick IIC gig worthy again!

Cheers guys,

alistair

 

[stokes]

According to the original poster it involves connecting the ground from the sheilded reverb cable to the ground on the other reverb RCA jack at the chassis.One cable has the shield,the other cable has no shield,but the jack itself has a ground connection.Cant verify if this works,but that is the idea.

 

[alistair]

hey, thanks very much for that.

maybe you could just answer one more question though- I have a lot of noise generated by both the presence and reverb controls (eg the controls on the back of the amp)- and just to test I unplugged the reverb tank and this noise is still generated by the knob.

Do you know if this mod will ground the reverb circuit and pots, or just the actual tank?

thanks very much for the info!

 

[stokes]

If by noise you are refering to noise when you adjust the knob,try cleaning the pot,could be dirty,These two knobs are rarely moved once the setting you like is found so dirt tends to accumulate and doesnt get wiped away by the motion of the internal wiper.Dirt problems are very common in a pot like this.A little contact cleaner should do the trick.

 

[alistair]

no its not dirty pots, its a constant hum. no noise when moving the pots, but the hum gets dramatically louder the higher the reverb and presence are, and mostly the higher the master 1 is.

my tech thinks it could be that the power transformer is so close to the speaker its throwing out noise? but surely that would make all mesa's this loud and I have certainly heard quieter ones.

 

[stokes]

More likely a noisy premp tube.Not really related to the pots themselves but to the higher gain when turning the pots higher.I would really doubt it is the transformer being too close to the speaker.These amps have very high gain and need quiet preamp tubes.A tube that would be quiet in a Fender type amp could be very noisy in a boogie.Could also be loose or dirty tube sockets.If you wiggle the tube in the socket while the amp is on and you get noise this could indicate a problem in the socket,could be dirty or need to be retensioned.

 

[alistair]

I don't think it is related to the pre-amp tubes because the hum is not affected by the volume 1 or lead gain knobs, (eg the clean and dirty gain controls) but by the overall master, which from my understanding controls the amount of power being used by the power amp. I will try wiggling tubes to see if that helps.

Also, I think the reverb and presence controls are located in a different section of the amp than the preamp yeah? like they are inbetween the preamp circuits (After the pre valves) but just before the power amp? The fact that they are noisy really makes me think power amp problems.


Thanks very much for the help!

 

[stokes]

Your master controls the amount of preamp signal that goes into your power tubes,not the amount of power being used by your power amp.Wiggling the tubes wont "help" anything,it is a test to see if the noise is coming from a loose or dirty socket.Your presence control adjusts the amount of NFB from your OT back into your PI,affecting the gain in the circuit.Your reverb circuit has some amount of gain as well.If you have a noisy preamp tube any increase in gain will amplify the noise.If you think it is the power amp pull all your preamp tubes and the noise will still be there.Your reverb,presence and volume controls are all before the power tubes so if it were the power amp the controls would have no effect on the noise.

 

[alistair]

thanks very much for that info, that really helps me understand what is going on.

is it safe to pull preamp tubes and turn the amp on though? should i pull them out then turn it on, or turn on and then pull them or what?
thanks very much for all your help!

 

[stokes]

Better yet,turn the amp on.Set your controls so you hear the noise clearly.Start pulling the preamp tubes at V1 and work your way down the line.When the noise stops that is your culprit.If it is noisy on the rhythm channel,V1 is the most likely suspect.

 

[alistair]

ah your a legend! 99% of the noise is all V2- there is still a touch of hum but i imagine thats just normal for a high-gain amp.

I tried a few different tubes/swapping the tubes around and that did nothing (if anything just mad V2 noisier).

according to my manual, V2 controls the mixer amplifier and effects return. If its not tube related (because switching them around did nothing), is it likely to be something in the amp getting amplified by the tube, or should i replace the tube socket or what?

thanks so much, you have been a great help!

 

[stokes]

Okay,there are alot of things besides that tube that could cause noise in the circuit.First of all are you using new tubes?Or are the subs you tried older tubes that have been in the amp a long time?Put that tube back and pull tubes 3 and 4 one at a time and see what if any effect they have on the noise.Tube v2 is actually the last stage before the PI and everything in the amp goes thru that tube into the power stage,so pulling that tube is "disconnecting" the entire preamp.Lets make sure it isnt 3 or 4 which I think is more likely the culprit.I am assuming V1 had no effect,so go ahead and pull 3 and 4 one at a time.

 

[alistair]

yep i pulled 3 and 4 independently and there was virtually no change. maybe a slight reduction, but it was so small it could have just been my imagination.

An audio technician friend of the family just came along, listened and just said 'dried out cap' and walked out of the room, but it was open at the tech's last week and he replaced all the filter caps and didn't notice anything else so I am not sure what to make of this. both are pretty experienced techs.

 

[stokes]

So if all the filter caps were changed,can I assume you have new tubes or are they old?Try rocking each tube in its socket while the amp is on and see if it produces noises and crackling.That would indicate dirty and or loose sockets.Also try bridging the effects loop as I described before.What did the tech who recapped it say?

 

[alistair]

yeah they are all new JJ tubes- both pre and power, and a balanced tube for the PI. rocking the tubes produces no noise, and bridging the loop doesnt change anything either.

The tech who replaced the filter caps said they looked ok, not too bad and that it didnt look like the other caps had dried out. He basically said the filter caps didnt NEED to be replaced, but in the long run its probably ok for safety and health of the amp.

 

[stokes]

Did the tech change all the caps?Or just the main supply caps?Caps dont always look dried out.What dries out is the electrolyte inside,not evident just looking at the cap.An otherwise fine looking cap could be ready to blow,especially if it is as old as your amp is.If he didnt change the filter caps in the preamp,that is the likely source of your noise.Since the noise stopped when you pulled V2 which is the last stage in the preamp,we know the noise is from the preamp.If he changed only the main caps,your power section is now operating more efficiently,sending more power down the line to the less than efficient preamp filters.If those caps are still the originals I would say that is most likely the cause of your noise.The amp is 20 yrs old,I would get those caps changed.And make sure he doesnt use anything but Sprague or a good quality cap,not the Jap caps, they suck.If all the caps were properly changed there are other sources that would be responsible,but unless you are technically qualified,I would not recomend you try the troubleshooting your self as you would have to work on an open "live" amp and could get yourself dead.If your tech doesnt know how to trouble shoot a noise problem,find another tech.

 

[alistair]

ok that makes a lot of sense. I will get in touch with someone to change the filter caps in the preamps today.
Thanks very much for that! I suppose I just assumed that if I had asked a tech to change the filter caps, then it wouldn't be an issue- maybe I did just pick the wrong tech though. Either way, thanks very much for all the help!

I'll report back when I've had it done.

 

[stokes]

It is a common practice to change all the caps when doing a recap,age is the caps biggest enemy,they are all the same age so it doesnt make sense to only change some and not the others,I am pretty sure you will be good once the rest of the caps are changed.