DSL50 Hum

[shortfuse]

My DSL 50 has started a very annoying hum almost all of a sudden.  When I turn it on I can actually hear the PT which I never did before when the stand by switch is thrown into the on position it get just a bit louder.
It almost does not matter where I stand or position myself with the guitar the hum is constant, worse in some positions better in others.  But persists even 20 feet away.  But the hum is not as bad when the toggle switch is in the middle position but in bridge or neck pick up mode alone it is unbearable at idle.
Question #1 can using the amp set at 16ohm and using a cab that is 8 or 4 ohm take out the PT?  I know it can take out a OT.  Sounds like a stupid question but the band room we are rehearsing at now has an older marshall JCM800 412 (single Jack) there I have been using says 16 ohm but I now have 2 amps doing the same thing after using them there.  I will post amp #2 after I resolve this one.  Also going to bring an Ohm mete this Wed to check the cab.  I originally thought it was my guitars (as I posted in the guitar section 1st) but took the guitars to GC (I am working out of town and did not have access to my other amps) and plug into several different amps and all was fine with the guitars P90's a bit noisy as normal les paul quiet.
Question #2 can a bad OT cause the PT to hum audibly and in turn create the noise I am getting.
Can a bad cap create this same problem?  I have somewhat checked the amps grounding points but not real sure which direction to head. 
Any suggestions on where to start would greatly be appreciated.

[shooter]

If your problem is there with *nothing* plugged in then keep that a constant, don't plug in till you figure it out.  If you can *hear* the tranny humming, can you "feel" it humming also?  Pull all your tubes and see if it still does. Hum  in the power section is usually too much current, or loose mechanical

[shortfuse]

So I am clear there are 2 hums 1 the actual audible hum at the trans itself and 2 the hum from the speakers.  Hum or audible hum form the transformer itself is there all the time.  Yes can physically feel trans hum.  Hum audible from the speaker is only if there when a jack is plugged in to the input jack.  It is louder as volume is turned up.  I did not try just the vol or gain to see if one affected it more than the other.  But it does not make a difference if I am on the red or green, Ultra or Crunch channels.  Only variable is if the guitar has the selector switch in the middle position it is 70% quieter, put on just the bridge or neck pick up and hums like a SOB.  Guitar not the issue fine in other amps.
Yes all tubes in, trans hum greater than with tubes out.  It is not the tubes tried tubes from another good working amp and vise versa fine in the JMP hum is DSL.  I disconnected the heaters and the physical transformer hum was like when the tubes are removed.  I then checked the 100ohm resistors and they read 52 ohms each.  Do i need to lift a leg of the resistor to get the correct reading on those?  Checking the heater wires there should be no reference to chassis ground?  Meaning I should have no voltage reading from either heater wire to chassis ground with the wires removed from the circuit and only connected to the PT.  I should only read 6v from across the 2 wires themselves at the trans?  I have not done this yet but was going to do some additional checking tonight.  I am going to pull voltage readings tonight as well.  Some of these will be tough as they are radial Caps but I do have the correct schematic downloaded now.  I also see BR-1 in the heater circuit as well?
My experience with the DSL's is it is most likely a Transformer but I have never had an issue on the PT side usually the OT side.  I think I am very fortunate to still have the stock OT in this amp.

[eleventeen]

If all of a sudden you have TWO SEPARATE AMPS doing the same thing, unless some massive freak power spike came in and lunched both amps, I would say it's almost a certainty that the problem is NOT with your amp(s)!


Did the owner of the studio install some new light dimmers? Yes, even in another room. Is he or another party working on something there where they have fluorescent lights as work lights?


If you have an amp > spkr impedance mismatch, usually this is not damaging unless your are running the amp full-out loud as it can get.


If the mounting bolts on a transformer get loose, it is possible for the tranny to produce a hum via eddy currents transmitted through the chassis and cabinet. That's really a long shot.


If I am understanding your description, my vote is for noisy dimmers (or some other electrical item) installed somewhere in the building.

[shortfuse]

Eleventeen I agree with everything you said but the common denominator is the 2 amps were used in that band room and they now do the same thing no matter where I plug them in.  Home, Apt, band room.  Coincidence Possibly, Attacked by black forest gnomes maybe, or maybe I am just getting wrapped around the axles that I have 2 amps with problems and assume they are related, maybe they are not.  I am trying to get myself unwrapped, methodically decipher and attempt to fix the problems.  DSL is 1st

Shooters post kinda steered me in the right direction, I think and here is what I have found so far.

I have heater voltage 2.9v at pins 4,5 & 9 of V3, V4, V6 & V7 (V5 & V8 are not used in the DSL50 I am using designations on the schematic).  V1 & V2 do not get heater voltage 0 volts but the elements glow???  I do have voltage at F1, F2, & F3 fuses on the board.
Also I think I have extremely high voltage at V3 pins 1, 2, 3.  See attached voltage readingshttp://el34world.com/charts/valve/ValveData.php?e=view&f=18267

Looking at the schematic heater voltage for V1 and V2 pass through the bridge BR1 and the bridge is fed from F1 and the heaters circuit from V6 fed from F2 and heater circuit V7 fed from F3.  To get to the Bridge is a chore and I have to remove the upper and lower boards.  Maybe just the tin cover on top going to take a break for a while.
 

[2deaf]

I have heater voltage 2.9v at pins 4,5 & 9 of V3, V4, V6 & V7 (V5 & V8 are not used in the DSL50 I am using designations on the schematic).  V1 & V2 do not get heater voltage 0 volts but the elements glow???

This seems perfectly normal if you measured AC to ground.  V3 and V4 have pins 4 and 5 hooked together and have AC heaters so that there should be 6.3 volts AC or so between 4 and 9.  V1 and V2 are wired the same, but have DC heaters.  They should have 6.3 volts DC or so between 4 and 9.  Don't tear the thing apart to get at the bridge if it is working fine.

Also I think I have extremely high voltage at V3 pins 1, 2, 3.

It is a cathode follower, so those voltages are normal.

[shortfuse]

2deaf you beat me to it I was just about to post that I was a dummy and tested the heater circuit voltages wrong.  I had just watched a You Tube Vid on testing Bridge rectifiers and yes Bridge is working fine ac heaters to V3, V4 V6, &V7 and Dc to V1 and V2 as you stated.
I never checked an amp with mixed ac dc heaters before.  And getting to the bridge and caps were not so bad 8 screws and the tin came off the top did not have to disconnect the board or plugs on board.

Guess I will have to read up on cathode followers as well.

I wish I still had my 2nd dsl to swap PT and or OT to eliminate that question. 

Can I utilize a listening tool to locate the audible hum noise within the circuit.  The same way you can see where the loss of signal is between stages?  If I get noise at V1 it would have to be something before V1 or after PI it would be in the power section, or am I reaching here?

[shortfuse]

Ok last thing I did tonight was plug directly into the effects loop with the guitar hum not as bad but still there.  It also does not have the gain back there but the same noise is still there would that put it into the power section?  Plugging direct into the loop bypasses the preamp puts it straight into the PI correct?

[shortfuse]

Well I finally found out what the problem with the dsl is.  After working on it giving up, working on it and giving up, I finally had some time off from work and got back into it.
Eleventeen was correct the 2 amps with the same type hum were un-related.
The DSL, when I got back home from being on the road I took it to a friend that works on these DSl and TSL's.  He told me that the main board I have in the amp is the old original ones and the board has deteriorated to the point where I have voltages beading through the board and creating hum.  He has ordered me a replacement board that marshall made to correct thses problems and is going to install it next week.  After he told me and showed me what was going on I did some more research and the old boards are inherit to this problem and or loss of power through bias drift.
Probably should have taken it to him in the 1st place.
The other amp was a Peavy 50/50 power amp I use with my rack unit.  It had a bad cap and was arcing from a lifted trace next to the cap to the cap.  Re-capped and re-tubed the amp, bypassed the burned trace with hard wire and that amp works just fine again.
Live and learn.  Gained some new knowledge with the DSL but those amps IMO are a PIA to work on.