What are symptons of a dying transformer on an old Marshall?

[Drew]

I have an old Marshall that was originally modded back in the late 80's. It has not been sounding like itself for a year or so. What would be I be hearing if the transformer was going?

 

[Digital Jams]

I have not experienced that but damn that would suck, those older transformers just seem to have something the new ones do not.

 

[glpg80]

the chances of you having a bad transformer without abusing it are very slim.

i would suspect its due for a mains/screens cap-job overhaul IMHO

 

[Drew]

Actually, the original transformer was swapped out in the late 80's on it's first mod--long before I owned the amp.

 

[thiswaythatway]

I had an output transformer crap out on an old Plexi i had. When that was going i was getting a steady reduction in volume. When i had a new trasnformer put in the amp actually sounded as good as it did with the old one.

 

[glpg80]

how hot is the PT getting?

it should get luke-warm AT MOST - if it is too hot to keep your hand on it then something is not right - i would suspect it is being driven hard by either ground loop problems, bias circuit issues, bad filter capacitors causing a false load, etc.

i seriously doubt you have a bad PT - they do go and all transformers do not have a limited timeline but they have to have a reason to be pushed beyond specs. either the paper insulations on the OLD models deteriorate or the windings become shorted due to one or some of the problems listed above.

 

[Drew]

how hot is the PT getting?

it should get luke-warm AT MOST - if it is too hot to keep your hand on it then something is not right - i would suspect it is being driven hard by either ground loop problems, bias circuit issues, bad filter capacitors causing a false load, etc.

i seriously doubt you have a bad PT - they do go and all transformers do not have a limited timeline but they have to have a reason to be pushed beyond specs. either the paper insulations on the OLD models deteriorate or the windings become shorted due to one or some of the problems listed above.

I haven't reached back there to tell about the heat--but I will.

The symptoms I have are a lack of power--it's a (modded) 100 watt Marshall and even with new power tubes installed and biased in the warm range, with the master volume around 11:00, it's loud but I can still be buried by a loud drummer and bass player. A year ago, at that volume that was not the case. Nothing else has changed in my setup--same fx, same guitar etc.

Other symptoms include a further reduction in power when I use the fx loop. That also didn't use to be the case.

The last symptom is over the last few months I have had to lower the presence and treble knobs and now run them completely off. A year ago I used to run them both around 11:00 to 12:00

 

[glpg80]

when was the last time the amplifier was re-capped properly.

does the bias drift after being on? if so, are you sure you are not biased too hot for the tubes you are using to take into account tube drift?

if you bias too warm you can bias the tubes into an always on state after hours of playing which will cook your OT primary - leading to the same problems listed as well. if it was the OT problem im mentioning, you would also have the amplifier go through tubes like nobodies business.

 

[Drew]

when was the last time the amplifier was re-capped properly.

does the bias drift after being on? if so, are you sure you are not biased too hot for the tubes you are using to take into account tube drift?

if you bias too warm you can bias the tubes into an always on state after hours of playing which will cook your OT primary - leading to the same problems listed as well. if it was the OT problem im mentioning, you would also have the amplifier go through tubes like nobodies business.

I haven't changed the caps on the amp in the 9 years I've owned it. I have no idea if they are original or not.

I ran the same power tubes (old Mesa branded Svetlana's) in that amps for 4 years--probably 150 gigs and who know how many hours of rehearsals. It was starting to sound a little off so I installed new power tubes--whatever Mesa sells for EL34's these days. The amp didn't sound right so I got some Winged C EL34's and had it biased to a "warm" setting (I think it was set at 32ma at 500 plate voltage but I'm not positive). The amp still seems off with the symptoms I described above.

 

[stratotone]

How old is the marshall? My 74 and 72 both had daly caps stock - blue tall caps. If you look inside the chassis, pay attention to the cap terminals (where they have wires soldered to them). Mine had a red dye on the solder joints, meaning they were original. If your amp is over 20 years old and on the stock/first set of caps, I'd replace those first.

Pete

 

[glpg80]

i believe you need to replace all of the filter and screen capacitors. if you have not done it in 10 years, and the amplifier was modified back in the 80's, i would say the stock filter capacitors are in that amplifier and are shot. look for a red mark on them or red colour on the solder.

after a recap job and a proper bias i believe you would be surprised.

i would also do the cap job as soon as possible, as the filter design and the secondary impedance load of the transformer is designed to be a capacitive load - if the capacitors are solidifying, the impedance is shifting, which putting strain on the PT every time you have it cranked.

this could lead to overheating, higher strain, and eventually frying the secondary of the PT.

 

[duesentrieb]

Measure it, Drew. Do you have a DMM which can read inductance (Henrys) ?

 

[Greazygeo]

Youve had these el34's over a year? If you play alot at band volumes they are probably whipped. El34's don't last very long if you use the amp alot.

 

[JerryP]

The transformers are one of the last things to look at in an amp when there are problems. One symptom of a failed OT is low volume and it sounds like you're playing thru a fuzz pedal and it won't gte a clean tone.
Jerry

 

[MrDan666]

I would also suggest having a tech put in new filter caps, if they are infact original. Most old Marshalls that i have worked on, sounded like crap if the original filter caps were still in the amp. A re-cap can make a big difference, especially if the originals are failing.

It's hard to say what it could be though, without it being on the bench to check out in person. I would get it to a tech and let them give it a thorough check up, so you can atleast see what action to take next.

 

[stratotone]

i believe you need to replace all of the filter and screen capacitors.

Screen caps? Why? They aren't electrolytic. You mean filter and bias and all electrolytic caps on the board, right?

Pete

 

[MrDan666]

i believe you need to replace all of the filter and screen capacitors.

Screen caps? Why? They aren't electrolytic. You mean filter and bias and all electrolytic caps on the board, right?

Pete

The filter caps for the screens on old Marshalls are electrolytic, usually two 50uF/50uF cans in series

 

[stratotone]

i believe you need to replace all of the filter and screen capacitors.

Screen caps? Why? They aren't electrolytic. You mean filter and bias and all electrolytic caps on the board, right?

Pete

The filter caps for the screens on old Marshalls are electrolytic, usually two 50uF/50uF cans in series

I got my caps confused - once I hear 'filter' I don't hear 'screen'. : ) I was thinking of the resistors, that's what I get for posting while at work.

Pete

 

[titanamps]

low volume is the primary symptom of a bad OT. they almost never fail on their own, though...OT failures are almost always preceded/caused by some sort of power tube failure or playing the amp unloaded (ask me how i know about that last one )

 

[Telephant]

The transformers are one of the last things to look at in an amp when there are problems. One symptom of a failed OT is low volume and it sounds like you're playing thru a fuzz pedal and it won't gte a clean tone.
Jerry

This is exactly like what my old TSL sounded like when I blew the OT.