Transformer problem?

[rfgordon]

OK, so I need the collective wisdom on this one. I modded a Holland Little Jimi amp for a guy by adding a gain stage and some local NFB to fatten up the tone and give it a wee bit more compression.

First, a bit of background. Before I ever worked on this amp, the owner said it was the only amp he'd ever had that seemed to change personality after it was on for 15 to 30 minutes. It would get a bit ballsier and gainier after it had been on for a while.

Now, post mod, it changes as well, only you hear a bit of PS noise after about 15 or so minutes of operation. I did nothing to the power supply other than tap into the V1 node to add one triode to the circuit.

I must also add that this amp uses a distributed PS filter network, with the filter caps for each tube on the board at that point. Some have commented on this board that they thought this layout method could be noisy. Personally, I don't have enough experience to weigh in on it, but I thought it was worth mentioning here.

My suspicion is that this amp's issue lies within the PT, and once it gets warmed up, internal resistance drops and voltages on the secondary go up. I also suspect that my mod, with a bit more gain, is amplifying noise that was already there. I haven't pulled the chassis yet to let it run a while an check the voltages, but I wanted to test the waters here first.

What do y'all think?

[unklmickey]

... adding a gain stage ...

...you hear a bit of PS noise after about 15 or so minutes of operation. I did nothing to the power supply other than tap into the V1 node to add one triode to the circuit...

...I also suspect that my mod, with a bit more gain, is amplifying noise that was already there....

...I haven't pulled the chassis yet to let it run a while an check the voltages...

i suspect you are right, that you are now hearing noise, that was previously there, because you now have more gain.

my suspicions, as to the source of the problem would be with filter caps (more likely near the preamp end of the PS chain), rather than the PT.

i have often seen electrolytics open, only when they get warm.

i'm going to guess that once you pull the chassis out, the offending cap (if this is the problem) might not get warm enough to fail.

you might need to apply some heat via a hair dryer, to cause the symptom to occur.

there are other possibilities, but my instints are telling me to look here first.


cheers,

unk