Mk III hum issue

[okgb]

Is there a better place for tech problem questions?
I have a mk III simulcass that I hadn't played for a couple of months, I power it up to play and it has this broad hum that sounded like a fluorescent light ballast , not a 60 cycle ground thing
after a minute or so it dies & blows a fuse. I check it no tubes, new fuse on a lightbulb limiter fine, check it with good tubes, fine but it still has the hum and I didn't leave it on long enough to see if it would blow a fuse. To add it had been recapped for the big electrolytics in the psu years earlier.
Anyone experience this before? Something common ?

 

[okgb]

Follow for anyone interested, It has been suggested to check the P.I. coupling caps for failure. Another well known source [ I trust ] recommends changing the signal caps like the coupling caps from 400v to 630v , obviously not the cathode caps.

 

[user 41014]

Follow for anyone interested, It has been suggested to check the P.I. coupling caps for failure. Another well known source [ I trust ] recommends changing the signal caps like the coupling caps from 400v to 630v , obviously not the cathode caps.

Mesa changed the specs from 400 to 630V for the two PI coupling caps due to over-voltage. These are the culprit, usually one of them for many of old Mark red plating.

I'm not aware of any other 400V orange drops that require the upgrade. That said it's simple and quick to check all caps for shorts. Open fails prevent AC as well as DC from passing and not as likely to cause amp and tube damage.

 

[quad48uF]

Follow for anyone interested, It has been suggested to check the P.I. coupling caps for failure. Another well known source [ I trust ] recommends changing the signal caps like the coupling caps from 400v to 630v , obviously not the cathode caps.

Interested to hear how this turns out. I experienced a similar issue with my Purple Stripe DRG before Mesa fixed it.