Fuse Blowing on Superamp AE 25

Tubes can short or open internally, as they heat up. There's a big difference between the(typical) 9 volts of a multimeter and the 400 to 600 volt B+, of an average tubed circuit.

Thanks for all the input. I have spent some more time reading and measuring. First, the Superamp does not have any bias adjustment - it is auto-biased - so I don't have to worry about that. Second, I checked the resistors in each bias circuit, and all are reading within a few percent of their stated values. Finally, and I don't know how this fits in, the Superamp has a 1/2 amp fuse at the bottom of the bias circuits (between the resistors and ground) and that fuse is intact. Nevertheless, I have just ordered a new set of power tubes, EL-34's, which the Superamp will take with no adjustments required (using KT-88's now) and when the bits arrive (fuses, and tubes), I'll try them out. Hopefully my problem will be solved.

If your 1/2A fuses are good; it's probably not an output tube problem. Hopefully, just a weakened fuse and not something power supply related.

Yes! The power supply is essentially a single transformer with a centre tap and two output windings. If that has gone it will be a bugger to replace. There are two chokes, so I will check their resistance now.

I think you are putting in considerable amount of money with new set of tubes etc just to find out cause for fuse blow . You may also consider sending your AE25 to : Mr.Dan Wemmer, Service/QC Manager
CARY AUDIO DESIGN
1020 Goodworth Drive | Apex, NC 27539

I sent mine ( AE25 Signature) two years ago and had all problems resolved ,thoroughly serviced , tested plus had all upgrades ( Jensen oil caps , fast acting Diodes ,RCA jack replaced ). It cost me a bit but I found great improvement in sound , extremely happy . Dropped the idea of upgrading to more costly amp !