Fuse Blowing on Superamp AE 25

It could have been a power surge. Why are you mail ordering a simple 2 amp fuse? Don't you have a radio shack or hardware store near?

You may have had a power tube short out.

Thanks for the idea. I will pull out all tubes before I put in the new fuse, then insert one at a time. Would that cause a problem?

No radioshack around here. No one I have shown the fuse to has ever seen one. Not an audiophile town.

2 caveats w/new tubes, 1)be sure to turn bias screws back (counter clockwise) to minimum before you insert new tubes and turn on. 2) Although it was not a problem for me with my Cary amps, it is not unusual for a power tube loss to be accommodated by a blown out resistor in the bias curcuit. You should note this when you bias new tubes if it exists.

Hope that solves your problem.

I will still be waiting for fuses for a while, but I have checked all tubes with an ohm meter, and checked to fuses on the current source tubes. Nothing unusual. of course, it may be that the low voltage of the ohm meter may show nothing that occurs at high voltage. Still, at only a few hundred volts, there wouldn't be any significant dielectric breakdown, I would think. Am I wrong?

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 !