72 SVT Blows Fuse

[AEB-1]

I have a blue line 6550 SVT that I totally recapped, replaced anything out of spec, and re-tubed. The amp came up fine and distortion free for about five minutes. Then it blew the 10 amp fuse in my variac. It will power up in standby but blows the fuse as soon as I flip the standby switch. The primary on the power transformer reads less than one ohm. Is that normal? Does anyone have vintage SVT transformer specs?

There wasn't any smoke, smell, or sparks. What's the best approach to isolate the problem?

[Hangman]

Interesting problem, Did it just blow the fuse in your variac? or did it blow the 10A fuse in the SVT too?

the best approach to isolate the problem (in my opinion) is to try disconnecting the center tap of the output transformer. Does your variac have a current meter on it? if not, it would be wise to find a way to see how much current is going into the amp. use a multi-meter and set it to measure the current going into the amp.... whatever you can do.

Then, with the variac still off, turn the SVT power switch on, and turn it off standby. slowly ramp up the current. you shouldn't be drawing more than 1 or 2 amps at idle (I honestly can't remember what the SVT draws) point is.. if the current meter is showing 2 amps and you have the only 10% of the way up, something is still wrong.

So, reconnect the center tap of the output transformer and disconnect the secondary of the power transformer (both the main high voltage supply and the screen supply) BE CAREFUL with those wires. they need to be individually covered... or capped or something so that when you turn the power on, they don't short to the chassis, or to one another. you can't just prop them up or something... inevitably one will fall, and you'll have problems.

SO please, cap them, and separate them. then... same thing with the variac. slowly turn it up and see if the current is running away. with the secondaries disconnected, you should see almost no current at all.

If you still see the current running away, then you likely have a problem with the power transformer.

I have a blue line 6550 SVT that I totally recapped, replaced anything out of spec, and re-tubed. The amp came up fine and distortion free for about five minutes. Then it blew the 10 amp fuse in my variac. It will power up in standby but blows the fuse as soon as I flip the standby switch. The primary on the power transformer reads less than one ohm. Is that normal? Does anyone have vintage SVT transformer specs?

There wasn't any smoke, smell, or sparks. What's the best approach to isolate the problem?

[AEB-1]

Fixed. Thanks to your reply. After reading your reply that suggested removing the OT center tap and also unhooking the Plate and screen supplies from the PT, I got cold feet on removing the center tap as the screen supplies would still be hooked up. So I decided to remove the output tubes. At the same time, I started to question my variac so I checked it out as well. I put an 8 amp slow blow in the variac. Turned everything back on (no output tubes) and voltages were ok, no fuses blow. Replaced the tubes one pair at a time. On the last pair, one tube flashed and the 10 amp fuse in the amp blew. The 8 amp slow blow held. Replaced the bad tube and the amp is solid again.

Cause: I have a small stash 60's vintage Tungsol 6550s. I tested them before putting in the amp. I guess one decided to give up after 5 minutes.

Questions: Is there a law of which fuse blows when two or more identical values are in series. The first one in the variac always blew first. Both 10 amp.

If you only remove the plate voltage, does that cause excessive screen current?

Comment: I forgot how much heat SVT's generate. Warmed my shop by several degrees.

Hangman, thanks for the reply. It got me on the right track.

[Hangman]

Nice work! I'm glad you figured it out. power/output transformers do go bad... but there are many more probable explanations for a fuse blowing. I have yet to replace a power transformer on an SVT, and I"ve worked on quite a few.

as for the fuses in series, I don't know. a fast blow of same or similar value will always blow first of course... but if you have two 8A slow blow fuses in series... I don't think one would be able to say which would blow first. I am not even sure how tight the tolerances are on fuses.