09-04-2014, 10:37 PM
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.
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.
(09-04-2014, 06:28 PM)AEB-1 Wrote: 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?