Is this power transformer bad?

[acorkos]

I'm working on restoring a Magnatone 260. With no tubes installed, the fuse holds. Installing a 5U4GB rectifier tube, the fuse blows after about 10 seconds with just the mains switch on...standby switch is in standby. I disconnected the rectifier socket connections to the first filter cap and output transformer....fuse still blows with power on, still in standby. I'm thinking these rectifier windings are shorted inside the transformer....? Thanks for your help.

Prowess Amplifiers - Magnatone - Schematics - Magnatone 260a

(I tried three different 5U4GB rectifier tubes....same results)

[jmaf]

I'm working on restoring a Magnatone 260. With no tubes installed, the fuse holds. Installing a 5U4GB rectifier tube, the fuse blows after about 10 seconds with just the mains switch on...standby switch is in standby. I disconnected the rectifier socket connections to the first filter cap and output transformer....fuse still blows with power on, still in standby. I'm thinking these rectifier windings are shorted inside the transformer....? Thanks for your help.

Prowess Amplifiers - Magnatone - Schematics - Magnatone 260a

(I tried three different 5U4GB rectifier tubes....same results)

With no tubes on it holds? If so, it isn't the transformer.

[Steve Conner]

Yup, sounds like the rectifier heater winding is shorted to something. Maybe another winding, or the core.

If you disconnect the 5V heater winding from everything else, and test with a DVM, you may be able to read some resistance between it and the chassis, or another winding. This would be more evidence that it's bad, a good one will show "infinity" ohms to everything else. But it's possible for faults like this to show OK under the low voltage of a DVM, but break down when full voltage is applied.

Some 5V windings have a centre tap. You can take the B+ from this to perfectly balance the load on directly heated rectifiers. A classic newbie mistake is to ground it, like all the other centre taps. This shorts out the B+ and blows the fuse.

[Prairie Dawg]

It doesn't sound promising. When you put a tube in you're loading the 5v winding which you were not doing before. I assume you have the correct value fuse installed? Of course, you can always try and use diodes in place of the rectifier tube and that'll unload the winding while still delivering the juice. Worth a try, otherwise you'll likely be shopping for a suitable power transformer.

[acorkos]

the transformer was in fact bad. Looking to buy a replacement. For a test, I swapped in one I had....all the correct taps were there, but the voltages were a bit low. 285-0-285VAC from the high voltage secondary, w/5U4GB rectifier I measured 380 VDC on the first filter cap, and 20VDC on the power tube cathodes. Will a transformer w/300-0-300 get me to ~425VDC high voltage and 26V on the power tube cathode? or should I get one with higher voltage, 325-0-325 or 350-0-350? Thanks fagain or your help!

[Steve Conner]

Does it sound good with the lower voltages? 420 is kind of scary for 6V6s, I think they would last longer on 380.

[jmaf]

Does it sound good with the lower voltages? 420 is kind of scary for 6V6s, I think they would last longer on 380.

I saw the schem and it read 425 VDC for the plates. That doesn't seem to be a 6V6 amp...I am unfamiliar with the Magnatones but wild guess would be 6L6GC as it requests 350 VDC on G2 which would already be high for the anode.

[acorkos]

it's blurry on the schematic, but the power tubes are 5881's

[Chuck H]

If the PT you have now is rated for enough mA you could always change the rectifier tube to a 5ar4 or even a diode rectifier for more volts.

[jmaf]

it's blurry on the schematic, but the power tubes are 5881's

I couldn't find the power tube model anywhere in the schem. Now that you say it's there, there is a 4 letter something to the right of the tubes. The Google also now tells me it's 5881, so I guess that's that.