Rectifier Arc, Bad tube?? help

[Macshaft]

Fired up my 2 hole rocket with no tubes and checked out voltages @ 122.5vac into a Hammond 370FX 275/0/275 150mA. - heater voltage 7vac Rectifier 5vac was 5.4vac - primary was 280vac to grnd. on both taps .after installing my start up tubes and powering it back up , 20 -30 seconds later it takes out the fuse (3a). No smoke , and nothing is hot . I had used an old fuse holder on this build so I suspected that . No , a second fuse and powered up again revealed the 5AR4 was arcing over . Is this just a bad tube ??? 280vac??

[UR12]

It is very possible to get a bad rectifier right out of the box and that is the way it usually manifests itself. I have had really bad luck with Sovtek 5ar4s. I tried 2 brand new ones in my Rocket build (I only owned two). One lasted about an hour and the other started arcing as soon as I installed it and flipped the standby switch. I threw in a NOS GE and it has worked great ever since. I now buy JJs

[M Fowler]

Wow, I pretty much use nothing but Sovtek 5AR4 and some GT with no problems but like you said Dana it can happen.

Mark

[Richie]

got 2 "NEW" valve arts gz34s bad out of the box for a 36 watter. looked like chinese 5ar4s.. I have better luck using the JJ myself for the current made tubes..but the NOS ones are really the best.

[Macshaft]

Hey thanks guys , I`ll try the one out of my first Rocket(Ellen Rachel) . That one is a JJ as well . Both bought from a local guy , maybe he`ll give me another one . Did not think a wiring issue would do that , and the PT voltages measured were within a few volts of the other Rocket . I think a 5AR4 is good for 550vac a side?? .

Thanx again , gotta fire up before the kids go to bed

Cam

[Macshaft]

5AR4 outa my other Rocket does the same thing in this amp . about twenty seconds or so after power up , tube arcs and takes out the fuse .Even before the standby switch is turned on . I used an 80uF Mallory TC-80 in the first tank . The other Rocket Has 2 - 40uF in this spot . Could I have a bad Cap ??

[UR12]

Since it fires up with no tubes installed then you might want to try a different set of tubes. 20 seconds is about the time it takes for output tubes to come up to temp and start drawing current. Sounds like something is shorting out.

[Macshaft]

Posted: Tue Apr 07, 2009 10:01 am Post subject:

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Since it fires up with no tubes installed then you might want to try a different set of tubes. 20 seconds is about the time it takes for output tubes to come up to temp and start drawing current. Sounds like something is shorting out.

I have not fliped the standby yet , so the only thing powered up is the first P.S. cap(80uF) . gotta be the cap , socket termination is good , no loose strands . I don`t have a meter that will measure this large cap . Maybe I will try a different cap . I have some 68uF`s i can try .Warm up time would apply to the 5AR4 as well I guess??

[Alexo]

Technically, an 80uf cap is too big for a 5AR4 to see right out of the gate. Usually, this sort of thing is not a problem, but you never know - I'd try a 40uf cap, or smaller, just for troubleshooting purposes.

[Structo]

Plus one, the 5AR4 has a max first filter value of 60uf.

That may or may not cause issues since you are only over spec a bit.

But it will be harder on the tube.

[Macshaft]

Plus one, the 5AR4 has a max first filter value of 60uf.

That may or may not cause issues since you are only over spec a bit.

Did not get enough time to check unloaded DC voltage on the first cap .
A.C. to the 5Ar4 is 560vac or 280vac a side . Maybe I am over the Mallory spec of 450vdc.?? and the cap is shorting ??

[Alexo]

I think you should be alright with a 450 volt cap with those kinds of voltages.

[Andy Le Blanc]

The size of that cap might be the issue. It'll make a current spike that exceeds
the peak rating on the rectifier plate. Ive seen plenty of shorts, and they
dont usually take out the rectifier.

[Macshaft]

I guess there is no need to ground the center tap on the 5vac , otherwise it makes the Rectifier tube make big arc across plates (ouch) . Stupid newbie mistake . aarrrgghhh!!!

[Structo]

The 5v tap has a center tap?