vox ac30 cc2 blowing fuses and rectifiers sparking????

[sdre]

Hello all!

My vox is having issues. Its blowing fuses. I have replaced the tubes twice this week thinking I just picked up faulty tubes. I noticed that whenever i put a new fuse in, the rectifier tube starts sparking then the fuse blows within a second. If I take the rectifier out the amp runs without blowing a fuse. Any ideas?????

Oh! One more thing. With the amp turned off and with my diode check on my multi meeter there is a short across every output jack. ?????????

Thanks in advance!

[Enzo]

The rectifier tube is sparking? Then I'd replace the rectifier tube and see what happens.

Try this. Leave in the rectifier tube, but remove the power tubes. Does the rectifier still spark?

Certainly it is possible there is a shorted filter cap, or a short in the output transformer. But I'd bet on the rectifier tube first.

[sdre]

The rectifier tube is sparking? Then I'd replace the rectifier tube and see what happens.

Try this. Leave in the rectifier tube, but remove the power tubes. Does the rectifier still spark?

Certainly it is possible there is a shorted filter cap, or a short in the output transformer. But I'd bet on the rectifier tube first.

Sparking- illuminating bluish/purple colors. When it happens the lights in my room dim. Its the second rectifier tube I bought for it. the power amp tubes are the second set this week that I purchased for it as well.

I tried pulling out the power tubes and the sparking still occurred.
Should all the output jacks be shorted?

[Robert M. Martinelli]

Sparking- illuminating bluish/purple colors. When it happens the lights in my room dim. Its the second rectifier tube I bought for it. the power amp tubes are the second set this week that I purchased for it as well.

I tried pulling out the power tubes and the sparking still occurred.
Should all the output jacks be shorted?

Hi sdre,
Are you sure your output jacks are shorted? OT's secondary windings have a pretty small DC resistance....so....do you have a dead short ( bad, indicating a possibly fried OT, though this is a rare event ) or a very low resistance ( normal )? Are you measuring with a jack plug or straight on the sockets? I' m asking this because some output sockets short the signal to GND when a jack plug is not inserted, to prevent the amp from working with the OT secondary open and the output tubes from arcing. Unlike SS amps, tube amps should never work without a load connected.

BTW I wouldn't measure DC resistance with a diode tester, which does not measure resistance, but rather gives a diode's forward voltage reading.

If your OT reads OK you can proceed as follows:

If you replaced the power tubes AND the rectifier, the rectifier is still sparking, and your room's light dim when you turn on the amp ( indicating the mains voltage is dropping due to excessive current, so it looks like a dead short ) I would check for dead shorts in the rectifier/filter caps area and the +B rail, like a bad solder joint, a solder joint touching the chassis somewhere, a bent socket lug or shorted filter caps.

Hope this helps

Best regards

Bob

[Enzo]

Your output is not shorted, the OT secondaries have a very low resistance. With the power tubes removed it wouldn;t matter at all if they were.

If any rectifier fails in there then you have a short across the B+ somehow. The secondary winding of the output transformer is not the issue, we need to check the primary. The center tap goes to B+ and the ends of the winding go to the power tube plates. Are the power tube plates - or their socket pins - shorted to ground? Is the main filter cap shorted?

The output transformer could have a short from the primary winding to the frame, or to the secondary windings. SInce the secondary has very low resistance and is grounded, it becomes the same thing.

[Robert M. Martinelli]

Oops, Enzo, looks like we just had a "simulpost" ( sorry if I dare to use this new word you created, hope you don't want royalties on that ).
Cheers
Bob

[sdre]

The short, which i'm testing with a diode test on the meter, is present no matter which output jack has a load plugged into it.

Is there a way of testing the O.T without power? I do not have access to a variac.

Im not familiar with vox's circuit, do any of you have a link to the schematics on a vox ac30 cc2? That would be amazing.

Im not sure where the mains filter cap is located. Even if I where to know, i'm not sure I can even get to the legs of the cap. They are all so close to the board and it looks like i may have to do some de-solidering to get the board off the chassis.

Enzo I checked for shorts on the power tube sockets and pins 3, 4 and 5 are shorted to ground on all of them. is this pointing to the mains filter cap being shorted? if so where is it?

You have to forgive my terminology as I am new to tube amps.
Thanks guys.

[Robert M. Martinelli]

I strongly advise you to use the proper instrument for your measurements.

A diode tester is NOT what you need for proper resistance measurements.

You need to set your DMM to Ohms to measure resistance.

The reason why you read a "short" ( which I believe to be a small resistance instead ) on pins 4 and 5 is that they're the heater pins, so, if your PT 6,3 VAC secondary has one side or the center tap to GND ( usually done to reduce hum ) you can think there's a "short" to GND there.

Also, as already stated, the OT secondary has a very low DC resistance ( a few Ohms ), but it's ( normally ) not a dead short to GND.

Pin 3 is cathode on EL84s - 6BQ5s ; if the AC30CC2 is cathode biased like the old ones ( these new lookalikes are quite different ) then you should read about 50 Ohms to GND with the tubes in, if it is a "fixed bias" then it's normal for the cathode to be connected to GND as the grid is made negative with respect to the cathode by external means ( a negative voltage from the PS section ).

Whatever the amp, the filter cap(s) are located right "after" the +B rectifier section.

Regards

Bob

[Fragger]

The schematic can be found here.

http://www.mzm.us/AC30CC_Schematics.pdf

[Robert M. Martinelli]

Thanks Fragger,


sdre,
So...the amp has a switchable cathode bias, so, depending on the bias switch position, you should read about 80 Ohms or 50 Ohms between pin 3 ( EL84s ) and GND.

C4 and C5 are the filter caps we were talking about. Be careful and make yourself sure to discharge them before working on the amp.

Good luck

Bob

[sdre]

Guys forgive me but when you say "B+" what are you referring to?

I set the dvmm to ohms and 50ohms is what is reading across pin 3 (with the cathode switch to 50ohms)

[trevorus]

Mine did this once. It was a bad rectifier. I have a Ruby Tubes rectifier in there now, and haven't had problems since. I wondered when it happened to mine how common of a problem it would turn out to be.

[sdre]

But what are the odds that I have purchased two bad rectifier tubes?

[sdre]

Thanks Fragger,


sdre,
So...the amp has a switchable cathode bias, so, depending on the bias switch position, you should read about 80 Ohms or 50 Ohms between pin 3 ( EL84s ) and GND.

C4 and C5 are the filter caps we were talking about. Be careful and make yourself sure to discharge them before working on the amp.

Good luck

Bob

VOXRULES, With a dmm what is the best way to test the caps?

[trevorus]

But what are the odds that I have purchased two bad rectifier tubes?

What brands were they?